An Unexpected Third Window in a Case of Advanced Cavitating Otosclerosis

Abstract

Otosclerosis is a disorder of the bony labyrinth characterized by an active metaplastic process within the endochondrial layer of the temporal bone, which normally undergoes very little bone remodeling after development. This bone remodeling is a highly regulated process that involves a simultaneous and continuing perivascular bone resorption (otospongiosis) and a deposition of immature and collagen-deficient bone (otosclerosis) (1). In the Caucasian population, the incidence of clinical otosclerosis is approximately 0.1%, but the histologic otosclerosis can reach 12%. To our knowledge, there are only a few references to cavity formation in otosclerotic foci, one described by Schucknecht (1) and one in a pericochlear cavity case report describing a pathologic “third window” (2). Pathologic third-window lesions may be anatomically discrete and, thus, classified by location (semicircular canals, bony vestibule or cochlea) or diffuse like in Paget disease. Clinically, these lesions are unique in that they have differing effects on air versus bone conduction and also may result in vestibular manifestations by making the vestibular sense organ sensitive to sound or mechanical stimulation (3). This is the first case of a third window effect because of a cavity in an otosclerotic focus in contact with the superior semicircular canal.