Non-invasive testing to diagnose the underlying auditory pathology of hearing impairment

Of the three major classifications of inner ear pathology, sensory, neural and metabolic, it is relatively easy to distinguish a sensory pathology from a neural or inner hair cell pathology using current clinical tools. Deficiencies in both distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) suggest a sensory pathology involving outer hair cells. Normal DPOAEs with deficiencies in ABRs suggest a neural pathology, involving inner hair cells, auditory nerve synapses or neural deficiencies. However, there are currently no non-invasive methods to distinguish a metabolic pathology from sensory or neural pathologies that may be suitable for translation into a clinical setting. Here, mutant mouse lines with defined sites-of-lesion in the inner ear are used to evaluate a method published by Mills (2006) to differentiate these main classes of inner ear pathology. ABR and DPOAE thresholds were recorded from mutant mice and littermate controls. Plotting normalised DPOAE thresholds against ABR thresholds resulted in a continuum of responses from normal to severe threshold elevations for both the ABR and DPOAE recordings. These responses did not fall neatly in the different pathology zones suggested by Mills, or into elliptical clusters suggested by the recordings made in exemplar mutant lines in the current study. Examination of extended parameters, such as response amplitude, did not provide any further differentiation between mice carrying different pathologies. We conclude that measurement of ABR and DPOAE thresholds and response amplitudes does not provide sufficient information to stratify metabolic inner ear pathology from either sensory or neural pathologies.