Pathology

Abstract

While some nail changes exhibit histopathologic changes also seen in the skin, others are specific to the nail. Pits are the most common signs of psoriasis. They originate from tiny psoriatic lesions deep in the nail pocket, which are probably analogous to the so-called pinpoint lesions of early psoriasis, i.e., small foci of spongiosis with some lymphocytic exocytosis and very few neutrophils but marked disturbance of the normal keratinization pattern. This translates into circumscribed areas of parakeratosis on the surface of the nail plate as long as this is covered by the ventral surface of the proximal nail fold. When they grow out, they break off leaving a shallow depression in the nail plate. However, they may remain stuck to the nail plate and are then clinically seen as small ivory to yellowish dots that are rarely seen beyond the proximal half of the nail. Histopathologically, they are like mounds of parakeratosis. Psoriasis lesions located in the mid-matrix give rise to parakeratosis that is included in the nail plate and clinically seen as a whitish area, called psoriatic leukonychia. Often, they also contain compressed neutrophils. These parakeratosis areas are often arranged in oblique columns from deep proximal to more superficial distal. The matrix epithelium beneath the intraungual parakeratosis is often remarkably unaltered; sometimes mild spongiosis is seen with exocytosis of lymphocytes and neutrophils.