A 32-year-old woman presents with unilateral anterior uveitis with mutton-fat keratic precipitates and posterior synechiae. On investigation, she is found to be tuberculin skin test positive with chest X-ray showing hilar lymphadenopathy. Which finding best distinguishes tuberculous uveitis from sarcoid uveitis in this clinical context?

Explanation

## Tuberculous vs. Sarcoid Uveitis: Discriminating Features ### Clinical Context Both tuberculous and sarcoid uveitis present with: - Granulomatous anterior uveitis (mutton-fat KPs) - Posterior synechiae - Systemic findings (TB: hilar lymphadenopathy; sarcoid: also hilar lymphadenopathy) - Positive tuberculin skin test (TB) or elevated ACE (sarcoid) The challenge is distinguishing them clinically. ### Comparative Table | Feature | Tuberculous Uveitis | Sarcoid Uveitis | | --- | --- | --- | | **Choroidal tubercles** | Present (pathognomonic) | Absent | | **Retinal vasculitis** | Caseating granulomas → branch retinal artery occlusion | Mild/absent | | **Snowball opacities** | Absent/minimal | Prominent (snowball vitreous opacities) | | **Epiretinal membrane** | Rare | Common | | **Anterior chamber** | Granulomatous (mutton-fat KPs) | Granulomatous (mutton-fat KPs) | | **Posterior synechiae** | Common | Common | | **Systemic findings** | Hilar lymphadenopathy, pulmonary infiltrates | Hilar lymphadenopathy, erythema nodosum, lupus pernio | | **Tuberculin skin test** | Positive (or negative in miliary TB) | Negative (anergy) | | **ACE level** | Normal | Elevated | ### Key Discriminator: Choroidal Tubercles **High-Yield:** **Choroidal tubercles** are the pathognomonic finding of tuberculous uveitis. These are small, yellowish, granulomatous lesions at the choroid, visible on fundoscopy. They represent caseating granulomas and are virtually diagnostic of ocular TB. **Key Point:** Retinal vasculitis (branch retinal artery occlusion, retinal hemorrhages) is more common in tuberculous uveitis due to vasculitis from caseating inflammation. Sarcoid uveitis rarely causes significant retinal vasculitis. ### Explanation **Tuberculous uveitis** causes: 1. Caseating granulomatous inflammation → choroidal tubercles (pathognomonic) 2. Vasculitis → retinal artery occlusion, retinal hemorrhages 3. Anterior granulomatous reaction (mutton-fat KPs) 4. Posterior synechiae **Sarcoid uveitis** causes: 1. Non-caseating granulomatous inflammation → snowball vitreous opacities (pathognomonic) 2. Epiretinal membrane formation 3. Anterior granulomatous reaction (mutton-fat KPs) 4. Minimal retinal vasculitis **Clinical Pearl:** If you see choroidal tubercles on fundoscopy in a patient with granulomatous uveitis, TB is the diagnosis until proven otherwise. Initiate anti-TB therapy after ruling out other causes. **Mnemonic: TB vs. SARCOID** - **TB**: **T**ubercles (choroidal), **B**ranch artery occlusion - **SARCOID**: **S**nowball opacities, **A**nergy (tuberculin skin test negative), **R**are vasculitis, **C**aseating granulomas absent, **O**ptic nerve involvement, **I**nflammatory membranes (epiretinal), **D**ry eye (Sjögren-like)