Neutrophilic dermatoses: Pyoderma gangrenosum and Sweet's syndrome

Pyoderma gangrenosum and Sweet's syndrome are classified as neutrophilic dermatoses as they exhibit intense dermal inflammatory infiltrates composed of neutrophils with little evidence of a primary vasculitis. They share several characteristics and respond to immunosuppressives. Aetiology is felt to represent a manifestation of altered immunologic reactivity. Patients with both conditions concurrently have been described. Diagnosis is based on clinical and histopathological findings. However, clinically the typical forms of the two conditions are quite distinct: pyoderma showing cutaneous ulceration with a purple undermined border and Sweet's syndrome having tender, erythematous, nonulcerated plaques and nodules. Approximately 50% of cases of pyoderma are associated with a specific systemic disorder. These include inflammatory bowel disease, rheumatoid arthritis, non-Hodgkin's lymphoma and myeloproliferative disorders. Many associations with Sweet's have been described, acute myeloid leukaemia, myeloma and adenocarcinomas, and haematological malignancy. There is overlap between the two conditions with lesions categorised as Sweet's syndrome being clinically more characteristic of atypical pyoderma and vice versa. We believe that pyoderma and Sweet's syndrome represent a continuum of spectrum of disease. The reason for the clinical differences between the conditions is unclear and merits further investigation but may be explained by varying levels of intensity and extent of the inflammatory process. This review will describe the pathogenesis, clinical features, diagnosis, associations and treatment of the two conditions.