Pyoderma gangrenosum:

Pyoderma gangrenosum is of unknown aetiology and presents with erythematous nodules or pustules which frequently ulcerate (Fig. 23.23). The ulcers can be large and grow at an alarming speed. The ulcer has a typical bluish black ('gangrenous') undermined edge and a purulent surface ('pyoderma'). There may be an associated pyrexia and malaise. Biopsy through the ulcer edge shows an intense neutrophilic infiltrate and occasionally a vasculitis but the diagnosis depends mostly on the clinical appearance. The main causes are:
inflammatory bowel disease
rheumatoid arthritis
myeloma, leukaemia, lymphoma
liver disease (primary biliary cirrhosis)
idiopathic (> 20% in some series).

Treatment
This is with very potent topical steroids or 0.1% tacrolimus ointment. High-dose oral steroids may be needed to prevent rapidly progressive ulceration. Oral dapsone and minocycline may help. Other immunosuppressants, such as ciclosporin, are useful in resistant cases. The underlying cause should be treated.