a. f.s.m.c. - core

ULTRASOUND AND PHAKOMETRY MEASUREMENTS

O F THE PRIMATE E Y E

Francis A. Young, Ph. D. George A. Leary, F.S.M.C.

Primate Research Center Washington State University

Donald N. F a r r e r , Ph. D.

I Distribution of this document is unlimited. 1

https://ntrs.nasa.gov/search.jsp?R=19660014825 2020-03-16T19:39:00+00:00Z

FOREWORD

This investigation was supported i n par t by Research Grant NB 05459-01 f rom the National Institute of Neurological Diseases and Blindness, Public Health Service, and in par t by the 6571st Aeromedical Research Laboratory, Holloman Air Force Base, New Mexico, under Project 6893 during August 1965.

The authors wish to acknowledge the assistance of K. Nolen Tanner, M.D., University of Oregon Medical School, Portland, Oregon, who performed the ophthalmoscopic and biomicroscopic examinations; Kenneth H. Oakley, M.D. , Bend, Oregon, who performed the refractions f r o m the sitting position; and Scott Oakley, Bend, Oregon, who assis ted in the performance of these examinations.

Appreciation is extended to D r . (Major) James R . Prine and the Veterinary Sciences Division for their support in the ca re and welfare as well as the administration of the general anesthesia for each subject used in this study. Specifically, the author wishes to thank Dr. (Captain) Phillip W . Day, Dr. (Captain) Donald C. Van Riper, TSgt Charles R. Teeters , J r , SSgt James C. Amos, A2C Kenneth H. Schult, A3C Barry J. Prandy, and A3C Wayne E. Johnson.

* * *<

This technical report has been reviewed and is approved for publication.

C.H. KRATOCHVIL, Lt Colonel, USAF, MC C omm and e r

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0

ABSTRACT

Results obtained on 160 eyes of 53 male and 40 female chimpanzees ranging in age from 2 to 15 years , using ultrasonography and photographic ophthalmophakometry to measure anterior chamber depth, lens thickness, and axial length, a r e compared with the results obtained on 140 human eyes of a comparable sex grouping using the same methods. methods a re not quite as high on the chimpanzees as on the humans, but the correlations between the measures of axial length and the vertical ocular refraction a r e virtually identical for the two groups. Either ultrasound or photographic ophthalmophakometry may be used successfully on primates and wil l yield results which compare favorably with those obtained on humans, but ultrasound is the method of choice since it does not require a s much time to make the measurement or to calculate the result as does phakometry. Fur ther , it does not require the rigid degree of control over the animal' s behavior that phakometry requires and i ts flexibility allows measurement in situations in which it would be impossible to obtain phakometry measurements. animal work, ultrasound is generally superior to phakometry.

The intercorrelations between

Thus for both human and

a

iii

I

INTRODUCTION

' .

Since 1962 Young and F a r r e r (Ref. 1, 2 , 3 , 4 , 5 ) have been carrying out a longitudinal study of the development of refractive characterist ics of chimpanzees' eyes as effected by age and environmental conditions. In order to gain more information, this study has recently been expanded to include ultrasonography (USG) and ophthalmophakometry, as well a s biomic r o s c opy , fundus photography , keratome t r y and ophthal- moscopy.

This paper presents a report of the methods and techniques used to obtain measures of the radius and power of the cornea, the depth of the anterior chamber, the radius and power of the lens surfaces , the equivalent power, the lens thickness, total power of the eye, and the axial length using ultrasound and phakometry. The results obtained a re presented in a preliminary form since the speed of transmission of ultrasound in the aqueous, lens, and vitreous of the chimpanzee eye have not yet been determined. The measurements presented a r e based upon the rate of transmission of ultrasound through the aqueous, lens, and vitreous of the human eye as determined by Jansson (Ref. 6). The phakometric measurements were made with equipment which, though modified in design, maintained the same optical principles and measurements as used on the human eye (Ref. 7). Since this equipment was used without alteration on the chimpanzee, the results obtained a r e subject to c o r r e c t i on.

The chimpanzee eye is slightly smaller than the human eye

It must be s t ressed that the results presented here a r e with a greater corneal curvature and a relatively deep anterior chamber. based on instruments and principles developed for use on human eyes which may require modification for use on chimpanzee and monkey eyes. The results presented here w i l l be corrected, i f necessary, after the equipment has been more efficiently designed for use on the chimpanzee and monkey eye.

.

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I1

APPARATUS

The ultrasound apparatus consisted of a Kretz Technik Series 7000 which provides a frequency range of 3 to 20 mega- cycles and a rectified t race to remove random small echoes. The baseline of the Kretz oscilloscope is not l inear and the degree of control over the signal and echoes is very limited. Fo r this reason the ultrasound unit of the Kretz was modified to allow simultaneous presentation of both the rectified and the unrectified echo t races . These t races were fed to a Tektronix type 547 oscilloscope equipped with a type 1Al dual-trace plug- in unit.

The type 547 oscilloscope features two identical t ime-base generators that can be used singly o r electronically alternated for viewing a single signal o r multiple signals at two sweep ra tes . The two time-base generators can also be used in "delaying'' and "delayed" sweep operation for highly accurate time measurements. Sweep rates of 0. 1 microsecond per cm. to 5 seconds per cm. in 24 calibrated steps with a displayed sweep-rate accuracycof 5 2 percent for both sweeps a re provided along with a sweep magnification feature which allows a 2X, 5X and 1OX magbification of the display horizontally with an accuracy of 6 5 percent i n the magni- fied positions.

Since distance is measured by ultrasound in t e r m s of the time required fo r a pulse to leave the crystal , s t r ike an inter- ference, and return to the c rys ta l a s an echo (the c rys ta l se rves as a transmitter and receiver) , the accuracy of measurement will depend upon the ra te of transmissions of sound through the medium and the accuracy with which the time intervals can be measured. The Kretz unit does not provide accurate measurement of time intervals nor does it provide an internal calibration whereas the Tektronix instrument provides both. Thus with the Kretz unit, it is necessary to use a piece of plastic which has been calibrated to provide an echo signal of a known distance interval on the screen of the scope. surface of the piece of plastic and the instrument controls a r e

The probe is placed iwcontact with the desired

2

adjusted until an interval of the proper magnitude is obtained on the scope. controls a r e merely set t o the desired values and provide much greater accuracy and reliability than the Kretz unit. recently substituted a Precision Sonics Company UTR - 1 ultra- sonic plug-in for the Kretz since the Precision sonics plug-in is designed to replace the 1Al plug-in in the Tektronix scope.

With the Tektronbc unit the sweep rate and delay

We have

This unit and the 547 scope provide a single package, highly versatile and accurate ultrasound unit which will display either a rectified or an unrectified t race with magnification of any portion of the trace o r both the rectified and unrectified t races at the same time without magnification. The complete unit weighs 3 1 . 8 kg. and is 33 cm. wide X 7 1 cm. long X 41 cm. high.

In the present study a six megacycle Kretz probe was The probe was fitted with a perspex tube made of acrylic used.

plastic with an internal diameter of 6 mm. and external diameter of 8 .5 mm. fi lm 0. 03 mm. (30 microns) i n thickness. When filled with water and attached to the probe, the tube provides a water column 31 mm. long. Recently, Saran plastic 0. 005 mm. (5 microns) thick has been substituted for the polyethylene as a membrane. Since the l imits of resolution of most ultrasound units in use today is 0. 2 to 0.3 mm. o r 200-300 microns, either of these membranes may be ignored as f a r as measurement is concerned and the front surface of the membrane may be taken as the front surface of the cornea.

The end of the tube was covered with a polyethylene

The phakometer, while based upon the one used by Sorsby, e t al. (Ref. 7 ) as far as dimensions a r e concerned, used two 32- candlepower, 12-volt automobile bulbs (GE 1073) as light sources instead of the electronic flash unit and m i r r o r arrangement used by Sorsby, These lights were operated at 6 volts for focusing and were llflashedlt by a short pulse f rom a 16,000 microfarad capacitor which provided a peak of 28 volts. The phakometer was mounted on the outer a r m of a Poser slit lamp base while the camera was mounted on the inner a rm. to hold the a rms at angles of 40 and 60 degrees while allowing the a rms to rotate as a unit. , F o r photography of the right eye the camera was to the left of the light sources and reversed for the left eye. p k s , which 1Ylashed" the l ights was controlled by the flash

The base was modified t o provide a locking device

The

2

synchronizer of the camera and provided an intensity of approximately 200 joules. A Miranda model F reflex camera fitted with an F 2 lens and extension tube to provide a magnification of 1: 1 was used on a rack and pinion focusing mount.

A Poser sl i t lamp with a Miranda camera equipped and mounted in the same manner as on the phakometer was used to photograph the slit lamp section. were mounted on a sliding board which permitted alternate use without moving the subject. Corneal curvature was determined through the use of a Bausch & Lomb keratometer with its range extended by supplementary lenses. The keratometer w a s cal i - brated against ball bearings throughout its normal and extended range .

The sli t lamp and the phakometer

I11

METHOD

Postmortem eyes were used initially to determine the control panel settings most suitable to provide adequate t races . These settings, which were maintained throughout the study a r e as follows:

The Kretz 7000

Reject - was left in "off" position (its use in an ear l ie r study resulted in modification of the magnitude of the intervals between echo peaks (Ref. 8)).

Filter - Ironrt position.

Gain - Trace expansion - when filled with water the probe

position t t 2 ' t of the arbi t rary scale prov.ided.

extension tube repeated the membrane signal every 31 mm. Since the chimpanzee axial length is less than this, all data were contained between the membrane signals. Setting at a maximum spread of the screen (Scale 6).

Calibrator - Set at zero and not used.

4

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The Tektronix 547

only time base A was used on the scope and the settings for this time base were: Mode, auto stability; slope, positive; coupling, AC; and source, plug-in. "AI1 was combined with a time/cm. of 10 and sweep magnification of 5X. set at "Calib.

A horizontal display setting of

The calibrator was

Dual t race 1Al plug-in

The mode w a s set at the alternating position in order to display the rectified t race on channel one and the unrectified t race on channel two at the same time. variable volts/cm. at maximum and input selector at "AC. ( I

Both channels had settings of 112f1 on volts/cm. ,

These settings gave a good amplitude to the lens surface peaks without evidence of "infolding" of the peaks when maximum amplitude signals were obtained. The Kretz instrument was used as a monitor f rom which observations of the form of the t race could be obtained. All measurements were taken f rom the film negatives obtained by photographing the t races displayed on the Tektronix scope.

Two calibrations were necessary, (a) the camera reduction ratio, sc reen size : film size, and (b) the screen magnification ratio, screen time interval: actual t ime interval or the ratio of the time interval represented by the length portrayed on the screen to the time interval in the eye.

( a ) Camera reduction - The interval between the two

By measuring this interspace on the film negative the extreme graduations on the face of the oscilloscope screen is 100 mm. camera reduction could be calculated from

loo/ C = R (camera reduction ratio)

where C = the microscope vernier reading in mm. for the measured interspace. The equivalent screen distances (S) between echo peaks could then be determined by measuring the distance between the p e d s 02 tkic G?rr; and rr,u?tip?ying 5.; the p,az-erz red-lrctioc ratin {R-!.

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5

The camera reduction was reassessed f o r each t race used. This ensured that so long as the camera w a s refocussed after removal for reloading, correct positioning was not a variable.

(b) Screen magnification - The time/cm and sweep magnification settings were recorded f o r each subject. purpose of the present investigation these settings were kept constant. seconds of t ravel represented by the screen to the actual number of microseconds in the media of the eye is obtained f rom the proportion

For the

Relative to the screen length, the number of micro-

TS/2M = (microseconds of t ravel)

where T is time/cm. setting on scope, S is the screen measurement o r the product on the camera reduction rat io and the measurement on the film, and M is the sweep magnification setting on the scope. to reach a surface and the echo to reach the c rys ta i , it must be halve d.

Since the screen interval represents the t ime for the pulse

Pending further investigation the following velocities were used in all calculations: aqueous and vitreous, 1 5 3 4 . 5 meters per second; the lens, 1 6 4 2 . 2 meters per second. These values a r e based on Jansson's values for human subjects of 1532 meters per second f o r the aqueous and vitreous and 1641 meters per second for the lens at 37' centigrade. the chimpanzee is approximately 3 8 . 4 centigrade, a temperature correction was made using Jansson's values for the aqueous and vitreous of 1 . 8 meters per second per degree centigrade and of 1. 0 meters per second per degree centigrade for the lens.

Since the rec ta l temperature of 0

Calculation of the t rue distances - The camera reduction (R), the screen measurement ( S ) and the number of microseconds of t rave l (U) in the respective t issues were calculated. To obtain the t rue depth of the anterior chamber and the thickness of the lens , the number of microseconds of t rave l in the respective t issue6 is multiplied by the velocity of the ultrasound within the t i s sues , i. e . ,

V U = t rue distance within the eye.

6

F o r the axial length, the posterior chamber depth f rom the r e a r lens peak to the retinal lens peak is calculated and added to the t rue anterior chamber depth and the t rue lens thickness with 0.5 mm. being added to account f o r the ret inal thickness (Ref. 8). < .

1, lialculation of the lens power - The lens power re fer red to the corneal vertex was calculated f rom a predetermined knowledge of the vertical ocular refraction under cycloplegia and the vertical corneal power, where

k'

K = vertical ocular refraction(from tables, Ref. 7 )

F1 = vertical corneal power (from keratometer or tables)

Fe = total power of the eye

Fv = power of the lens at the corneal vertex

= dioptric equivalent of the axial length (n = 1. 3333)

The. vertical ocular refraction is algebraically subtracted f rom the dioptric equivalent of the axial length taking into account the sign of the refraction (k' is always positive).

Photographic ophthalmophakometry combined with a photographic slit lamp technique was used to obtain the following data on the components of ocular refraction: chamber, the radius and power of the front and back surfaces of the I I I , lens , the equivalent power of the lens, the total power of the eye and the axial length. The method is that used by Sorsby, et aL I '

(Ref. 7 ) and requires mydriasis and complete cycloplegia.

the depth of the anterior

Comparison ophthalmophakometry utilizes three of the Purkinje images: cornea, and the third and fourth images f rom the front and back surfaces of the lens respectively. If each of these images could be duplicated by using two vertically-aligned light sources , the separation of each individual pa i r wil l be proportional to the radius of curvature of the parent surface when compared with the predetermined radius of curvature of the cornea.

the first image from the anterior surface of the

In the present investigation each pair of images was photographed separately since they do not fo rm i n the same plane. The f i r s t and fourth images were photographed at 40" to the visual axis and the third image at 600,do avoid obscura- tion by the bright corneal images (Ref. 9). A photograph of the sl i t lamp section was obtained at 40° to provide data for the apparent depth of the anterior chamber and apparent lens thickness from which the t rue depth and t rue lens thickness could be calculated.

The separation of the Purkinje images and the components of the slit lamp section were measured on the film negative under a Pye two-dimensional microscope. f rom a knowledge of the cycloplegic ocular refraction, the corneal data and observations on the depths and curvatures of the lens is by conjugate foci relationships employing the paraxial equations of optics.

Computation of the resul ts

A. Subjects

The animals used in this study comprised of 93 available chimpanzees (53 males and 40 females) f rom the total population of 119 animals at the colony of the 657ist Aeromedical Research Laboratory, Holloman Air Force Base, New Mexico. The subjects were divided according to age, based on dental eruptions , and sex as shown in Table 1 .

B. Procedure

On the day of the study the chimpanzees were not fed until the examination had been completed. 2 2 . 5 kg. ) were brought into the preparation room and strapped to a portable operating table. hydrochloride (Cyclogyl) was placed in each conjunctival sac followed by a second drop ten minutes la ter . second drop the chimpanzees were given 1.65 mg. phencyclidine hydrochloride (Sernylan) per kilogram of body weight, intramus - cularly. Twenty minutes la te r , a third drop of Cyclogyl was placed i n each conjunctival sac. Sernylan was given before the first drop of Cyclogyl was placed i n the sac.

Small animals ( less than

One drop of 1 percent cyclopentolate

Following the

For the animals over 2 2 . 5 kg. ;

8

TABLE XI. Age and Sex Determination of the Chimpanzee

Number Number Age of of

(Years 1 males female s Total

4

10

6

6

17

9

5 12 8 20

6

7

8

9

10

1 1

13

15

9

8

2

1

1

0

0

0 -

7 16

4 12

2 4

2

2

3

3

1 1

1 1

1 - 1 - Total 5 3 40 93

9

After the administration of the third drop of Cyclogyl the animals were strapped into chairs which consisted of the base and back par t s of an Armed Forces medical field examination chair mounted on a hydraulic bumper jack. animal w a s taped to the headrest of the chair which was then rolled into the f i r s t examination room. ination with a Thorpe sli t lamp was usually performed f i r s t , followed by an ophthalmoscopic examination and fundus photography with a Nikon fundus camera. head and of the chair were adjusted by the examiner and an assistant to permit these examinations. inations the animal was moved to a Bausch and Lomb keratometer for determination of corneal curvature. keratometry. In a few cases keratometry preceded the biomicroscopic and ophthalmoscopic examinations.

The head of the

A biomicroscopic exam-

The position of the animal 's

Following these exam-

Phakometry followed

After phakometry the animal was moved to a second examination room where it was refracted in a sitting position while still in the chair . f r o m the chair and returned to a table in the supine position. The chimpanzee was then moved to a third examination room for a second refraction by a different refractionist and for ultrasound measurements and Schiotz tonometry. All refractions were made with Copeland s t reak retinoscopes and trial case lens using a procedure described in detail in Reference 10. At this point the animal w a s removed f rom the table and if small, taken t o a recovery room o r , i f large, to his cage. Using this procedure, between 10 and 17 animals were examined each day.

At this point the animal was removed

IV

RESULTS

The measures of anterior chamber depth, lens thickness, and axial length as determined on 160 eyes by ultrasonography and photographic ophthalmophakometry a re presented in Table I1 along with the differences between paired measures and the ver t ical ocular refraction.

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Table 111 presents a comparison of the results obtained on the chimpanzees with those obtained on human subjects by Leary, Sorsby, Richards and Chaston (Ref. 8). The compar- isons a r e made in te rms of correlation coefficients between comparable m e a s ~ r e s on the two groups of subjects. chimpanzee group is somewhat younger than the human group since the youngest human is seven years of age compared with two years of age for the chimpanzees. 18 nf the 140 eyes w e r e f rom children aged 7 to 11 and the remainder were 25 o r less except for one sugject 34 and one 47 years old. virtually the same as those obtained on the older subjects. sex breakdown - - males, human 59 percent, chimpanzees 57 percent and females , human 41 percent, chimpanzees 43 percent - - is virtually the same i n both groups.

The

In the human population

The resul ts obtained on the younger subjects are The

V

DISCUSSION

The results presented in Table I1 do not show as c l b e an agreement between the measurements made by photographic ophthalmophakomet r y and ultrasonography on the chimpanzee eye as the agreement found on human eyes by Sorsby, Benjamin, and Sheridan (Ref. 7). subjects were able to cooperate and did s o by maintaining f i x - ation during the measurement process. were under Sernylan anesthesia, some still showed slow eye movements and occasional body movements which made it difficult to hold the eye in the proper position for photographic phakometry. A fur ther complication a rose f rom the necessity of having to adjust the position of the body of the animal i n order t o bring the eye into proper position for photography. For these reasons photographic ophthalmophakometry proved to be much more time consuming and difficult t o use on the chimpanzees than on humans, and the validity of the results decrease in proportion to the increase i n difficulties encountered.

This might be expected since the human

While the chimpanzees

2 1'

TABLE 111. Cor relations Between Comparable Measure me nt s Obtained with Phakometry and Ultrasonography

in Humans and Chimpanzees

Anterior chamber - 0 . 2 0 USG' ( -0 . 33,)3

Anterior chamber -0 .23 t o . 72 I phak. ( -0 . 31) (+O. 4 5 )

Lens thickness +0 .08 -0 .35 USG ( t o . 30) ( -0 . 23)

Lens thickness io. 03 -0 .44 t 0 . 7 1 phak. (io. 25) ( t o . 0 6 ) ( t o . 2 4 )

Axial length -0 .73 t 0 . 4 9 -0. 17 USG ( -0 .77 ) ( t o . 61) ( - 0 . 2 9 )

Axial length -0 .76 -0. 17 t o . 97 phak. ( - 0 . 7 0 ) ( -0 . 06) ( t o . 83)

(All values 0. 17 and higher for humans and 0. 18 and higher for chimpanzees a r e significant at the 5 percent level of confidence. )

Ultrasonography Phakometry Chimpanzee values shown in parenthesis

~ ~ ~ ~

22

On the other hand ultrasonography on chimpanzees does

This can not require as fixed a position of the animal since only the t ip of the probe need be brought in contact with the cornea. be done while the animal is in the supine position and strapped to 2 table so that gross body rnovem-ents are few and movements of the head can be controlled to a great degree by holding with the hands or a chin holder. is possible t o follow movements of the eye to some extent and still obtain measures on the visual axis.

Since the probe is light and flexible, it

While the correlations between the paired measures of the same element such as the anterior chamber are all higher in the human subjects as would be expected if the measures a r e more valid and reliable, the axial length correlation between the two methods is quite high in the chimpanzee. In fact , the correlation between the vertical ocular refraction and the two measures of axial length is approximately the same as that found i n the human. The largest discrepancies between the human and chimpanzee correlations occur in correlations involv- ing the anterior chamber depth and the lens thickness. In the human subjects the lens thickness is unrelated to the vertical ocular refraction whereas it is significantly related i n the chimpanzee. animals have thicker lens than the myopic animals and this relationship holds with either ultrasound o r phakometry.

The relationship suggests that the hypermetropic

In general, the correlations between the two types of measurements on the chimpanzees a r e high enough to suggest that either method can be used but that ultrasound appears to have a slight edge in validity and reliability over phakometry as fa r a s the animal is concerned. over phakometry in t e r m s of flexibility and in t e r m s of t ime required for i ts use. ofkrs; the greater promise for research on the development of refractive character is t ics i n animals and in humans.

Ultrasound has a large edge

Thus it would appear:that ultrasound

2.3

REFERENCES

1. Young, F . A . , and D. N. F a r r e r . The Refractive E r r o r and Intraocular P r e s s u r e of Immature Chimpanzees Under S e rny 1 Ane s the s ia, AR L Technic a1 Documentary Report Number ARL-TDR-63-3, Holloman AFB, New Mexico, February 1963.

2 . Young, F. A . , and D. N. F a r r e r . "Refractive Characterist ics of Chimpanzees. ' I Amer. J. Optom. , 41, 81-91, 1964.

3. Young, F. A . , and D. N. F a r r e r . The Refractive E r r o r and Intraocular P r e s s u r e of Immature Chimpanzees Under - Sernvl Anesthesia, A R L Technical Documentarv Report - Number ARL-TDR-64-9, Holloman AFB, New Mexico, August 1964.

4. F a r r e r , D. N., and F. A. Young. A Longitudinal Analysis of the Refractive Characterist ics of Chimpanzees. ARL Technical Documentary Report Number ARL-TR-65- 1, Holloman AFB, New Mexico, March 1965.

5. F a r r e r , D. N . , and F. A. Young. The Refractive Character- ist ics and Intraocular Tensions of Colony Chimpanzees, ARL Technical Report Number ARL-TR-65-23, Holloman AFB, New Mexico, November 1965.

6. Jansson, F. "Measurements of Intraocular Distances by U l t r a - sound.'' Acta ophthal., Kbh. Supp. 74, 1963. --

7. Sorsby, A . , B. Benjamin, and M. Sheridan. Refraction and Its Components During the Growth of the Eye. Spec. Rep. Ser . med. Res. Coun., Lond. No. 293, H. M. Stationery Office, 1961.

8. Leary, G. A . , A. Sorsby, Joan M. Richards, and Jennifer C haston. Components of Ocular Refraction in Life 1. If Tech- nical Considerations. Vision Res. 3 , 4877498. 1963.

Ultras onographic Measurement of the

24

t

*

9 . Thomson, L. A. The Anatomy of the Human Eye. Clarendon Press , Oxford, 1912.

10. Young, F. A. “The Distribution of Refractive Errors in Monkeys. Exp. Eye R e s . , 3, 230-238, 1964. ---

25

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ULTRASOUND AND PEAKOMETRY MEASUREMENTS OF THE PRIMATE E Y E

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3 A B S T R A C T Results obtained on 160 eyes of 53 male and 40 female chimpanzees ranging in age from 2 to 15 years , using ultrasonography and photographic ophthalmophakometry to measure anter ior chamber depth, lens thickness, and axial length, a r compared with the resul ts obtained on 140 human eyes of a comparable sex grouping using t h e same methods. not quite a s high on the chimpanzees as on the humans, but the correlations between the measures of axial length and the vertical ocular refraction a r e virtually identical for the two groups. Either ultrasound or photographic ophthalmophakometry may be used successfully on pr imates and wi l l yield resul ts whic compare favorably with those obtained on humans, but ultrasound is the method of choice since it does not require as much time to make the measurement o r to calculate the result a s does phakometry. Fur ther , it does not require the rigid degree of control over the animal 's behavior that phakometry requires and its flexibility allows measurement in situations in which it would be impossible to obtain phakometry measurements. sound i s generally superior to phakometry.

The intercorrelations between methods a r e

Thus for both human and animal work, ultra

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security Classification 1 4 .

K E Y WORDS 0

Primate

Ultrasound Measurements Phakom et r y Mea su rem ent s

Eye

Human Eye Anterior chamber depth Lens thickness Axial length

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