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.
Higher power view of the granulomatous process reveals histiocytes, lymphocytes, and fibroblasts radially arranged around a focus of degenerated collagen fibers with central mucin deposition.