The Rarebit fovea test : a new measure of visual function

Abstract

Visual acuity (VA) measurement by the use of letter charts is the most frequently used method for testing foveal function. However, since a decimal visual acuity of 1.0, often used as limit for normality, can be achieved with less than two-thirds of the normal number of optic nerve axons, the letter chart test cannot be regarded as sensitive to low-degree damage. The reason why letter charts are unable to detect small and subtle defects is due to the high receptor density and the proportionally large stimuli used in VA testing.

The Rarebit technique was developed with the explicit aim to improve detection of small defects. The method is built on the principle of detection of very small and bright stimuli, corresponding to half the normal minimum angle of resolution in the tested retinal location. The test principle is easy to understand, the results are easy to interpret, and no expensive equipment is required. The system includes two tests Rarebit Perimetry (RBP) and Rarebit Fovea Test (RFT), the first for evaluation of the 30º visual field (with the possibility to test out to 60º), and the second for evaluation of the foveal function i.e., the most central 4º VF.

Several studies have shown that conventional perimetry and vision function tests are insensitive to minor neuro-visual damage. In Rarebit improved detection of small defects, is intended to be obtained due to two deliberate deviations from standard procedures. Target information content is minimized (i.e., small dots compared to the receptive field at the tested retinal location, briefly exposed) and thresholding is replaced by simple probing of the completeness of the retino-cortical detector matrix.

The aim of the current thesis was twofold. Paper I evaluated the physiologic properties of the test, i.e., the effect of age, binocular summation and different luminance levels in the test stimuli, in normal subjects. In Paper III the fixation stability during the test was studied. In order to determine the potential clinical value, patients with diabetes and cataract were examined. In the patients with diabetes, the results from the RFT were compared with fundus photographs and OCT measurements. The cataract patients were examined with both RBP and RFT prior to and after cataract surgery (Paper II and IV).

The findings from the studies support the idea behind the RFT, i.e., that small stimuli might be useful for detecting small defects in the neuro-retinal architecture and can be expected to give additional information about the visual system, compared to findings from conventional tests, e.g., VA and funduscopic examinations. The RFT can be expected to be of clinical value in patients where early identification of damage is relevant and sensitive methods for follow-up are required.