Subcutaneous emphysema.

A medical emergency, the detection of subcutaneous emphysema requires thorough evaluation to exclude the multitude of disease processes that may demonstrate this clinical finding. Gas gangrene must be considered in the differential diagnosis of all forms of subcutaneous emphysema and infections with some species, such as C. novyi, may not produce gas at all. Isolation of C. septicum from the blood is almost always associated with colon cancer or hematologic malignancies. Nonclostridial gas gangrene in diabetic patients is indistinguishable clinically from clostridial gas gangrene. A unique and true dermatologic emergency is the detection of nontraumatic subcutaneous emphysema of the thigh with or without associated erythema, tenderness, or bullous lesions. This finding is associated with perforated viscus in a retroperitoneal location. Infections with gas-producing organisms continue to be a source of significant morbidity in modern times.

Generalized perforating granuloma annulare.

Generalized perforating granuloma annulare is characterized by 1 to 4 mm umbilicated papules on the extremities, and is most commonly seen in children and young adults. Transepithelial elimination of mucinous, degenerating collagen fibers and surrounding palisading lymphohistiocytic granulomas are important histologic features. Perforating sarcoidosis and perforating granuloma annulare may be difficult to differentiate because of a similar clinical appearance and the presence of sarcoidal granulomas in biopsy specimens. A particularly high incidence of perforating granuloma annulare has been reported in the Hawaiian Islands.