The Human Cochlea and Cochlear Implantation : Morphological Characteristics and Clinical Correlations

Abstract: The most common sensory deficit in the world is sensorineural hearing loss. Cochlear implantation (CI) can majorly contribute to restore hearing, not only in patients with severe to profound hearing loss, but also in hearing-impaired patients with residual low-frequency hearing. The overall aims of the present thesis were to study human cochlear anatomy in order to improve structural preservation during CI surgery. An archival collection of temporal bones underwent micro-computer tomography and synchrotron radiation phase-contrast imaging (SR-PCI) with 3D reconstructions, new techniques to digitally image and reproduce the human inner ear. Studying the anatomy of the facial nerve and its interaction with the cochlea revealed that a fusion of the two was found in 1.4 % of the specimens (cochlear-facial dehiscence). This may cause facial nerve excitation after CI. CT-scans and intraoperative electrically auditory brainstem response (e-ABR) measurements were analyzed in patients with cochlear-facial dehiscence. A large evoked late myogenic potential at low stimulation levels during intraoperative e-ABR measurements, can foresee excitation at CI activation. The 3D anatomy of the fundus of the inner acoustic canal was also studied, helping to interpret preoperative imaging of the VIIIth nerve before CI. In a subsequent study, SR-PCI reproduced the soft tissue anatomy at the round window region. Results indicated a high risk for trauma at cochleostomy. For optimal preservation, the round window approach was recommended. In a long-term follow-up the first 21 consecutively operated patients in Uppsala, that underwent hearing preservation CI-surgery, data could be retrieved in 15 patients. Pure tone audiometry was assessed preoperatively and at one, three and >5 years following surgery. Insertion angle, number of electrodes inside the cochlea, user-time of the processor, and stimulation strategies were documented. Results showed that long-term preservation of hearing is possible in most cases. There was a high correlation between insertion depth and preservation of residual hearing. Also, patients with complete hearing loss experienced good performance in speech discrimination and user time.