Visual Loss: Overview, Visual Field Testing, and Topical Diagnosis

The afferent visual pathways encompass structures which perceive, relay, and process visual information: the eyes, optic nerves (cranial nerve II), chiasm, tracts, lateral geniculate nuclei, optic radiations, and striate cortex. Lesions anterior to and including the chiasm may result in visual acuity (clarity) loss, color deficits, and visual field defects (abnormal central or peripheral vision). From a neuro-ophthalmic standpoint, unilateral retrochiasmal (posterior to the chiasm) disturbances can present primarily with homonymous (both eyes involved with the same laterality) visual field defects without acuity loss. Higher-order processing, instrumental in interpreting visual images, occurs in extrastriate association cortex. Abnormalities in these areas can cause, for instance, deficits in object recognition, color perception, motion detection, and visual attention (neglect of visual stimuli in left or right hemifields). This chapter provides an overview of these structures, details methods of visual field testing, then describes a framework for the localization and diagnosis of disorders affecting the afferent visual pathways. Determining where the lesion is first, then finding out what it is second is the advocated approach. Further details regarding these structures' anatomy, blood supply, organization, and neuro-ophthalmic symptoms, as well as the differential diagnosis of lesions affecting them, are detailed in Chapters 4 through 12.