[Campimetric changes in optic radiation lesions].

Abstract

Visual field defects may be congruous or not in lesions affecting the optic radiation. This fact, and the macular sparing found in some cases, can be explained if the optic radiation is envisaged as formed by three layers, the superficial corresponding to homolateral peripheral vision, the intermediate to contralateral peripheral vision, and the deepest to central vision from both hemimaculae. The fibers of all these layers fan out sequentially, first the more superficial, then the deeper, around the posterior part of the lateral ventricles, following a longitudinal plane of divergence in their way to area 17. The progressive displacement, upwards and downwards, of the thick fibers transmitting peripheral vision, leave out finally the thin fibers for central vision as the sole constituents of the central and terminal part of the optic radiation. As supporting evidence of this conception we have the fact that if the optic radiation is encroached on from its outer side (as it generally happens for reasons stated in the full text) visual field defects tend to be larger in the homolateral field. This holds true for temporal and parietal lesions, and to a lesser degree for occipital lesions. Deep lesions (for instance intraventricular tumours) tend to affect first the deeper strata of the optic radiation (macular vision and/or contralateral quadrants of peripheral vision). The schematic drawings presented allow us to understand how a temporal lesion may produce: a) no visual field defect, b) an upper homonimous quadrantopsia, c) a lower homonimous quandrantopsia, d) a homonimous hemianopsia; and that these field defects can be either congruent or uncongruous. Deep parietal lesions will produce visual field defects in the lower quadrants, which may be congrous or not. As the visual fibers for macular vision are deeply located in the optic radiation, forming a thin but wide layer in the vertical plane, they cannot be affected to a significative extent without the more superficial fibers for peripheral vision being also affected to an even greater extent. Due to the width and depth of this macular layer even in extensive lesions of the optic radiation some of its fibers can escape injury. This can explain macular sparing in some cases without resorting to hypothetical bilateral macular representation which is deemed unacceptable. Anatomical data and clinical examples are given which lend support to all these contentions.