Progressive outer retinal necrosis is a differential diagnosis for CMV retinitis.

• A. Histoplasmosis
• B. Progressive outer retinal necrosis
• C. Toxoplasmosis
• D. ARMD

Answer: (B)

Progressive outer retinal necrosis is considered in the differential for CMV retinitis because both can cause necrotizing retinal lesions in severely immunocompromised patients, such as those with advanced HIV/AIDS. PORN is typically due to varicella-zoster virus and tends to progress rapidly, but it often presents with different signs than classic CMV retinitis.

Key distinction is how the disease presents. PORN shows rapid outer retinal and photoreceptor layer destruction with little to no intraocular inflammation (little vitritis) and minimal retinal vasculitis or hemorrhage early on. The necrosis often starts in the outer retina, can involve the posterior pole and periphery quickly, and may spare the inner retina initially.

CMV retinitis, in contrast, characteristically produces fluffy, gray-white retinal lesions with retinal hemorrhages—the so-called “pizza pie” appearance—and is associated with more conspicuous vitritis and sometimes retinal vasculitis. The pattern of involvement and the inflammatory response help separate the two.

Understanding these differences is important because the treatments differ: PORN often requires antiviral coverage that targets VZV (and may respond less well to standard CMV-directed therapy), whereas CMV retinitis is managed with CMV-directed antivirals.