Objective Measurements and Cochlear Implants Imaging

Abstract

In Norway, about one out of 2000 babies born is deaf, which means up to 20–30 children are born deaf every year. Some become deaf later due to diseases like meningitis or because of disease during pregnancy. In recent years, in particular, the Cochlear Implant (CI) has become a well-established treatment for deaf children and adults. The Department of Otorhinolaryngology at Oslo University Hospital (OUS) is responsible nationally for all deaf children in Norway. A CI offers the possibility to deaf people of partially restoring their hearing. A CI consists of two major parts - a sound processor (SP) and a stimulator/implant. The SP’s microphone picks up sound and analyses it in terms of frequencies and volume. This information is sent via radio frequencies to the implant inside the head. The implant stimulator transfers a biphasic pulse to one of its electrodes along the electrode array inside the inner ear (cochlea). These pulses bypass damaged hair cells and directly stimulate the hearing nerve sections/fibres. Objective Measurements in Cochlear Implants: During electrical stimulation and when sending information via radio frequencies to the implant, a huge electrical artefact is caused. The artefact is several 1000 times larger than the small electroencephalogram (EEG) response of a few µV that we want to measure. Use of filters, triggering, averaging of the signal, and subtraction methods make it possible to also measure these responses for CI recipients. None of these objective measurements are regularly implemented in the clinical routine or frequently used for SP programming. So far, no objective measurement method has been found to find out how much a patient is hearing, or how loudly. For CIs, two further objective measures were implemented which are only possible with a CI, because an implant is required to take these measurements. Electrically evoked stapedius reflex threshold (ESRT) is a measurement carried out during surgery. Certain electrodes get stimulated while the surgeon observes the reflexes of the stapes muscle. The threshold can be determined by lowering the current or charge delivered to the electrodes and therewith to the hearing nerve fibres. The other measurement is the Evoked Compound Action Potential (ECAP) which measures the response of the nerve fibres inside the cochlea after electrical stimuli from the implant. Imaging in Cochlear Implants: Before surgery, all paediatric and adult patients with expected complications at OUS have to undergo a Magnet Resonance Imaging (MRI) and all patients a Computed Tomography (CT) scan, which gives the surgeons an anatomical overview. After surgery, an intra-operative X-ray picture is performed to verify the correct placement of the electrode array inside the cochlea. Conventional X-ray imaging gives a general overview without an exact picture of electrode placement, such as for example displacement into scala vestibule. Project Outline: This project shall investigate if the combination of various objective measures for CI programming can be of help or even improve the programming. Hereby the following investigations need to be done. Starting with the surgery ESRT, ECAP and electrical evoked auditory brainstem responses (EABR) measurements can be carried out. The ESRT can give information if the whole auditory loop is functioning. ECAP measurements may indicate more sensitive regions, flip over and distance to the modiolus or nerve fibres. An intra-operative X-ray examination can give only an approximate indication about the electrode placement. A post-operative flat panel CT scan may give more detail about the electrode placement. The combination of ECAP measurements, such as sweep, spread of excitation and recovery function and flat panel CT scans may make it possible to detect problem areas or an electrode dislocation. This could provide valuable information, because problem electrodes may be excluded during SP programming or handled with special care. EABR measurements may indicate the coupling of the electrodes to the nerve fibres. In addition, this could be a valuable measurement for auditory neuropathy spectrum disorder (ANSD) patients, where a dissynchrony of the nerve fibres is assumed. Data Analysis: ECAP, EABR, and ESRT levels will be compared with subjective speech recognition tests, in quiet and noisy conditions. Flat panel CT scans and electrode placement will be compared to speech performance and objective measure levels. Project Goal: This project aims to find new procedures/implementations for programming a CI SP. Better programming produces better hearing, which leads to better social integration. There is a need for research on whether objective measures can be a predictor of speech recognition performance. This could be used to suggest different therapy approaches. Conclusion: The studies have shown that there is a significant relationship between observed intra-operative EABR measures and post-operative speech recognition. Both the FD-CT scan and per-operative fluoroscopy improved the CI electrode placement during CI surgery. These methods have helped us minimize poor clinical results by monitoring the exact position of the electrode array during surgery. ECAP and EABR measurements can also help identify in correct placement of the electrode array. Unfortunately intra-operative objective measures in our study, such as ECAP, ESRT, and electrode impedances did not provide statistically significant correlations that may help to predict the programming T- and C-levels for all patients.