Glaucoma Functional Damage and Comparative Psychophysical Studies

C l i n M e d International Library Citation: Elgohary AA, Elshazly LHM, Fahmy IA (2015) Glaucoma Functional Damage and Comparative Psychophysical Studies. Int J Ophthalmol Clin Res 2:039 Received: June 28, 2015: Accepted: September 21, 2015: Published: September 23, 2015 Copyright: © 2015 Elgohary AA. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Elgohary et al. Int J Ophthalmol Clin Res 2015, 2:5 ISSN: 2378-346X


Introduction
Many clinical tests have been developed to determine the damage of visual function caused by glaucoma.These tests help in early detection of glaucoma, quantification of glaucomatous damage and progression.Currently, perimetry remains the measure of choice for studies of glaucoma progression.Visual field loss is the most prevalent and characteristic form of visual function loss associated with glaucoma [1].High visual centers can combine the two disparate, inhomogeneous visual fields.Therefore it would be best to stereothreshold was the measure of stereoacuity .Data were recorded as yes/no ,then scoring was done to facilitate statistical analysis.

Contrast Sensitivity
For each subject contrast sensitivity function (CSF) was determined using the Vistech Contrast Sensitivity Test chart (VCTS Vistech Consultants, Dayton, OH, USA) the chart is model 6000.It is similar to VCTS 6500 chart but is used to test near sensitivity.The chart consists of 5 rows of sine-wave gratings.The rows increase in spatial frequency from the top to bottom of the chart, and on each row the gratings decrease in contrast from left to right; it was viewed at 40 cm at a luminance level of 100 cd/m2.Patients were wearing their best distance correction and were tested binocularly.The lowest contrast grating determined the sensitivity score that used to plot a CSF for the patient.The test was repeated three times for every patient to ensure reproducibility.

Statistical analysis
The Students t-test and analysis of variance (ANOVA) test were used to analyze continuous variables such as age, vision, cup /disc ratio, IOP and MD.The Kruscalwalis test was used to determine differences among dichotomous variables, such as sex and answers to yes /no questions which were treated as dichotomous categorical variables in the analysis.When examining the scores of color and depth perception tests, we used the results for both eyes.The Spearman correlation coefficient was used to determine the relationships within the entire study population between binocular contrast sensitivity, mean deviation (both eyes), and stereopsis.Color vision was compared using wilcoxon signed ranks to compare between the two tests; CUCV and D15.The Kruscalwalis test was used to compare between normal, mild and advanced cases in each test type.Odd ratio and sensitivity and specificity were done using WinPepi statistic software version 11.44.A P-value less than 0.05 was considered statistically significant.

Results
A total of 106 participants, comprising 56 glaucoma patients and 50 non-glaucomatous normal controls, were included in this study.Glaucoma groups comprised 27 men and 29 women, mean age of 46.05 ± 10.89 years.Control cases comprised of 20 men and 30 women, mean age of 43.17 ± 9.46 years.Mild glaucoma group showed significant difference in cup/disc ratio, as compared to control group (mean of right eyes was 0.45 ± 0.05 and mean of left eyes was 0.46 ± 0.03 (P = 0.001, t test).The [8].Automated VF analysis using the 24-2 SITA-standard program was chosen using Humphrey Field Analyzer [HFA II, Zeiss, Dublin, California].Functional defects of glaucoma eyes were classified based on their HFA mean deviation (MD),as previously mentioned [9], as follows: mild , MD < −6 dB ; moderate , MD > −6 dB <−12 dB ; severe, MD > −12 dB .In the present study glaucoma cases were divided into two groups; group 1 represented mild cases with MD <−6 dB (early glaucoma), and Group 2 that included advanced glaucoma cases with MD > -12 dB.We excluded moderate cases because they were few cases.
Exclusion criteria for all subjects included high refractive errors, ocular disease such as cataract, corneal opacity and retinal disease.Also, cases with any systemic disease and medication that might impair vision or pupil responses were excluded.

Color vision
Color vision testing was carried out in good daylight conditions, supplemented when necessary with artificial lighting.Two color vision tests were used.

Farnsworth-Munsell D-15 test
This hue discrimination arrangement test uses 15 colored caps which are arranged in sequence from a fixed pilot cap.The results are plotted on a hue circle diagram, which allows tritan defects, red-green defects or generally poor hue discrimination to be identified.

The city university color vision (CUCV) test
The test consists of 10 plates in a book.Subjects were instructed to indicate which of the four test colors the closest match to the central color was.Each plate provides a single unique response corresponding to protan, duetran, tritan or normal responses.Number of subjects who were normal or tritans were detected and comparison of the two tests was done.

Stereoacuity (depth perception)
The stereoacuity was measured with the Lang-Stereo test; a random-dot stereo test uses a panographic technique to present disparity.Lang I: Disparity = Car 550", Star 600", Cat 1200" Lang II: Disparity = Moon 200", Car 400", Elephant 600".Subjects were tested under the same condition and they had no previous experience with this type of test, no glasses were required.The test cards were viewed binocularly at 40 cm distance.Patients were asked if they see a picture in the card and to point to its site.The lowest disparity which the patient can reliably discriminate was recorded and this mean deviation was 2.45 ± 1.5 and 2.56 ± 1.1 for right and left eyes respectively and it was significantly reduced as compared to the control group (P < 0.001, t test, (Table 1)).
Advanced glaucoma group showed significant increase of C/D ratio (mean of right eyes was 0.78 ± 0.08 and mean of left eyes was 0.77 ± 0.07 (P = 0.001, t test).There was significant deterioration in visual fields (Right eye MD was 15.93 ± 0.80 and left eyes MD was 13.65 ± 0.72 (P = 0.001, t-test), as compared to the control group (Table 1).

Color vision
Responses to color testing are shown in table 2.

Stereoacuity
Patients in mild glaucoma group showed statically significant depth perception defects at all tested disparities [200, 550,600 and 1200 seconds of arc] (p < 0.05) as compared to the controls.The depth perception defects were increased in advanced glaucoma group.There was a statically significant difference between mild and advanced groups (Table 2).

Contrast sensitivity results
The mean values of binocular CSF of controls, mild and advanced glaucoma patients were summarized in table 3. The binocular CSF of mild and advanced glaucoma patients were significantly low at    all spatial frequencies, as compared to control subjects.There was significant difference between mild and advanced cases at low and mid spatial frequencies (1.5, 3 and 6c/d), as shown in table 3 and figure 2.
There was significant correlation between the deterioration of visual field mean deviation and the reduced depth perception and contrast sensitivity .Also, as the contrast sensitivity was reduced, the depth perception was decreased (Table 4).

Discussion
Glaucoma pathologically leads to the progressive damage of both large (magnocellular) and small ganglion cells (parvocellular and konicellular) retinal ganglion cells [10,11].Also, glaucoma effects involve the lateral geniculate nucleus (LGN), geniculocortical pathway and visual cortex [4,12].The LGN has three distinct visual channels, namely the magno-, parvo-, and koniocellular pathways for motion, red-green color and blue-yellow modalities, respectively [10].The neural mechanisms that binocularly combine the compromised monocular inputs of contrast sensitivity, color and depth perception are integrated at the level of the striate and extrastriate visual cortex [13].
Sensitivity for detection of fine spatial detail, motion and colour signals can be selectively damaged in glaucoma and may precede visual field loss [14,15].Various tests were used to detect the binocular contrast sensitivity function as well as stereoacuity and color vision deficits in early glaucoma [16].In this study, we used Vistech contrast sensitivity test chart to explore for binocular contrast sensitivity reductions at different spatial frequencies.We observed deterioration of the contrast sensitivity at all spatial frequencies in early and advanced glaucoma cases.This was in agreement to work done by McKendrick et al. [11].They observed that glaucoma patients demonstrated reduced sensitivity across the spatial frequency range, for which they suggested a combination of reduction of both magnocellular and parvocellular processing [11].
In the present study, we obtained reduced stereoacuity in both early and advanced glaucoma groups, and this was significantly related to reduce mean deviation in perimetry.Our study confirmed that stereopsis was reduced in glaucoma similar to previous studies that used the random dot [14], the line stereograms [2] and the Frisby depth perception tests [13].Reche-Sainz JA et al. [17] found that in advanced glaucoma cases depth perception was reduced as examined by Titmus, and TNO depth perception tests, they did not find such changes in early cases with ocular hypertension [17].The neuronal basis of stereovision depends on disparity cells sensitive to binocular disparity, located in the primary visual cortex and extrastriate areas.The explanation of loss of binocular vision was suggested to be due to relative delay of input from one eye compared to the input from the other eye, which may affect binocular interactions at the level of the visual cortex [12].
In the present study, there was a significant increase in number of tritan patients as detected by D15 test.This suggested that color vision testing is essential for every glaucoma patients.In glaucoma, blue color affection is related to involvement of small bistratified ganglion cells, which are fewer in number and comprise about 1% of ganglion cells and receive their input from the blue-cone bipolar cells [18].The explanation for tritan defect in POAG was suggested to be due   selective damage to blue-yellow sensitive ganglion cells or their axons either due to their larger receptive fields [19], or their relative scarcity [20].In the present study we did not observe red green chromatic deficits.This is in agreement to the work done by Karwatsky et al. [21].However, Rauscher et al. [16] described losses of the red-green chromatic mechanism in advanced POAG cases [22].
Many tests are used to detect tritan defect in POAG included H-R-R, Lanthony, F2 plates, D-15, and the City University tests.In general, the CUCV and D15 tests are useful for acquired color vision defects [23].However, in the present study we observed low sensitivity of the CUCV test and moderate sensitivity of D15 test in glaucoma patients.This is relatively in agreement with previous studies suggested that these tests individually are not considered to be very sensitive for screening POAG.The results from a combination of these tests may be a useful addition to other data collected in glaucoma screening programs [18].

Conclusion
We found that glaucoma patients showed significant reduction in binocular contrast sensitivity scores and depth perception which may have utility in identifying early glaucomatous nerve damage.Also, color vision screening using D15 test may help in discriminating patients with glaucoma.Future use of a simple office test that may combine color, contrast and depth perception in one test may provide an easy way for screening of early glaucomatous damage.

Figure 1 :
Figure 1: The predictive value of a positive test (or positive predictive value) is the post-test probability of the disease if the test is positive.A = CUCV test, B = D15 test.
significant difference between the two test CUCV and D15

Figure 2 :
Figure 2: CSFcurves (mean values) of control group (the black line), group 1; patients with mild glaucoma (the red line) and group 3; patients with advanced glaucoma (the blue line).

Table 1 :
The Characteristics of the Examined Groups.
IOP: Intraocular Pressure, C/D cup disc ratio, MD: Mean Deviation in Perimetry Data expressed as mean ± SD, P1=difference between mild and normal, P2 = difference between normal and advanced *P significant < 0.05, anova test a Significant difference from the normal cases, t test • Page 3 of 5 • ISSN: 2378-346X Elgohary et al.Int J Ophthalmol Clin Res 2015, 2:5

Table 3 :
Contrast sensitivity results in controls, mild glaucoma and advanced glaucoma cases.
Data expressed as mean ± SD.CS; contrast sensitivity, P1 difference between normal and mild, P2difference between normal and advanced, P3 difference between mild and advanced.*P significant if < 0.05, t test.

Table 4 :
Correlations of the studied variables.