Red Amsler Grid

Sir,

I read with interest the paper by FH Zaidi and colleagues titled ‘The Amsler chart is of doubtful value in retinal screening for early laser therapy of subretinal membranes’ (Eye 2004; 18: 503–508).

Their work deserves commendation for bringing confirmation to our clinical observations and experience that 87% of threshold scotomas were not detected by black and white Amsler grid when the field defect is less than 6° (6 squares) on the grid. Authors report that a mere 29% of eyes with subretinal neovascular lesions were detected by high-contrast black-and-white Amsler grid testing. Unfortunately, it is not known what real visual deficits, both in terms of a scotoma as well as visual distortion, that is metamorphopsia, their 100 patients actually had at the time of presentation. In other words, the Amsler grid would detect vision deficit only if there is one, and there may not be anything to detect in some of their patients who passed the Amsler test for they were not checked with threshold perimetry or ideally with Fundus Scanning Laser Projection Perimetry. We also do not know the extent and severity of the organic lesions that was detected by the Amsler grid against the ones that passed the test. I believe it is important for this reason not to conclude that subjective vision tests with alternative macular test grids and colour-contrast tests would continue to be of limited usefulness in the future.

The red-on-black chart, the classical colour-contrast test version, of the Amsler grid is too difficult to be seen (very low contrast) by most patients and creates an unacceptably high false-alarm rate. Furthermore, none of the Amsler grid variations made available to date fully utilized the testing potential of their background at the same time as the foreground grid lines as the stimuli. Most retinal and macular lesions, such as macular degeneration or medication toxicity, cause quantifiable contrast sensitivity loss^1, 2 and a blue–yellow defect, whereas optic nerve, chiasmal, and postchiasmal disorders (with the exception of dominantly inherited optic atrophy associated traditionally with a blue–yellow defect) cause a red–green defect.^{3, 4, 5, 6, 7} Colour field test cards based on the colour desaturation test technique utilizing the background at the same time as the grid lines with low and intermediate calibrated contrasts of blue-on-yellow, blue-on-red, and white-on-red combinations are available (www.ixm.us; Figure 1a–d). These versions of the Amsler charts may detect early visual field defects of 5 decibels depth that may be coassociated with approximately 25% washed-out or darkened colour appearance easily noticeable by an average observer, also providing over 90% detection sensitivity and 90% specificity in neuro-ophthalmic practice.⁶ The authors correctly point out the need for an alternative to the conventional high-contrast achromatic Amsler to improve its limited detection sensitivity for screening and the above-described new versions may just be the answer for this need.