MULTIPLE-SCLEROSIS LESIONS OF THE AUDITORY PONS ARE NOT SILENT

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 o r definite multiple sclerosis and normal or near normal hearing. Behav ioural measures included (i) general hearing tests (audiogram, speech discrimination) and (ii) hearing tests likely to be critically depende nt upon brainstem processing (masking level difference, and level disc rimination). Brainstem potentials provided the electrophysiological da ta. Multiplanar high-resolution MRI of the brainstem provided the anat omical data. Interaural time discrimination for high-frequency sounds was by far the most sensitive of all tests with abnormalities in 71% o f all subjects. Whenever any other test was abnormal this test was alw ays abnormal. Interaural time discrimination for low-frequency sounds and evoked potentials were closely related and next most sensitive wit h abnormalities in similar to 40% of all subjects. Interaural level di scrimination and masking level difference were least sensitive with ab normalities in <10% of subjects. Speech discrimination scores correlat ed significantly with the masking level differences, as well as with i nteraural time discrimination for high-frequency sounds. Pontine lesio ns were found in five of the 16 patients, in whom an objective method for detecting magnetic resonance lesions could be applied. All four wi th lesions involving the pontine auditory pathway had marked abnormali ties in interaural time discrimination and evoked potentials. None of the other 12 had evoked potentials abnormalities. We conclude that neu rological 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 a t the level of the pens is probably the most sensitive to multiple scl erosis lesions, because of its exceptional dependence upon neural timi ng in the microsecond range and the lack of redundancy in the encoding of high-frequency sounds. Precise neural timing may be critical for s ome aspects of speech processing.