AN ANALYSIS OF METHODS FORMEASURING DIPLOPIA FIELDS ANDAN INTRODUCTION OF A DEVICEFOR MEASUREMENTS OF THIS SORT

BY Albert E. Sloane, M.D.

ONE of the most exciting chapters in the history of ophthalmologypertains to the work on motor anomalies of the eye. The studentrepeatedly meets the same names which are like giant guide postsindicating and directing in logical steps the accumulating knowl-edge in this field. When one considers the limited technologicalequipment available to these men and then realizes that thenewest of our techniques were basically described before the be-ginning of the century, an appreciation for their stature is ob-tained. Certainly the names of Alfred Graefe, Hirschberg, Mauth-ner, Bielschowsky and Hering give Germany its share of giants.In like manner, England has produced its Maddox, and Franceits Javal, and Landolt with his genius for establishing method.And no less do the early great names stand out in this country:Savage, Stevens, Duane, Howe, White, and now Lancaster. Asthe results of the teachings of these masters many new names willappear especially in this country, where in the past twenty years,leadership in the field seems to have been established. Contribu-tions have not only stemmed from ophthalmologists but also andin a notable degree from physiologists and mathematicians.

It may be stated that when the actions of the various extra-ocular muscles are understood, there will be no difficulty inevaluating the diplopia fields by any of a number of differenttechniques. Since the middle of the nineteenth century variousauthors have described methods for the measurements of diplopiafields. It would appear that an analysis of these various methodsis indicated so that the reader may decide which technique isapplicable under average circumstances. The importance of ease

Measuring Diplopia Fieldsin carrying out a test, the duration of time required to make it,and the comprehensibility of the test would tend to make certainprocedures preferable to others.

Since the actions of various ocular muscles had for a long timebeen an enigma to the average ophthalmologist, many authorshave attempted to make their tests such that a minimal workingknowledge of the individual actions of the various muscles wasrequired. One only had to follow a particular rote and to analyzethese findings by a few simple rules, in order to arrive at a properdiagnosis. Many of the men who devoted considerable amounts oftime to devising such tests were also pioneers in educating oph-thalmologists in a better working knowledge of motor anomaliesof the eye.The earliest works stressed the importance of observing the

movements of the eye in order to identify the lagging musclesin cases of diplopia produced by paresis.

Mackenzie (34) in 1833 stated in speaking of total third nervepalsy: "If with the finger we lift the upper lid in such a case, andtell the patient to look to the ground we see that he attempts todo so but is utterly unable to accomplish his intention." In thefourth volume (35) of his book published in 1854 he notes:

by passing the finger from side to side and upward and downward be-fore the patient and desiring him to follow it with his eyes withoutmoving his head, we detect which eye and which muscle is affected,even when the loss of power is very slight. In general it is the ad-ductor or abductor of one or the other eye that is defective so that theorgan can not be completely inverted or everted. Thus diplopia is ex-perienced in general when the patient endeavors to look straight be-fore him and always when he tries to look in the direction opposite tothe deviation of the affected eye. If the levator oculi for instance ispalsied, diplopia occurs whenever the patient attempts to look upward.In cases of palsie of the fourth nerve or of the inferior rectus the rotarymotion of the eye will be interfered with as may perhaps be detectedby laying hold of the patient's head and bending it from shoulder toshoulder.

Carter (8) in 1876 described the use of a colored glass and anelongated fixation testing object as for example, a roll of whitepaper; thus one could see the inclination as well as the doublingof the images.

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Albert E. SloaneSoon it became apparent that evaluating the character of the

subjective diplopia field and proper identification of the imagebelonging to each eye was a more satisfactory method. No doubtthe influence of accumulated knowledge and clinical informationwas largely responsible for the acceptance of this method. It setthe background for the better understanding of diplopia. Thus itappears that there came a shift in emphasis from that of evaluat-ing the lag in the paretic muscle and then explaining the diplopia,to that of first interpreting the subjective diplopia and thenidentifying the involved muscle. The most popular methods ofmeasuring diplopia fields became those employing s'ubjective re-sponses by observing the doubling of images and identifying theimage of one or both eyes by the use of an appropriately coloredfilter. The simple red glass 'test'whereby the observer watches amoving light and describes the field of greatest diplopia is perhapsthe oldest and still most widely used. Hering's law (io; 1879) ofequal and symmetrical movement of the two eyes did much toexplain the difference between primary and'secondary deviation.

Hirschberg (20) was one of the early investigators who meas-ured the angle of devia'tion. He employed a wall painted in cali-brated degrees of arc, using a red 'glass and plotting his findingsin terms of separation of images for the various positions tested.He first worked at a two meter distance, then used one meter. Hisassistant,'Ohm, later indicated his findings in a similar manner(Fig. i6). Hirschberg noted that the method "is based upon thepatient's subjective measurement of the reciprocal distances ofthe images. To render the results more apparent, some authorshave introduced diagrams or divisions."A division into numbered squares is described in "Mr. Bader's

Text Book (21; 1868)" and "a similar'division of a wall intosquares of 0.25 meters in length was adopted by Heiberg (22) ofChristiania in 1872." Hirschberg in 1875 improved on thesemethods by using an angular division rather than the longitudinaldistance an "innovation [that] is not a trifling one, as it may atfirst appear, but rather essential for our purposes, as in perimetryof the visual field." He commented on the method of testing fordiplopia with prisms: "By the use of prisms two sources of errorare introduced. The first is a faulty position of the prism; the

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Measuring Diplopia Fieldssecond and more important is the tendency to a fusion of the twoimages." In experiments he showed that "at first the deviation isequal to 30, after a few seconds 20 and shortly it becomes zero.That is to say, single vision is obtained by the tendency to thefusion of both of the images."

Landolt (31) in his book published in 1879 refers to the use ofa red glass before one eye as being most important. (It is interest-ing to note how many of the leading ophthalmologists used thered glass test in evaluation of diplopia purely as a qualitativemethod and ignored the quantitative aspects.) This simple test isreferred to in most of the standard (43) textbooks up to recentyears; it appears that the differences described only occur in themethod of depicting the nature of the diplopia. It is a good testwhen one is dealing with an intelligent patient, but only to thepoint that one gets a qualitative rather than a quantitativemeasurement.

In 1879 Landolt (32) described a technique for measuring theactual amount of diplopia. He set up a tangent cross made up oftapes calculated at five-degree intervals for a three-meter range.A red glass was placed before one eye of the patient. The positionof the red image indicated directly the angle of the squint by itsposition in relation to the white image in distance of tapes. Thepatient's head was kept still and then the candle was moved inthe different positions of gaze. At this point another importantstatement was made by Landolt:

We come now to an important method of correcting and at the sametime measuring strabismus; that is the use of prisms. In lookingthrough a prismatic glass we find that it produces an effect analogousto that of the deviation of the eye; that is to say it causes displacementof the objects looked at. It follows from this that employed in a con-trary sense that prism would neutralize the diplopia caused by the devi-ation of the eye.

Thus over seventy years ago, were described two techniques notunlike that practiced by many ophthalmologists at this time. Thedeviation was measured (i) by interpolating the linear deviationinto degrees of separation of images; (2) by neutralizing thediplopia with prisms.

In i893 Ziegler (64, 24) described the use of a scale calibrated

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in arc-degree intervals containing both a vertical and horizontalcomponent so that diplopia could be measured directly. Thisprotractor-like device was primarily used for the measurement ofthe power of prisms.Maddox (38) in his memorable work, Tests and Studies of the

Ocular Muscles, published in 1898, described the use of a tangentscale and his glass rod. A cross was constructed containing a space

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;1O 9 8 7 6 5 4 3 2 0 2 3 4 5 6 7 8 9 1042 40 38 36 34 32 30 28 26 2422 164 2 0 8 6 4 2 0 2 4 6 B 0 12 141618 0224 26 2 3 3 36 M 4 42

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FIGURE 1. MADDOX TANGENT SCALE (LYLE AND JACKSON)

for a central light, and degrees of arc were then indicated on thecross calibrated as a tangent scale for five meters. The patient wasseated five meters away and readings were taken with the head innine different positions. Unlike Landolt, who moved the test ob-ject, Maddox moved the head in order to get the effect of rotationof the eyes. A Maddox rod was held before one eye and the patientwould indicate where the streak passed through the scale; therod was held appropriately to get the vertical deviation and wasmoved go9 to get the horizontal deviation.Maddox stated:

It is always a good plan, when time permits, to take as accurate meas-urements as we can of the diplopia in all nine areas of the motor field.By repeating such measurements from time to time an excellent idea

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Measuring Diplopia Fieldsof the progress of the case is formed and difficulties in the diagnosismay be cleared up.

Maddox also specified a special way of indicating the degreeof diplopia. He suggested that one make a square for the positionof gaze that is to be depicted, place a dot in the center of thissquare to indicate the position of the true image, then place asecond dot to indicate the false image. A faint line is drawn hori-zontally and vertically to connect the two dots, indicating thedegrees of displacement, and wherever possible the inclination ofthe rod is drawn, if it is not seen perfectly vertical or horizontal.One of the greatest influences in the measurement of diplopia

was exercised by Duane (9). He advocated using the reverse (white)side of a curtain tangent screen; the other side (black) was turnedtoward the patient, who sat thirty inches away with his eyesopposite the center. A red glass was placed before the right eye, anda light was placed in six cardinal positions. The patient turned hiseyes, not his head. The location of the images was then indicatedwith pins, thus noting the displacement in terms of degrees. Onthe white side of the curtain one could evaluate the diplopia.Duane (15) suggested indicating the image of one eye as a ring, theother eye as an X. The circles on the tangent screen were at inter-vals of five degrees, calibrated for a distance of thirty inches; diam-eters passed through the center of the circle of fifteen-degree inter-vals; and the chart was further subdivided into a checkerboardpattern of squares, each square being approximately one degree. Inthis way it was easy to transfer the findings on to a reduced chartfor record purposes.

Stevens (56) in i906 introduced a perimeter method of measur-ing the diplopia. A rider which could be moved on the arm ofthe perimeter until it seemed to obstruct the image of the deviat-ing eye would be moved to such a position, and the arc degreesof diplopia could then be read directly. That is, the patient fixedthe center of the perimeter and indicated the direction of thesecond image. Then the obstructing rider would be moved intothis position, thus appearing to be where the second image was.Stevens condemned this method himself on the basis that it was"clumsy, rather troublesome and above all not of great utility."

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He also noted that prisms when "properly used gave the mostuniform result, although no measurements are entirely satisfactorysince they may vary from hour to hour." While deprecating hisquantitative-perimeter method and paying a somewhat reservedtribute to prisms, he stated that he preferred his Tropometer,because "this offered a perfect means of measuring the rotatingability of an affected muscle. By this means one could estimatethe real extent of the disability."The Tropometer (Fig. 2) presented in i894 was an instrument

designedto measure the various rotations of the ey..I ws e.:

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FIGURE~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2. ..........TE

of a telescope containing a forty-five degree prism or mirror atits objective end so as to reflect at right angles. The images of theeye were seen against a scale. As the eye was made to move, thedeviation could be expressed quantitatively by reference to thescale. Stevens was the last of the great contributors to give pref-erence to a measure of rotation of an eye as the best index of thepresence of a paresis. It is interesting to note that several yearslater Weeks (58) commented: "Instruments for measuring limita-tion of movement are not as reliable as the measurement of the

Albert E. Sloane642

Measuring Diplopia Fieldsfield of binocular vision with the candle which gives the amountof diplopia in prisms in different directions of gaze."

Tiffany (57) in 1902 used the light and the red glass at a distanceof fifteen to twenty feet from the patient. Even at this distancehe permitted no movement of the head, but instructed the patientto follow the light, thereby denoting the position of the imagesand separation. Thus he depended on a qualitative rather thanquantitative analysis; on the other hand he measured the degreeof strabismus with the perimeter. In spite of the valuable con-tributions of Landolt (31,32,33), Maddox (38), and Duane (9),many of the authors of the day continued to describe diplopiafield testing in a qualitative rather than quantitative way. Nettle-ship (41) used a candle and a red glass at six feet, moved the light,and merely recommended a different way of recording the find-ings.Edward Jackson (25) in 1goo described the red glass and a

flame for a diplopia test and saw that one "might use a handle ofthe reflecting ophthalmoscope to arrive at the decision as towhether there is a torsion type of deviation." De Schweinitz (53)quotes Duane's strong influence, but gives no indication of theimportance of quantitative measurements, simply referring to thered glass and the candle. Parsons (48) deviated a bit by suggestingthat the lighted candle be kept four feet from the patient andthe patient move the head rather than the light, and indicated aslightly different way of recording his findings. All the recordingsup to this period were placed with the right side on the right sideof the chart, the left on the left side of the chart.

Bielschowsky (29), an authority in the field of motor anomalies,was made Professor of Ophthalmology and Director of the EyeClinic of the University of Marburg in 1912. His influence onthe ophthalmologists of the United States was tremendous, as aresult of his many lectures and demonstrations on ocular motility.Many of his audiences were composed of ophthalmologists inade-quately trained in the basic knowledge of the eye movements, andhe did much to bring enlightenment. His great contributions areof increasing significance since they paved the way for renewed in-terest in such related fields as orthoptics and made possible a deeperappreciation of the work of our own ophthalmologists such as

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Albert E. SloaneDuane, Stevens, White, Savage, Howe, and Lancaster. He no doubtinspired many to work in this field.

Bielschowsky (3) recommended and used the red glass and theMaddox tangent cross in measuring diplopia fields. He advocatedturning the patient's head to get the various positions of gaze thathe wished to test:

The patient is seated, his head in the normal position facing the lightin the center of the tangent scale at a distance of two and one-halfmeters (21/2M) on a level with his eyes in the median plane of his head.A dark red glass is put first in front of the normal eye, then in front ofthe paretic eye. The red glass should be so dark that the eye behind itsees nothing but the colored light. Instead of moving the light fixatedby the patient, the doctor turns the patient's head so that he has tomove the eyes in the opposite direction in order to maintain fixation.The passive rotation of the patient's head to induce the various ocularmovements instead of shifting the light through the whole field offixation has the advantage that the doctor is able to control the posi-tion of the patient's head and therefore his eyes during the whole ex-amination.

Thus the patient is given an opportunity to note the position ofthe red image in relation to the tangent scale on the cross. Thepatient then indicates this position by describing it in terms ofnumbers to the right or left of the light, or by going up to thechart and pointing. In this way the diplopia is measured quanti-tatively in the various positions of gaze. Since the eye behind thered* glass is the fixing eye, the primary deviation is measured whenthe filter is held before the non-paretic eye, and the secondarydeviation is indicated when the glass is held before the paretic eye.

*In the choice of the red glass, Bielschowsky (3) states: "The red glass must be sodark that the eye behind it does not see anything but the red light otherwise thepatient would see double images of the whole tangent scale and be unable to makeexact statements.'

Scobee (54) notes that the red glass should be "no darker than is necessary to im-part a reddish tinge to the white light." A very dark lens tends to disrupt fusion anduncovers a heterophoria which may influence the findings.The writer feels that assuming there were a constant critical dividing line between

a heterophoria and heterotropia, the use of a graded colored glass to determine thispoint would still be subject to many variables, such as intensity of illumination, dis-tance, technique. It is evident that Bielschowsky's method of testing would be con-fusing to the patient if he were to visualize a second Maddox cross through a lightred glass. Certainly, it is not usual to have an anatomic heterophoria of such anorder as to disguise the nature of a paresis when the diplopia is evaluated in thefield of the paretic muscle.

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Measuring Diplopia FieldsIn cases requiring the evaluation of the obliquity of double

images Bielschowsky (4) made an important contribution. Hecontended that one should avoid the use of a vertical test objectsuch as the cpnventional candle. (The same reasoning of courseapplies to the bar light (13) of Anderson or a roll (8) of paper heldvertically.) Torsion of the vertical meridians of double images willappear different according to whether there is, beside the verticaldeviation, a homonymous or crossed diplopia. Thus, in spite of

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_̂3FIGURE HEAD-TILTING TEST

(BIELSCHOWSKY)

the same paresis, the two images will diverge or converge upward,by the influence of the horizontal element of the diplopia. Thiscan be avoided by using a horizontal object.The head tilting test (5) is a qualitative test that brings out

effectively the influence of the position (2 I) of the head on thediplopia. Bielschowsky used a simple apparatus based on theprinciple of Helmholtz' Visierzeichen. The relationship of thepatient's head to the object of his attention is fixed by havinghim bite on a small plate at the end of a rod to which is connected,thirty inches (75 cm.) away, a piece of white cardboard containinga black horizontal line. Thus the tilt of the patient's head will

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Albert E. Sloanedetermine the tilt of the cardboard; a tubular hand support issupplied to hold this rod without interfering with its rotation.Doubling of the horizontal line viewed will occur if the head istipped one way, and will be less or absent if tipped the other.We shall consider the right eye in a right trochlear palsy. In

tipping the head to the right there is a compensatory vestibularinnervation to intort the right eye and extort the left eye (lae-vocycloversion). The intorsion of the right eye would be broughtabout normally by the additive action of both superior obliqueand superior rectus. These two muscles would, however, neutralizetheir respective elevating and depressing actions. In the absence ofthe depressing function of the paretic superior oblique therewould be no resistance to the elevation produced by the superiorrectus and thus the eye would assume an elevated position produc-ing a doubling of the line.Kestenbaum (26) in 1946 described an interesting way of meas-

uring the diplopia without the use of a colored glass. In additionto a light, a second object (for example, a pencil) is held before theeyes. Thus the patient sees two objects doubled: two images of thelight, two images of the pencil. The pencil is now moved untilone of its images is superimposed over the image of the light; thedistance between the pencil and the light therefore is the measureof the diplopia and can be estimated by the examiner or measureddirectly by him. The test may be carried out in the various posi-tions of gaze; thus, without equipment other than a small ruler,the examination can be made in an effective way. Kestenbaumalso suggested what he calls the "Fixation Shift" test, in whichthe patient is asked to estimate the distance between the twoimages. In order to do this he looks from one image to the other,making a number of changes of fixation; one notes the movementthe eye makes and for each millimeter some five degrees wouldbe the amount estimated. It would seem that this technique mightbe better in theory than in actual practice. Kestenbaum also sug-gests the use of a non-calibrated screen, whereon the patient indi-cates the position of the second image and the examiner measuresit directly with a ruler.

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Measutring Diplopia Fields

THE RED-GREEN TEST

Ewald Hering (17) in 1899 used a red filter before the right eyeand a green filter before the left eye, and moved red and greentargets seen monocularly by each eye until they appeared super-imposed. He also used a faint red and green light. Thus by firstusing the Red-Green Test to demonstrate and measure the angle

(A) (B)

FIGURE 4 (BURIAN)A: The double image test. B: The red-green test (Lancaster). In (A) the fixation lightimaged on the fovea of the fixating eye and a peripheral element of the deviated eye.Result: uncrossed diplopia. In (B), each test line is imaged on fovea of the eye with

which it is seen. Result: crossed diplopia.

of squint by the displacement of the two images, he gave thebackground of all such methods by using: (i) a darkened room;(2) red-green filters; (3) red-green lights.

Duke-Elder (i i) comments to the effect that the Red-Green Testis a type of cover test where one eye is "functionally covered" inas far as the other eye is concerned. An ingenious deviation fromthe simple red glass test is this use of complementary red-green

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targets. Essentially (6) the difference between this type of test andthe simple red glass test is that, in the latter, there are doubleimages of one light: the patient is looking at one fixation object,that is, one light which is visible to both eyes. The red glass simplyhelps to identify the image of one eye. Since the two eyes are notlooking directly at the same object the image will fall on thefovea of one eye and will be in an extrafoveal position in theother eye; thus the image of the eye in the extrafoveal position willbe projected according to the distance from the fovea. A personwho has an esotropia, while in actuality the eyes converge, willproject images that are uncrossed, and in a similar way a personwho has an exotropia will have crossed diplopia. Since the opticalunderstanding of this is not nearly so important as the clinicalreport of the patient, it has not been hard for the ophthalmologistto appreciate that a crossed diplopia meant a divergence of thevisual axes, and an uncrossed diplopia meant a convergence of thevisual axes. Thus it has become a sort of second nature to acceptthis finding. Now when complementary red and green filters areused before the two eyes, two lights, or two test objects are used,one is red and seen by only the red-covered eye, the other is greenand seen only by the green-covered eye. Since the person will seeeach test object with the fovea of each eye, the images will be pro-jected along the true direction of the eye. Thus a person withconvergent strabismus, looking at red and green similarly locatedtargets, will project the image of each eye as though it were con-vergent or crossed in relationship to the other, whereas in diver-gent strabismus the images will appear in the form of a homony-mous relationship. Trhis is in keeping with the direction of theeyes; but because it is different and diametrically opposite towhat one has been familiar with in the use of the other types ofmuscle tests, such as the Maddox rod, where the reverse is true,confusion sometimes arises. Nevertheless if the situation is under-stood, this test is valuable.By the turn of the century the red-green method had found

many uses in ophthalmology, particularly in testing for malinger-ing (FRIEND) and in the Worth (63) four-dot test for measuringbinocular vision. An American ophthalmologist, Charles Williams(62), constructed a cross made of four hollow tubes arranged with

Measuring Diplopia Fieldsincandescent bulbs and red filters. In front of this cross he arrangeda series of illuminated green numbers each separated by thedistance of an angle of deviation calculated for twenty feet. Thepatient wore red-green complementary glass goggles and the devia-tion for the primary position of gaze could be read directly. Bycontrolling the switches so as to illuminate first the vertical armof the cross which was red and the horizontal green numbers, onemeasured the horizontal deviation. Then with similar switchesthe horizontal red line was made to appear along the green verticalnumbers giving the vertical deviation. Williams did not speak ofusing the test in the various positions of gaze but it could well beused that way.

Walter R. Hess (42) of Zurich, who in 1949 was awarded theNobel Prize for his discoveries concerning certain functions ofthe brain, was one of the first (18) to popularize the use of thered-green test for the measurement of diplopia. He believed thatthe Hirschberg technique which employed a candle and a co-ordinate system painted on a wall was not practical for clinicaluse. He utilized a tangent screen made up of red lines on a blackcloth (19) with red spots at intersections of the fifteen and thirtydegree lines with themselves along the horizontal-vertical axes(Fig. 5).The patient held in his hand a wand with a green test object

(later shaped like an arrow) at its end. The head was held absolutelyquiet and only the eyes followed the object. A pair of red andgreen goggles were placed before the eyes. Thus, one eye lookingthrough the red filter could see only the red spots and the lineson the chart; the other eye wearing the green filter could seeonly the green test object on the end of the stick. It was now thepatient's task to indicate with his pointer the position of each redmark on the tangent screen. Since he saw the red mark with oneeye and had to project the green arrow with the other eye, truesuperimposition of the green arrow on the red dot would onlyoccur if there were no deviation. If a deviation was present, theseparation could be easily demonstrated and measured. Hess thenplotted the relative position of the green arrow on the screen ontoa similarly reproduced recording card (Fig. 6). Thus the test wasmade for the position of the nine red spots. The positions of all

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the red targets were then connected and similarly the positions ofall the green targets, thus forming two quadrilateral figures. Now,if the red-green goggles are reversed, then the opposite eye wouldtake up fixation and the diplopia fields would then indicate thesecondary deviation (if the first had measured the primary). Ineach instance, the size of the figure for the paretic eye would besmaller than that of the non-paretic eye. Thus one could dif-ferentiate a concomitant from a paralytic type of strabismus. Hessworked at a distance of one-half meter (1/2 M) and he was carefulin the construction of his tangent screen to make sure that he was

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FIGURE 5. THE HESS RED-GREEN TEST

Measuring Diplopia Fieldsworking with true tangents; therefore the lines appear curvedand convex towards the center.

Sattler (51) criticized the method of Hess as being deficient inthat it made no measure of the torsion component; he also statedthat too small a field was tested and therefore one was likely tomiss a weak paresis. He objected to the distance of one-half meter,because he felt that accommodation might play too important arole, and criticized the recording charts as inadequate because the

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REDGREEN

FIGURE 6. DIPLOPIA FIELDS (HESS): PARESIS RIGHT MEDIAL RECTUSSecondary deviation: O.D. fixing (red); O.S. (green). Note paretic (O.D.) eye hassmaller field (red). Primary deviation: O.S. fixing (red); O.D. (green). Note paretic

(O.D.) eye has smaller field (green).

names of the muscles, printed on them, left too little room forrecording. Ohm (65) also believed that the one-half meter distanceinfluenced the accommodation too much.John Ohm, an assistant of Hirschberg, produced a number of

tests for diplopia field measurements. In one of his earliest works(44) he utilized the principle of the complementary red-greenfilters by constructing a- complicated apparatus made up of avertical and horizontal arm with movable green and red coloredarrows. It measured one and one-half meters in height and twometers in width (45 degrees to each side of the zero point). Thepatient sat one meter away. Thus bv wearing the red-green glassesthe patient saw one arrow with each eye. It was the task of the

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patient to indicate when the points of each arrow touched andwhen the bodies of the arrows were in alignment. The red arrowwas so mounted that it could be rotated according to the torsionaldisplacement. With this Ohm was able to measure the angle ofdeviation of the eye in the primary position and thereby indicatethe extent of the diplopia. The primary and secondary deviations

GREEN

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FIGURE 7. APPARATUS FOR MEASURING DIPLOPIA (OHM)

could be measured by causing fixation to be obtained with thenon-paretic or the paretic eye (Fig. 7).

In 1907 (45) he referred to this apparatus as being too clumsyand substituted a method in which he used a tangent screen afterthe method of Hirschberg (18; 1875) arranged in a coordinatemanner. He worked at a one meter distance and used a red andgreen paper arrow. rhe red arrow pointed down, and the greenarrow pointed up. Thus he could pin one paper to any part ofthe field that he wished to test, and the patient would then directthe position for placing the second paper seen by the other eye

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Measuring Diplopia Fieldsso that the two arrows pointed at each other. He utilized ninecardinal points on which to place the arrow. In 1906 Ohm (45)also devised a screen on which a black cloth was faced with bluestrings spaced at intervals of 5 degrees of arc. The patient worered-blue goggles and indicated the position by using the bluetip of a wand to touch the part of a red arrow seen by the red

FIGURE 8. WIRE-STRUNG DEVICE FOR MEASURING DIPLOPIA (OHM)

covered eye. Ohm (65) was impressed by the work of Krusius andliked the idea of the patient being directly observed through theglass screen. In 1916 he reported a modification which permittedhis apparatus to be used at one meter (i M) distance. Krusius (28)tested at one-half meter.Ohm stated that the accommodation influenced the test at one-

half meter and even influenced the labyrinth and thus was tooshort a distance for his purpose. He accomplished his aim by con-structing a scaffold measuring 5o degrees to the right and left ofthe zero point, and 25 degrees above and 45 degrees below the

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zero point. He strung this with wires separated at the estimatedtangents of stated arc degree intervals. Thus he could stand on thefarther side of the screen and by looking between the wiresobserve the movements of the eyes and also measure the angle ofsquint both objectively by observing the corneal reflex, and sub-jectively by utilizing the red glass and a white arrow in thevarious positions of gaze. The scaffold measured 250 cm. inwidth, and i6o cm. in height and required a chin and head rest.

Later, in 1921 and 1925, Ohm advocated the following, whichmay represent what in his mind was the best of his tests from apractical and clinical viewpoint. He utilized a chin rest and ahead rest to keep the patient's head in a fixed position. The work-ing distance was one meter. He suspended (46) a white paperarrow or made a small cross with white chalk at the zero pointof the tangent screen, and placed a red glass before the deviatingeye. The patient was then told to look at the point of the whitearrow; this of course was done by the fixing eye since the deviatingeye was apparently sufficiently obscured by the red filter. Ohm nowheld a small white illuminated bulb which appeared red to thedeviating eye. The bulb was so placed that the red light appearedto be on the point of the arrow; its position and distance from thearrow then gave immediately the angle of deviation of the squint-ing eye. The patient was told to ignore the white light, thus in-suring bifoveal fixation. The arrow could then be used in dif-ferent fields of gaze and the flash-light positioning repeated; ineach case the patient disclosed the amount of diplopia by thedistance between the light and where it appeared at the point ofthe arrow or on the cross. Ohm (47) suggested that these findingsbe recorded on charts indicating the position of the deviating eyerelative to the position of the fixing eye. After the manner ofHirschberg (20), he marked the distance between these positionsin degrees designating a convergent position by (+) and divergentposition by (-); vertically, a right hypertropia was (+) and aleft hypertropia (-). By connecting the relative positions of thesevalues on a graph by a line he demonstrated his so-called strabis-mus curve (Figs. 9, io).

Sattler (5 1) used the complementary red-green test in this man-ner. He employed a black wall, 2.4 meter square, on which was

654 Albert E. Sloane

-40-35-30-25-20 -15 -10 -5 0+5 410+15+20+25+30435+40+35 i r ' ' ' ' n ' ' ' ' ' ' '

* =RED GLASS BEFORE 0. D.FIGURE 9. STRABISMUS CURVE (OHM): PARESIS LEFT

SUPERIOR RECTUSLeft eye fixing; red glass before O.D. (secondary deviation).

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-50 - - - - - - - - ..

FIGURE 10. PARESIS LEFT INFERIOR RECTUS (OHM)Left eye fixing; red glass before O.D. (secondary deviation).

A

6Albert E. Sloane

ruled a coordinate system painted with pure green lines. (This initself offered great difficulty since complementary green is noteasy to produce.) The wall was ruled off very much like graphpaper in that the lines at the 100 intervals were a bit thinner thanthe two zero lines, and the lines indicating 50 separations werethe thinnest. The zero point was placed 120 cm. from the floor,approximately at eye level. He placed green marks (crosses) at the200 crossings and at the zero point. The patient's head was securedby a chin rest and by an occipital rest so that the head was verticaland erect and fixed, a point stressed by Sattler as most important

FIGURE 11. SATTLER S RED-GREEN TEST

656

Measuring Diplopia Fields 657for evaluating reexaminations. The patient held in his hand a

long black stick at the end of which was a red cross which measured1o by 5 cm. Red-green glasses large enough so as not to restrictthe field of vision were placed before the eyes. The spectacleframe was so arranged that it could be reversed for either eye,

thus allowing a measure of primary and secondary deviations. Theexamining doctor held a green stick in his hand with which hedirected the patient's attention to the target (Fig. I i). rhe patientindicated this position with the red cross at the end of his stick.Since the cross was made up of vertical and horizontal components

L R

REDGREEN

FIGURE 12. PARESIS RIGHT LATERAL RECTUS (SATTLER)O.S. (green) fixing; primary deviation.

the patient would unknowingly demonstrate torsion if suchexisted. The examiner recorded the findings on ordinary milli-meter graph paper. Sattler then connected with a red line thepoints seen by the eye looking through the red filter and with a

green line those by the green filter (Figs. 12, 13). He suggestedthat with a weak paresis one must test beyond a 200 field, but witha more marked paresis a smaller field was sufficient. He noted thatnot much intelligence was required by the patient to obtain an

accurate result. It is obvious that such a large screen offers a dif-ficult problem for the ophthalmologist in the usual office.

In 1939 Lancaster (30) described his red-green test. This was

actually a stream-lined application of the well-established principleemployed in the previously described tests using complementary

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Albert E. Sloane

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FIGURE 13. PARESIS RIGHT LATERAL RECTUS (SATTLER)O.D. (green) fixing; secondary deviation.

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FIGURE 14. EQUIPMENT FOR LANCASTER RED-GREEN

TEST (WELCH ALLYN, NEW YORK)

colored targets and goggles. It is a practical apparatus combiningthe best features of the others (Figs. 14, 15).

Lancaster used a white tangent screen which could be rolledup out of the way. It was ruled into squares 7 cm. wide, so that at

a distance of two meters each square subtended approximately

658

L

23

Measuring Diplopia Fields 659

two degrees of arc or four prism diopters. The squares were allexactly the same size and extended sixteen degrees of arc to bothsides of the midline. If a greater area is required this effect maybe obtained by bringing the patient to a point one meter fromthe screen, in which case the degree values would be doubled.Lancaster made no allowance for the fact that the tangents wouldrequire the squares to be wider as one came away from the

r l K t1 v1 1816-.4-4-.."~~~~~~~~~~~~~~~~~~~~~~

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UGREEN GLASS BEFORE O. S.

FIGURE 15. METHOD OF RECORDING (LANCASTER RED-

GREEN)Red glass O.D.; green glass O.S. Note direct representation

of extorsion O.S. down and to left.

center. He stated that at two meters, rectangular squares were

sufficiently accurate. The miniature reproduction of the screen on

a card was used for recording. The technique suggested is asfollows: The patient holds a flash light which projects a red linearlight, and wears red-green goggles, the red glass before the righteye. The examiner projects a green linear light in the variouspositions of gaze. The patient then covers this projected greenlight with his red light. He sees the examiner's light with his

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Albert E. Sloane

left eye and projects his own light with his right eye. The readingsare made direct, the displacement between the two lights beingrecorded on the chart. T'he lagging eye will actually lag, thedivergence of the eye will give an uncrossed position of the twolights; one will project exactly in the direction of the eye. Lan-caster states that if one wishes a more extreme position of gazethan the chart will allow, the head may be turned in the ap-propriate direction. Thus we have a patient wvearing a red andgreen filter before the right and left eye respectively; his head isfixed so that all movements must be from his eyes. The examinerthrows one colored linear light onto the screen and the patientcovers that light with a similar but opposite colored target. Seeingthe examiner's light with only one eye and projecting the secondlight with the other eye he is indicating the degree of his devia-tion. This test has strong advantages in that it first allows theexaminer great liberty in finding the field of fixation; second,by the use of the linear target, the effect of torsion is clearly andimmediately indicated; third, it allows a measure of the secondaryangle of deviation by exchanging the lights.

Lancaster deserves great credit for reducing the study of ocularmotility to a level capable of popular reception and acceptance.His many papers (50) on this subject support this statement. Hisgreatest work has been that of continually and repeatedly stressing,basic facts in this field.Anderson (1) modified the Lancaster red-green test in that he

projected a chart rather than using a curtain screen. His chartwas plotted in squares of 7 cm., each to subtend an angle of 20 at2 meters. A red horizontal line is projected onto the center of thechart and the patient is asked to cover this line with a green linethat he projects from a torch he holds. Of course he wears red-green goggles so that each eye sees only one image. After the read-ing is made, the entire chart is projected to the various positionsof gaze to be tested and the previous procedure is repeated. Thusthe red image is always at the center of the scale providing foreasy measurement. The results are plotted on ordinary graphpaper.

Burian (7) in a memorable paper on fusional movements instrabismus described a modified diplopia test using polaroids to

66o

Measuring Diplopia Fieldsproduce monocularly seen targets. He employed two projectioninstruments provided with polarizing plates, one polarizing thelight in the vertical direction and the other in a horizontal direc-tion (Fig. i6). One instrument projected a tangent scale properlycalibrated in arc degrees for 41/2 meters or 15 feet. The other instru-ment projected a colored dot; this projector was movable so thatthe dot could be placed anywhere on the screen that was desired.The patient wore before each eye a square polarizing plate eachso oriented that one eye saw the dot and the other the tangentscale. The frame carrying these polaroid plates could be reversed

FIGURE i6. DIPLOPIA FIELD TEST (BURIAN)

so that either polarizing agent could be placed before either eye:thus one could change fixation to the opposite eye. At the begin-ning of the examination the dot is placed on the screen so that itcoincides with zero of the scale. If both eyes are not directedtoward the same point one image will fall on an extrafovealposition and a spot will appear on the scale at the angle of squint;thus it may be measured directly. By moving the head to variouspositions of gaze, the angle of squint or measurement of diplopiamay be obtained in all fields. This may be done either by thepatient's noting the apparent position of the dot in relationshipto the scale or by moving the dot so that superimposition of thedot on the center of the scale appeared to occur; in either casethe examiner reads the amount of diplopia directly. The amountof equipment and the setup,required for this test would seem tomake it one that might prove not too popular with most practition-ers who would have little or no other use for this apparatus.

Lees (34) recently described a rather complicated apparatus for

661

662 Albert E. Sloanemeasuring the binocular relationship. It employs the principle ofbifoveal imaging and is said to be superior to the Hess test in thatit does not depend on a color-separation system. Lees states thatif the colors "are not correct, the dissociation obtained is only of

SCREEN "A".

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General view of apparatus. Screen "A" with virtual Screen "A" with tangentScreen "A" on the left. image of screen "B" pattern suppressed readyScreen "B" on the right. seen superimposed in for plotting projection of

the mirror. left eye. The right eyefixing the mirror image.

FIGURE 17. V. T. LEES, PLAN OF MIRROR SCREEN TEST

(Photographs by courtesy of the Department of Medical Photography, Manchester Royal Infirmary)

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Measuring Diplopia Fields

relative degree and errors then are liable to occur in the test."In his instrument dissociation is obtained by a plane mirror placedbefore one eye. It has two reflecting surfaces so that it may be usedbefore either eye. The mirror is exactly half way between twoidentically printed Hess tangent scales erected at a go9 angle toeach other. Each scale is so constructed that its pattern may bemade to appear or disappear as required during the test. Thus thepatient looking at one scale directly and the other in the mirrorsees "different but identical scales exactly superimposed." Theeye being plotted has its scale disappear leaving only a blank sur-face in order to avoid fusional stimuli (Fig. 17).

METHOD OF TESTING

The patient sits close to the mirror facing one screen directly(50 cm. away) while the other eye sees the second screen in themirror. The mirror itself allows each eye to see only one screen.The examiner places his pointer at the various positions of gazeon the screen visible in the mirror. The patient in turn places asecond pointer where it appears to cover the examiner's pointer.Of course, he indicates these positions on the second screen fromwhich the scale has been made to disappear. By making the scaleappear after each position is noted, a reading can be made directly.The findings are transferred onto the Hess chart for recordingas with the red-green test. It is of paramount importance that theapparatus be accurately aligned. Lees states that

It will be realized that, as the apparatus requires to be installed as asemi-permanent fixture, and as its manufacture is more exacting, theinitial cost will far exceed that of the original Hess screen test. Theadvantages offered in rapid, accurate and trouble free results far out-weigh the initial difficulties and expense involved.

At this time I should like to introduce for consideration adevice which I have been using since June, 1949, and which hassubsequently become more valuable to me with daily use.

It consists simply of a board measuring some i 8" x 12", or30 cm. x 45 cm. and made of a transparent piece of plastic material.It is mounted on a light weight handle; contains a small centralbulb and has a Maddox cross indicating the tangents up to 240

663

Albert E. Sloane

FIGURE i8. DEVICE FOR MEASURING

CERTAIN BINOCULAR FUNCTIONS

(SLOANE)

of arc either side of the horizontal zero and i20 above and 160below the horizontal zero. The instrument is held at the easyworking distance of a half meter or 20 inches from the patient.The bulb is the type normally found in a May ophthalmoscope;it is incased in a condensing lens so that a bright corneal lightreflex is obtained. I have added a movable target so that certainother tests can be made. Thus in effect we have a Maddox cross,calculated in degrees for 1/2 meter on a transparent medium and

FIGURE 19. APPLICATION OF DIPLOPIA TEST (SLOANE)

664

Measuring Diplopia Fieldscontaining a special source of illumination in its center (Fig. i8).For measuring the diplopia the patient's head is fixed securely

against a head rest, although this is not absolutely necessary. Adark red glass is kept before one eye, the patient holds an un-sharpened lead pencil using its rubber eraser end as a pointer. Thedevice is then kept 1/2 meter from him, the observer standingdirectly behind so that he may watch the movements of the eye(Fig. 19). The patient then indicates the position of the red lightby placing the eraser at the end of the pencil onto the device: itsposition is easily noted and plotted directly, indicating the degreesof diplopia both vertically and horizontally. The patient's headis then turned in the various positions of gaze. It is quite easyafter a short period of practice to determine how far the patient'seyes can be turned without obstruction -from the nose or face.This is possible because the observer stands directly behind hislight and he notes the presence of the light reflex on the corneasof both eyes. When the eye is turned too far to one side so that,for example, the nose obstructs, he does not see the light reflectionon this eye. The head of the patient is then rotated until the lightjust appears. The working distance is very convenient, the patientindicates easily and quite surely the position of the red light. Themeasurements of diplopia are made in the various positions ofgaze and the test is accomplished in very short order. In the matterof diagnosing an ocular paresis, I have found no disadvantage inusing the test at 1/2 meter as compared with the 2 meter distancethat I had been using with the double image type of test for thepast eleven years.

In a patient such as an arthritic who is unable to move his head,the device may be moved in the various positions of gaze. Itsportability makes it convenient to use outside the office. I havefound it particularly helpful in examining patients in the hospital.

After having used this device for some months, I found asimilar one described by Krusius (28) in 1908. His Scheiben-Deviometer consisted of a pane of glass 120 by 6o cm. or 48 by24 inches, upon which was calibrated a cross computed for tangentsin degrees of arc. The screen was further divided into smallsquares. A working distance of 6o cm. or 24 inches was employed.He used two luminous bulbs for fixation and watched the corneal

665

reflex to determine the angle of strabismus for which the instru-ment was intended. The alignment of the corneal image wasexecuted by flashing off and on the bulbs, of which one wasmovable and could be centered with the angle of squint. Hesaid an allowance of 60 should be made for the convergence at6o centimeters. On the basis of Krusius' work, Ohm (65) made hiswired tangent scale previously described.The cover test utilizing so-called parallax is a subjective test

that may measure diplopia by use of prisms. In this techniquethe patient will observe a movement in the object viewed whenthe cover is changed from one eye to the other. The amount ofthis movement is, of course, determined by the amount of excur-sion that the eye must make to recover its position of fixation.Prisms will neutralize the apparent movement of the object whenalternate occlusion takes place. The end point is determined whenthere is no movement of the target as the two eyes are alternatelycovered. Duane (g) comments: "These movements are noticeableeven when the deviation is very slight: a hyperphoria of one-tenth(i/io) degree being made appreciable by a distinct up and downmovement of the object." Graefe (16) stresses that the prisms thatwill just neutralize the diplopia will vary and that the prismswhich will hold the vision single easily is the true measure.Landolt (32) also suggested prisms used in this way as a measureof diplopia, although most of his experiments (33) in this fielddealt with the use of a tremendous sized tangent screen employedat a distance of 3 meters. White (60) describes this cover test asdependent on the cooperation and on the observations of thepatient. "Many patients are quite stupid about being able tointerpret the movement of the test."Maddox (39) believed his objective method of measuring the

angle of squint combined the accuracy of the perimeter with thespeed of the Priestly-Smith (55) method. He used a tangent crosscalibrated for one meter, in the center of which was a lightedcandle. While the patient fixed the candle Maddox noted theposition of the corneal image in the deviating eye, giving op-portunity to estimate quickly the angle of squint by the Hirsch-berg (59) method. The sound eye was then covered to allow thesquinting eye to fix the candle and when it did this the position

Albert E. Sloane666

Measuring Diplopia Fieldsof the corneal image was then noted. Now the dominant eye wasagain uncovered and the patient was told to look at the numberssuggested by the Hirschberg estimation and if this were correctthe corneal image was noted to be in the center of the deviatingeye. If this were not the case, the dominant eye could fix on theproper number until the candle light was reflected in the centerof the deviating eye. This figure gave the measure of the squint inits primary deviation. By causing the paretic eye to fix, thesecondary deviation was measured in a similar manner. Maddoxdid not enlarge upon the use of this technique for the differentpositions of gaze but this same application to such measurementsis of course obvious.

In the device that I described (Fig. i8), I can obtain a reason-able determination of the angle of squint without the use ofprisms. First the patient's head (as in the diplopia field) is keptin a stable fixed position if possible. Then looking directly behindand sighting along the light of this device standing at 1/2 meterfrom the patient I observe the position of the reflection of thislight on the cornea of the deviating eye while I have the fixing eyeoccluded.This at once establishes several facts: First, angle Kappa is not

a factor since I am determining my point of reference for thecorneal reflex by the visual axis. Now, I have my light centeredwhere it should be centered if the eye is in a fixing position.This will be my point of reference, where I shall try to placethe light when I make my measurement. Second, since I am sight-ing along my light, rays reaching my eye from this corneal imagepractically retrace thr- hi path from the light at least in the horizon-tal plane. In this way I shall not be confused by any considerationfor the angle of reflection and no allowance need be made for that.Now I uncover the fixing eye. Of course this light reflection is nowdisplaced according to the angle of squint away from the center. Ithen move a target, a colored button, across the face of this boardasking the patient to watch the button. This of course he doeswith his fixing eye. I move the button until I note that the lightreflex is again in the center or at that point of the cornea whereI noted it originally in determining my point of reference on thedeviating eye. The deviation will then read directly from the

667

Albert E. Sloaneposition of the button. If I wish to check this, I simply cover thefixing eye which had been looking at the button and direct theattention of the patient to the light. Since the deviating eye isnow directed towards the light, there should be no movementnoted; if there is, than I am not absolutely accurate and I canmake my adjustments by moving the target to one side or the other.This does not differ from the technique as presented for any ofthe major amblyoscopes whereby the target for each eye is setfor the objective angle of squint, and one has arrived at the meas-ure when one obtains no movement when each eye is alternatelycovered. Now, to measure the angle of squint in the variouspositions of gaze, I turn the patient's head in the appropriatedirections.

Krimsky (27) observed the displacement of the corneal lightreflex from the fixation position in the deviating eye. He thenmeasured this deviation by the amount of prism necessary to re-store this light reflex to its fixation position. This test was thenrepeated in the various positions of gaze. Thereby the field ofgreatest separation of visual axes was found and evaluated.The cover test is associated with the name of Duane. Here one

looks for the field of greatest recovery required (when the screenis removed) as the field of action of the paretic muscle. One canmeasure with prisms the amount required for neutralization ofthis movement and thus obtain a quantitative, accurate reading.This test must be made in the six cardinal fields and is mostimportant in the diagnosis of paresis even of a slight amount. Aconvenient working distance suggested by White is between 15and i8 inches. It is frequently found that the deviation measuredwith prisms is greater when the prisms are placed before one eyethan when placed before the other. This is due to the differencebetween the amount of the primary and secondary deviation. Anindication as to which is the paretic eye may be obtained by notingthat a paretic eye moves more slowly than the sound eye, whichmoves faster because of the greater secondary deviation. White(4) recommends that for a good successful use of the cover testthe following are requisites:

i. There must be central vision in each eye.2. Sufficient light must be present, so that the examiner may note

668

Measturing Diplopia Fieldsthe least movements of the eye, but the light must not be so bright orglaring that the patient or observer is blinded by it.

3. The screen or cover should be large enough to cover an eye with-out undue attention to its position. White prefers a card 6 by 15 cm.

4. The cover should not be moved more rapidly than the uncoveredeye can take up fixation. The square prism is most satisfactory, andthe usual size of 4 cm. is sufficient. White states that round prisms areunsatisfactory since they are hard to hold separately and impossibleto hold in combination. He does not advocate the rotary prism. Ifcooperation is good and fixation normal, an "amount of strabismus (6i)eighty (80) to one hundred (ioo) prism diopters may be accuratelymeasured."

Cover testing with prisms is simple in theory but not alwayseasy in practice, particularly for those who are inexperienced intheir use and who do not have the dexterity required to hold anumber of strong prisms in the proper position before the eye.This excellent method of measuring diplopia requires that theexaminer have some practice with the use of prisms. Only afterrepeated use do the manipulations become less awkward. Onemust be able to use the cover before the alternate eye, at the sametime making sure that the patient keeps his fixation in the properdirection. He must also be deft in the handling of prisms so thatin a minimal amount of time they may be put into the rightposition. The error produced if they are not so held is not alwayseasy to avoid. The awkwardness of these manipulations will some-times make the occasional user omit this excellent test. No doubtthe prism bar such as described by Berens (2) is doing much tooppose this tendency. Great credit must be given to White's in-fluence in making this method a popular one in this country.

In regard to this cover test, Ludvigh (35) points out certainlimitations in noting small movements of redress:

Under ideal conditions, excursions of much less than one diopter totwo diopters cannot reliably be perceived by the unaided eye. In usualclinic conditions and with much less cooperation from the examinee,it seems advisable to adopt at least the upper limit of two diopters. Thismeans that if it is desired to detect eye movements of one degree orless, either a subjective test should be employed or the examiner usemagnification.

669

Albert E. Sloane

CONCLUSIONS

The following thoughts represent the impressions of the authoras to the relative merits of the various diplopia field tests.

1. Any person who has a working knowledge of ocular motilityand the principles of projection of images will have no difficultyin analyzing and evaluating diplopia fields by any of the tech-niques.

2. A personal interest in a given test makes that test particu-larly desirable, encouraging the protagonist to endure the ob-jectionable features and to stress the favorable aspects.

3. The following should be noted in regard to the subjectivemethods.A. The Testing Distance. In the literature studied, this has

varied from 1/2 meter (20 inches), to 6 meters. The former hasbeen criticized as stimulating accommodation, and a consequenteffect upon the angle of deviation and on convergence. The longerdistance is too great to permit observation of the eye if one isusing a movable light with the head in a fixed position, or thescale would be too small if the Maddox cross is used. A meanaverage of fifteen methods studied was i.8 meters or 72 inches.Thus on the basis of desirable distance arrived at by such means,one could choose the Bielschowsky technique of the tests employ-ing double images (unifoveal and extrafoveal imaging) and 21/2meters and the Lancaster red-green of the complementary colortests (bifoveal imaging) at 2 meters. The Anderson modificationof Lancaster's test would also apply.

B. Type of Scale. All but one of the group studied used tangentsof the arc degrees, thus providing larger intervals for equal arcdegrees as one moved away from the zero point. Lancaster (30)however insists that, for practical purposes at two meters, rectan-gular squares are sufficiently accurate. It seems to the writer thatfrom a clinical viewpoint the difference would not be appreciable.Since we think in terms of exactness even though we know thatit is unattainable it might have been a better idea for the Lan-caster chart to have been ruled into tangent equivalents. Thishowever is an insignificant criticism of an excellent test.

C. Moving the Head, or Moving the Test Object. Of a group

670

Measuring Diplopia Fieldsof i6 methods, 1 1 showed preference for moving the target. Fourchose a fixed test object and moved the head. Lancaster movedthe target but stated that if one wanted a wider excursion thanhis chart offered, the head (30) could also be turned but turned aprecise amount, namely, to another fixation on the chart whichbecomes the new zero point. Bielschowsky was a powerful forcein advocating the head movement, because in this way he couldcontrol the situation. Actually head turning makes repeatedmeasurements less accurate then does moving the target. Thepossible objection of labyrinth stimulation in head movementshas been raised but appears to be unfounded. The writer useshead movements most of the time, but may feel prejudiced inits favor by Bielschowsky's influence and for the second reasoncited above.

D. Unifoveal and Extrafoveal Imaging Versus Bifoveal Imaging.The red-green complementary test has found many disciples,and its use in this country has been stimulated by Lancaster. Thered glass double-image test associated with a Maddox scale hasmany followers too, largely through the influence of Bielschowskyand his school. One weakness of the red-green test is paradoxicallyone of its principal reasons for superiority. The positioning of theimages is direct: that is, esotropia produces crossed images; exo-tropia, uncrossed images. It is true that such a concept is muclheasier to teach to the neophyte, for whom the apparently reversedpositioning of the images is often so difficult to grasp. On theother hand, the diplopia with the red glass test is in keeping withthe experience of the patient who has double vision: a personwith a paralytic sixth nerve has homonymous diplopia even thoughhis visual axes cross. This source of confusion is readily dissolvedif one considers the deviation only and does not think in terms ofdiplopia. Actually in the red-green test there is no subjectivediplopia; the patient superimposes one image on the other. Never-theless one must be on guard, for if one carries out tests for otherfunctions utilizing double images (example: Maddox rod, Graefenear phoria, square prism) the reverse positioning must be inter-preted properly. The alternative is to substitute all these tests forthose employing the complementary red-green principle.

E. Size of the Scale. In the literature reviewed, the scales have

671

6 Albert E. Sloanevaried in size fromi an entire wvall to a plastic board 18 ly 24inclhes. Thie longer the test distance, the greater the scale arearequired. Actually normal eye movements are those that neverexceed a 120 (14) excursion; but in testing for a paresis wve go ivellbeyond these limnits in order to isolate a given field as much aspossible to the action of single muscles. Nowv if one moves thehead, a relatively small chart is required, wvlhereas if one favorsmoving- the object of fixation then a muclh larger clhart is neces-sary. For examiiple, note the size of the coordinate type of screenused by Sattler and Olhmwnvhiclh is designed for amplitudes of asmuich as 50o to either side of the zero point. From a practicalstandlpoint lhead movement is a desirable method for aetting- themost ocular exctursions for the size of chart used.

F. The Cover-Test Neuitralizationi of Apparent Aovemnent of the7Test Object by Prisms. This wvas one of the earliest tests described,so certainly enougol years have passed to permit proper evaluia-tion. WhIiite, whlIo ha-id a greater experience wvitlh cover tests thanmost people, puts it reasonably whilen lhe states (6o): "It is a verydelicate test wvhien properly observed and again is quite tedious andvaltueless whllen cooperation is lackinog" He found that he "usedit muclh less than formerly."

G. Dissociatiotn of the Images by Polaroids. This type of testusually lhas been employed in the laboratory as part of the in-vestigation of other plhases of binocular functions. A true evalua-tion of it in this field lhas not been available. Howvever there isno reason to believe it would not prove uiseful. One must con-sider the expense of providing the projection apparatus andequipment required, althouglh the cost might well be less if theinstruments wvere prodtuced in quantity.H. Overlapping of Douible Images and Then Mleasuring the

Separation of the 7Twvo Test Objects as Siuggested by Kestenbautm.Not enough experience has been had in using this technique.One wvould imagine that the influence of torsion might offersome confusion, since dissimilar test objects are employed. From astandpoint of a qualitative rather than quantitative method andof one that requires no special equipment, it has merit.

I. The Shape of the Fixation Object. The issue resolves itself

6729

Measuring Diplopia Fieldsinto an arrangement where in addition to the separation of theimages the influence of torsion may be demonstrated, or wherethe separation of the double images alone will offer the necessarydiagnostic clues. The linear test objects such as Sattler's cross,Ohm's arrow, and Lancaster's line all satisfy the first requirement.Only the round source of light satisfies the second requirement.Of course, if one uses the spot of light for the diplopia and, whenindicated, a horizontal line for the torsion as suggested by Biel-schowsky, then both conditions are satisfied, but two tests arerequired as compared to one when a linear test object is used. Thereason the spot of light has survived as a practical test is probablythe fact that, in the evaluation (40) of a paretic muscle, diplopiais of prime importance and the torsion is of secondary importance.It is not my intention to underestimate the great importance oftorsional disturbances.

J. A Test Which Can Be Made at an Easy Working Distancesuch as one-half Meter. The device that I have described seemsto offer a considerable amount of information when used at theone-half meter distance. The practicability and convenience ofthis test may offset the disadvantage of some convergence andaccommodation. This does not imply that other tests for distanceshould be omitted, but that one may augment the other in every-day practice.

4. A discussion of the objective methods must include:A. Utilization of the Corneal Reflex in a Centered Position as

a Measurement of the Angle of Deviation. The Krusius apparatuswith its switch-controlled lights apparently was not used enough,if at all, in the different positions of gaze. Its weight, size, andfragility must have proved a handicap in performing this test.Ohm's wire-strung coordinate system was so large that it wouldprobably not appeal to the clinician, if another good method wereavailable. Also, the one-meter distance between examiner andpatient makes it difficult to determine the position of the cornealreflex. The Krimsky technique of cover and prisms for centeringthe reflex is good within the limits of any corneal reflex technique.It has the advantage of allowing both distance and near fixation.The test described by the writer also lends itself to measurement

673

Albert E. Sloaneof the deviation by observing the centered corneal reflex of thelight, but is limited to near. The corneal reflex method may notbe quite accurate, but it is quick and easy to do.

B. Neutralization with Prisms of the Movement of RedressWhen the Eyes are Alternately Covered. This test appears to bethe most accurate of the objective tests, since in the alternateocclusion each eye must take up fixation, and thus neutralizationof the movement represents the measure of the deviation separat-ing the fixations of each eye. Were it possible to reduce the awk-wardness of handling a large number of strong prisms in theirproper positions, a great service would be rendered ophthalmol-ogists.

5. Concerning the method of recording data, practically all thewriters until the period of Peter (49) plotted their charts so thatthe patient's right was also at the right on the chart. White (likePeter) plotted his chart as he looked at the patient, thus thepatient's right is on the left side of the chart. Advocates of thered-green test plot the right eye on the right side of the chart.Some official agreement might tend to lessen confusion. (A similarsituation previously existed in plotting visual fields and axis ofastigmatism.)From the above, it would appear that of the subjective tests

the red-green test as presented by Lancaster and the red glassused and advocated by Bielschowsky more nearly represent thebest points of all the tests reviewed and that the cover test forneutralization of movement of redress with prisms as advocatedby Duane and stressed by the White school is the best acceptedof the objective tests. Duke-Elder (I 2) states: "A measurement ofthe primary and secondary deviation can be obtained by neutrali-zation with prisms, more easily and accurately by the doubleimages of the tangent scale and most easily by the Lancaster pro-jection test." The author cannot help but feel that in averagehands the subjective tests are the most effective.The conclusion is inevitable that familiarity with the basic

principles and the use of more than one method are necessary.

674

Measuring Diplopia Fields 675

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