The Melbourne Speech perception scores of group A and group B at 1 yr and 2 yr postimplant are given in Table 2. The mean speech perception scores in the control (Group C) group at 1 and 2 yr are 3.93 (SD 2.36) and 5.37 (SD 2.1). It is interesting to note that the speech perception outcome in group A are statistically better than the control group (C) (two tailed p value <0.005, Mann Whitney U Test). Analysis of the 39 children with OAE rather than all 60 children with APP did not alter the conclusions of this paper but it was decided not to include ears with APP in the control group C.
Further statistical analysis was performed ANOVA comparing the three groups which shows that the speech perception scores of the three groups are significantly different, but overall ANOVA does not give the order of the three groups. One problem with ANOVA and t-test is that they require a normal distribution of samples (or near normal). Our distributions do not fit these criteria as our samples are not normally distributed. Because ANOVA does not give any specific ordering, a multiple comparisons approach was employed. To perform multiple comparisons procedure, t statistics for all pairs of means were computed and compared with the critical t value for that pair. In these computations pooled standard deviation of all populations was used which increases the power of the tests. Table 3 shows the multiple comparisons t statistics of paired groups in the study.
From the table one can safely conclude that, speech perception scores of group A (APP normal EABR) are better than speech perception scores of groups B (APP abnormal EABR) and C (No APP normal EABR). Also it is clear that speech perception scores of group C are better than speech perception scores of group B.
Power calculations of t statistics in comparing groups was undertaken as there are only 15 subjects in category B which showed that despite this discrepancy, the statistical results are valid (Table 4).
Forty five of the 60 ears that had APP and abnormal or absent ABR had normal EABR on intraoperative testing and the outcome of cochlear implantation was statistically better than a control group of deaf ears which did not have APP. The EABR findings suggest that for these children, the afferent conduction in the auditory pathway to the level of the inferior colliculus was not disordered—in other words, there was no brainstem neuropathy. The finding that the 45 children with APP and normal EABR performed better using cochlear implants than a group of children with no APP and normal EABR suggests that the higher cortical areas are not adversely affected. Fifteen of the 60 ears (25%) which had APP and absent or abnormal ABR had abnormal EABR and these children only gained limited benefit from a cochlear implant. It would appear that these 15 children did have a neuropathy.
The presence of OAE and the absence of ABR can be explained by the presence of OHC when there is a significant loss of IHC. Animal studies support this theory. Studies performed in chinchillas (Harrison, 1998) with carboplatin (an anticancer drug from the same group as cisplatin) have shown an extensive loss of IHC while the OHC remain intact. In these animals, the preserved outer hair cells produced both an electrical signal recorded as the cochlear microphonic (CM), and an acoustic signal recorded as the OAE while the lack of IHC produced a reduced or absent ABR. There are also genetic animal models: the Bronx waltzer mouse (Bock, Yates, & Deol, 1982) and the Beethoven mouse (Bussoli, Kelly, & Steel, 1997) are mouse mutants which have extensive loss of IHC in the presence of OHC. Chronic hypoxia can also produce a similar loss of IHC in guinea pig (Harrison, 2001). This is of interest as many premature infants with auditory neuropathy have suffered periods of hypoxia (Rea & Gibson, 2003) and may explain why a few infants pass OAE screening tests when they have a profound hearing loss on ABR testing (Davis & Wood, 1992).
An alternative suggestion is that there is a disorder of the synapse rather than loss of IHC. There is a recent discovery of hair cell synaptic ribbons required for faithful synaptic transmission (Khimich, Nouvain, & Pujol, et al., 2005). Mouse mutants (Basoon and Piccolo mutants) with disordered hair cell synaptic ribbons have been studied but the ABR findings are not similar to auditory neuropathy as the wave V is present but delayed in a manner similar to when the nerve is affected by an acoustic neuroma.
Aso, S., & Gibson, W. P. R.. (1994). Electrocochleography in profoundly deaf children: comparison of promontory and round window techniques. American Journal of Otology
Bock, G. R., Yates, G. K., & Deol, M. S. (1982). Cochlear potentials in the Bronx waltzer mutant mouse. Neuroscience Letters
Buss, E., Labadie, R. F., Brown, C. J., Gross, A. J., Grose, J. H., & Pillsbury, H. C. (2002). Outcome of cochlear implantation in pediatric auditory neuropathy. Otology & Neurotology
Bussoli, T. J., Kelly, A., & Steel, P. (1997). Localisation of the bronx waltzer (bv) deafness gene to mouse chromosome 5. Mammalian Genome
Davis, A., & Wood, S. (1992). The epidemiology of childhood hearing impairment: factors relevant to planning of services. British Journal of Audiology
Dowell, R. C., Blamey, P. J., & Clark, G. M. (1995). Potentials and limitations of cochlear implantations in children. Annals of Otology, Rhinology, & laryngology
, 166 (Suppl)
Gibson, W. P. R., & Sanli, H. (2002). Auditory neuropathy: the use of electrophysiological tests. In T. Kubo, Y. Takahashi, & T. Iwaki (Eds.), Cochlear Implants—An Update
(pp. 53–58). The Hague: Kugler Publications.
Harrison, R. V. (1998). An animal model of auditory neuropathy. Ear & Hearing
Harrison, R.V. (2001). Models of auditory neuropathy based on inner hair cell damage. In Y. Sinninger & A. Starr (Eds), Auditory Neuropathy: A New Perspective on Hearing Disorders
(pp. 51–66). Cambridge, MA: Singular Press.
Khimich, D., Nouvain, R., Pujol, R., Tom Dieck, S., Egner, A., Gundelfinger, E. D., Moser, T. (2005). Haircell synaptic ribbons are essential for synchronous auditory signalling. Nature
Madden, C., Hilbert, L., Rutter, M., Greinwald, J., & Choo, D. (2002). Pediatric cochlear implantation in auditory neuropathy. Otology & Neurotology
Mason, J. C., De Michelle, A., & Stevens, C. (2003). Cochlear implantation in patients with auditory neuropathy of varied etiologies. Laryngoscope
Mills, D. M. (2006). Determining the cause of hearing loss: differential diagnosis using a comparison of audiometric and otoacoustic emission responses. Ear & Hearing
Miyamoto, R. T., Kirk, K. I., Renshaw, J., & Hussain, D. (1999). Cochlear implantation in auditory neuropathy. Laryngoscope
O’Leary, S. J., Mitchell, T. E., Gibson, W. P., & Sanli, H. (2000). Abnormal positive potentials in round window electrocochleography. American Journal of Otology
Ray, J. D., & Gibson, W. P. R. (2004). Role of auditory stimulation in maturation of the auditory pathway. Acta Oto-laryngologica
Rea, P. A., & Gibson, W. P. R. (2003). Evidence for surviving outer hair cell function in deaf ears. Laryngoscope
Shallop, J. K., Peterson, A., Facer, G. W., Fabry, L. B., & Driscoll, C. L. (2001). Cochlear implants in 5 cases of auditory neuropathy: postoperative findings and progress. Laryngoscope
Sininger, Y. S., Hood, L. J., Starr, A., Berlin, C. T., Picton, T. W. (1995). Hearing loss due to auditory neuropathy. Audiology Today
Starr, A., Picton, T. W., Sininger, Y., Hood, L. J., Berlin, C. I. (1996). Auditory neuropathy. Brain
Trautwein, P. Hofstetter, P., Wang, J., R., Salvi, Nostrant, A. (1996). Selective inner hair cell loss does not alter distortion product otoacoustic emissions. Hearing Research
Trautwein, P. G., Sininger, Y. S., & Nelson, R. (2000). Cochlear implantation of auditory neuropathy. Journal of the American Academy of Audiology