Nail matrix hypergranulosis.

Nail pathology shares some common features with skin pathology, but it also has its own peculiar aspects. The anatomical and physiological characteristics of the nail unit probably play a major role in determining these pathological differences. Although the presence of keratohyaline granules is a normal feature of the skin, there is no granular layer in the normal nail matrix. As a consequence, nail matrix hypergranulosis should be considered a separate entity from skin hypergranulosis. In our review of 150 longitudinal nail biopsy specimens, keratohyaline granules were seen in the nail matrix of 24 cases of lichen planus, 29 cases of spongiotic trachyonychia, 10 cases of psoriasis, and three cases of Hallopeau acrodermatitis. In all cases, the presence of keratohyaline granules was associated with the absence of the normal keratogenous zone. Similar nail matrix features were detectable in three cases of malignant melanoma, two cases of primary systemic amyloidosis, and one case of histiocytoid hemangioma compressing the nail matrix. Our data suggest that inflammatory and compressive insults to the nail matrix cause both disappearance of the keratogenous zone and matrix keratinization with the formation of keratohyaline granules. Skin hypergranulosis reflects a hyperplasia of a normal skin component. In the nail matrix, however, hypergranulosis represents the appearance of structures not normally present. Nail matrix hypergranulosis should be considered a pattern of nail matrix reaction to different inflammatory insults. It is therefore more analogous to epidermal parakeratosis than to epidermal hypergranulosis.