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ELECTROPHYSIOLOGICAL AND OBJECTIVE MEASURES
Dr Steve Mason
Evoked Potentials Clinic, Medical Physics Department
Queen's Medical Centre, Nottingham, England
Overview
Electrophysiological and objective measures have a valuable role to play in the management of patients receiving cochlear implants, in particular young children, complex cases, and difficult-to-test patients. The number of young implanted children is increasing world-wide as the benefits of early implantation become apparent. These objective tests can provide assistance in the following tasks:
- Assessment of the level of hearing loss
- Guidance in the selection of the ear and patient for implantation
- Monitoring the functioning of the implant and identification of possible faults
- Assist with the tuning of the device
Before Implantation
Electric response audiometry (ERA), employing sound stimuli, can be performed to confirm a profound hearing loss and support the results of behavioural audiological assessment. Again this investigation is more often required in young children using techniques such as the auditory brainstem response (ABR) and electrocochleography (ECochG).
Responses can also be evoked by electrical stimulation presented at the promontory or round window in the middle ear, such as the electrical auditory brainstem response (EABR). These tests can be performed on patients where there is doubt as to whether the auditory nerve is intact, fortunately this situation exists in only a very small number of patients. The effectiveness of a cochlear implant is entirely dependent on the presence of the auditory nerve since it is designed to by-pass the damaged hair cells in the cochlea and stimulate the auditory nerve directly. Conventional ERA can also help in the assessment of the auditory nerve, if there is a residual ABR present then this confirms that the nerve is intact and transmitting activity into the brainstem. MRI imaging is also an essential and primary part of this assessment process.
Testing with the cochlear implant
A wide range of electrophysiological and objective measures can be performed using the cochlear implant during and after implantation.
- Electrical Stapedius Reflex (ESR)
- Electrically evoked sensory potentials (eg EABR)
- Electrically evoked Event Related Potential (EERP)
- Integrity Testing (IT)
- Neural Response Telemetry (NRT)
- Electrode Impedance Telemetry (EIT)
Thresholds of the electrically evoked potentials (eg EABR, NRT) can be used to guide the audiologist towards appropriate threshold levels of electrical stimulation for the tuning map and threshold of the ESR the highest comfortable levels (ie the dynamic range). In particular NRT is a recent exciting development that enables electrically evoked action potentials to be recorded from within the cochlea. The techniques of EIT and IT can be used to identify faults with the implant and the electrode array.
Most implant centres apply at least one or more of these tests at the time of implant surgery with the following aims:
- Immediate reassurance to the surgeon, adult patients and parents of implanted children that the implanted receiver and electrode array are functioning normally and that the auditory nerve is being stimulated.
- To provide valuable information to assist the initial fitting and tuning of the implant, particularly in young children and complex cases.
- Establish a reference set of data to assist with subsequent management as required
After implantation these tests are valuable tools for the identification and management of electrode faults, device failure and difficulties with tuning. In particular IT is an essential tool for monitoring functioning of the device in children who were implanted a few years ago when the technique of EIT was not available.
What the future holds
- Increased dependency on electrophysiological and objective measures as more and more younger children receive cochlear implants.
- Exciting developments and application of techniques associated with NRT.
- Valuable insights into objective assessment of perception and cognition of stimuli and speech sounds through the use of electrically evoked event-related potentials.
- Probing the survival and regeneration of neural structures and pathways in the auditory system
Bibliography
Brown CJ , Hughes ML, Luk B, Abbas PJ, Wolaver A, Gervais J (2000) The relationship between EAP and EABR thresholds and levels used to program the Nucleus CI24M speech processor. Ear & Hearing 21: 151-163.
Cheour M, Leppänen PHT, Kraus N (2000). Mismatch negativity (MMN) as tool for investigating auditory discrimination and sensory memory in infants and children. Clinical Neurophysiology 111: 4-16.
Garnham J, Cope Y, Durst C, McCormick B, Mason SM (2000). Assessment of aided auditory brainstem response thresholds prior to cochlear implantation. British Journal of Audiology 34: 267-278.
Hodges AV, Butts S, Dolan-Ash S, Balkany TJ (1999). Using electrically evoked auditory reflex thresholds to fit the Clarion cochlear implant. Annals of Otology, Rhinology & laryngology - Supplement 177: 64-68.
Kileny PR, Zwolan TA, Zimmerman-Phillips S and Telian SA (1994). Electrically evoked auditory brain-stem response in paediatric patients with cochlear implants. Archives of Otolaryngology - Head & Neck Surgery 120: 1083-1090.
Mason SM (1993). Electric response audiometry. In: McCormick B ed. Paediatric Audiology 0-5 years. London: Whurr Publishers, pp187-249.
Mason SM (1994). Electrophysiological tests. In: McCormick B, Archbold S, Sheppard S, eds. Cochlear implants for young children. London: Whurr Publishers, pp103-139.
Mason SM (1997). Objective Measures. American Journal of Otology 18: S84-S87
Mason SM, O'Donoghue GM, Gibbin KP, Garnham CW and Jowett CA (1997). Perioperative electrical auditory brainstem response in candidates for paediatric cochlear implantation. American Journal Otology 18: 466-471.
Nikolopoulos TP, Mason SM, Gibbin KP, O'Donoghue GM (2000). The prognostic value of promontory electric auditory brainstem response in pediatric cochlear implantation. Ear & Hearing 21: 236-241.
Ponton CW, Moore JK, Eggermont JJ (1999). Prolonged deafness limits auditory system developmental plasticity: evidence from an evoked potentials study in children with cochlear implants. Scandinavian Audiology 28: 13-22.
Shallop J K (1997). Objective measurements and the audiological management of cochlear implant patients. In: Alford BR, Jerger J, Jenkins HA (eds). Electrophysiological Evaluation in Otolaryngology. Advances in Otorhinolaryngology. Basel, Karger, 53: 85-111.