H syndrome: recently defined genodermatosis with distinct histologic features. A morphological, histochemical, immunohistochemical, and ultrastructural study of 10 cases.

This study analyzes the histopathological findings in H syndrome, a recently recognized autosomal recessive genodermatosis characterized by indurated, hyperpigmented, and hypertrichotic skin in well-defined anatomical areas accompanied by various systemic manifestations. So far, descriptions of the histopathological skin changes in this disorder, as reported in a few small case series, were inconsistent, leading to diverse clinical interpretations. In an attempt to define standardized, diagnostic, morphological criteria that will distinguish this disorder from other fibrosing conditions, we studied skin biopsies from 10 patients with H syndrome. The characteristic morphology included widespread fibrosis (moderate in dermis and severe in subcutis); striking mononuclear infiltrates consisting mainly of monocyte-derived cells (small CD68 histiocytes and CD34 and FXIIIa dendrocytes) and plasma cells; and thickened, fragmented, and partially calcified elastic fibers, admixed with well-formed psammoma bodies, a previously unrecognized feature in nonneoplastic skin and subcutaneous conditions. In addition, the ultrastructure of CD68 small histiocytes exhibited distended endoplasmic reticulum and scarcity of lysosomes, features typical for fibroblasts but unusual for histiocytes. These unusual findings in the histiocytes pose a question as to their possible role in the fibrotic cascade in this disorder. We conclude that the above findings are essential for the diagnosis of H syndrome and that incisional biopsies are mandatory for recognition of the full spectrum of histopathological findings.