Original Article Radiological Dehiscence of the Mastoid Portion of the Facial Nerve after Posterior Tympanotomy: Does It Lead to Increased Risk of Nerve Injury?

OBJECTIVE: We aimed to establish the frequency of postoperative bony dehiscence overlying the mastoid portion of the facial nerve after round window membranous cochleostomy using high-resolution computer tomography, and to ascertain whether this was associated with a higher incidence of facial nerve injury compared with other studies. We also evaluated the usefulness of the St Thomas ’ classification in predicting the possibility of performing round window insertions. Study Design: Retrospective case series. Setting: Tertiary auditory implant centre. MATERIALS and METHODS: Twenty-five patients who underwent cochlear implantation at our institution, two of whom were operated on bilater-ally, yielding 27 ears for analysis. We conducted submillimetric analysis of postoperative high-resolution computer tomography images to ascertain the prevalence of bony dehiscence overlying the mastoid portion of the facial nerve. The type of round window found during surgery was also determined in medical records. RESULTS: We found a radiological dehiscence rate of 40%. None of the patients in our series sustained a facial palsy. We propose some possible reasons for differences between our findings and those of others. All 18 patients with a Type Ia round window, two of the three patients with a Type IIa round window, and none of the four patients with a Type III round window had a successful round window insertion. The classification was found to be a useful predictor of performing round window membranous cochleostomy. CONCLUSION: Thinning of the bony cover of mastoid potion of the facial nerve to the point of radiological dehiscence does not put it at risk. Novic