CLINICAL PROBLEM

Intra-operative monitoring of CM and AP can record objectively the results of stapedectomy operations at every step of the procedure.

Intra-operative monitoring of cochlear potentials, in particular the SP, has been suggested, in cases of endolymphatic sac decompression and in drainage operations for Ménière’s disease, in order to record the effective reduction of SP.

In an effort to preserve hearing, intra-operative monitoring of cochlear and brainstem potentials has been used in neuro-otologic surgery.

Continuous monitoring provides feedback about the condition of the auditory nerve and the auditory pathway in the brainstem, and may identify the mechanism causing loss of response during suboccipital operations for CP-angle tumours, during retrolabyrinthine or retrosigmoid vestibular nerve section, and when the neurosurgeon moves the vessels during microvascular decompression procedures of the facial, trigeminal, and vestibular nerves.

Vascular compromise, especially to the cochlea, can be detected during surgical procedures. Monitoring changes in the latencies of the ABR allows surgeons to modify their techniques, for example, in retraction and surgical manipulation of the brain. The sooner they are notified of the changes, the more effectively they can control the responses. This is particularly relevant when procedures are aimed at preservation of hearing. However, in spite of the immediate preservation of the ECochG potentials, in a proportion of cases the hearing may deteriorate postoperatively as a result of various mechanisms, especially vascular compromise.

Small tumours are harder to diagnose than large ones, but easier to remove; the operation causes fewer deaths, less morbidity, and less residual neurological deficit; the facial nerve often can be spared; and in up to 30% of cases, the hearing can be preserved. In a small study by the National Hospitals for Nervous Diseases, 5 out of the 7 patients with tumours of less than 1.5 cm had their hearing preserved, but 15 out of 17 patients with tumours greater than 1.5m became deaf (Prasher et al 1987, Sabin et al 1987).

CHOICE OF ERA TESTS

The effects of general anaesthesia and continuous stimulation should be taken into consideration when choosing the EP for intra-operative monitoring, and early EPs are more suitable for that reason.

General anaesthesia depresses the activity of the CNS, and the kind of general anaesthesia is of particular importance when choosing an EP for intra-operative monitoring. Lader & Norris (1969) showed that the EP components appearing after 50 ms are significantly affected by nitrous oxide. However, early auditory-evoked potentials, including ECochG and ABR, are not affected significantly, in man, by commonly used anaesthetics, such as thiopentone, nitrous oxide, and halothane (Duncan et al 1979). Neuromuscular blocking agents, such as succinylcholine and pancuronium bromide, used prior to intubation also do not affect the ABR or MLR (Harker et al 1977, Kileny 1983). A considerable drop in body temperature during anaesthesia may prolong the latencies of the ABR and MLR, and have a confounding effect on the responses.

Another essential requirement of intra-operative monitoring is repetitive stimulation, and habituation may affect the response. Picton et al (1976) have demonstrated that the late vertex cortical potentials decline with repeated stimulation, but the MLR and ABR do not.

The early EPs are used in neuro-otologic surgery to measure both hearing sensitivity and the condition of the brainstem when necessary. Arterial hypotension, hypocarbia, and, especially, surgical manoeuvres may affect the ABR (Grundy et al 1981).

ECochG is a very sensitive indicator of the cochlear function, and, in particular, may detect hydrops in Ménière’s disease when the SP/AP ratio is measured. Reduction of the endolymphatic pressure during the operation of endolymphatic sac drainage may be reflected in ECochG as a reduction of SP amplitude.

Both ECochG and ABR are useful in acoustic tumour surgery to preserve hearing. However, transtympanic ECochG gives a better resolution and uses less stimulation, hence giving more rapid feedback on changes in cochlear function than does ABR. Recording of the cochlear function is compromised further by the presence of a hearing loss in acoustic tumours and by ambient noise in the operating theatre.

ABR is of some value when more rostral changes are monitored during neuro-otologic surgery, especially latencies of brainstem components such as intervals I–III, I–V.