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Spatial Contrast Sensitivity and the Diagnosis of Amblyopia

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Spatial Contrast Sensitivity and the Diagnosis of Amblyopia British Journal of Ophthalmology, 1987, 71, 58-65 Spatial contrast sensitivity and the diagnosis of amblyopia A C W VOLKERS,' K H HAGEMANS,2 G J VAN DER WILDT,' AND P I M SCHMITZ' From the 'Department ofBiomedical Physics and Technology, Erasmus University, PO Box 1738, NL-3000 DR Rotterdam; 2Health Centre, De Linie 2, Capelle a. d. IJssel; and the 'Institute ofBiostatistics, Erasmus University, Rotterdam, The Netherlands SUMMARY The relationship was studied between spatial contrast sensitivity function and type of pathology of the visual system. Two characteristics were found to be typical for amblyopia: (1) there is a discrepancy between the high-frequency cut-off ('grating acuity') and the Snellen acuity; (2) the contrast sensitivity is strongly dependent on the width of the stimulus. In optic nerve degeneration a decrease in contrast sensitivity is found at low spatial frequencies. Decreased contrast sensitivity at high spatial frequencies is atypical but occurs in those disorders associated with decreased Snellen visual acuity. From these findings a relationship is defined which enables spatial contrast sensitivity tests to be used to differentiate between amblyopia and optic nerve degeneration. The ophthalmologist is faced with the task of diag- contrast or, in other words, how well coarser struc- nosing the cause of any visual impediment. After tures can be seen at low contrast.' optimal correction and examination by slit-lamp or The study of contrast sensitivity function as it is ophthalmoscope there remain some patients in related to several types of pathology is not new. whom the diagnosis is not clear. Their visual loss may Various workers have reported on contrast sensi- be due to optic nerve degeneration or to amblyopia tivity in amblyopia''9 and in optic nerve disorders.45 ('lazy eye'). It would be helpful if a simple method The CSF has been described in optic nerve lesions'0-'2 were available to distinguish between these two and in cataract. 13 Most of these publications deal with possibilities. Determination of spatial contrast sensi- rather small numbers of cases, and almost all concern tivity function (CSF) might be such a relatively our understanding of specific disorders of the visual simple method. system, but there are no reports on the evaluation of The Snellen optotype chart is a measure of the CSF as a diagnostic aid in neuro-ophthalmological minimum angle of resolution at high contrast. In CSF practice. the maximum discernible spatial frequency can also The aim of this study was threefold: first, to be interpreted as a measure of the resolving power: determine to what extent the shape of the CSF is the 'grating acuity' can be calculated from this related to the particular type of pathology; secondly, frequency. Although the stimuli with which the visual to evaluate the effectiveness of spatial contrast sensi- acuity and the grating acuity are determined are quite tivity tests for the diagnosis of amblyopia, especially different (optotypes and line patterns respectively), with regard to the differentiation between amblyopia it is interesting to compare the visual acuity and the and optic nerve degeneration with decreased visual grating acuity because both values are measures of acuity; and thirdly to compare the visual function in the spatial resolution at high contrast. The CSF optic nerve degeneration combined with normal provides more information than the Snellen acuity, visual acuity with visual function in a group of normal while it also estimates the spatial resolution at low eyes. Correspondence to A C W Volkers, Erasmus Universiteit Rotter- In a pilot study by Hagemans and Van der Wildt' in dam, Postbus 1738, 3000 DR Rotterdam, The Netherlands. a small group of patients it was found that the 58 Spatialcontrastsensitivity and the diagnosis ofamblyopia 59 dependence of contrast sensitivity on the width of the The CSF was determined for spatial frequencies stimulus was much greater for amblyopes than for ranging from 0-1 to 25 6 cycles per degree (c/deg) in normal subjects or patients with other pathology. steps of a factor 2. Up to 0-4 c/deg the viewing distance was 50 cm, resulting in a stimulus width of Subjects and methods 320; from 0-8 to 25-6 c/deg the viewing distance was 200 cm, which gave a width of 8°. For lower fre- The subjects who took part in this study were chosen quencies the stimulus had to be wider, because if from among the outpatients of an ophthalmological there are fewer than 5 cycles within the stimulus the practice. They were divided into several groups number of cycles has an influence on the CS.'5-8 The according to the type of pathology, thus: amblyopia CS is defined as the reciprocal of the value of the (n=98), optic nerve degeneration (n=42), and contrast at the threshold. cataract (n= 10). A group of normal persons (n=26), When the CS was determined as a function of the that is, without any known pathology or complaints, width of the stimulus, the monitor was at a viewing were also examined. distance of 100 cm; the width of the stimulus was The group of patients with optic nerve degener- changed from 160 to 10 in steps of a factor 0.5. The ation comprised those who had suffered from optic width dependence was determined at a spatial fre- neuritis in the previous three years, with or without quency of 1-6 c/deg. other evidence of multiple sclerosis. Optic neuritis The CS data were automatically plotted in double was electrophysiologically diagnosed in that period. logarithmic form against spatial frequency or against This group was divided into two subgroups, those the width of the stimulus, depending on the type with normal visual acuities (greater than or equal to of determination. The apparatus with which the 1 0; n=16) and those with diminished visual acuities measurements were performed was specially de- (less than 1 0; n=26). The group of cataractous eyes signed for clinical use."9 The measuring procedure was examined to enable a comparison to be made runs automatically and is controlled by a micro- between groups of eyes with decreased visual acuity processor system. The whole measuring sequence originating from different causes. including patient instruction takes about 25 minutes Cases with evidence of multiple pathology were for both eyes. excluded. From a statistical point of view each For each of the CSFs the corresponding grating patient was allowed to occur only once in the series. acuity was calculated by extrapolating the high fre- Hence in cases of binocular disease only one eye quency part of the CSF to the maximum contrast (chosen at random) was used. Five amblyopes had (CS= 1). A quadratic function was used for. the binocular disease, while 93 had monocular. The extrapolation, as this type of curve yielded a good fit numbers of binocular and monocular cases of optic for the appropriate part of the CSF. The grating nerve degeneration were 7 and 35, and of cataract 4 acuity is expressed as the reciprocal of the minimum and 6, respectively. angle of resolution in minutes of arc, as is the Snellen The mean age was 35 years, standard deviation visual acuity. The acuity A (1/min of arc) is equiv- 10 years, for both normal and amblyopic subjects; 45, alent to a spatial frequency of 30xA [c/deg]. SD 17, years for subjects in both optic nerve degener- The slope of the CS between 10 and 40 width was ation subgroups; and 59, SD 11, years for the subjects taken as the measure for the width dependence of the in the cataract group. CS. This was expressed as: The visual acuity of all subjects was determined by S=(log(CS(4°))-log(CS(10))/log(4). the Snellen optotype chart. Results DETERMINATION OF CONTRAST SENSITIVITY The contrast sensitivity (CS) was determined by Point by point averaging of the data points of the presenting vertical sinusoidal gratings on a television CSFs within each of the groups yielded the group- monitor. The contrast of the stimulus was variable; specific CSFs. The curves for the group of normal the contrast threshold was determined for each eye eyes and for the group ofamblyopic eyes are depicted separately by a modified von B&k6sy tracking tech- in Fig. 1. The mean visual acuity for the group of nique.'4 The spatial frequency of the grating and the normal eyes was 1 34; the mean grating acuity was width ofthe stimulus could be adjusted. The height of 1-18. the stimulus always subtended 50 visual angle; the As is to be expected from the lower mean visual average luminance of the stimulus was 5 cd/m2. The acuity in the amblyopic eyes (0-39), contrast sensi- examinations were performed in a dark room. The tivity at the higher spatial frequencies (CS(HF)) was grating was counterphased every 0-6 s to prevent low. This was associated with a decrease in the after-images. grating acuity, which had a mean value of 0-80. 60 A C W Volkers, KHHagemans, GJ van der Wildt, and PIM Schmitz 1. O - 0 'Z 0. 5 - ce 0. 3- Fig. 1 Lower left: Averaged CSFs 0. 2- for26 normal visualsystems andfor 500 98 amblyopic eyes. The dashed 300 lines atthehighfrequency endofthe 200 CSFs indicate extrapolation to the ,5 100 grating acuity. Lower right: The 4-0 v 50 relationships between contrast C sensitivity and width ofthestimulus UA 30 ('width dependence')for normal * 20 eyes andfor amblyopic eyes. Upper tau left and right: Ratio ofthe two w 10 curves in the lowerpart with the 0 5 normal one as the reference. The 3 o normal (n=26) curve on the left (the ratio ofthe two 2 eyes CSFs) is called the 'visuogram'.
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