In posterior uveitis, which layer of the retina is primarily affected in acute retinal necrosis (ARN)?

Explanation

## Acute Retinal Necrosis (ARN) — Pathology **Key Point:** ARN is a fulminant viral retinitis characterized by full-thickness retinal necrosis, typically beginning at the periphery and progressing centripetally. ### Histopathological Features of ARN | Feature | Details | |---------|----------| | **Extent** | Full-thickness retinal necrosis | | **Distribution** | Peripheral retina; centripetal progression | | **Viral inclusions** | Intranuclear and intracytoplasmic viral particles (HSV-1, HSV-2, VZV) | | **Inflammation** | Minimal to moderate anterior chamber reaction; vitritis typically mild relative to retinal involvement | | **Vascular involvement** | Vasculitis; retinal hemorrhages; arterial sheathing | | **Progression** | Rapid (days to weeks) if untreated | **High-Yield:** ARN differs from cytomegalovirus (CMV) retinitis in that ARN causes full-thickness necrosis with prominent vasculitis, while CMV typically shows a "cottage-cheese and ketchup" appearance with granular opacification and hemorrhage, sparing the inner retina initially. ### Differential: ARN vs. CMV Retinitis | Feature | ARN | CMV | |---------|-----|-----| | **Retinal layers affected** | Full-thickness | Outer retina (granular) | | **Vasculitis** | Prominent | Minimal | | **Vitritis** | Mild to moderate | Minimal ("quiet retinitis") | | **Immune status** | Any (CD4 > 50) | Immunocompromised (CD4 < 50) | | **Progression rate** | Rapid (weeks) | Slow (months) | | **Causative virus** | HSV-1, HSV-2, VZV | CMV | **Clinical Pearl:** The hallmark of ARN is the **white granular border** advancing from the periphery, often with a hemorrhagic leading edge — this is full-thickness necrosis, not just outer retinal disease. **Mnemonic:** **ARN = Acute Retinal Necrosis = All Retinal layers Necrosed** (full-thickness) — remember this to distinguish from CMV's outer-retina-only pattern.