Detection of dead regions in the cochlea: relevance for combined electric and acoustic stimulation.

A dead region is a region in the cochlea where the inner hair cells and/or the auditory neurones are functioning very poorly, if at all. People who are being considered for a combination of a cochlear implant and a hearing aid typically have a dead region in the parts of the cochlea that normally respond to medium and high frequencies, but have some functional hearing at lower frequencies. For such people, it may be useful to determine the edge frequency, f(e), of any dead region. This may be relevant to choosing the most appropriate insertion depth of the electrode array, and to the way that frequencies in the input signal are mapped to acoustic and electric stimulation. It may also be helpful in interpreting the results of research studies. This paper reviews methods for diagnosing dead regions and defining the value of f(e). It is argued that the value of f(e) cannot be determined reliably from the audiogram, although a dead region is likely to be present at a given frequency when the hearing loss at that frequency is 70 dB or more. When a sinusoidal signal is reported as sounding highly distorted or noise-like, a dead region may be present at the signal frequency, but again this is not a reliable indicator. The TEN test is a simple clinical method for diagnosis of dead regions. Where this test gives a positive diagnosis, it is recommended that psychophysical tuning curves be measured to define the value of f(e) more precisely.