Nonarteritic anterior ischemic optic neuropathy is caused by infarction of the laminar or retrolaminar portion of the optic nerve head supplied by the short posterior ciliary arteries. Clinically NAION is characterized by sudden, usually painless, loss of vision in one or both eyes. Examination findings include decreased visual acuity, a visual field defect, decreased colour vision, a relative afferent pupillary defect, and optic disc swelling.