Follicular occlusion due to hyperkeratosis: a new hypothesis on the pathogenesis of mammillary fistula.

Mammary fistula (MF) is a disease characterised by recurrent draining abscesses around the areola. The aetiology of MF remains unclear. The most common cause is duct obstruction by squamous metaplasia. The clinical aspects and histological findings of MF are very similar to those observed in hidradenitis suppurativa (HS). We propose a new hypothesis on the pathogenesis of MF and suggest that occlusion of hair follicles by keratinous plugging may relevantly contribute to the development of MF.