Previous Research
The normative properties of the auditory brainstem response (ABR) were studied and we produced both clinical baseline values and statistical operations which are now used to provide diagnostic criteria. Later developments included further applications of ABR in multiple sclerosis, both in the diagnosis and the evaluation of experimental treatment.
Sounds to the ear produce a travelling wave which sweeps along the basilar membrane. We developed an ABR technique that measured the travelling wave velocity (TWV) and basic physics predicted that if the pressure on the basilar membrane increased, it would stiffen and the TWV would increase. The inner ear pressure is increased in endolymphatic hydrops which occurs in Ménière's disease. The TWV was found to increase accordingly. The downstream application of this work was an objective test for the diagnosis of endolymphatic hydrops and for the objective evaluation of various drug treatments for Ménière's disease. The last area included a dose ranging study that showed most benefit came from moderate doses rather than the large dose recommended by the drug company.
Prof. Thornton served as secretary to the MRC Working Group on Artificial Auditory Stimulation and played a significant role in starting cochlear implant work in this country. A survey was carried out in the Wessex Region which enabled us to estimate the number of potential implant patients in this country. These data were influential and used by the government in setting up cochlear implant centres.
A further technique which used the ABR to estimate the loudness discomfort level (LDL) was developed and a clinical methodology achieved. The application is to enable the fitting of hearing aids to very young children so that speech development can be aided and progress as near to normally as possible. The danger in fitting very young children is that they cannot tell you when LDL is reached and so this technique can be used to ensure that the cochlea is not further damaged by overstimulation from the hearing aid.
The next stage of work focussed on Otoacoustic Emissions (OAEs) which are sound waves generated by the outer hair cells in a normally functioning cochlea. The main application is in screening new born babies for hearing loss. A large RCT study, in which over 21,000 babies were tested demonstrated the worth of the screen and led to the instigation of the current neonatal screening programme. The neonatal OAEs are very small immediately following birth and increase in amplitude over the next 4-5 days. The pressure for early discharge from maternity units is such that testing has to be carried out on day 1 post partum with the subsequently high false alarm rate using the conventional method. We took a technique, used in RADAR, involving maximum length sequences (MLS) to address this problem. This enabled us to develop a much faster stimulation rate and produce a system that can detect OAEs that are only 20% of the amplitude of those that are detectable by the conventional technique. This significantly reduces the screening false alarm rate and the needless referrals to clinics for hearing evaluation.
The MLS technique also enables us to record not only the conventional responses but also a family of nonlinear responses (NLRs). In OAEs we have shown that the NLRs are much more sensitive to small pathological changes in hearing status than the conventional response and we are looking for a translational outcome in industrial noise monitoring where this technique could give the earliest warning of hearing damage. We have also shown that the NLRs exist for evoked responses and, for the ABR, have demonstrated they are a much more sensitive indicator of the effects of age than is the conventional response.
