Tone pips of suprathreshold intensities elicit an acoustic nerve response (I) and six low amplitude brainstem potentials (II-VII) during the initial 10 ms. Seven waves were studied in 40 control subjects and 5 waves (I-V) in 47 patients with MS. The results suggest involvement of the auditory pathway of 24 of 27 patients in the clinically "definite", of 5 of 9 cases in the "probable" and in none of 5 patients in the "possible" MS groups. EAEPs were normal in 6 cases with a spinal form with one exception where changes of potential were indicative of a midbrain lesion. Dysfunction within the acoustic pathway was observed at the level of the acoustic nerve and in the medulla oblongata, pons and midbrain. The significance of the bilateral EAEP abnormalities found in some patients at different levels is discussed with regard to a polytopic location of the underlying lesion.
Stimulation with a short tone pip elicits an acoustic nerve compound action potential (I) and different waves (II--VII) in the inital 10 ms. Seven waves have been studied in 40 control subjects and five waves in 12 patients with vertebral-basilar insufficiency. Abnormalities of the different waves were observed at levels such as cochlea and/or acoustic nerve, medulla, caudal pons, rostral pons, and midbrain. The recording of early auditory evoked potentials (EAEP) is a noninvasive method of confirming impairment of the auditory pathway caused by a reduced vascular supply of vertebral and basilar arteries.
Within the first 10 ms after a sine-shaped sound wave (tone pip) seven small-amplitude potentials can be recorded in persons with normal hearing and normal brainstem functions. These components, in the nanovolt range, correspond to the electrical activity of various pathways of the auditory tract. In accordance with this view the resulting potentials were assigned to the following structures in the region of the periphery and the brainstem: component I corresponds to the cochlea or acoustic nerve (receptor, II to the cochlear nucleus (medulla), III to the upper olive (caudal pons), IV to the lateral lemniscus (rostral pons), V to the inferior colliculus (midbrain), VI to the medial geniculate body (diencephalon), VII to the acoustic radiation (cortex). Clinically well defined lesions of the acoustic nerve and brainstem indicate that there is a close topographical relationship between the clinical localisation and absence or delay of the individual components.
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