Visual scotomata with intracranial lesions affecting the optic nerve.

Central or paracentral scotoma is an occasional manifestation of certain space-occupying intracranial lesions. The reason for the development of this defect is not yet clear. Traquair (1946) defines a scotoma as " an area of depressed vision within the field margin surrounded by an area of less depressed or normal vision." Either a depressed area or a channel of varying width may connect the scotomai with the periphery or with the blind spot or with both. Extension of a scotoma to the margin of the field is referred to as " breaking through." When such an extension has occurred careful quantitative perimetry will determine to what extent the central defect predominates in the visual field. Sudden consciousness of failing vision usually indicates a macular defect, and may bring the patient to an ophthalmologist before pallor of the discs has developed. Bitemporal defects are of gradual onset, and as about 120 degrees of the binocular field remain unaffected the patient's orientation is not seriously disturbed. Hence, except in the case of retrochiasmal lesions when pallor of the disc may never appear, patients whose macular vision is good do not present themselves for examination until they have reached the stage when pallor of one or both discs is evident.