Case of an evolving porokeratoma: histologic insight into a hyperkeratotic zebra

Summary: Porokeratomas are rare, acanthotic, solitary nodular plaques characterized by cornoid lamellae. Due to their veruccoid appearance, porokeratoma may be clinically mistaken for other warty proliferations e.g., verruca vulgaris. Notably, the mechanisms underlying porokeratoma formation — as well as how porokeratoma relates to other forms of porokeratosis versus representing distinct histologic entities — is not well understood. Here, we present temporal insight into a case of an evolving porokeratoma from the posterior heel, initially treated as verruca vulgaris. Patient’s first superficial biopsy showed subtle, non-diagnostic features of hyperkeratosis and parakeratosis. After failing conservative treatment, a re-excision of the lesion 2 years later revealed classic features of porokeratoma including cornoid lamella overlying vacuolated epidermal dyskeratosis. Interestingly, during this time interval, patient also developed disseminated actinic porokeratosis (DSAP). Ultimately, this case provides unique histologic and temporal data into the evolution of a bonafide porokeratoma; we suggest that some porokeratomas may share a similar pathophysiological mechanism with other forms of porokeratosis, such as DSAP. By comparing our case to those previously reported in the literature, we submit a refined series of criteria delineating porokeratoma from other histologic mimics.