Necrobiosis lipoidica and necrobiotic xanthogranuloma have many findings histopathologic in common. Among them are involvement of the entire sweep (usually in breadth and depth) of reticular dermis, lymphoplasmocytic infiltrates around blood vessels, histiocytes arranged in palisades, presence of both mono- and multinucleate histiocytes, and degeneration of collagen. Necrobiotic xanthogranuloma differs from necrobiosis lipoidica, however, by sheer extensiveness of the process, which is diffuse throughout the reticular dermis and extends far into the subcutaneous fat (often to the base of a deep biopsy specimen), vastly greater degeneration of collagen, clefts of cholesterol that often punctuate zones of degeneration of collagen, ubiquitous foam cells, and greater density of lymphoplasmocytic infiltrates. The infiltrate in the subcutaneous fat affects lobules mostly, unlike the situation in necrobiosis lipoidica which affects septa mostly.
Necrobiotic xanthogranuloma is a rare disease, but one highly distinctive. Once familiar with its signature histopathologic, the condition can be recognized for what it is at a glance at scanning magnification. The changes most engrossing are dramatic degeneration of collagen that tends to involve most of the reticular dermis and striking necrosis of adipocytes that may alter much of the subcutaneous fat. Those changes, in conjunction with histiocytes in palisaded array around zones of degenerated collagen and necrotic fat, are virtually diagnostic of necrobiotic xanthogranuloma. When, added to those components are foam cells and cholesterol clefts in the dermis and subcutaneous fat, a diagnosis of necrobiotic xanthogranuloma can be made with assurance. Foam cells and cholesterol clefts are not attributes of necrobiosis lipoidica.
Necrobiotic xanthogranuloma involves skin and other organs of middle-aged persons especially. Lesions in the skin present themselves as papules and plaques, especially in the periorbital region, but also on the trunk and extremities. In time, lesions tend to ulcerate. When the process extends into an eye, the result may be uveitis, iritis, and keratitis. Not uncommonly, patients with necrobiotic xanthogranuloma exhibit a paraproteinemia marked by IgG monoclonal gammopathy. Other correlates with necrobiotic xanthogranuloma include cryoglobulinemia, multiple myeloma, lymphocytic leukemia, and an increase in C1 esterase activity.
Various clues permit specific diagnosis histopathologic to be rendered in necrobiosis lipoidica, necrobiotic xanthogranuloma, and other diseases that resemble them vaguely or closely. Necrobiosis lipoidica, despite similarities in nomenclature to necrobiotic xanthogranuloma, is different entirely from the sound-alike-clinically, histopathologically, and pathogenetically. Clinically, necrobiosis lipoidica usually presents itself as yellowish plaques traversed by telangiectases, the lesions having a predilection for the anterior tibial surface of young women; histopathologically, it is devoid of foam cells and cholesterol clefts, and mechanistically it is related to overt presence of or covert tendency to diabetes mellitus.
In sum, foam cells and/or cholesterol clefts in the setting of what seems at first blush to be necrobiosis lipoidica is a clue to necrobiotic xanthogranuloma.