Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION)

NAION is acute ischemia of the laminar and prelaminar optic nerve head supplied by the short posterior ciliary arteries (paraoptic branches), producing axonal infarction with disc edema and progressive optic atrophy. Pathophysiologically, it represents a compartment syndrome in a structurally crowded optic nerve head (small disc with no cup, the disc-at-risk) compounded by hypoperfusion (nocturnal hypotension, sleep apnea, vasculopathic small-vessel disease).

Clinical presentation: sudden painless monocular vision loss noticed on awakening or first thing in the morning (because of nocturnal hypotension), altitudinal visual field defect (60 percent inferior, 20 percent superior, 20 percent central or other patterns), reduced color vision and contrast sensitivity, relative afferent pupillary defect (RAPD), and acute disc edema (sectoral or diffuse, often hyperemic) with peripapillary hemorrhages; second eye involvement occurs in 15–20 percent over 5 years (rare bilateral simultaneous presentation).

Critical differential — arteritic AION (giant cell arteritis): occurs in patients > 70, severe vision loss often counting fingers or worse, pale disc edema (chalk-white), associated headache, jaw claudication, scalp tenderness, polymyalgia rheumatica symptoms, very high ESR (often > 100) and CRP, normal disc-at-risk anatomy may not be present; emergent high-dose IV methylprednisolone (1 g/day for 3 days) prevents fellow-eye blindness and confirms with temporal artery biopsy. Workup of all AION includes immediate ESR, CRP, CBC, and same-day temporal artery palpation; biopsy if any GCA features.