Multiple sclerosis lesions of the auditory pons are not silent

Author(s): Gardner, J. C., Fullerton, B. C., Stufflebeam, S. M., Furst, M., Rosen, B. R.

Journal/Book: Brain. 1994; 117: Walton St, Journals Dept, Oxford, United Kingdom OX2 6DP. Oxford Univ Press United Kingdom. 1127-1141.

Abstract: To understand the relationship between brainstem lesions and auditory neurology in patients with multiple sclerosis, we compared behavioural, electrophysiological and imaging data in 38 patients with probable or definite multiple sclerosis and normal or near normal hearing. Behavioural measures included (i) general hearing tests (audiogram, speech discrimination) and (ii) hearing tests likely to be critically dependent upon brainstem processing (masking level difference, and level discrimination). Brainstem potentials provided the electrophysiological data. Multiplanar high-resolution MRI of the brainstem provided the anatomical data. Interaural time discrimination for high-frequency sounds was by far the most sensitive of all tests with abnormalities in 71% of all subjects. Whenever any other test was abnormal this test was always abnormal. Interaural time discrimination for low-frequency sounds and evoked potentials were closely related and next most sensitive with abnormalities in similar to 40% of all subjects. Interaural level discrimination and masking level difference were least sensitive with abnormalities in <10% of subjects. Speech discrimination scores correlated significantly with the masking level differences, as well as with interaural time discrimination for high-frequency sounds. Pontine lesions were found in five of the 16 patients, in whom an objective method for detecting magnetic resonance lesions could be applied. All four with lesions involving the pontine auditory pathway had marked abnormalities in interaural time discrimination and evoked potentials. None of the other 12 had evoked potentials abnormalities. We conclude that neurological tests requiring precise neural timing can reveal behavioural deficits for multiple sclerosis lesions of the auditory pens that are otherwise 'silent'. Of all neurological systems the auditory system at the level of the pens is probably the most sensitive to multiple sclerosis lesions, because of its exceptional dependence upon neural timing in the microsecond range and the lack of redundancy in the encoding of high-frequency sounds. Precise neural timing may be critical for some aspects of speech processing.

Note: Article RA Levine, Massachusetts Eye & Ear Infirm, Eaton Peabody Lab, 243 Charles St, Boston, MA 02114 USA

Keyword(s): interaural discrimination; speech discrimination; masking level difference; evoked potentials; HUMAN BRAIN-STEM; EVOKED-POTENTIALS; NERVE FIBERS; CONSONANT RECOGNITION; BACKGROUND-NOISE; RESPONSES; SPEECH; VOWELS; REPRESENTATION; PATTERNS

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