A 28-year-old man with known syphilis presents with floaters, photopsia, and blurred vision. Dilated fundus examination reveals vitritis inflammation with focal retinal infiltrates and hemorrhages in a hemorrhagic retinitis pattern. Which investigation is most appropriate to confirm the diagnosis of syphilitic posterior uveitis?

Explanation

## Clinical Context The patient presents with posterior uveitis (vitritis, retinal infiltrates, hemorrhages) in the setting of known syphilis. The clinical picture suggests **neurosyphilis with posterior uveitis**, which may be associated with meningitis or vasculitis. ## Why CSF VDRL and FTA-ABS? **Key Point:** CSF VDRL is the gold standard for diagnosing neurosyphilis, including syphilitic posterior uveitis. A positive CSF VDRL confirms CNS involvement and mandates IV penicillin therapy [cite:Harrison 21e Ch 207]. **High-Yield:** While serum VDRL may be positive in any patient with syphilis, **CSF VDRL is specific for neurosyphilis**. CSF FTA-ABS (fluorescent treponemal antibody absorption) has higher sensitivity for detecting treponemal antibodies in CSF and helps confirm active CNS infection. **Mnemonic:** **CSF VDRL = Neurosyphilis confirmation**. Always perform lumbar puncture in any patient with syphilis and uveitis to rule out CNS involvement. ## Diagnostic Workup for Syphilitic Uveitis | Test | Specimen | Interpretation | Role | |---|---|---|---| | Serum VDRL | Blood | Positive in active syphilis (any stage) | Screening; non-specific | | Serum FTA-ABS | Blood | Positive in all stages; remains positive after treatment | Confirmatory for syphilis | | CSF VDRL | Cerebrospinal fluid | Positive = neurosyphilis (CNS involvement) | Gold standard for neurosyphilis | | CSF FTA-ABS | Cerebrospinal fluid | High sensitivity for CNS treponemal disease | Supports neurosyphilis diagnosis | | Dark-field microscopy | Lesion exudate | Direct visualization of spirochetes | Not useful in uveitis | **Clinical Pearl:** Syphilitic posterior uveitis is a form of neurosyphilis and often presents with vitritis, retinal vasculitis, and hemorrhagic retinitis. Lumbar puncture is mandatory to confirm CNS involvement and guide treatment (IV penicillin vs. IM benzathine penicillin). **Warning:** A positive serum VDRL alone does NOT confirm neurosyphilis; CSF examination is essential. Many patients with secondary or tertiary syphilis have positive serum serology but negative CSF, indicating no CNS involvement. ## Treatment Implications ```mermaid flowchart TD A[Syphilis + Posterior Uveitis]:::outcome --> B[Perform Lumbar Puncture]:::action B --> C{CSF VDRL Result?}:::decision C -->|Positive| D[Neurosyphilis Confirmed]:::outcome D --> E[IV Penicillin G 18-24 MU/day x 10-14 days]:::action C -->|Negative| F[Non-neurosyphilitic syphilis]:::outcome F --> G[IM Benzathine Penicillin G 2.4 MU weekly x 3 weeks]:::action ```