Vestibular-dependent surface potentials can be recorded from over the scalp following stimulation with intense air- (AC) and bone-conducted (BC) sound. However, sound-evoked responses may be confounded by parallel stimulation of the auditory system. To demonstrate the pure vestibular origin of the cortical potentials, patients with severe to profound bilateral hearing loss were stimulated with AC and BC sound. The responses had the same amplitude as those recorded in normal subjects, and were only present in patients with preserved vestibular function, confirming their vestibular origin. One negative surface potential, the N15, was largest when measured over the forehead, and detailed mapping of this potential localised it to the eyes. This extraocular response had the same polarity on each side of the eye and was altered by changing gaze direction, suggesting an extraocular muscle origin (i.e. an ocular vestibular evoked myogenic potential, or OVEMP). Galvanic vestibular stimulation (GVS) produces large eye movements with horizontal and torsional components directed away from the cathode. A modified electrode montage was used to characterise the OVEMPs produced by GVS. OVEMPs recorded from beneath the eyes had the appropriate polarity to produce the torsional eye movement and likely originated in the inferior oblique muscles. Sound-evoked OVEMPs were investigated in patients with superior canal dehiscence (SCD), as they have vestibular hypersensitivity to sound. The SCD patients had large sound-evoked OVEMPs with low threshold, similar to the VEMP. OVEMP amplitude was much larger in the patients than controls and could be an additional diagnostic marker for this condition. Although SCD patients have large VEMPs and eye movements evoked by AC sound, little is known about other vestibular reflexes. It was shown that patients also have large sound-evoked vestibulo-spinal reflexes, similar to those evoked by GVS. However, despite these large reflexes, there was little consistent whole body sway. Finally, a case is reported in which the combination of VEMP and OVEMP results indicated the location and nature of a central nervous system lesion. The patient had delayed potentials when stimulated on the left side, indicating a demyelinating lesion in the root entry zone of the left vestibulocochlear nerve.
| Identifer | oai:union.ndltd.org:ADTP/212662 |
| Date | January 2008 |
| Creators | Rosengren, Sally Marie, Clinical School - Prince of Wales Hospital, Faculty of Medicine, UNSW |
| Publisher | Publisher:University of New South Wales. Clinical School - Prince of Wales Hospital |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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