Erythema induratum: a case of mistaken identity

• Erythema induratum (EI) can easily be mistaken for e nodosu , perniosis or cutaneous polyarteritis nodos • The failure to detect Mycobacterium tuberculosis on or by a polymerase chain reaction test on the biopsy does not exclude the diagnosis. azi illu sio the lowe B n 1 first described erythema induratum (EI) in 1861 to strate chronic, painful, violaceous, indurated and occanally ulcerative nodules that occurred predominantly on r limbs of patients (mainly women) with tuberculin hypersensitivity. Its appearance can easily be confused with erythema nodosum, polyarteritis nodosa or perniosis. The diagnosis of EI could easily have been missed, as the cutaneous lesions were similar to those characteristic of erythema nodosum. In addition, there was a partial response to corticosteroids. A diagnosis of EI is based on cutaneous characteristics, a positive Mantoux test, evidence of tuberculosis and histological findings. The diagnosis of EI in this patient was established with three of the four criteria. Furthermore, the patient responded clinically to antituberculous therapy. The negative polymerase chain reaction (PCR) result is not surprising. In other studies, only 56%–88% of patients previously diagnosed with cutaneous tuberculosis had a positive PCR result using the IS6110 primer. Schneider et al found a positive PCR result in only 5 of 20 patients with EI. Shimizu et al failed to isolate Mycobacterium tuberculosis by either culturing cutaneous EI tissue or inoculating the tissue into guinea pigs. The association of EI with tuberculosis is still a matter of debate. Some have considered EI to be a type III or type IV hypersensitivity reaction to M. tuberculosis antigens. This may explain the patient’s partial response to corticosteroids.