Fourteen patients with hereditary motor and sensory neuropathy (HMSN), 12 of Type I and 2 of Type II, were assessed for auditory dysfunction. Five patients complained of hearing loss and all had pure-tone audiograms outside the normal range, while one patient who did not complain of hearing impairment also had an abnormal pure-tone audiogram. Assessment of loudness function, speech audiometry and brainstem auditory evoked potentials (BAEP) suggested that the hearing loss was the result of VIII nerve dysfunction, a conclusion supported by the abnormality of the electrocochleogram (ECochG) in one patient.
Pulses of white noise each of uniform duration but adjustable in interpulse spacing were presented to subjects at a comfortable loudness level. Reduction of pulse spacing below a critical interval resulted in a subjective impression of continuous noise. Eighty-four per cent of patients with sensori-neural hearing loss had a longer critical interval than normal subjects. Tentative explanations for this phenomenon are put forward in terms of desynchronisation or impaired interaction between inner and outer hair cells. Longer critical intervals found predominately in neural cases are explicable in terms of a reduction in neural conduction velocities. Interesting similarities in respect of end organ and nerve fibre lesions are apparent between critical intervals and reported electrocochleographic responses.
The decision to fit an induction loop system in many buildings is often dictated by cost, in particular that of the amplifier. In order to accommodate the peaks of speech the amplifier must be 4 times as powerful as that required for average speech. This often makes a profound difference to the cost and tips the balance against a loop being installed. Experience with two loops fitted for an audience of hard of hearing people where the peaks were clipped without any apparent loss of discrimination initiated this study. The findings showed that although high fidelity is to be preferred, peak clipping in loop systems can be tolerated without serious loss of intelligibility.
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