Evaluation of contrast threshold measurements and simultaneous brightness ratios in the diagnosis of glaucoma
There is considerable dissatisfaction with the reliability and sensitivity of the methods used to assess the glaucomatous visual field. Two types of visual field test, which have been proposed as having potential in diagnosing glaucomatous visual field defects, have been modified and tested on a group of patients from a glaucoma clinic, a group of age-matched control subjects and a younger control group. 1. A grating pattern was generated using a laser interferometer which projected a large diameter image onto the retina independent of the subject's refractive error. The experimental set up which produced the most reliable and consistently low contrast threshold values in normal subjects was sought. The display characteristics which were examined included different orientations for the field quadrants as projected to the subject; stationary and flickering patterns using a variety of flicker generation methods; red and green light sources; and concentric or vertical sinusoidal grating patterns. Ultimately the optimal display was found to be a stationary image consisting of a green, vertical sinusoidal grating pattern. Arcuate regions of the visual field (at 10 to 20° from fixation) were stimulated in 4 distinct, obliquely oriented quadrants and a low spatial frequency (one cycle per degree) was chosen. 2. Normal limits were obtained from age-matched control subjects for comparison with the results for the patients from the glaucoma clinic. In the patient group, of the 13 who completed the test, 9 individuals were identified as abnormal with one or more of their contrast threshold scores exceeding that limit. The patients' Friedmann visual field plots were analysed and the amount of loss in each quadrant was quantified. There was a positive correlation between the quantified visual field loss and contrast threshold scores in 6 patients, a statistically borderline correlation in 2 patients and the absence of a correlation was found in 5 patient's results. The results for a subgroup of 6 visually abnormal eyes (not affected by glaucoma) excluded from the age-matched control group are also described. Their visual defects included mild cataract, amblyopia and retinal detachment. There were no clear abnormal results in 5 of the eyes in this group; however, in one subject with retinal scarring due to an infection, there was a distinct elevation of contrast threshold in the affected eye. Humphrey visual field plots were obtained for all but one of the age-matched control subjects. 3. Simultaneous brightness ratios (SBR) have previously been shown to provide an indication of'glaucomatous damage. The same subject groups as described above were tested. SBRs were obtained for central vision in both eyes of subjects (inter-ocular ratio). This technique was now extended for the first time to paired regions within each eye (intra-ocular ratios) producing 'nasal I temporal' and 'upper I lower' ratios. In each test the subject controlled the brightness ratio which was changed in a smoothly graduated and continuou8 way. The most effective procedure for recording repeatable SBRs was first explored, and it was determined that these could be best obtained by alternating the start point of the graduated filter position. For each subject, 5 ratios were obtained: inter-ocular SBR; upper I lower intra-ocular SBR for right eye and left eye; and nasal I temporal intra-ocular SBR for right eye and left eye. 4. Normal limits were obtained from age-matched control subjects for comparison with the results for the patients from the glaucoma clinic. In each of the 5 SBR tests carried out, these limits were wide, reflecting considerable variation in the normal results. Of the 14 patients who completed the tests, 5 were identified as abnormal by one or more of their SBRs being outside normal limits. Three of these were identified as abnormal by their inter-ocular SBRs alone, one was abnormal according to his upper I lower intra-ocular SBR alone and one patient had an abnormal inter-ocular SBR and an abnormal intra-ocular SBR. The corresponding regions of the patients' Friedmann visual fields were quantified, and these values were used to calculate visual field loss ratios. There was a positive correlation between the visual field loss ratios and SBRs in 3 patients, but no correlation in 11 patients. In the sub-group of 6 visually abnormal eyes without glaucoma, mild cataract appeared not to adversely affect SBR. Mean SBRs were normal in the subject with retinal detachment but there was evidence of an enhanced amount of variation in the readings. Two subjects with a damaged retina and one with an amblyopic eye did produce abnormal inter-ocular SBRs, with the normal eye being significantly more sensitive in both cases.