Erythema induratum: a case of mistaken identity

To the Editor: In a recent issue of the Journal, Chew et al described a woman from Vietnam with skin nodules that, on histological examination, showed lobular panniculitis with granulomatous inflammation.1 No mycobacteria were visible and a polymerase chain reaction test for Mycobacterium tuberculosis was negative. Two months after starting quadruple antituberculous therapy (including rifampicin), her lesions had resolved. Erythema induratum (ostensibly due to hypersensitivity to M. tuberculosis) was diagnosed, despite the absence of evidence of tuberculosis. Other possible diagnoses were considered, but leprosy was not mentioned.

In regions of Australia where leprosy is not endemic, the disease is frequently overlooked.2 Birrell3 described a man from Malta with recurring skin lumps. Biopsy showed panniculitis with giant cells, and the man was initially misdiagnosed as having “Weber–Christian syndrome” or “relapsing febrile non-suppurative nodular panniculitis”. Soon after, another Maltese patient presented similarly. This time, leprosy was suggested, and a biopsy revealed the presence of Mycobacterium leprae.4 Re-examination of slides from the first case showed similar organisms, confirming leprosy.5

The patients described by Chew et al and Birrell had migrated from countries in which leprosy was endemic, and biopsies revealed granulomatous panniculitis. Weber–Christian syndrome and erythema induratum are rare, ill-defined conditions with confused aetiologies, and both lack a specific diagnostic test. Therefore, cases of leprosy can be easily misdiagnosed as one of these conditions. That the biopsy in this patient did not show visible M. leprae is against a diagnosis of leprosy. But in my experience, even in lepromatous (multibacillary) disease, occasionally a skin smear of a lesion or (more rarely) a biopsy specimen may fail to reveal bacilli. Of course, this would be likely if the patient had received specific treatment for leprosy previously.

Respectfully, I suggest that Chew et al should attempt to exclude lepromatous leprosy in their patient by looking for possible missed stigmata of leprosy, enquiring whether she has ever been treated for leprosy, asking whether any close acquaintances have had the infection or a chronic skin condition, and following up the patient in the long term.