|Year : 1997 | Volume
| Issue : 2 | Page : 99-103
VISTECH contrast sensitivity testing in primary open angle glaucoma
A Mathai, R Thomas, A Braganza, S Maharajan, T George, J Muliyil
Schell Eye Hospital, Christian Medical College, Vellore, India
Schell Eye Hospital, Christian Medical College, Vellore
Source of Support: None, Conflict of Interest: None
Contrast sensitivity has been recommended as a screening and diagnostic test in primary open angle glaucoma (POAG). We tested contrast sensitivity (CS) using Vistech charts in 184 eyes of 95 patients. Three groups were examined--established primary open angle glaucoma, glaucoma suspects and age matched controls. The distribution of contrast sensitivities amongst the three groups were similar. The median contrast sensitivity of glaucoma suspects and controls were well within normal limits while that of the POAG group fell along the lower limit of normal. In all three groups the younger subjects scored better than the older, indicating a depression of contrast sensitivity with increasing age. Even if depression of any one spatial frequency was considered abnormal, the test yielded a sensitivity of 55.4% and specificity of 69.5%. Similarly contrast sensitivity testing was found to be of little use in detecting field defects a maximum sensitivity of 47.3% and specificity of 73.3%. Vistech contrast sensitivity testing is not a useful test in POAG screening or diagnosis.
Keywords: Contrast Sensitivity Testing - Vistech Charts - Primary Open Angle Glaucoma - Glaucoma Suspects
|How to cite this article:|
Mathai A, Thomas R, Braganza A, Maharajan S, George T, Muliyil J. VISTECH contrast sensitivity testing in primary open angle glaucoma. Indian J Ophthalmol 1997;45:99-103
|How to cite this URL:|
Mathai A, Thomas R, Braganza A, Maharajan S, George T, Muliyil J. VISTECH contrast sensitivity testing in primary open angle glaucoma. Indian J Ophthalmol [serial online] 1997 [cited 2019 Oct 18];45:99-103. Available from: http://www.ijo.in/text.asp?1997/45/2/99/15009
Psychophysical studies have shown abnormalities in contrast sensitivity (CS) in primary open angle glaucoma (POAG) and ocular hypertension. Arden and Jacobson showed that CS in the higher spatial frequencies for their plates (3.2 to 6.4 cycles /degree) were depressed in glaucoma. In addition they also found that the degree of CS loss correlated well with the extent of visual field damage. Stamper et al obtained similar results except that they did not find any specific depression in the spatial frequencies 3.2 to 6.4 cycles/degree. Teoh et al found that only 39% of patients with glaucoma had abnormal contrast sensitivity with generalized depression at all spatial frequencies except the highest. Reduction of CS at all spatial frequencies has been reported by Adams, Heron et al, whereas Ross, Bron et al showed that the spatial frequency depression depended on the severity of glaucoma. Sponsel et al using Vistech charts found that CS depression in the middle spatial frequencies for this system (3-6 cycles/degree) offered the best combination of diagnostic sensitivity and specificity, 71% and 50% respectively. The purpose of this study was to evaluate the usefulness of Vistech contrast sensitivity testing in POAG as well as to determine its association with field defects.
| Materials and methods|| |
Fifty-six eyes of 31 patients with established POAG, 74 eyes of 37 glaucoma suspects(GS) and 54 eyes of 27 age matched controls formed the three study groups. All patients were above 39 years and phakic with unaided vision of 6/6 and a best corrected near vision of J1 in the study eye. Absence of media opacities as well as macular and other ocular pathology were confirmed by a detailed ophthalmic evaluation including slit lamp biomicroscopy, examination of the posterior pole with a 78D lens and binocular indirect ophthalmoscopy. Intraocular pressure (IOP) was measured using the Goldmann applanation tonometer.
The disc features suggestive of glaucoma included in the study in addition to cup-disc ratio were: (a) vertically oval cup (b) neuroretinal rim pallor (c) baring of circumlinear vessels (d) notching of disc rim (e) nerve fibre layer defect as determined by slit lamp biomicroscopy with a 78D lens and red free filter. Field defects were evaluated using the 30-2 program on the Humphrey's field analyzer (HFA).
The control group had IOP less than 21 mmHg, normal discs and normal fields. They were invariably selected from patients who sought presbyopic correction.
Glaucoma suspects had an IOP of more than 21 mm Hg in both eyes on at least two occasions and/or cup-disc ratio of 0.7:1 or more with glaucomatous features, or a cup-disc asymmetry of more than 0.2:1 between the two eyes with glaucomatous features in the eye with the larger cup. There were no demonstrable glaucomatous field defects according to established criteria.
Eyes with POAG had IOP consistently above 25 mmHg in both eyes prior to treatment as well as glaucomatous optic disc changes. HFA 30-2 program showed typical glaucomatous field defects. A field defect was considered glaucomatous if the pattern deviation plot showed three or more non edge points depressed below the 5th percentile for the normal population with one of these points depressed to the first percentile; in addition the corrected pattern standard deviation had a value found in less than 5 % of the normal population.
We performed contrast sensitivity using distance Vistech charts (VCTs 6500) in which contrast threshold at five spatial frequencies (1.5, 3, 6, 12 and 18 cycles/ degree) were determined. The testing distance was 10 feet. The test procedure and specifications followed were as recommended in the manual supplied by the manufacturers.
| Results|| |
Age in the POAG group ranged from 45 to 75 years, in the glaucoma suspects from 39 to 68 years and from 39 to 71 years in the control group.
The contrast sensitivity distribution in the three groups is shown in [Figure - 1]-[Figure - 3]. The scatter diagrams shows that the majority of the values in all three groups fall within the normal range (gray shaded area). The median lines for controls and GS fall along the upper limit of normal range while that of the POAG group falls along the lower limit of the normal range. To study the effect of age on CS, each group was subdivided by their median age into older and younger subgroups [Figure - 4][Figure - 5][[Figure - 6]. In all three study groups the older subjects scored less than the younger ones.
| Validity of contrast sensitivity for Diagnosis of Glaucoma:|| |
To evaluate sensitivity and specificity, cut-off values with any 1, 2, 3, 4 and all 5 spatial frequencies depressed were used. Each eye was given a score of one or zero at each spatial frequency: score one if contrast threshold was below normal (below the gray area) and score zero if it was normal. The sum total of the five individual scores at the five spatial frequencies would give the final score. Thus an eye with an absolutely normal CS would get a score of 0 and one with all five spatial frequencies depressed would score 5. Eyes with 1, 2, 3, or 4 spatial frequencies depressed would score 1, 2, 3, 4 respectively. The results are shown in [Table - 1], and the resulting receiver operating characteristic (ROC) curve in [Figure - 7].
| Positive predictive value (PPV):|| |
The positive predictive value was calculated at a cut-off score of 1 (i.e. with any one spatial frequency depressed), which has the most acceptable combination of sensitivity and specificity. For 1% prevalence of POAG the PPV is 1.8%; for a 50% prevalence it is 64.5%.
| Validity of contrast sensitivity for detection of mean deviation of p<2% at various spatial frequencies:|| |
To study the usefulness of CS in detection of field defects only POAG and GS groups were chosen. (The control group was not included as they all had normal 30-2 program on the HFA). The subjects were divided into two subgroups using a cut-off of p<2% for the mean deviation. Thus the subjects with significant field defects had p<2% (p<2%, <1%, <0.5%) and those with insignificant field defects had p>2% (p<5%, <10%). The results are shown in [Table - 2][Table - 3]
| Discussion|| |
Various reports have been published about contrast sensitivity (CS) abnormalities in POAG.,, Sponsel et al noted contrast sensitivity depression in the middle spatial frequencies. They found that the mean Vistech sensitivity levels were not significantly different in the three groups studied (normals, suspects and defects-divided on the basis of perimetric criteria). According to the Vistech manual POAG patients show a typical mid spatial frequency depression. Our study showed a minimal depression of CS in the POAG group especially in the highest spatial frequency (18 cycles/degree). This may be explained by the fact that the median age of the POAG group was 55 years while that of the other two groups was 46 years. Singh et al using the Arden gratings demonstrated contrast sensitivity depression in plates 6 and 7 (3.2 and 6.4 cycles / degree) in patients above 50 years of age. The minimal depression seen in our POAG group is probably a function of age. Also as the higher spatial frequencies are affected by cataracts, refractive errors and macular degeneration, all of which are confounding factors in the older age group, depression of this spatial frequency would not be of much use in the diagnosis of POAG.
In all three groups it was seen that the younger patients scored marginally better than the older ones. Arden found that age had no effect on contrast sensitivity. Our finding is consistent with the data of Stamper et al and other studies.,
To study the sensitivity with any one spatial frequency depressed we used the scoring system as already described. The sensitivity with any one spatial frequency depressed is 55.4% with a specificity of 69.5%. Thus even if we use a very lax diagnostic criterion of depression of any one spatial frequency, 44.6% of glaucomas would be missed and 30.5% of the normals would be labeled falsely positive. To determine the best cut-off for diagnostic discrimination, true positives (sensitivity) are plotted against false positives at each cut-off value in the ROC curve. An ideal ROC curve would be close to upper left hand corner. The ROC curve [Figure - 7] we obtained lies close to the central diagonal line indicating that the test is of poor diagnostic value. Further, with such low sensitivity and specificity the positive predictive value of the test would be too low to be of any use for screening in glaucoma. Assuming that the prevalence of glaucoma in the community is 1%, the probability of a test positive individual having glaucoma would be 1.8%. In a clinical setting, if we assume that the prevalence (pretest probability) of the disease has been increased to 50%, the predictive value of a positive test would only be 64.5%. The value of any diagnostic/screening test can be maximized if considered as a function of the pretest probability (prevalence) and the likelihood ratio. The likelihood ratio in this case is only 1.82.
Eyes with field defects were evaluated to determine the percentage amongst them who had CS abnormalities. Mean deviation was taken as the criterion for this analysis because like contrast sensitivity it is affected by diffuse glaucomatous damage. The highest sensitivity achieved was 47.3% in the spatial frequency 18 cycles /degree. Association of Vistech contrast sensitivity and visual fields in glaucoma was studied by Sponsel et al. They did not find any significant difference in the mean CS levels among their three study groups. The highest sensitivity and specificity in their study was 71% and 50%, respectively, with the best association at 3 to 6 cycles/ degree. If contrast sensitivity testing (CST) were more sensitive to glaucomatous damage than HFA (or identified another pathway that later leads to classical defects), more subjects would be classified as abnormal by CST compared to the current gold standard (HFA). As this did not occur it is unlikely that CST is capable of detecting disease earlier than visual field analysis.
We have evaluated the usefulness of contrast sensitivity testing in diagnosing glaucoma and found that the sensitivity and specificity were too low to be of any diagnostic use. Similarly we attempted to evaluate the diagnostic ability of CST with respect to field defects and the sensitivity and specificity obtained was poor. We conclude that spatial CST with the Vistech charts is not useful either in diagnosis or screening of POAG.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7]
[Table - 1], [Table - 2], [Table - 3]