Infectious DiseasesTier 2Disease (DEADMAN)

Symptoms of syphilis that can mimic other conditions and how this can be considered in the differential diagnosis

Description

Syphilis is classically described as "the great imitator" given its capacity to mimic numerous other conditions across its clinical stages - a painless genital ulcer (primary), a non-specific rash involving palms/soles (secondary), and a wide range of organ-specific presentations (neurological, cardiovascular, gummatous) in tertiary disease

Syphilis incidence has risen in many high-income settings in recent years, increasing the importance of maintaining a broad differential that includes syphilis for compatible presentations

Treponema pallidum causes a chronic, staged infection with distinct pathological mechanisms at each stage - direct local tissue invasion (primary chancre), disseminated immune-complex-mediated vasculitis (secondary stage rash/systemic symptoms), and, in untreated tertiary disease, granulomatous (gummatous) or direct vascular/neural invasion (cardiovascular and neurosyphilis)

Secondary syphilis classically causes a maculopapular rash involving the palms and soles (a distinctive feature narrowing the differential), often with systemic symptoms (fever, lymphadenopathy, malaise) that can mimic a viral exanthem or other systemic illness
Neurosyphilis can present with a wide range of neurological/psychiatric symptoms (mimicking other neurological or psychiatric conditions) at any stage of infection, not only in classical "tertiary" late disease

A low threshold for syphilis serology testing in any patient with an unexplained rash (particularly involving palms/soles), unexplained neurological or psychiatric presentation, or genital ulceration is appropriate given syphilis's capacity to mimic other diagnoses and the availability of curative treatment (benzathine penicillin) if identified
Full STI screen and HIV testing should accompany any confirmed syphilis diagnosis given shared risk factors and elevated co-infection rates

Neurosyphilis (can mimic diverse neuropsychiatric presentations), palmar/plantar rash differential (secondary syphilis alongside other causes), congenital syphilis (vertical transmission if maternal infection untreated)

Syphilis's reputation as "the great imitator" is not just historical trivia - it should actively lower the threshold for syphilis serology testing in unexplained rashes (particularly palms/soles), unexplained neurological/psychiatric presentations, and genital ulceration, given both its capacity to mimic other diagnoses and its full curability with timely treatment

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