The paracentral visual field in multiple sclerosis: evidence for a deficit in interneuronal spatial summation?

A visual complaint such as blurred or "washed-out vision" can be one of the early signs of multiple sclerosis (MS). Although visual deficits are commonly attributed to optic nerve demyelination even with preserved visual acuity, the results of a considerable number of visual studies are inconsistent with this interpretation [Camisa, Mylin, & Bodis-Wollner, Annals of Neurology 10 (1981) 532-539; Regan & Neima, British Journal of Ophthalmology 68 (1984) 310-315]. However, a retinal axonal (nerve fiber layer) defect can be detected in some eyes, this is not the rule. Routine visual field (VF) tests, with a low sampling rate may also be non-informative in MS and optic neuritis, possibly because the VF abnormalities may be small and spotty or they can be found between tested points. The present study combined the advantages of VF and contrast sensitivity (CS) testing by applying contrast perimetry (CP), to the central 16 degrees of the VF. Four paracentral VF quadrants were tested in clinically affected and unaffected eyes of 31 MS patients and 26 controls. The stimuli were vertical Gaussian apertured sinusoidal gratings (Gabors) of 1 cpd. CS was obtained as a function of the diameter of the Gábor ranging from 1 to 7.4 degrees. The CP data of controls and definite and probable MS groups were significantly different for each pattern size, but the largest difference was found at diameters 2.5-3.7 degrees. Our study adds to previous evidence showing that optic nerve pathology does not explain "subclinical" and manifest visual dysfunction in MS. Given previous studies revealing orientation dependent monocular visual deficits and our study results, parsimony suggests that MS affects a network relying on myelinated lateral axonal branches of the visual cortex, binding monocular columns of neurons with like-with-like specificity.