A 72-year-old woman from Bangalore presents with sudden onset of a dark shadow in the upper half of her left visual field. She denies eye pain or diplopia. On examination, visual acuity is 6/6 in both eyes. Pupils are equal and reactive. Extraocular movements are intact. Perimetry shows a left superior altitudinal visual field defect. Fundoscopy reveals a pale, swollen optic disc with superficial flame-shaped hemorrhages at the superior margin. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Anterior Ischemic Optic Neuropathy (AION) ### Key Clinical Features **High-Yield:** The combination of **sudden onset altitudinal visual field defect**, **pale swollen optic disc with flame hemorrhages**, and **age >60 years** is pathognomonic for AION. ### Pathophysiology **Key Point:** AION results from ischemia of the optic nerve head (optic disc) due to occlusion of the short posterior ciliary arteries (SPCAs) that supply the optic nerve head. This is a vascular event, not inflammatory. ### Why Altitudinal Defect? The optic nerve head has a segmental blood supply: - **Superior segment** supplied by superior SPCA - **Inferior segment** supplied by inferior SPCA Occlusion of one SPCA causes infarction of that segment, resulting in an **altitudinal (sectoral) visual field defect** — typically superior altitudinal (more common) or inferior altitudinal. ### Distinguishing AION from Other Causes | Feature | AION | PION | Optic Neuritis | Retrobulbar ON | |---|---|---|---|---| | **Onset** | Sudden | Sudden | Subacute (hours–days) | Subacute | | **Visual field defect** | Altitudinal (sectoral) | Altitudinal or central scotoma | Central scotoma or generalized | Central scotoma | | **Optic disc appearance** | Pale, swollen, flame hemorrhages | Normal initially (ischemia is posterior) | Swollen (papillitis) | Normal (inflammation is posterior) | | **Pain with eye movement** | No | No | Yes (90%) | Yes (90%) | | **Age of onset** | >60 years (giant cell AION) or >50 years (arteritic AION) | Any age | 20–50 years | 20–50 years | | **Associated systemic disease** | Giant cell arteritis, diabetes, hypertension | Syphilis, sarcoidosis, Lyme disease | Demyelinating disease (MS) | Demyelinating disease (MS) | **Clinical Pearl:** **Arteritic AION** (associated with giant cell arteritis) is a medical emergency requiring immediate ESR, CRP, and temporal artery biopsy to prevent blindness in the fellow eye. Non-arteritic AION (most common) occurs in patients with vascular risk factors (diabetes, hypertension, hyperlipidemia). ### Why NOT the Other Options? **Posterior Ischemic Optic Neuropathy (PION):** The optic disc appears normal initially because the ischemia is posterior to the optic nerve head. The optic disc does not swell or show hemorrhages. This patient has a pale, swollen disc with hemorrhages, ruling out PION. **Optic Neuritis:** Typically presents with **pain on eye movement** (90% of cases), **subacute onset** (hours to days), and a **central scotoma** (not altitudinal). The optic disc is swollen (papillitis) if the inflammation is anterior, but the visual field defect pattern is different. This patient has no pain and an altitudinal defect, making optic neuritis unlikely. **Retrobulbar Optic Neuritis:** The optic disc appears normal because the inflammation is posterior to the optic nerve head. Patients present with **severe eye pain on movement**, **central scotoma**, and **subacute onset**. This patient has no pain, an altitudinal defect, and a swollen disc, ruling out retrobulbar ON. ### Mnemonic: AION-SUDDEN **A**ltitudinal field defect **I**schemic (vascular) **O**ptic nerve head **N**o pain **S**udden onset **U**nder 60 (non-arteritic) or Over 60 (arteritic) **D**isc swollen with hemorrhages **D**iabetes, hypertension (risk factors) **E**SR/CRP (if arteritic suspected) **N**o central scotoma