Epidermoid cyst as differential diagnosis for spherical keratoma: What do we know?

Summary

This article briefly presents the case of a space‐occupying coronary band mass that raised the question of how one can differentiate between two uncommonly seen but now commonly paired differential diagnoses and prompted a review of the relevant literature. For over 20 years, spherical keratoma and epidermoid cyst (EC) have been given as differential diagnoses for benign space‐occupying masses of the foot with the understanding that the two must be and could be differentiated only by histological examination. However, nowhere in the literature is it specified how these two structures differ histologically, leading to a diagnostic conundrum for pathologists and surgeons alike. This article reviews the relevant literature in both equine and human medicine, on which the equine world's understanding of ECs is based, finds the two histologically indistinguishable and suggests these terms are in fact not differentials, but rather that a spherical keratoma may be a type of EC, and that histological evidence from the scarcely documented subungual epidermoid inclusion cyst in humans could provide insight into the pathogenesis of spherical keratoma formation in horses. The authors do not offer unequivocal answers but seek to highlight the complexity of the topic, suggesting categorisation may be influenced by location and suspected aetiology more than by histological characterisation.