Interdigital syphilis: a rare form of secondary syphilis

Introduction

Syphilis is a sexually transmitted infection (STI), well known as the “great imitator”. A papular eruption is a common presentation in secondary syphilis. Split (fissured) papules may occur at sites such as the nasolabial folds and angles of the mouth. ¹ Atypical forms of syphilis are more often seen in people living with HIV (PLWH)s, malnourished or immunocompromised individuals as well as, chronic alcohol users, children, and pregnant women.^2,3

Case report

A married white man, 39 years old, and a regular alcohol user complained of painful erosions in the fourth interdigital space of both feet. A clinical diagnosis of Gram-negative toe web infection was made and the patient was prescribed oral cefuroxime 250 mg twice a day. After 10 days with no improvement, he was referred to a dermatologist. For almost a year, he and his wife had been separated. He recalled group sex with two female partners six months earlier during this separation period.

On examination bilateral, shallow ulcers with pale, macerated surrounding skin were evident in the fourth interdigital spaces of both feet with some coagulated blood on the lateral sides of toes (Figure 1(a) and (b)). The patient had severe pain and walking difficulties. His genitalia had multiple erosions on the shaft of the penis and enlarged inguinal lymph nodes bilaterally (Figure 1(c)). The rest of the body was unaffected; there were no other signs or symptoms. Microscopic examination for tinea pedis was negative, as was culture for Candida albicans infection. Blood analysis was positive for rapid plasma reagin (4+), Treponema pallidum hemagglutination assay (4+; 1:2560), and anti-Treponema pallidum IgM (EIA; 181 U/ml). Secondary syphilis was diagnosed on the basis of clinical and serological findings. HIV and STI tests (Chlamydia, gonorrhoea, and herpes) were negative. At first, he received doxycycline 100 mg twice daily and later one injection of 2.4 million IU of benzathine penicillin G intramuscular. Treatment was started with doxycycline due to absence of benzathine penicillin G in the local pharmacy. After 10 days of doxycycline treatment, the lesions on the feet had healed. Serological tests of the patient’s wife were also positive for syphilis, but she had no clinical signs and was diagnosed with early latent syphilis.

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Figure 1.

(a) Interdigital ulcer. (b) Multiple erosive lesions on the shaft of penis. (c) Circular erosion on the fifth digit of the right foot.

Discussion

Pedal syphilis can take three main forms: plantar (syphilitic) papules, condylomata lata, and interdigital syphilis. Plantar papules are relatively common in secondary syphilis, although the interdigital area is rarely affected. Condylomata lata of the toe webs with or without genital lesions have been reported, and are regarded as an uncommon manifestation.^4,5 Clinically, it is characterized by the so-called “split-papule”. ⁶ Multiple genital lesions can also be seen in syphilis including atypical lichenoid ones. Papular genital lesions usually coalesce to form condyloma lata, which were not seen in our case. Multiple erosive lesions are more characteristic of genital herpes rather than primary syphilis. Erosive lesions in secondary syphilis are typically seen on mucous membranes rather than the skin.

Syphilitic lesions, both primary and secondary, are typically painless, although the reason for the absence of pain is unclear. In contrast, syphilis in the toe web area is usually painful, ⁷ perhaps arising from mechanical trauma.

The differential diagnosis of interdigital pedal syphilis includes tinea pedis, Gram-negative toe web infection, erythrasma, macerated corns, verrucae, tropical mycoses (chromomycosis, mycetoma), trophic and neuropathic foot ulcers, decubitus ulcer, leprosy, bullous diseases, and lues maligna.^8,9 The latter is a rare form of secondary syphilis, characterized by nodulo-ulcerative lesions mainly seen in PLWH, malnourished and immunosuppressed patients, or in chronic alcoholics. ² At least theoretically, the differential diagnosis can include primary chancre or even tertiary syphilis in the pedal web space.

Benzathine penicillin G is the drug of choice for uncomplicated early syphilis. ¹⁰ Several other options including amoxicillin and second generation cephalosporins have been used as alternatives. ¹¹ In our case, there was no improvement using a lower than recommended dose of a second generation oral cephalosporin for 10 days, but clinical cure was achieved with appropriate dosing of oral doxycycline for the same duration.

Failure to recognize rare clinical presentations of syphilis may delay diagnosis and treatment, and moreover enable further transmission.^2,10,12 We conclude that syphilis screening should be recommended in erosive interdigital lesions negative for fungal infections, especially in individuals at risk of acquiring HIV and STIs.