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History:  26 yo female with a 6 month history of a left posterior calf lesion which began as a red hard indurated and erythematous papule.  2 weeks later it spontaneously drained purulent material and began to ulcerate.  3 months ago an outside dermatologist empirically began potassium iodide therapy after an inconclusive biopsy.  She was transferred to our care when the lesion continued to progress.  No fever, cough, weight loss or systemic symptoms.

Epidemiology:  Patient is a nun.  Born in the Andean highlands but has lived for many years in Lima.  Works in poor areas of Lima.  1 month prior to the illness she had briefly visited the cloud forest in the high jungles of Per&uacute.

Physical Examination:   Afebrile.  No lymphadenopathy or organomegaly.  Ulcerative non-tender lesion with wide erythematous reaction on the left calf as shown in the image at right.  The upper smaller ulcer is the site of the biopsy by the referring dermatologist.

Laboratory Examination:  Normal CXR.  Hematocrit 40.  WBC 5.2 with 63 neutrophils, 28 lymphocytes, and 5 eosinophils.  Liver function tests normal.  Bacterial and fungal cultures were sterile.

The patient was placed on 4-drug anti-TB therapy and within 15 days the lesions were healing with good granulation tissue and within 1 month the ulcer was completely healed over.  A follow-up image of the scar taken at the present visit is shown (image at right, C).

Erythema induratum is classified as one of the tuberculids.  The tuberculids represent a group of nodular and vasculitic reactions which are thought to represent hypersensitivity reactions to antigens and soluble products of M. tuberculosis.  As with erythema nodosum which may also be associated with tuberculosis, organisms are not found in the lesions themselves.  In Erythema nodosum the inflammatory infiltrate is restricted to the septal regions surrounding the adipose lobules and does not extend into the adipose tissue.  The tuberculids are distinct from the more common primary tuberculous lesions of the skin or the post-primary tuberculous infection of the skin such as lupus vulgaris or tuberculosis verrucosa cutis from which organisms can be directly isolated by culture.  Diagnosis of Erythema induratum is made by a combination of clinical presentation (see below), markedly positive TB skin test, response to anti-TB therapy, and characteristic histologic findings (see above).  PCR amplification of biopsy tissue from patients with lobular granulomatous panniculitis has demonstrated M. tuberculosis DNA in up to 77% of patients (Arch Dermatol 1997;133:457-62).  The histologic findings by themselves are indistinguishable from what is called nodular vasculitis in areas where TB is not highly prevalent so that the clinical spectrum outlined above is a necessary complement to the histology.

95% of cases of Erythema induratum are in young women and almost all lesions are on the posterior calves. The lesion is characteristically a 1-3 cm nodular lesion which eventually ulcerates.  Other nodular lesions of the legs, such as erythema nodosum or other forms of panniculitis, never ulcerate and some forms of vasculitis, such as polyarteritis nodosa or nodular vasculitis, only do so rarely.  The posterior calf location is less characteristically seen in other nodular leg lesions.

Other tropical ulcerative lesions that might be preliminarily considered based on visual inspection prior to biopsy results being known would include: leishmania, sporotrichosis, or Buruli ulcer.  None of these is purulent and generally they don?t have the surrrounding erythematous reaction seen here.  None, including Buruli, would respond to standard TB therapy and in our case all were ruled out by biopsy and culture.  Anthrax is also not purulent and an eschar would be expected to form quite quickly.

The prognosis in these cases is guarded as there tends to be a recurrance of lesions in other locations over a protracted period of years at irregular intervals even after modern 4-drug chemotherapy for 6 months.  Most clinicians would not repeat TB treatment for a recurrence if the initial course of therapy had been completed.  The lesions may also sometimes involute spontaneously.  A primary focus of tuberculous infections is sometimes but not necessarily found.