Diagnosis and Treatment of Secondary Syphilis Mimicking Palmoplantar Psoriasis
When confronted with palmoplantar lesions resembling psoriasis, you must obtain serologic testing for syphilis (both nontreponemal and treponemal tests) before assuming a diagnosis of psoriasis, as secondary syphilis has nearly 100% sensitivity on nontreponemal tests and frequently presents with psoriasiform palmoplantar lesions.
Diagnostic Approach
Key Clinical Features to Distinguish Secondary Syphilis from Psoriasis
The palmoplantar presentation of secondary syphilis can be remarkably deceptive 1, 2, 3:
- Morphology: Look for pinkish-red papules and plaques with thick silvery scale—virtually identical to psoriasis 1
- Distribution: Palms, flexor wrists, and medial longitudinal arches of feet are characteristic locations 1
- Biett's collar: A peripheral collarette of scale is suggestive but present in only 45.5% of cases 4
- Tenderness on vertical pressure: This finding can help differentiate from psoriasis 2
- Associated findings: Examine for genital lesions, though 72.8% of palmoplantar syphilis cases have no history or current genital involvement 4
Critical Diagnostic Pitfalls
Do not rely on the presence or absence of genital lesions—the majority of patients with palmoplantar secondary syphilis will not have concurrent genital manifestations 4. Additionally, comorbid conditions like chronic psoriasis or liver disease can obscure the diagnosis and lower clinical suspicion 5.
Laboratory Testing Algorithm
Nontreponemal tests (RPR or VDRL): These have 100% sensitivity in secondary syphilis 6, 7
- Multiple studies consistently demonstrate 100% sensitivity across different nontreponemal tests in secondary syphilis 6
Treponemal tests (TPHA or FTA-ABS): Use for confirmation 2
HIV testing: All patients with syphilis should be tested for HIV 7
Histopathology when serologic findings are indeterminate: Key features include:
Important caveat: Initial serologic testing can yield indeterminate results, particularly in patients with liver disease 5. If clinical suspicion remains high, repeat testing is essential.
Treatment
First-Line Treatment
Benzathine penicillin G 2.4 million units IM as a single dose is the definitive treatment for secondary syphilis 7.
Alternative Regimens for Penicillin-Allergic Patients
For nonpregnant, penicillin-allergic patients 7:
- Doxycycline 100 mg orally twice daily for 14 days (preferred alternative due to better compliance)
- Tetracycline 500 mg orally four times daily for 14 days
- Ceftriaxone 1 g daily (IM or IV) for 10-14 days
Avoid azithromycin due to documented resistance and treatment failures, particularly in men who have sex with men 7.
Special Considerations
- Penicillin-allergic patients with unreliable follow-up: Desensitize and treat with benzathine penicillin 7
- Close follow-up is essential for all patients receiving alternative therapies 7
- Treatment response: Palmoplantar and genital lesions should resolve with appropriate antibiotic therapy 9
Post-Treatment Monitoring
Monitor nontreponemal titers to confirm treatment response. An 8-fold decrease in VDRL titer indicates successful treatment 9.
Clinical Pearls
The prevalence of palmoplantar involvement in syphilis is 47.85%, and palmoplantar lesions may be the only clinical presentation 4. Maintain high clinical suspicion even in patients with established psoriasis, as the two conditions can coexist or be misdiagnosed 5. The key is to test for syphilis in any atypical or treatment-refractory palmoplantar eruption, regardless of sexual history or presence of genital lesions.