Neurosyphilis
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Direct answer
Neurosyphilis is infection of the central nervous system by Treponema pallidum, and it can occur at any stage of syphilis, often years after the primary infection. It presents as asymptomatic, meningovascular, parenchymatous or gummatous disease, with general paresis and tabes dorsalis as the classic parenchymatous forms. Diagnosis rests on cerebrospinal fluid examination, and treatment is intravenous crystalline penicillin G for 10 to 14 days.
What you must remember
- General paresis: Personality change, grandiosity, memory loss, tremor and dysarthria, often with Argyll Robertson pupils and seizures.
- Tabes dorsalis: Lightning pains, sensory ataxia, Charcot joints, bladder dysfunction and optic atrophy from posterior column and root damage.
- Argyll Robertson pupil: Small, irregular pupils that accommodate but do not react to light, a light-near dissociation.
- Meningovascular syphilis: Strokes and cranial nerve palsies from inflamed small arteries, with meningitic features in the CSF.
- CSF VDRL: Highly specific but insensitive, so a negative result does not exclude neurosyphilis.
- Serum treponemal tests: Almost always positive in neurosyphilis, so a negative serum result makes the diagnosis unlikely.
- HIV coinfection: Lower the threshold for lumbar puncture in any HIV-positive patient with syphilis.
The forms of neurosyphilis and their time course
The forms differ in timing and in the part of the nervous system affected. Timing matters because early neurological involvement can appear within the first years, while parenchymatous disease follows a long latent period. The table summarises the four recognised categories.
| Form | Typical onset | Key features |
|---|---|---|
| Asymptomatic | Any stage | Abnormal CSF, no symptoms |
| Meningovascular | Early to mid-term | Stroke, cranial nerve palsy, meningitis |
| Parenchymatous | Years to decades | Paresis with dementia, or tabes with sensory ataxia |
| Gummatous | Late, rare | Focal granulomatous mass lesion |
The parenchymatous forms represent classic late disease, whereas meningovascular disease tends to appear earlier in the course. Because the forms overlap, any new neurological or psychiatric picture in a person with syphilis deserves a lumbar puncture. Treatment is the same penicillin-based regimen for every form, and outcomes are better when it starts before irreversible damage occurs. Follow-up CSF examination after treatment shows whether the inflammatory changes are settling.
A penicillin-allergic patient needs a planned alternative, with desensitisation considered, rather than a casual substitute.
Where the exam tries to catch you
Examiners test the choice of test, the drug and the drug route.
- Trap: Relying on a negative CSF VDRL to exclude neurosyphilis Correct: the test is specific but insensitive, so a negative result must be weighed against cell count, protein and the clinical picture.
- Trap: Treating neurosyphilis with a single intramuscular benzathine penicillin dose Correct: intravenous crystalline penicillin G is the treatment of choice because it achieves adequate CSF levels.
- Trap: Attributing a light-near dissociated pupil to diabetes alone Correct: the classic Argyll Robertson pupil points to neurosyphilis, although other causes such as diabetes occasionally produce it.
Frequently asked questions
What is the treatment of choice for neurosyphilis?
Intravenous crystalline penicillin G is the treatment of choice for neurosyphilis, given for 10 to 14 days. Standard regimens use 18 to 24 million units daily in divided doses. Ceftriaxone is an alternative in penicillin-allergic patients, although penicillin remains the preferred drug.
Which CSF findings support a diagnosis of neurosyphilis?
A raised CSF white cell count, mainly lymphocytes, and a raised CSF protein support neurosyphilis in the appropriate clinical setting. A reactive CSF VDRL is highly specific when present. Because neither finding is sensitive, a normal CSF result does not completely exclude neurosyphilis.
Why is tabes dorsalis associated with Charcot joints?
Tabes dorsalis destroys the dorsal columns and dorsal root ganglia, abolishing deep pain and position sense in the legs. Patients then injure their joints repeatedly without feeling it, leading to neuropathic Charcot joints, commonly the knee or ankle. Lightning pains and loss of ankle reflexes often precede the joint changes.
How should HIV-positive patients with syphilis be assessed for neurosyphilis?
Clinicians should keep a low threshold for lumbar puncture in HIV-positive patients with syphilis, because neurosyphilis is more frequent in this group. CSF cell count, protein and VDRL guide the diagnosis. If the CSF is abnormal, the standard neurosyphilis regimen is given and follow-up CSF examination documents the response.