Collagenolytic (necrobiotic) granulomas: part II--the 'red' granulomas

A collagenolytic or necrobiotic non-infectious granuloma is one in which a granulomatous infiltrate develops around a central area of altered collagen and elastic fibers. The altered fibers lose their distinct boundaries and exhibit new staining patterns, becoming either more basophilic or eosinophilic. Within the area of altered collagen, there may be deposition of acellular substances such as mucin (blue) or fibrin (red), or there may be neutrophils with nuclear dust (blue), eosinophils (red), or flame figures (red). These color distinctions can be used as a simple algorithm for the diagnosis of collagenolytic granulomas, i.e. 'blue' granulomas vs. 'red' granulomas. Eight diagnoses are included within these two groupings, which are discussed in this two-part article. In the previously published first part, the clinical presentation, pathogenesis and histologic features of the 'blue' collagenolytic granulomas were discussed. These are the lesions of granuloma annulare, Wegener's granulomatosis, and rheumatoid vasculitis. In this second half of the series, the 'red' collagenolytic granulomas are discussed; these are the lesions of necrobiosis lipoidica, necrobiotic xanthogranuloma, rheumatoid nodules, Churg-Strauss syndrome, and eosinophilic cellulitis (Well's Syndrome).