Eighth nerve aplasia and hypoplasia in cochlear implant candidates: The clinical perspective

Objective: To identify the clinical and radiologic charaeteristics of aplas ia and hypoplasia of the eighth nerve. Study Design: Retrospective case-note review. Setting: Cochlear implant program. Patients: All children at the authors' institution in whom the cochlear imp lant assessment failed because of absence or hypoplasia of the eighth nerve . Intervention: Computed tomography of petrous bones and magnetic resonance i maging of the brain. Main Outcome Measures: Presence or absence of eighth nerve and other radiol ogic factors contraindicating implantation. Results: Of 143 cochlear implant candidates, 237 were judged ineligible for cochlear implantation. The preimplant assessment failed in 10 candidates o f 143 because of bilateral aplasia or hypoplasia of the eighth nerve (7 cas es) or unilateral aplasia or hypoplasia of the eighth nerve and a contraind ication to operation on the other side (3 cases). The aplasia or hypoplasia of the eighth nerve was confirmed by magnetic resonance imaging in seven c ases (5%): six were syndromic (3 CHARGE, 1 VATERRAPADILLINO, 1 Mobius, 1 Ok ihiro), and one was nonsyndromic autosomal-recessive. All seven children ha d delayed motor milestones and absence of auditory brainstem responses. Conclusion: Aplasia and hypoplasia of the eighth nerve are not uncommon in pediatric cochlear implant candidates, particularly in the presence of a sy ndrome such as CHARGE. Magnetic resonance imaging of the brain is mandatory before implantation because it can identify the presence or absence of the eighth nerve. Parents of children with profound hearing loss, delayed moto r milestones, absence of auditory brainstem responses, and a syndromic diag nosis, should be made aware of this possible abnormality.