The use of the term “auditory neuropathy” (AN) has probably steered clinicians away from considering cochlear implantation in the past, as a “neuropathy” usually implies abnormal neural function and leads to the assumption that a cochlear implant is unlikely to be successful.
In a typical audiology clinic setting, the main tools available for the differential diagnosis of sensory and/or neural hearing losses include OAEs and ABR testing using acoustic stimuli (acABR). Unfortunately, OAEs provide information only about outer hair cell function and can be easily masked when there is middle ear pathology. It can also be difficult to make a differential diagnosis using acABR testing alone because when the ABR waveforms are absent or abnormal it may be unclear if this resulted from poor/abnormal cochlear stimulation of the auditory nerve or it is the dysenchronus hair cells. Determining the site of lesion in these cases on the basis of these two tests alone is not possible.
At Sydney cochlear implant centre, A test battery for those group of cases is adopted,
SCIC uses a pre-operative battery of three tests:
(1) Electrocochleography (ECochG) to test cochlear function,
(2) ABR testing to acoustic stimuli (acABR), and, ifneeded,
(3) ABR testing to electrical stimuli (EABR)
These tests are performed under a general anesthetic and have been chosen to improve the ability to make separate evaluations of cochlear versus auditory nerve and brainstem function. Also to show abnormality along the nerve and/or brainstem pathway.
Of the 45 pediatric patients evaluated in this study the combination of ECochG, acABR, and EABR results showed the following diagnostic population breakdown:
1) APP group—auditory dys-synchrony (AD)
Thirty-one (69%) of the children presented with an APP on ECochG bilaterally. These cases also showed an abnormal or absent acABR, but normal waveforms were seen when an electrical stimulus was used (EABR). This indicates that the auditory nerve and brainstem were capable of synchronous neural activity.
This combination of results also suggests that the main site of lesion was within the cochlea and/or the initial synapse with the auditory nerve. This was defined as “auditory dys-synchrony” (AD), as the absent/abnormal acABR demonstrated that the impairment to the auditory pathway
was sufficient to prevent synchronous neural firing in the auditory nerve and brainstem when sound was used. However, this could be overcome when an electrical stimulus was used.
2) TrueAuditorynerve Neuropathy
11 children (24.4%) showed abnormal acoustic and/or electrical ABRs bilaterally. In these cases, the waveforms were either absent or showed a significant delay in the wave IIII interval. These children were classified as having a true auditory neuropathy, as the neural pathways continued to show an abnormal response even when electrical stimulation was used.
3) Brainstemauditory neuropathy:
Three children (20%) bilaterally showed the presence of earlier waves, but wave V was either absent, significantly reduced in amplitude or significantly delayed. This was classified as “brainstem auditory neuropathy” (BAN), as the abnormality was seen only for the later waves that should have been generated by the brainstem.
OUTCOMES WITH COCHLEAR IMPLANTATION
All patients had been implanted with the Nucleus cochlear implant and only those with a full insertion of 22 electrodes were included. A full insertion was confirmed by performing a Common Ground artifact test during surgery. When all 22 electrodes have been successfully inserted an artifact can be seen at the beginning of each trace as each electrode is stimulated
separately. The waveform reverses in phase at around electrode 9 as the array takes the first turn around the cochlea. A Stenvers view x-ray is also evaluated postoperatively to confirm the correct and full insertion of the cochlear implant array.
For those reasons, in our study we used information from pre-operative test battery to counsel families about predicted postimplant outcomes. By using this combination of diagnostic tests in conjunction with functional evaluations and CT and MRI scans it has been possible to place
patients into four categories with general predictions of outcomes with cochlear (CI) implantation:
(1) Sensory hearing loss (SHL): Primarily cochlear pathology. Good outcomes predicted with CI.
(2) Auditory dys-synchrony (AD): Believed to be primarily cochlear pathology. Good outcomes predicted with CI.
(3) Auditory neuropathy (AN): Consistent with auditory nerve pathology. Less optimal outcomes predicted with CI.
(4) Brainstem auditory neuropathy (BAN): Consistent with auditory brainstem pathology. Less optimal outcomes predicted with CI.
The decision to proceed with surgery is always a significant one for families. But knowing what can be realistically expected beforehand helps them to weigh the risks versus the benefits and ensure that the most appropriate type of intervention is in place for optimal language development
Posted by: Ahmed Mohamed Mehanna, Formerly fellow at the Sydney Cochlear Implant Australis, , now lecturer of otolaryngology, Alexandria school of MEDICINE, Egypt (06-Nov-2011)