Psoriasiform keratosis.

Presented herein are 18 cases of erythematous, scaly papules or plaques with microscopic features of both seborrheic keratosis and psoriasis. There was, however, no known clinical diagnosis of psoriasis in any patient, neither at initial presentation nor on follow-up examination. Most lesions were solitary, present for 6-7 months, and identified on the upper or lower extremities. Other sites included the scalp, neck, shoulders, and back. Men were affected slightly more often than women. The mean age at diagnosis was 66.8 years. The most common diagnoses, clinically, were seborrheic keratosis, followed by basal cell carcinoma, Bowen's disease, actinic (solar) keratosis, and squamous cell carcinoma, among others. The lesions averaged less than a centimeter in diameter and were dome shaped, scaly, and yellow to gray-tan. Histologic examination revealed irregular verrucous epidermal acanthosis, with hyperkeratosis, parakeratosis, hypergranulosis, and intracorneal collections of neutrophils, often in alternating tiers. Vascular dilatation and lymphocytic chronic inflammation were present in the superficial dermis. Periodic acid-Schiff (PAS) stain for yeasts or dermatophytes was negative in all cases. There was no clinical evidence of disseminated psoriasis in any patient; the mean follow-up duration was 22.6 months. We have coined the term psoriasiform keratosis as a provisional appellation until the nature of these lesions is determined more definitively. It is unclear whether a psoriasiform keratosis is a rudimentary manifestation of psoriasis or a lesion sui generis.