A 59-year-old white man presented with a 1-month history of a mildly itchy rash on his arms, upper thighs, and back that began shortly after clearing brush around the exterior of his house. A physical examination revealed erythematous papules and plaques on his arms with scattered papules on his trunk and upper legs (Fig 1). Some of the plaques on his arms had an annular configuration, with scattered thin crusts. A medical history revealed that he was currently being treated for Crohn disease of his terminal ileum and colon with 6-mercaptopurine (100 mg daily); he had also completed a course of prednisone and metronidazole 3 months before onset of the rash. He had no history of previous cutaneous lesions. The initial clinical diagnosis was contact rhus dermatitis, and a skin biopsy specimen was taken from a plaque of his left forearm before treatment. The patient was initially treated with topical steroids, including fluocinonide and clobetasol creams. The skin biopsy revealed noncaseatng granulomas with perivascular, interstitial, and periadnexal lymphocytic inflammation. Special stains were negative for fungi, acid-fast bacilli, and bacteria. Polarizing microscopy did not reveal foreign bodies. A chest radiograph was normal, and a tuberculosis skin test was nonreactive. Pathologic review by the Armed Forces Institute of Pathology supported a diagnosis of MCD.2 Because there was no improvement with topical steroids, the patient was placed on a tapered course of prednisone for 2 weeks. He partially responded to this therapy, but relapsed soon after completion. Treatment was changed to metronidazole 500 mg three times daily for 2 weeks, then twice daily for 6 weeks. He noticed improvement within 2 weeks, and at 6 weeks was completely clear of lesions (Fig 2). No side effects, including paresthesias, were reported. Metronidazole was discontinued, and he remained clear of rash for 3 months. He then developed ten 4-mm, scaly, mildly itchy papules on the dorsal surfaces of his forearms, and metronidazole 500 mg twice daily was restarted. He was completely clear of lesions in 10 days, at which time he discontinued treatment on his own because of an upset stomach. He remained clear of rash at follow-up 2 months later.
Numerous therapies for MCD have been used, including surgical excision, topical steroids, oral prednisone, azathioprine, cyclosporine, sulfasalazine, tetracyclines, oral tacrolimus, infliximab, mesalamine, and oral metronidazole.1, 3, 4 In a study of 26 patients with perineal Crohn disease, all patients cleared with oral metronidazole 20 mg/kg/day. Tapering of this dose often led to recurrences that responded to retreatment with metronidazole. The main side effect was paresthesia.5
The cause of MCD remains unknown, but may involve antibody cross reactions between skin and gut antigens with deposition of immune complexes in the skin.1, 2 The antibiotic effect of metronidazole against anaerobic bacteria in the intestinal tract may also improve MCD by reducing bacterial antigens that might cross react with skin antigens.6
Oral metronidazole was an effective treatment for MCD in this patient without significant side effects, and a minor flare responded rapidly to retreatment.