Identify the role of the retina or of the optic nerve as an etiology of visual decease is sometimes difficult and requires eliminating obvious causes, including toxic or inherited ones. These two last causes are responsible for bilateral forms, though sometimes asymmetrical. The existence of an abnormality of the retinal examination and pupillary movement is an important diagnostic argument that must be systematically looked for, as well as the existence of an afferent pupillary defect in unilateral visual deficit. It is often required perform complementary examinations and to compare their results. However, in some cases, only the disease progression and the repetition of examinations allow to settle between these two etiologies.