Cochlear Microphonic Responses to Pure Tones in Man Recorded by a Non-Surgical Method

Abstract

A non-surgical method of recording the cochlear microphonic (CM) responses in man has been developed, using new active electrodes in combination with an average response computer. One of these electrodes was a needle electrode placed in the external acoustic meatus, another a fine 80-micron stainless steel wire electrode associated with a special cannula, which was placed on the promontory or the round window niche.

The procedure of the electrode placement was carried out completely without pain or bleeding. The acoustic stimuli were continuous pure tones or short bursts of pure tone. Subjects were volunteers with normal hearing and patients with conductive and sensorineural hearing loss. All the procedures of this experiment were made in a sound-proof room.

The results are condenced into the following:

(1) All the subjects with normal hearing showed consistently and stably the cochlear microphonic response, recorded either from the external acoustic meatus or from the promontory.

(2) When the short bursts of pure tone were presented, a complex response consisting of cochlear microphonic, action potential (AP) and summating potential (SP), was recorded from the promontory, and a complex response of the cochlear microphonic and action potential was recorded from the external acoustic meatus.

(3) When the continuous pure tones were presented, beautiful records of the cochlear microphonic responses were obtained both from the external acoustic meatus and from the promontory.

(4) The form of the intensity function measured from the promontory in the normal subjects was in essential agreement with that reported for the round window recording in animals.

(5) Some of the intensity functions obtained from patients with severe sensorineural hearing loss showed the following abnormal patterns: (a) reduction in the magnitude of the cochlear microphonic response, (b) distortions in the form of the intensity function (the nonlinear relation), and (c) a parallel shift of the intensity function to the right side (reduced sensitivity).

(6) The sensitivity of the cochlear microphonics seemed to have some relation to the degree of the hearing loss in the audiogram.

(7) The non-surgical method is applicable to routine clinical audio-metry for differential diagnosis of sensorineural deafness, but empirical correlations must be established between the form of intensity functions and different types of sensorineural deafness.

(8) In middle ear impairment the less sensitive extratympanic method must be employed and the cochlear microphonics do not offer any advantages over other audiometric and otologic methods of diagnosis.