Cochlear implantation after acoustic tumour resection in neurofibromatosis type 2: impact of intra- and postoperative neural response telemetry monitoring
The present paper reports about a 16-year-old male with neurofibromatosis type 2 (NF-2) of the Wishart type with bilateral deafness who had undergone cochlear implantation after resection of the acoustic neuroma (AN) of the same side. Neural response telemetry (NRT) recordings are essential in those patients during cochlear implantation where no stapedial reflexes can be electrically elicited due to the resection of the AN. In the present case, amplitude growth function and a type II pattern of the NRT waveforms could be well established. The comparison of the N(1) response intra-operatively and after 2 years showed a decline in latency by 50% and an increase in absolute amplitude by 10 times at the same current level of electrical stimulation. This improved auditory nerve transduction suggested a change to a 'faster' encoding strategy to improve speech understanding. The change from SPEAK to ACE 18 months after the operation led to an increase in the open-set sentence recognition test from 52 to 88%. Thus, NRT recordings monitor the intra-operative success of electrode placement and help to assess the integrity of the auditory pathway. Moreover, they can reliably be used in programming the speech processor postoperatively as objective tool. In patients with NF-2, the restoration of hearing can be successfully achieved in several ways. The indications for hearing implants (auditory brain stem and cochlear implants) should be carefully considered with respect to the remaining, functional integrity of the auditory nerve and the technical possibilities to monitor the success of these procedures.