Auditory neuropathy (AN) was first reported by Starr et al1 in 1996. There were 10 patients, including adults and children, whose auditory brainstem response (ABR) were absent or abnormal while the response of cochlea otoacoustic emission (OAE) or cochlear microphonic (CM) existed. The AN was then defined with those features.
Clinically, AN is defined as: (1) any degree of hearing loss, usually bilateral; (2) presence of OAE or CM; (3) abnormal auditory evoked potential (I wave of ABR); (4) poor speech discrimination score; (5) disappearance of ipsilateral and contralateral acoustic reflex.
At present, whether the cochlear implantation surgery should be performed on patients with AN was controversial.2 It was reported that cochlear implantation could provide good speech discrimination for patients with severe sensory neurological hearing loss.3,4 The postoperative debugging in patients with AN is not the same as the patients without AN. It is very important to screen the patients with AN accurately and objectively.
Round window electrocochleography (RW ECochG) can be used to estimate the residual function of cochlea. Compound action potential (CAP) is more conspicuous than ABR. The threshold of CAP is more easily estimated.5 Our study was designed to investigate whether the patients of sensory neurological hearing loss suffered from AN using the intraoperative RW ECochG test during cochlear implantation surgery and to help us select adequate postoperative debugging parameters in order to improve the speech discrimination ability.
Thirty-two patients with severe sensory neurological hearing loss underwent intraoperative RW ECochG tests. There were 21 males and 11 females. Their age was from 1 to 31 years and the average age was 7.3 years. Their inner ears were well generated, as detected by high resolution computed tomography (CT) scan. There were 14 left ears and 18 right ears involved in the operation. CI24R was implanted in 24, MedeL in 3 and CI24M in 5 patients. All patients were admitted for prelingual hearing loss. After hearing aid, there was no significant effect on 14 patients and no effect on 18 patients. Four patients had a history of hypoxia during birth, two patients had a history of jaundice, and 10 patients had a history of ear toxic medicine administration. One had suffered from rubella, one from measles and five from common cold during pregnancy. The mothers of two patients had used toxic medicine during pregnancy, and 2 had been exposed in radioactive ray. The auditory meatus of all patients was unobstructed, no fluid was found in their middle ears, and eustachian tube functioned well. CT scan showed no abnormality.
GSI Audera ASSR and auditory evoked potential test system and GSI TIP50 inserted earphone (GSI Company, USA) were used.
All patients took supine position with the operative ear upward and received general anaesthesia at the operating room. The transmastoid facial recess was used to approach the round window. The active electrode was placed into the round window niche under a microscope. The reference electrode was placed on the ipsilateral mastoid region.
The CM and CAP before the cochlea windowing were recorded. The CAP was recorded again after the cochlea windowing.
Tone pips were used for recording CM at frequencies of 0.5, 1, and 2 kHz in RW EcochG. High and low bandpass filters were at 30 Hz and 3 kHz, respectively. Repetition rate was 8 Hz. Sweep limit was 198. The responses were displayed on a 10-ms window. The maximum simulating intensities were at 115.6 dBnHL for 0.5 kHz; 114.9 dBnHL for 1 kHz; 115 dBnHL for 2 kHz.
Click (100 μs) was used for recording CAP. High and low bandpass filters were at 3 Hz and 3 kHz. Repetition rate was 15 Hz. Sweep limit was 138. The responses were displayed on a 10-ms window. The maximum simulating intensities were at 106.4 dBnHL.
Tone pips (2:1:2 Black man pip) were used for recording CAP. High and low bandpass filters were at 3 Hz and 3 kHz. Repetition rate was 15 Hz. Sweep limit was 138. The responses were displayed on a 20-ms window. The maximum simulating intensities were at 118.4 dBnHL for 0.5 kHz, 117.6 dBnHL for 1 kHz, 117.8 dBnHL for 2 kHz.
The CM of 32 patients at 0.5, 1 and 2 kHz was recorded. In the 32 patients, the CAP of 12 patients (12/32) was not recorded both before and after cochlea windowing; the CAP of 20 patients (20/32) was recorded. Of them, the CAP of 9 patients (9/20) was recorded before cochlea windowing; the CAP of 11 patients (11/20) was recorded after the cochlea windowing. We diagnosed the patients whose CM could be recorded but CAP could not be recorded before and after cochlea windowing as AN. The incidence was 37.5% (12/32).
The CM at 0.5, 1, and 2 kHz was respectively showed in Figure 1. The CAP generated by click was showed in Figure 2A and generated by tone pips at 1 kHz and 2 kHz was respectively showed in Figure 2 B and C.
Davis and Hirsh6 had reported that one in 200 patients who suffered from sensorineural deafness could be diagnosed as AN. The incidence was 0.5%. The incidence reported by Kraus et al7 was 15% and reported by Rance et al8 was 11%. In this study, 12 (12/32) patients who suffered from profound sensorineural deafness with normal cochlea were diagnosed as AN because of the presence of CM but absence of CAP. The incidence was 37.5% (12/32). The higher incidence in the present study might be related to that the fewer patients were enrolled in this study.
Rance et al8 found that in 20 AN patients, 10 patients' bilirubin level was more than 350 μmol/L and deduced that this might be the most common risk factor for AN. Of those 10 patients, 3 patients had hydrencephalus caused by anoxia. In this study, of 12 patients whose CAP did not elicit, 2 patients had obvious hypoxia when they were born. The proportion of anoxia (2/12) was similar with that (3/10) reported by Rance et al. Six patients (6/12) had administered ototoxic drug such as streptomycin. It requires to be further studied whether the administration of ototoxic drug such as streptomycin is related to AN.
Clinically, AN is characterized by the presence of OAE or CM and absence of CAP or ABR. The presence of OAE or CM indicates the normal function of the cochlear outer hair cells.9 The absence of CAP or ABR means the lesion may be at the inner hair cells, the synapse between the inner hair cells and auditory nerve fibers, spiral ganglion cells, the VIII auditory nerve fibers or combination of all regions above.1,10
Traditionally, AN is usually diagnosed by sensorineural hearing loss, the deficiency of ABR, the presence of OAE and the normal imaging.1,11-13 It is worth noting that OAE test is not enough to diagnose AN because there is a specific condition that CM is normal but OAE cannot be elicited. Although OAE is generated from outer hair cell, its transmission is subject to the influence of middle ear. OAE cannot be recorded when patients have AN together with middle ear disease. The patients with middle ear deuto-clinical pathology may show normal tympanogram, however, OAE cannot be recorded in them. It is testified that OAE is sensitive to feeble cochlea damage. OAE cannot be recorded because of outer hair cell (OHC) active movement affected while the CM can be recorded in some special patients who suffer from AN together with inner ear disease. So it was suggested that CM should become a routine test for the infants with abnormal ABR.8 In this study, OAE was not recorded in 32 patients, while CM of intraoperative RW ECochG of all the patients was recorded.
ABR wave I was not usually clear when patients have moderate to severe sensorineural hearing loss.14 Both CAP of electrocochleography and ABR wave I are derived from cochlear auditory nerve active potential. The occurrence rate of CAP is 100%. The latency of ABR wave I is similar to CAP of electrocochleography when it is stimulated with loud and intense sound and the waveform and latency are stable. So the CAP of electrocochleography can take the place of ABR wave I for clinical diagnosis.15 It is apparent that to diagnose AN objectively and accurately through OAE and ABR test is difficult in the past.
In this study, the intraoperative RW ECochG test during cochlear implantation surgery was performed, CM and CAP were used to screen the patients with AN. It is well known that RW EcochG belongs to a near-field record. The recording electrode at round window niche is near to cochlear hair cell and auditory nerve, the maximal amplitude of CAP can reach 10 mV or more. So if the CAP is not recorded after cochlea windowing, it can be confirmed no CAP response occurs. In our study, the CAP test which was done before and after cochlea windowing was very rigorous. The patients who had CM response but no CAP response before and after cochlea windowing were suspected to be AN.
Of the 20 patients who had CAP response, 11 patients (11/20) had CAP after cochlea windowing but no CAP before cochlea windowing. It may be cochlear basal membrane, spiral ganglion cell and auditory nerve etc. synchronism enhancement after cochlea windowing or because the active electrode was nearer to the cochlear basal membrane. This needs a further study.
The effect of cochlear implantation on AN patients was not clear. It might be related to the position of lesion. Cochlear implantation might be successful if the lesion was localized in the inner hair cell or synapse of auditory nerve fibers because the electrical signal directly stimulated the spiral ganglion cell without passing by a part of auditory passage. If the lesion of cochlear nerve fiber was severe enough, the electrical signal could not spread to centre and the cochlear implantation would not have any effect on the patients with AN. It was reported that 20 patients with AN received cochlear implantation.16-18 It was exceptional that the patients showed a good even the best outcome after cochlear implantation. The early speech recognition test showed improvement to some extent.2 Cochlear implantation could bring prominent advantages to the patients with AN. However, like wearing traditional hearing aid, there might be some patients who have not had improvement.2 On the whole, it should be cautiously considered before performing the cochlear implantation to the patients with AN.8 In our study, the patients with AN who were screened objectively through intraoperative RW EcochG test underwent cochlear implantation surgery and showed a good effect.
It was reported that the patients with AN who had received cochlear implantation would have a better outcome if the stimulating velocity were slowered when their implanted device began to work.19 In our study, 12 patients screened to be AN had a good outcome through slowering stimulating velocity while their implanted device began to work.
In this study, intraoperative RW EcochG test was performed under general anesthesia in an electrically noisy operating room, and after the round window niche was exposed through transmastoid facial recess approach. This method was different from that reported by Gibson et al.20 The test took about 8 minutes. It was safe, rapid, painless and easy to be accepted by the patients. This provided an objective and accurate method for screening the patients with AN. It can also provide a meaningful guidance for the working of implanted device and is worth being recommended. The similar report about the research has not been found yet.
Application of intraoperative RW ECochG during the cochlear implantation surgery may objectively and accurately screen the patients with AN. It can give a meaningful clue for the working of implanted device.
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