A Solitary Annular Plaque on the Jaw: Atypical Presentation of Secondary Syphilis

Secondary syphilis, a stage of infection caused by Treponema pallidum, is classically characterized by widespread mucocutaneous eruptions. However, atypical presentations can lead to diagnostic uncertainty. This case report highlights an unusual manifestation of secondary syphilis in a 33-year-old male who developed a solitary annular plaque on the left jaw, emphasizing the importance of clinical suspicion, histopathology, and specialized testing in confirming the diagnosis.

Clinical Presentation

The patient presented with a single annular plaque on the left jaw that appeared 4 weeks prior to consultation. The lesion initially emerged as a small, asymptomatic spot and expanded centrifugally to reach a diameter of 5.5 cm. Physical examination revealed a well-demarcated, raised, infiltrated annular plaque with fine scaling along its periphery [Figure 1A]. Notably, no additional skin lesions were observed elsewhere on the body, though left submaxillary lymphadenopathy was detected. The absence of systemic symptoms, such as fever or malaise, further complicated the initial clinical assessment.

Histopathological and Immunohistochemical Findings

A punch biopsy of the lesion revealed significant histopathological abnormalities. The epidermis exhibited acanthosis and orthokeratosis with exudates of polymorphonuclear neutrophils. The dermis demonstrated superficial and deep perivascular inflammation involving hair follicles, with a dense infiltrate of plasma cells, lymphocytes, and eosinophils [Figure 1B and 1C]. These findings, particularly the prominence of plasma cells, raised suspicion for syphilitic involvement.

To confirm the presence of T. pallidum, immunohistochemical staining using an anti-T. pallidum antibody was performed. This revealed abundant spirochetes within the epidermis [Figure 1D], providing definitive evidence of syphilitic infection.

Laboratory Investigations

Serological testing was consistent with active syphilis. The T. pallidum particle assay (TPPA) returned positive, and the toluidine red unheated serum test (TRUST) showed a titer of 1:64, indicating recent infection. Screening for hepatitis B, hepatitis C, and HIV antibodies yielded negative results.

Epidemiological History

Upon further questioning, the patient disclosed multiple episodes of unprotected extramarital sexual contact 8 weeks before the onset of the skin lesion. This timeline aligned with the typical incubation period of secondary syphilis, which usually manifests 3–12 weeks after primary infection.

Diagnosis and Differential Considerations

The diagnosis of secondary syphilis was established based on the clinical presentation, histopathology, immunohistochemistry, and serological evidence. The atypical morphology and solitary nature of the lesion initially prompted consideration of other conditions:

Granuloma annulare: Characterized by annular plaques, but typically lacks scaling and plasma cell-rich infiltrates.
Tinea faciei: A fungal infection that often presents with central clearing and active borders, but potassium hydroxide (KOH) preparation or fungal culture would demonstrate hyphae.
Plaque psoriasis: Exhibits well-demarcated erythematous plaques with silvery scales but lacks plasma cell infiltrates.
Subacute cutaneous lupus erythematosus: Presents with photosensitive annular lesions but shows interface dermatitis on histopathology.

The histopathological identification of plasma cells—observed in 74%–86.4% of secondary syphilis cases—provided critical diagnostic leverage. Immunohistochemistry further distinguished syphilis from mimics by directly visualizing spirochetes.

Treatment and Follow-Up

The patient received benzathine penicillin G at a dose of 2.4 million units via intramuscular injection weekly for three consecutive weeks, adhering to the World Health Organization (WHO) guidelines for secondary syphilis. Four weeks after completing therapy, the plaque resolved completely. The patient remained under follow-up to monitor serological response and potential relapse.

Discussion

Atypical Cutaneous Manifestations in Secondary Syphilis

Secondary syphilis is renowned for its diverse cutaneous presentations, often termed “the great imitator.” While macular and papular eruptions are most common, atypical lesions—including pustular, nodular, and annular forms—account for approximately 10%–15% of cases. Annular lesions, though rare, have been documented in areas such as the genitalia, trunk, and extremities. This case is exceptional due to the solitary facial location, a site reported in fewer than 2% of annular syphilitic lesions.

Diagnostic Challenges and Pitfalls

The absence of classic symptoms, such as palmoplantar involvement or mucous patches, delayed recognition in this case. Clinicians must maintain a high index of suspicion for syphilis when evaluating sexually active individuals with unexplained cutaneous lesions, regardless of morphology or distribution. Histopathology plays a pivotal role, with plasma cell infiltration being a hallmark feature. Immunohistochemistry offers superior sensitivity compared to silver stains (e.g., Warthin-Starry), particularly in early or paucibacillary lesions.

Public Health Implications

The rising global incidence of syphilis underscores the need for heightened awareness of atypical presentations. Misdiagnosis or delayed treatment perpetuates transmission and increases the risk of tertiary complications. Screening for co-infections, such as HIV, remains essential, as syphilis enhances viral transmissibility and acquisition.

Conclusion

This case illustrates the diagnostic complexity of secondary syphilis, particularly when presenting as a solitary annular plaque on the face. The integration of clinical history, histopathology, and immunohistochemistry proved indispensable in confirming the diagnosis. Clinicians must consider syphilis in the differential diagnosis of unusual skin lesions, especially among high-risk populations. Early recognition and treatment are critical to preventing complications and curtailing disease transmission.

DOI: org/10.1097/CM9.0000000000000807

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