Visual Acuity and Cataract Surgery

Visual Acuity and Cataract Surgery George W. Weinstein, J . Vernon Odom, and Robert R. Hobson

Visual acuity is the prime motivation for cataract surgery. Yet there· may be a discrepancy between the appearance of the opacity and Lhe visual acuily. It is not unusual fo1· some patients to ha"e markedl)' decreased visual acuit)' with what appears to be an insignificant degree of nuclear sclerosis, while other patients may have urprisingl) good acuit} despite the presence of advanced posterior sub-capsular opacities, a viewed b) the slit lam p. In addition, patient ymptoms of poor visual function may be out of keeping with their performance in the usual office seuing where visual acuity is tested with a Snellen chart. Jt is not unusual for patients to complain of trouble with reading or driving a car \\'ith still imact or nearl) intact performance as measured with Snellen acuity testing. Visual tasks may be particularly difficult under other conditions, such as night Lime driving or reading outdoors in bright sunshine. In addition, visual disability from co-existing ocular disea es, such a!> macular degeneration, glaucoma, or other optic neuropathy, mar be difficult to son out from that produced by the patient's cataract. It is common ophthalmic parlance to refer to a patient ,,·ith a "20/40 cataract" but o nly "count fingers" vision, or to speak of a "20/100 macula" or a "20/50 disc." All ophthalmologists are fully aware that visual acuity i but one aspect of visual function and Lhat good visual function demands the integrity of the entire visual system. This manner of speaking represents an auempt on the pan of the clinician to fractionate the various components of the visual process and to assign levels of performance to each of them, by

26

G.W. WEINSTEIN, J .V. ODOM ANO A A. HOBSON

arbitrarily de ignating a Snellen acuity notation to indicate their respective abiUties or disabilities. The problem of evaluatin g visual fu nction in the presence of media opacities is one of panicular importance in clinical practice. In the absence of media opacities, the evaluation of visual [unction rests primarily o n examination of the f undus with an ophthalmoscope and d etermination of corrected visual acuity. The presence of media opacity often makes ophthalmoscopic examination of the fundus difficult o r impossible. The reduced visual acuity a ociated with media opacities may be attributable to the opacity or to a disease posterior to the opacity. In cases of advanced disease, litcle if any improvement in visual function can be expected following removal of the opacity. The purpose in attempting to assess visual functions in cases of media opacities is to determine the likelihood of substantial improvement of visual acu ity fo llowing surgical removal of an opacity so that the patient and ophthalmic surgeon ca n discuss the risks and likely benefits of removing the media opacity. Therefore, appropriate medical management of the patient with cataracts is improved if one can accu rately determine the po rtion of the visual deficit auributable to the opacity and that attributable to the disease. The importance a nd magnitude of the problem ma)' best be understood by examining the statistics associated with cataract extractions (CEs). In 1985, there were approximately 1,200,000 pe rformed and the projected n umber for 1990 is 2,100,000. 1 Of all CE , 8.8 percent, or more than 40,000 cases annually, have a visual result of 6/12 (20/40) or poorer attributable to fact0rs other than surgica l complications.2 One assumes these patients had an addi 1ional visual disability which was undiagnosed at the time of surgery. Certainlr some of those patientS whose post-operative acuity is poorer than 20/40 still have considerable improvement in visual acuity and are better able to care for themselves. Simple, reliable test which can determine the functional integrity of th e macula would aid the o phthalmologist in idemifying those case with media opacities that are least likely to benefit from surgery and in providing the patient with appropriate counseling. Such improved diagnostic testing would save some patiems the risk of surgical complications. If one assumes a reduction of even 10 percent in the 8.8 percent of patients who have no complicatio ns but who do not attain 20/40 fo llowing surgery, testing has the potential of saving $12 miJlion annually in medical coses (4000 cases x $3000 surgery and hospital costs}. T he single largest category of cataracts extracted is senile cataract and the percemage of those with visual resu lts of 20/40 o r better is reduced in those over 65. 2 Medicare pays the costs of approximatel)' 60 percent of all CEs performed in hospitals (I lealth Care Finance Administration, Center for Health Care Statistics, personal communicatio n). Therefore, any substantial reduction in the percentage of CEs with poor results because of prio r d i ea e would reduce the demands on the Medicare/Medicaid budgets. For example, a 10 percent

6

VISUAL ACUITY AND CATARACT SURGERY

27

reduction has the potential to reduce Medicare/Medicaid costs by at least $7,200,000 annually. The Retinal and Choroidal Diseases Panel of the National Eye lnstituce recognized the magniLUde of these issues and recommended that a priority of that section should be improvement of present techniques and the development of new non-invasive techniques to evaluate visual function behind opacities.3 A media opacity diffuses, absorbs, and scatters the light reaching the retina. Consequently, the luminance and contrast of stimuli used to assess visual function are reduced relative to that which would reach the retina of an eye without an opacity. The utility and validity of standard tests of visual function a re reduced, which often complicates the decision to surgically remove the cataract. Despite these difficulties, several tests have proved useful or have been suggested as being useful in evaluating the appropriateness of surgery to remove cataracts. Each has its Limitations, however. For example, ultrasonography may be used to provide information about the structural integrity of the retina, e.g., detect retinal detachments, 4 '5 but it fails to provide information about retinal function.

TESTS OF VISUAL ACUITY

Visual discrimination depends on the optical resolving power of the eye and the integrity of the retina and visual pathways. A variety of tests of visual acuity have been developed, some of which provide subjective, qualitative information, while others are subjective and quantitative in nature. Electroph)'Siologic tests provide objective information about visual acuity, but there remains a problem of their accurate quantification. Snellen Acuity

This is the most commonly performed clinical test of visual function. It is based on a visual angle of l minute of arc which each component of a standard optotype, such as a letter E, with three horizontal bars, one vertical bar, and two horizontal spaces subtend the standard visual angle, while the entire optotype subtends a 5-minute angle (Fig. 1). For Snellen acuity testing, small high comrast optotypes are utilized, either printed with a mode1·ate degree of ambient illumination or projected wiLh a low ambient illumination. Snellen acuity may be measured both for distance (usually 20 feet or 6 meters) or near (usually 14 inches or 1/3 meter), and while there usually is correspondence between these two readings, there is sometimes a discrepancy, particularly in patients with nuclear cataracts (where near acuity may be better tlrnn distance acuity) and posterior sub-capsular opacities (where the reverse is true).

28

G.W. WEINSTEIN, JV. ODOM ANO A.A. HOBSON

---

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Figure 1. Snellen visual acuity chart. The Snellen

chart uses small, high contrast optotypes, predicated on the concept of the visual angle of one minute of arc.

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Other Tests

The tests \\ hich have been proposed to assess 'isual function in the presence of media opacities include behavioral detection of pauern (laser inlcrferometry, entoptic phenomena, and projection of eye charts through holes in the opacity omo the retina), electrophysiologic responses to light (electroretinogram and visual evoked potentials [VEP ]) and to pattern (VEPs), and discrimination of relative patte1 n location(' ernier acuity). The red uced luminance and contrast of stimuli employed to elicit psychophysical or electrophysio logic responses result in alterations of these re ponses which are characteristic of those stimulus alterations but which may also be confounded with disease. Therefore, the ideal response to test visual function behind media opacities would be a response which remains unaltered as luminance or contrast are reduced, but is altered by retinal d i ease. Each


29

of these tests and its accuracy will be discussed briefly. The degree to which it meets the dual criteria indicated will be assessed, especially as a function of the cataract density. Also an indication of the test's accuracy will be presented. To compute accuracy, we have reviewed the available literature and constructed a 2 x 2 contingency table of the results of each article reviewed. Accuracy was the number of true positives and true negatives divided by the total sample. So the levels of accuracy reported would be as similar as possible, a criterion of 20/50 or better as good acuity was employed, if the data provided permitted. Blue Field Entoptoscope (BFE). The BFE measures a person's ability to appreciate an entoptic phenomenon. Patients view a deep blue light (430 nm) which stimulates approximately the cemral 20 degrees. If patients report seeing 20 or more moving spots of light, which correspond to the white corpuscles in the perifoveal circulation, they have a normal retina and are predicted to have a good post-operative visual function, i.e., 20/40 or better.6 Three brightness levels are provided. If the phenomenon is not appr eciated at lower luminance levels, one may increase the luminance. Pupils are dilated. Sinclair et al. 6 introduced the Blue Field Entoptoscope (BFE) as a test of retinal function behind media opacities. Its accuracy ranged from 72 to 95 percent depending on the light level of the stimulus used to induce the phenomenon. Subsequent assessments of the instrument have indicated an accuracy of 55 percent7 to 78 percent,8 •9 the reduced accuracy being attributable to the reduced Light transmitted through more dense cataracts in which the patients had poorer pre-operat.ive acuity. 1- 9 ln Skalka's8 sample, a flas h VEP was performed in a sub-group of the patients with poor pre-operative acuity. lls accuracy was 95 percent.

Several manufacturers make instrumems for assessing acuity using interferometry. Double images of collimated light source or a laser are focused in the vicinity of the pupil. The interference of the two light sources forms sinusoidal gratings or fringes on the retina in the area of over-lap of the two beams. The spatial frequency of the fringes is a function of the two point separation. Orientation of the gratings may be varied. Although there are no precise procedures for determining the highest spatial frequency for which patterns are no longer discriminable, staircase procedu res are often used. Testing is conducted with dilated pupils. Green and Cohen 10 employed LIA as a means of assessing visual function behind cataracts, indicating its accuracy as 88 percent. The tendency of the procedure Lo over-estimate the pose-operative acuity of amblyopes was noted early, 11 as was its difficulty in use with patients with dense cataracts. 12 Subsequent reports have indicated an accuracy of 64 to 85 percent. 1' - 17 Faulkner 16 had identified seven conditions in which the Laser Interferometric Acuity (LIA) .

30

G.W WEINSTEIN, J.V. ODOM AND A.A. HOBSON

retina is incorrectl y id en tified as normal: serous detachment, cyscoid macular edema, glaucoma, amblyopia, macular ho le or cy ts, geographic atroph y o f the macular pig ment e pitheliu m , and early post-operati\•e detachment; and two conditions in which the retina is incorrecliy iden tified as abnormal : mature cataracts or testing with undilated pupils. Faulkner notes tha t if one exclude these n ine conditions accuracy is 100 percent. Halliday and Ross 17 note that if one could un ifo rmly identi fy these condition prio r to the test, the test would be largely unneces ary an d argue that LIA is not useful clinicall)'. Potential Acuity Meter (PAM). The PAM p rojects an eye chart subtending 6

degrees throug h an 0. 1-mm aerial pinhole aperture which passes th rough a clear region of Lhe cacaracto us le ns and is imaged on the retina (Fig. 2). Snellen acuity is measu red by asking patients to read the chart as they would a wall chart. The instrument fits on a slit lamp so the beam may be aimed using the lit lamp. Adjustmen ts for spherical cor rection can be made. T esting is conducted wilh pu pils dilated. Minkowski, Palese, and Guyton 18 introduced the PAM, which like LIA, projects a stim ulus into the eye. Rather than a gratin g, the stimulus is a Snellen eye chart. PA f accuracy was 84 percent in Minkowski's study.

Figure 2. Potential acuity meter (PAM). The PAM projects a Snellen acuity chart onto the patient's fun· dus through a 0.1-mm en· trance pupil.


31

Faulkner 16 compared LI A and PAM. He found thal in those seven categories of cases in which the LIA predicts beuer acuity than is obtained I 00 percent of the time, the PAM predicts better acuity than obtained in only 55 percent of the cases. This apparent advantage of the PAM may be a consequence of the fact that it consistently provides estimates of acuity which are poorer than those obtained using LIA, making false over-estimates of acuity less probable. 16 The most frequently used electrophysiologic response is the ERG (Fig. 3). As a mass response of the entire retina, abnormal ERGs tend LO represent diffuse retinal disease, but are unaffected by localized d isease, such as macular degeneration. Therefore, ERGs are useful in identifying generalized rod or cone degenerations or in identifying retinal detachments. 5 •19 However, it is quite possible to have a minimal or non-recordable ERG and have macular function intact or only minimally impaired. 5•20 Therefore, even the bright Rash ERG, while indicative of the general health of lhe retina, is not useful as an indicator of post-operative visual acuity.

E/ectroretlnography (ERG).

Figure 3. Electroretinogram (ERG). The electroretinogram is generated from the outer retinal layers. It is a mass response of the retina and, as ordinarily performed, does not provide specific information about the macula.

32

G W WEINSTEIN, J.V ODOM AND A A HOBSON

Visual Evoked Potential (VEP). The VEP is comprised of the electrical respon!>es LO visual 'ltimuli recorded from the occipital cortex. These respomes are picked up b}' scalp electrodes and then proce sed bra signal averager (Fig. 4). Although Lhe macular region of the retina represents only approximately 5 percent of its surface, there is a disproponionatel)' large area of rhe surface of the occipital cortex devoted to the macula, whereas the periphery of the fundus achieves onl} a much smaller topographic area. Also, Lhe large area represented by the macula is at the posterior tip of the occipital cortex, whereas the f undus periphery is represented above and below the calcarine fissure, along the medial aspect of the cerebral hemispheres. As a result, the VEP primaril) reflects visual function of the central retina. Becau~e the VEP is large!}' determined by the cemral 6 to 12 degrees of the retina it has been suggested as a means of e\·aluating central retinal function in the presence of media opacities.21 -' 1 A number of investigators have reported considerable accu raC} predicting the visual acuities of patients folio'' ing surgical remo\'al of media opacities 1 ~~ ba ed on pre-

Figure 4. Visual evoked potential (VEP). The VEP reflects electrlcal activity of the visual cortex In response to visual stimulation. The normal VEP In response to 10 Hz stimulation shows a double peaked response, with a small peak related to central visual function (visual acuity). The upper tracing shows the normal pattern, while the lower tracing Is obtained from a patient with a macular lesion, showing preferential loss of the small wave.

VISUAL ACUITY ANO CATARACT SURGERY

33

surgical recordings of flash-elicited VEPs. The accuracy of these reports has ranged from 48 to 95 percent. Because of saturation ofVEPs as flash intensity increases, VEPs are little affected in amplitude or latency as intensity is increased from 2 to 4 or 5 log unitS above threshold, at which point amplitude decreases. 32 Therefore, it has been reasonable LO assume that VEPs elicited by Aashes would be minimally influenced by dense opacities. The criteria for response evaluation have varied widely, including (1) absolute response amplitude, (2) signal-to-noise ratio, (3) response time, (4) appropriate variation of the response with manipulation of some stimulus parameter, (5) response morphology, and (6) comparison with the normal fellow eye. Some criteria are associated with greater accuracy than are others. Absolute amplitude of a single transient stimulus condition appears the least accurate (48 to 62 percent), as it is easily subject to individual variation. 24•26- 28 Similarly, latency or phase angle of a V£P elicited by a single stimulus condition is not particularly useful. 23•26 Morphology or VEPs elicited by transient stimuli (3 or 4 c/s) tends not to be useful as it varies dramatically between normal individuals. 24 - 26 However, relative changes in phase angle as a function of Aash rate~·22•2 :i o r the relative changes in amplitude with increasing illuminance 14 or Hash rate"·23 appear to be quite predictive. At faster rates of stimulation (i.e., eight flashes per second or more), morphology is more constant across individuals and can be a reliable indicator or response normalcy. 23 Accuracy is often improved when there is a normal fellow eye for comparison23•24 or multiple objective criteria are used. 22·27 Several methods have been used to maximize response validity, including scimulation through closed eyelids to minimize differences between normals and those with media opacities22- 2 ~ and the use of red stimuli. 2'1.26 One major advantage of the Aash VEP is that its accuracy should be unimpaired by cataract density. Our preliminary dala show thaL accuracy is approximately the same in patients whose pre-operative acuity is 20/200 or better and in patients whose pre-operative acuity is 20/400 or worse. Despite its established effecLiveness in predicting post-operative acuity in patientS with media opacities, the utility of flash-elicited VEPs has some limitations. Because they are elicited by luminance changes, Aash VEPs are indirectly related to visual acuity. Only to the degree that the pattern and luminance systems rely on the same neural substrate of the macular region will Rash response inform one as to the integrity of pattern processing. Secondly, neither standard conditions nor easy-to-use objective criteria for discriminating normal versus abnormal responses have been established. l!I Vernier Acuity (VA). 1-l yperacuity refers to acuity tests which produce better

performance than that which could be predicted on the basis of the spacing of central retinal cones. This includes both stereoacuity and vernier acuity. VA is of particu lar interesl with regard LO evaluation of patientS with opaque media (Fig. 5). Although VA is one of the finer spatial discriminations of the visual

34

G.W. WEINSTEIN, J.V. ODOM AND R.R. HOBSON

Figure 5. Vernier acuity. Vernier acuity relies upon the ability to detect visual alignment or misalignment of two points In space. This highly sensitive test can be performed even through opacities of the ocular media.

system, major alterations in the stimuli result in relatively small changes in vernier threshold. The effects of blur on VA have only recently been of interest and re earch on the clinical usefulness of VA is even more recenl. luch of the data has been available only in the last }ear. VA is minimally affecLed by blur or diffusionss and yet is severely impaired by retinal disease. 54 The robustness of VA is surprising at first glance, because it is one of the finer pauern discriminations of the human visual system, frequently less than IO" (arc econd). Presumabl)', the resi tance of position discrimination req uired by VA task to disturbance by optical degradation is accounted for br the physical basis of the d iscrimination. A possible stimulus used to align two objects is the alignment of the centroids of their light d istributions. 3~- 37 Because the centroid of a light di tribution is unaffected by opticaJ degradation, the alignment of the centroids of the light di tributions of two objects should remain quite accurate. Recent efforts to use VA to evaluate patiems with media opacities appear successful.!18 Pattern VEPs. Pauern-elicited VEPs ha,·e become a

tandard means of examining the integrity of the visual system (Fig. 6). Pattern reversal

VISUAL ACU ITY AND CATARACT SURGERY

35

Figure 6. Visually evoked potential in response to pattern stimulation. The pattern VEP can be used to provide quantitative assessment of visual acuity. However, it demands that the subject maintain fixation and attention and be properly refracted.

stimulation elicits a waveform which varies relatively little in normal individuals. Moreover, when low spatial frequency patterns are employed ,, optical degradation of the image affects the response only slightly, if the normal subject can resolve 10' (arc minute). 39 Moreover, retinal disease reduces VEP amplitude and delays its latency. 40 In a sample of 39 patients.

36

G.W. WEINSTEIN , J .V. ODOM ANO R.R. HOBSON

who e opacitie had been removed, those who e poi.t-operathe acuity was normal (20/30 or better) had larger ampliwde VEPs elicited by pauc111ll reversing ten times per second than those whose post-operative acu ity was poorer than 20/30.2' Rather than low patial frequency pauerns, laser speckle may also be employed. 41 Contrast Sensitivity. The visual system can respond to timuli over a wide

range of contrast. This characte ri!.tic, kno" n as contrast sen itivit}, ma} be evaluated by a number of different methods. Narrow, medium, or wide (high, medium, or low spatial frequencies respectively) patterns may be used . These patterns consist of bars of various widths; the test i'I referred to as grating awzty. The edges of the bar may be sharp or blurred. Contrast patterns would then be referred to as square wave or sinusoidal gratings respectively. Clinical tests are available in print form such a Arden plates manufactured by American Optical Company (Fig. 7) or Grating Acuity Plates

Figure 7. Contrast sensitivity tested with Arden gratings. The Arden gratings consist of a series of plates of various bar widths, from narrow (high spatial frequency) to wide (low spatial frequency). The top of each plate Is low contrast and, as each plate Is progressively exposed by drawing It from Its holder, greater and greater contrast is appreciated. The end point is reached when lhe subject can perceive the stripes.

37


VISION CONTRAST TEST SYSTEM 2

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•

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•

"

• c

D

I

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Figure 8. Contrast sensitivity testing with Vistech optotypes. A printed chart showing gratings at various orientations, of different spatial frequencies and contrasts Is viewed by the subject. A slmllar chart has been developed for near. There Is also a sllde avallable for projection testing.

by VisTech (Fig. 8), as well as video-generated pacLerns manufacLUred by Optronix (Fig. 9). Administration of the test requires monocular viewing of the test places at specified distances (both near and distance tests are available), with the subject reporting a Lhresholcl al which dist.incl bars are seen, as compared lo a unifo1·mly gray field. By testing contrasL sensitivity with gral.ing pauems of varying spatial frequencies and contrasts, it is possible Lo develop a two-dimensional representation of comrast sensitivity. This plot is known as the visuogram (Fig. 10). It is analogous co the audiogram, a well-accepted means of depicting hearing sensation. When tJ1is concept was first developed by Arden, it was thought that characteristic abnormalities might be found in various ocular disorders, such as high frequency loss with cataract, medium frequency loss with glaucoma, and so on. However, while this has not proved to be the case, the visuogram is still useful in illustrating abnormalities of visual function better than any of our other current means. For example, the Snellen acuity notaLion simply refers LO the lower right hand ponion of the curve, with high comrasc and high spatial ~rcq~cncy. The visuogram has been shown LO correlate better with r eal bfe visual performance than does Snellen acuity.

38


Figure 9. Conlrast senslllvity testing with Optronlx video-generated device. The subject views a video screen which shows gratings of various spatial frequencies, whose contrast Increases with time. When lhe subject can appreciate lhe presence of slnpes, a button is pressed and an automatic plot of threshold is oblained.

CLINICAL EXAMPLES Patient with Early Cataract Com monly in cataract a dinical problem i!> that of the paLient who seeks medical atten tion becau c of poor visua l acuiL}'. fir L thoughL to be the result or a need fo r new eyeglasses. O n examination the patien t is fo und to bavc early changes in the lens, o ften in the posterior sub-capsular area. Snellen visual acuity is good and the ophthalmologist is 1·eluctam LO re-

39


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Uch as abnormal visuogram (see Figs. 7 through I 0). Thus the visuogram may have provided the justification fo r earlier ('ataract su rge 1-y than wou ld have othe rwise been auempted. The generally good results of cacaract surgery have encouraged more patients to seek early attention and tests of contrast sensitivity allow the ophthalmologist to better documenl the true nature of the patient's symptoms.

Patient with Advanced Cataract A patient with opaque or nearly o paque lens or other significam opacity of the ocular media may defy full evaluation by conventional methods. lf ophthalmoscopy is impossible or if fundus d etails, including the appear•mrc of 1he macula and optic disc, are inadequate for evaluation, objective

40


means of evaluation may be indicated. In such a case, the VEP becomes very useful (see Figs. 4 and 6). The VEP provides the patient with information about the best achievable acuity, assuming that surgery is done without complications and that there is no other inte1·vening problem. Patient with Immature Cataract and Macular Degeneration or Optic Neuropathy

In Lhis situation, a patient pre ents with both cataract and some other ocular disorder, such as macular degeneration, glaucoma, or some other co-existing abnormality. The ophthalmologist is faced with the dilemma of determining how much visual loss results from each of these conditions and, in panicular, whether cataract surgery could be expected to produce a significant improvement in the patient's visual function . In such a situation, the various qualitati\'e or semi-quantitati\'C objective devices such as the PAM (see Fig. 2), BFE, or LIA tests are especially useful. Vernier acuity tests may also be very helpful (see Fig. 5). As for the patient with advanced cataract, VEP ma} also be very helpful (see Figs. 4 and 6). SUMMARY

Visual performance in patients with cataracts and other conditions characterized by opacities of' lhe ocular media is better characterized by contrast sensitivity studies than by visual acuity tests. A variety of subjective and objective tests of visual function is now a\'ailable to evaluate the patient witJ1 both immature cataract alone, immature cataract combined with other ocular disorders such as macular degeneration and optic neuropathy, and advanced cataract, including VEP, vernier acuity, potential acuity meter, laser interferometr}'. entoptic imagery, as \\ell as VEP testing using na hes or pauerns. Earlier surgery for cataract and eventuallr for binocular vi~ion is frequently a justifiable and desirable goal.

REFERENCES I. Arons IJ : 1 he future in sight: Ophthalmology. Ophthalmol Managemem, July/A ugust, 1985 2. Emery JM, Mcintyre DJ: Ex1racapsular cataract surgeq•. St. Louis, "-losby, 1983, pp 359-36 I 3. Retinal and Choroidal Diseases Panel. Vision research: A national pla11 19831987. Vol. 2. Pan I. Repon of the Retinal and Choroidal Diseases Panel. Washington, D.C., NationaJ Institutes of HealtJ1, 1983. NIH Publication 16. 83-2471, p 174

VISUAL ACUITY ANO CATARACT SURGERY

41

4. O g uc hi \' : Elcctrophysiological a nd cchograph ical examination of ver)' yo un g children. J Pedi:ur Ophthaln1o l Strnbismus 17:399, 1980 5. Fulle r DG , Huuo n WL: Presurgical evaluatio n of eyes with opaque media. New Yo rk, Grune & Strauon, 1982 6. Sinclair S H , Loeb! M, Rh•a CE: Bluefield e ntoptic phenomenon in cataract patients. Arch Ophthalmol 97: 1092, 1979 7. ~!iris R, i\lissonen L: Evaluation of the macular function in cataractous eyes by means of the blue field e nto ptoscope. Dull Soc Beige d'Ophthalmol 20 I: 12 1, 1982 8. Skalka HW : l~lu e field e nto ptoscopy and VER in pre-operative cataract evaluation . Ophthalmic Surg 12:642. 198 1 9. Murphy GE: I.imitations or blue fi e ld entopto~cop)' in evaluating macular function. Ophthalmic Surg 14: 1033, 1983 IO. Green DC, Cohen M~I : Laser interferometry in the ew1luation of potential macular function in the presence of opacities in the ocular media. Tra ns Am Acad Ophtlrnlmo l Otolaryngol 75:629, 1971 11. Gstalder R.J , Green DG: Laser inte rferometric acuity in a mbl yo pia. J Pediatr Ophthalmol 8:25 1, 1971 12. Maumenee AE: Discussion of Green DG, Cohe n MM: Laser inte rferometr y in the evaluation of potential macula1 function in the presence o r opacitie~ in the ocular media. T rans Am Acad Ophthalmol OLOlL Ophtha lmo l Vi~ Sci 2 1:92. 1983 Essock EA, Williams RA , Enoch JM , Raphael S: The effects of image clegradaLion by cataract in vernier acui1y. Inves t Ophthalmol Vis Sci 25(9): I 043. 1984 van Lith ( :t I ri1, He kke1L-Wicbcnga \V: Cataract, pattern stimula 1io n and visually evoked potentials. Doc O phthalmol 55: 107, 1983 ath S, herman J, Bass SJ: VEP delays in macul.1r disease. l m el>l O phthalmol Vis l i 22 (A RVO Suppl): 60. 1982 Fukuhara .J . Uozato H, Nojima S. et al.: Visual-evoked potentials elicited by laser s peckle pa uerns. Studies 011 the charac1cris1ics and assessm e nt o l visual fun ctio n in patiems with owlar media opacitic . Invest Opthalmol Vis Sci 24 : 1400, 1983