Clinical Use of Cochlear Nerve Action Potential Responses in Man for Differential Diagnosis of Hearing Losses

Abstract

The whole-nerve action potential response (AP) of the cochlear nerve in man was stably recorded either from the external acoustic meatus or from the promontory by a non-surgical method, using an average response computer. Acoustic stimuli were either clicks or tone pips. Subjects were 8 normal volunteers, 15 patients with sensorineural deafness, and 2 patients with conductive deafness. Both the intensity function of the latency of N1 and of the amplitude of N1 were measured from all the subjects. There were many variations in pattern of these curves that might be produced by conductive or sensorineural hearing loss. On the basis of the working hypothesis of a double organization, these results were interpreted. The general conclusions are as follows (1) For normal hearing, the input-output function of N1 included the two distinct segments, the L-curve and the H-curve, independent of the position of the electrode. Each of the segments might be generated by a different population of neurons and stimulated by a different class of sensory cells. (2) For conductive hearing loss, the change in the latency and the amplitude seemed to be proportional to the degree of the hearing loss. (3) For sensorineural deafness, three points may have clinical significance: (a) the overall reduction in amplitude of N1, (b) the monotonicaly increasing form of the input-output function of N1, in other words, the disappearance of the L-curve from the input-output relation, and (c) the prolonged latency of N1. (4) Our findings on the AP in man seemed to be practically identical with those reported for guinea pigs and cats.