Learning Objectives:

Introduction: Open-type congenital cholesteatoma (OCC) of the middle ear is relatively rare. It is also difficult to diagnose correctly preoperatively because the clinical presentation is very similar to that of congenital ossicular anomaly (COA). Here, we present a case report of middle ear OCC masquerading as COA.

Case presentation: A 12-year-old Japanese boy with a history of imperforate anus and spinal anomalies presented with a 1.5-year history of left-sided hearing loss. The external auditory canal and tympanic membrane were normal on both sides. Pure tone audiometry showed a 44-dB conductive hearing loss.High-resolution temporal bone computed tomography (TBCT) showed a defect in the long process of the incus and the stapes suprastructure, and a small shadow near the stapedial tendon. An endoscope-assisted microscopic transcanal approach was used to identify the ossicular defect (as expected) and an OCC extending from the stapedial tendon to the promontory. We removed the entire OCC and reconstructed the ossicular chain with tragal cartilage. The postoperative air–bone gap was 9.4 dB.

Conclusions: OCC often presents in a way very similar to COA, with conductive hearing loss and an intact tympanic membrane. However, a history of progressive hearing loss and the presence of a subtle soft tissue density on TBCT are suspicious of OCC, rather than COA, in which the hearing loss is of a congenital nature. In this patient, the cholesteatoma was located in the sinus tympani around the stapedial tendon and was difficult to assess with an operating microscope. In lesions of the sinus tympani and facial recess, endoscope-assisted microsurgery can facilitate cholesteatoma removal and reduce the risk of recurrence.