This 80-year old woman presented with a solitary white nodule on her elbow. The perforating variant of granuloma annulare (GA) is characterized by the transepidermal elimination of degenerate (necrobiotic) collagen fibers. It typically presents in the extremities as a localized group of papules with umbilicated crusts. The pathogenesis of GA is currently unknown although immune-mediated or infectious mechanisms are suspect. GA must be differentiated from necrobiosis lipoidica (NLD) and rheumatoid nodule. NLD most commonly occurs on the shins of diabetic patients and is characterized by a deeper and more diffuse degeneration of collagen fibers that commonly lacks mucin deposition. Rheumatoid nodule is also a deep process and is usually markedly fibrotic.
Histologic section of skin shows a prominent perforation through the epidermis, adjacent to which is a focal, palisaded granulomatous dermatitis surrounding a central area of connective tissue degeneration.