Eccentric fixation, Investigation and Management

By: Vineela.Cherukuri

11IMMO07

• A uniocular condition in which some part of retina other than fovea is used for fixation, but in which the fovea retains its normal straight ahead projection (in most of the cases). It may be associated with a defect of central vision caused by a stuctural lesion of fovea.

• Eccentric fixation can be present during both monocular and binocular viewing conditions, but it is best diagnosed under monocular viewing conditions. This is important to keep in mind to avoid confusion with anomalous correspondence.

• Anomalous correspondence is relevant only during binocular fusion and must be measured in those conditions.

• It is only shown when the better eye is covered (Exception = microtropia with identity)

• Why is it important to know about eccentric fixation??????

• For a long time it has been known that there is a class of amblyopic patients with eccentric fixation.

• Such patients do not assume central fixation when the fellow eye is covered; the amblyopic eye remains more or less deviated.

• This observation assumed great importance when subtle modification of the fixation behavior had to be carefully monitored to chart the progress during active (pleoptic) treatment of amblyopia

• Most patients are examined and treated regardless of their fixation behavior and before they reach an age at which a reliable diagnosis of eccentric fixation becomes possible and this pose many tantalizing questions and clinical problems.

Prevalence

• In a study published 40 years ago we found eccentric fixation in 44% of 433 amblyopic patients with strabismus.

• In anisometropic amblyopia, eccentric fixation was encountered only rarely except in patients with microtropia.

• Recent data on the prevalence of eccentric fixation are lacking and as we are seeing and treating amblyopic patients at a much younger age than before it is our impression that eccentric fixation has become less frequent.

Classification

• Bangerter’s classification of fixation patterns in amblyopia is as follows:

I. Central fixation II. Eccentricfixation (nonfoveolar) III. No fixationNonfoveolar fixation may be divided into a

number of classes, depending on the retinal area with which the eye appears to fixate-

Parafoveolar (adjacent to the foveolar reflex), Parafoveal (outside but close to the foveal wall) Peripherally eccentric (somewhere between the edge of the

fovea and the disk and occasionally even beyond the disk)

• The general term eccentric, though logically applicable to all nonfoveolar patterns of fixation, is often restricted to peripheral fixation.

• Nonfoveolar fixation occurs not only with horizontal but also with vertical eccentricity.

• Nonfoveolar fixation is not as precise as foveolar fixation. Nonfoveolar fixation always covers an area that is larger the farther it is from the fovea

• Successively repeated ophthalmoscopic observations or, better yet, successive fundus photographs show this very clearly.

Fixation photographs of three amblyopic patients withA, foveolar,B, parafoveolar,andC, peripheral eccentric fixation

Each circle represents a fixation during which a photograph wastaken. The different fixation locations on 9 to 12 consecutive photographs were superimposed onthe photographs shown in this figure. Note the increasingly greater scatter with increasingly greatereccentricity. All three patients had a visual acuity of 6/60 despite the great difference infixationpattern. (From Noorden GK von, Mackensen G: Phenomenology of eccentricfixation. Am J Ophthalmol 53:642, 1962.)

• Central as well as nonfoveolarfixation may be steady or wandering. Wandering fixation, which occurs only upon covering the sound eye, must be distinguished from the monocular, spontaneous, pendular, and vertical oscillations that are occasionally found in deeply amblyopic eyes.

• This condition has been designated as the Heimann Bielschowsky phenomenon. It is clinically similar to other forms of monocular nystagmus that may occur in connection with a posterior fossa or brain stem disorder.

• Generally patients with esotropia, one would expect a nasal eccentricity, and in patients with exotropia, a temporal eccentricity offixation.

• There are, however, esotropic patients with temporal eccentric fixation and exotropic patients with nasal eccentric fixation (paradoxical fixation behavior).

• This type offixation occurs most frequently in patients who have consecutive deviations following surgery; for example, a formerly esotropic patient with long-standing amblyopia becomes exotropic, or vice versa.

• Paradoxicalfixation may also appear as a primary anomaly.

• Oppel noted that 15 out of 50 untreated esotropic patients (30%) fixated temporally, whereas von Noorden and Mackensen found 6 of 40 esotropic patients (15%) who fixated temporally, but only two of these had not been treated surgically.

Investigations

• Grossly eccentric fixation is readily established by holding a small light source in front of the patient’s eyes in the midline of his head; the fixating eye is covered, and the patient is asked to fixate the light with the amblyopic eye. If the eye has grossly eccentric fixation, it will make no movement of redress, or perhaps only a small one, but the reflection of the light will not be centered in the pupil.

Temporally decentered light reflex OS. Visual acuity OS: 5/200.B, Light reflex. OS remains decentered when OD is occluded. C, Series of fixation photographs of OS reveal fixation with a retinal area one disk diameter nasal to the optic disk

• Corneal Reflex Test:• Assuming that the angle lambda is the same in

both eyes, eccentric fixation can be obtained by comparing the position of corneal reflection of pentorch in amblyopic eye with that of the fellow eye.

• Relative displacement of reflex by 1mm would indicate the eccentric fixation of 11⁰(or20prism) approximately

• Ophthalmoscopic methods/Visuscope:• Project the ophthalmoscopic target onto the patients retina so that

it can be seen by the practitioner and its position judged in relation to the retinal details.

• Dilated pupil may be necessary, as the opthalmoscopic light causes puplillary constriction, also young patients accommodate about 4D looking at near targets causing examiner view blurred. Larger pupil increases field of view of examiner.

• With the fixating eye occluded the examiner observes the amblyopic eye noting the point or area of retina upon which the star is projected when the patient is attempting to look at the star straight at it.

• If the point or area is eccentric to the fovea the patient is asked whether the star is straight ahead or to one side of the light.

• If the patient sees the star straight ahead it indicates that the spatial value of retina has been centered at the eccentric point of fixation.

• If however the patient sees the star to one or the other side of the light, it must be then asertained whether the fovea has retained its straight ahead projection.

• To do this the patient is instructed to try and look directly at the star and see it straight ahead.

• This will result in a movement of the eye in an attemt to fix with the fovea often accompanied by the statement that star has dissapeared.

• This indicates that the normal spatial value of the fovea has been maintained (Eccentric viewing).

• The centre of projection of the amblyoscopic eye may also be discovered visiscopically whilst the patient has both eyes open.

• He is asked to look at the star and try to see it straight ahead . In order to do this the amblyopic eye may:

(1) maintain fixation with an eccentric point or area, but this may not be the point of eccentric retinal fixation and only represent the point or area of retina where the greatest visual acuity is obtained.

(2) make a movement to take up fixation with another point or area of the retina other than the fovea thet is with the point of eccentric retinal fixation. This point may be found to nasal to the fovea, whereas the point of greatest visual acuity is temporal to the fovea.

(3) makes a movement to take up fixation foveally

• After Image Transfer Method:• This test assumes that an after image in one eye will be

transferred to the normally corresponding point in the other eye.

• Photographic flash gun, masked to provide a bright strip of light, produce line after image in the non amblyopic eye while the amblyopic eye is occluded.

• The occluder is then changed to the other eye and patient made to read snellen chart or any other small fixation target.

• After a few seconds the after image appears have been transferred at a cortical level to the amblyopic eye.

• The patient is asked to indicate the position of the after image in relation to the fixation point.

• In eccentric fixation the after image will appear slightly to the side of fixation letter, few cases a long way away(Indicates deeply ingrained ARC)

• It can often over come by encouraging the patient to see the after image close to the fixation letter or passing through it.

• Entopic phenomena:• Haindinger’s brushes and maxwell’s spot are

entopic phenomena which occur due to characteristics of central foveal area of retina.

• They can be seen only by the foveal area under the right conditions.

• Haidinger’s brushes:• In case of Haidinger’s brushes a brightly illuminated blue

polarized field is used when the direction of polarization is rotated, two darkened and opposing sectors of the central field can be seen to rotate: the patient sees a dark blue rotating propeller.

• These circumstances are created by the use of specially designed apparatus which comprises a disc of polarised material rotating with a blue filter before a bright field.

• Haidinger’s brushes are yellowish, brushlike shapes that seem to radiate from the point of fixation when polarized, preferably blue light is viewed.

• With central fixation the center of the brushes is superimposed on the fixation point; with eccentric fixation the brushes appear peripheral to the fixation point.

• Its due to birefringence induced by the xanthophyll which is radially polarizing.

• Some structure in can behave as radial analyzer of blue filter- yellow macular pigment(xanthophyll) radial analyser.

• Vertical vibration falls on analyzer and horizontal on the plane of transmission perpendicular to the plane of incidence.

• Vertical element travel more blue light so blue brush brush is seen.

• Dichroism: The effect of absorption of light polarised in one direction and transmission in plane at right angles.

• Maxwell’s spot:• Maxwell’s spot is another entoptic phenomenon

whereby the macular region is represented by a dark spot appearing in the blue region of the visible spectrum. Its position, relative to that of a fixation mark, should be a sensitive index of the retinal area used for fixation.

• If a blue filter is quickly placed in front of your eye as you view a bright uniform white or yellow background a dark disc appears in the macular area.

• Why does this occur?????

• This is due to xanthophyll pigment (zeaxanthin) in the macula, which acts as a yellow filter which excludes more of blue light than the surrounding retina does so that relatively dark spot appears in the part of visual field that corresponding to the macula.

• Can also be used to measure the density of macular pigment. The darker maxwell spot the denser the pigment.

• Perimetry Method:• This method depends on the fact that the

physiological blind spot is same angular distance from the fixation point in both eyes in normal subjects; which is only true for isometric eyes and similar axial length.

• The position of blind spot is plotted in the non-amblyopic eye, and the position of its centre from the fixation point horizantally and below the horizontal are both carefully measured.

• The same measurements are repeated on the amblyopic eye.

• Any discrepancy between the position of the blind spot relative to the fovea in the two eyes indicates eccentric fixation.

• Amsler Charts:• Can be used only in case of foveal scotoma’s.• Patient with foveal scotoma reports an interruption

in the pattern of squares corresponding to the scotoma.

• In amblyopia with central fixation(Eg- anisometropic amblyopia) this distubence in the lines will be at the point of fixation and extended for about 1cm or more depending on the extent of amblyopia.

• In eccentric fixation the scotoma will be to one side of the point fixed by the patient- the projection of fovea.

• Mallet called this lang’s one sided scotoma and stated that it will only present in microtropia.

• Interestingly this scotoma is on the opposite to that side which would expected from strabismus (Eg- temporal in the esotropia, which is charecteristic of microtropia)

• Past pointing test:• This will give an indication if the localisation of objects in space

has been disturbed with the amblyopic eye.• This procedure is first tried with the good eye so that the

practitioner can see the normal ability of the patient to perform the test.

• The amblyopic eye is covered and the patient is asked to place a finger on the forehead just above the eye.

• A pen torch is held just above the eye at a distance of about 25cm.

• The practitioner explains the word ‘Go’ the patient moves the finger to touch the light.

• The occluder is then changed to the good eye and the test is repeated with the amblyopic eye, the patient being to touch the light with the tip of the finger.

• If this cannot be done the finger goes a few centimeters to one side. Past pointing is demonstrated.

• This indicates that the eccentric area, upon which the object of regard is imaged, is not being used to estimate the principal visual direction.

• The innate association between the principal visual direction and the fovea is maintained.

• RELATION TO VISUAL ACUITY: • There is normal decrease in visual acuity as a function of the

distance of the object image from the foveola.• While there is a general trend for eyes with low visual acuity

to have greater eccentricity of their fixation pattern.• Since visual acuity decreases in the peripheral retina, the

magnitude of the eccentric fixation will determine the best possible visual acuity that the patient may obtain in a case of eccentric fixation.

• EFFECT OF DARK ADAPTATION:• The fixation movements were considerably larger

when the eyes were observed in bright light through the ophthalmoscope.

• Which result in improved functioning of the amblyopic eye in low luminances

• CLINICAL SIGNIFICANCE:• The attention directed to the fixation pattern of

amblyopic eyes is justified by the importance of that pattern for the prognosis and selection of treatment.

• Peripheral eccentric fixation is an unfavorable sign and unsteady and wandering fixation are more favorable prognostic signs.

• It does not follow that standard vision can be more readily and permanently restored to an eye that already has central fixation.

• On the other hand, as it is often difficult to be sure of the presence of central or eccentric fixation at an early age, it is always appropriate to start treatment with direct occlusion.

• Inverse occlusion is sometimes indicated at the beginning of therapy in cases of steady eccentric fixation.

• Monitoring the fixation behavior in addition to checking visual acuity may be helpful during the treatment for amblyopia.

Treatment

• Red filter:• As the fovea contains only cones, a red filter placed before the

eye with non-central fixation while the dominant eye is occluded, may have the effect of inducing central fixation by preferential stimulation of the fovea.

• This is because under red illumination only the photopic mechanism is active so that the cones of the fovea should be preferentially stimulated.

• At other times when red filter was not being used, the patient was required to use an occluder in front of the amblyopic eye. The red filter was used for a period of two months to eighteen months.

• Projectoscope:• Modified keeler opthalmoscope which may be used for diagnosis and

treatment.• The Nut Auto-disc is provided with three graticules.• Stage 1: Localisation of the fovea With the Auto-disc position 1 The linksz star in green light may be focused on the retina and

accurately placed on the patient’s fovea.• Stage 2: Dazziling the extra macular retina With the Auto-disc position 2 By pressing the trigger on the projectoscope the second graticule

consisting of a 3ᴼ or 5ᴼ black spot automatically rotates to occupy the same position as the inner and the outer ring of linksz star.

At the same time a brilliant light of predetermined duration and intensity dazzles the retina whilst the macular area is occluded by the spot.

This technique inhibits the false fovea, and produses a strong after image within few seconds.

• Stage 3: Foveal stimulation by flashing light With the Auto-disc position 3 The 3ᴼ black spot is automatically replaced by a disc of white

light, whilst the extra macular retina is projected by a green filtered light. When the trigger is released to produce this effect, an alternate

is also automatically activated, produsing a flashing light. The duration of the light and dark phases may be pre-set by dails

on the control unit.

• This can be used in treatment which involves only the first two stages.

• The procedure is as follows-1)The pupil of the amblyopic eye is dilated.2)The patient is seated in such a way that the fixating eye

can observe a red spot light reflected in a mirror set at a suitable angle of about 45ᴼ so that the amblyopic eye is directed straight ahead or a slightly divergent position. The amblyopic eye must be maintained steadily in this position during the application of the after image.

3)Stage 1 and 2 are carried out.

4)The mirror is then removed, the fixing eye then occluded and the patient is instructed to look at a white screen which is illuminated with a flashing white light. The patient should then appreciate the after image. His responces could be one among the following:

(і) Appreciation of correct negative after image- that is an after image which is appreciated in the light and consists of a colored circle with a clear centre.

All persons with normal vision will appreciate a negative after image in the light and a positive after image in the dark.

(іі) Appreciation of negative after image but with no centre, i.e. no central spot- the after image being the same color all over.

(ііі) Appreciation of negative after image with a black centre.(iv) No appreciation of after image The aim of the treatment in the early stages, after the

patient is able to appreciate a after image is to obtain a correct negative after image.

• This is done by repeated application of the stimulus and also by regulating the dark and light phases of alternoscope which illuminate the white screen.

• The patient should be taught how to project the image straight ahead.

• Eg: In any case of convergent strabismus with eccentric fixation the patient will project the after image incorrectly in the direction of deviation.

• The patient is therefore taught to move his eye outwards and attempt to place the after image straight ahead.

• This may be facilitated if there is an object within which the patient may place the after image– i.e. a cross with a gap in the centre.

• Euthyscope:• This is modified opthalmoscope by cuppers and is used

for diagnosis and treatment of eccentric fixation.• The beam of light illuminates an area of approximately

30ᴼ at the posterior pole of the eye.• A green filter is incorporated and also two black discs of

5ᴼ and 3ᴼ diameter which can be placed in the centre of the beam of the light.

• Procedure:• The pupli of amblyopic eye is dilated.

• The fovea is located with the green filter and then the 5ᴼ or 3ᴼ black disc is projected on to the macular area and the light allowed to stimulate the circular paramacular zone for 20 to 30 seconds.

• This produses an after image.• Encourage the patient to appreceate correct after image.• He is then instructed to look through one of the

instrument incorporating haidingers brushes and encouraged to find the brushes within the clear centre of the after image.

• Bilateral eccentric fixation:• May occur in patients without strabismus and

with bilateral central scotomas caused by macular disease

• References:Binocular Vision And Ocular Motility Gunter K. von Noorden, MD Emilio C. Campos, MDPractical orthoptics in the treatment of squint T.Keith LylePickwell’s binocular vision and anomalies Bruce J. W. Evens