Neurophysiology of the auditory system: basics and ION techniques

5.1

The auditory nerve

Introduction of intraoperative monitoring of the hearing function in the 1980s has reduced the incidences of hearing loss as a complication to surgical operations that affect the auditory nerve considerably. The auditory nerve was one of the first cranial nerves to be monitored routinely in neurosurgical operations .

There are three ways in which the function of the auditory nerve can be monitored: one is by recording the auditory brainstem response (ABR). The other method used for monitoring neural conduction in the auditory nerve makes use of recording auditory-evoked potentials directly from the exposed CNVIII or the surface of the cochlear nucleus (CN) in response to stimulation with transients sounds.

Recording of far-field auditory-evoked potentials has been referred to as brainstem auditory-evoked potentials (BAEP) or brainstem auditory-evoked responses (BAER). The term ABRs is currently the most commonly used name for the recording of far-field (short latency) auditory-evoked potentials.

The ABR is usually recorded differentially from an electrode placed on the mastoid on the side that is stimulated and another electrode placed on the vertex. A sequence of peaks and valleys characterizes the response that occurs during the first 10 ms after a transient sound such as short tone bursts, strong tone bursts or, more commonly, strong click sounds ( Fig. 5.1 ). Typically, only the vertex positive peaks are labeled, and the labels are usually Roman numeral from I to VI or VII. These components of the ABR are the results of the sequential activation of structures of the ascending auditory pathways.

The symptoms and signs of hearing loss from damage to the auditory nerve are different from hearing loss caused by conductive hearing loss and hearing loss from pathologies that affect the cochlea. Damage to the auditory nerve gives proportionally much worse effects on speech discrimination than the same hearing threshold (pure tone audiogram) caused by the conductive apparatus or the cochlea. Thus the reduction in speech discrimination for a specific threshold elevation as reflected in the pure tone audiogram is much worse than the loss of speech discrimination caused by cochlear damage with the same change in the pure tone threshold. Hearing tests after surgical operations are often limited to obtaining a pure tone audiogram, which is a poor predictor of a decrease in the ability to understand speech.

5.2

History of recordings of the auditory brainstem response

The basis for intraoperative monitoring of the function of the ear and the auditory nerves was the development of the signal averaging technique by Dawson and work by the Communication Biophysics Group at the Research Laboratory of Electronics at MIT under the leadership of Walter Rosenblith . These developments made it possible to use electrical activity that was generated in the central nervous system to assess the functions of sensory systems such as the auditory system. Analog techniques were first used for that and later digital techniques. The digital signal averager, now a part of many kinds of neurological equipment including equipment used in intraoperative neurophysiological monitoring, was developed at MIT .

Physiological work that was done in the Eaton Peabody Laboratory at the Massachusetts Eye and Ear Infirmary and the Communication Biophysics Group by Kiang laid the ground for the use of these techniques, first in studies of the auditory system and later in other sensory and motor systems . Kiang wrote, “The significance of these potentials is unknown,” and further, “I believe that such recordings and processing can be made so simple and convenient that validation for diagnostic purposes may well be within our grasp. With these features, the computers may well find good use in monitoring the effects of operative procedures on auditory responses directly.” These predictions were all fulfilled, both regarding their use in diagnosis and for intraoperative neurophysiological monitoring.

Later, many investigators contributed to development of methods for intraoperative neurophysiological monitoring of auditory-evoked potentials , first, electrocochleographic (ECoG) potentials from the ear and later far-field-evoked potentials from the auditory nerve and structure in the brainstem (now known as the ABR) .

Recordings of the ECoG indicate the function of the ear, but it has limited usefulness in monitoring auditory function during surgical operations, because it does not represent the function of the auditory nerve and therefore cannot be used to detect impairment of the function of the auditory nerve.

Averaged auditory-evoked potentials recorded from electrodes placed on the mastoid (or earlobe) and the vertex appeared as a series of peaks and valleys. The early investigators of auditory far-field potentials placed attention (only) to the vertex positive peaks, and they labeled these with Roman numerals ( Fig. 5.1 ). The convention of labeling vertex positive peaks with Roman numerals that were introduced by Jewett, and it is still in use. Using electrodes placed at the mastoid (earlobe) and vertex yield evoked potentials that are generated in the auditory nerve where it exits the ear, the intracranial portion of the auditory nerve, auditory nuclei in the brainstem but probably not the thalamus. Different conventions of recording have been used: some investigators have shown vertex positive potentials as an upward deflection (top trace in Fig. 5.1 ), some have shown vertex positive potentials as a downward deflection (middle trace). Some studies present the ABR after digital filtering (lower traces in Fig. 5.1 ).

5.3

Generation of far-field-evoked potentials

The generation of far-field potentials is complex and depends on factors that are incompletely understood. The far-field potential generated by a nerve or fiber tract in which a volley of neural activity travels relies not only on the neural activity that is propagated in the nerve. The anatomical organization is also an important factor that affects the evoked potentials that can be recorded from a location that is distant from the nerve (i.e., far-field potentials). The end of a nerve where propagation of neural activity stops generates far-field potentials . The same is the case for a fiber tract. A location of a nerve or fiber tract where it is bent or where the electrical conductivity of the surrounding medium changes may also generate far-field potentials from propagated neural activity. Changes in the electrical conductivity of the surrounding medium of a nerve or fiber tract are essential for the generation of far-field potentials from a nerve or fiber tract.

5.4

Intraoperative neurophysiological monitoring of the auditory brainstem response

Recording of far-field auditory-evoked potentials is the most commonly used method for intraoperative neurophysiological monitoring of the function of the auditory nerve in surgical operations in the cerebellopontine angle . The primary purpose of monitoring the ABR has been to assess the function of the auditory part of CNVIII. The auditory part of CNVIII was one of the first cranial nerves to be monitored intraoperatively . Similar monitoring is now also in use for other purposes, including monitoring of the effect of manipulation of the brainstem .

Recording of the ABR involves recordings of responses that have a minimum amplitude and often appear in a strong background of biological electrical activity [electroencephalographic (EEG) activity] and sometimes electrical and magnetic interference. Because of that, many responses must be added (averaged) to obtain an interpretable record, and that may take a relatively long time. When ABR is used in the clinic for diagnostic purposes time is not a noticeable problem, mostly a nuisance, but when used for neurophysiological monitoring, it is a severe obstacle, because it is vital to have an interpretable response within a short time.

The other method used for monitoring neural conduction in the auditory nerve makes use of recording auditory-evoked potentials directly from the exposed CNVIII or the surface of the CN . Direct recordings from the CNVIII and the CN have the advantage of providing almost instantaneous information about changes in the neural conduction in the auditory nerve. However, these methods can only be used when the appropriate structure is exposed surgically, and they require that the surgeon position a recording electrode on the exposed auditory–vestibular nerve or near the CN, in the foramen of Luschka.

In some descriptions of techniques for monitoring the auditory system in operations in the cerebellopontine-angle, monitoring of the evoked potentials from the ear (ECoG) has been included. However, ECoG potentials are not affected by changes in the neural conduction in the auditory nerve, but ECoG potentials are affected by the function of the ear and impairment of the blood supply to the cochlea causes changes in the ECoG potentials.