Vestibular evoked myogenic potentials in multiple sclerosis patients
Introduction
Vestibular evoked myogenic potentials (VEMPs) are obtained through the delivery of acoustic (or galvanic) stimuli that are able to trigger a vestibulo-spinal response (Bickford et al., 1964) by stimulating the saccular macula (Murofushi et al., 1995, Murofushi et al., 1996). The response may be recorded from surface electrodes placed over various neck or limb muscles. The first two peaks, labelled P13 and N23 on the basis of their polarity and their latency, depend on the integrity of vestibular afferents, and are almost invariably detectable (Colebatch et al., 1994).
VEMP recording is a simple, inexpensive and rapid method that is well tolerated by subjects, and easily implementable in a laboratory equipped for recording evoked potentials. Moreover, VEMP recording and subjective visual vertical (SVV) evaluation, the latter a measure of the ability to judge the orientation of the gravity vector without allocentric visual cues, provide reliable information about otolith function without very sophisticated equipment, such as that used to test the linear vestibulo-ocular reflex.
When recorded from the surface of the sterno-cleido-mastoid (SCM) muscles, VEMPs check the integrity of the saccular afferences to the brainstem vestibular nuclei, and then to SCM through the upper cervical segments and the accessory nerve.
To date the use of VEMPs in clinical settings has mainly involved patients suffering from vestibular end organ or vestibular nerve dysfunction, whereas there are only a few reports all very recent, concerning patients with brainstem lesions (Murofushi et al., 2001, Itoh et al., 2001, Shimizu et al., 2000).
This retrospective study aimed to evaluate the occurrence of abnormal VEMPs in a group of 70 MS patients, and to compare such occurrence with data both from other neurophysiological tests and from clinical history and examination.
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Patients
Seventy unselected patients (mean age: 35.5 years; range 19–61 years) suffering from MS according to the diagnostic criteria recommended by the International Panel on the Diagnosis of MS (McDonald et al., 2001) underwent VEMP recording during hospitalization either for diagnostic purposes or for a relapse. At the time of VEMP recording the diagnosis of MS could be made in most of the patients on a clinical basis, and in those (6/70) in which this was not possible the diagnosis was confirmed by
Results
The past medical history of our patients suggested the occurrence of brainstem symptoms in 31.9% and of cerebellar symptoms in 14.5% of the patients. The current evaluation of the patients showed brainstem symptoms in 4.3% and signs in 23.2% of the patients, and cerebellar symptoms in 10.1% and signs in 27.5% of the patients. In particular, 14.5% of the patients complained of ‘dizziness’ as one (or the only) of their current symptoms.
VEMPs were detectable from both SCMs in 65 patients; in 3
Discussion
VEMPs are a muscular response obtained by galvanic or acoustic stimulation of the saccular macula. In cats both the sacculus and the utriculus show inhibitory connections to the ipsilateral SCM motoneurons (or to both ipsi- and contralateral motoneurons in the case of the utriculus) through the medial vestibular spinal tract (Kushiro et al., 1999). In humans also, the saccular connections are ipsilateral (Li et al., 1999) and inhibitory (Wu et al., 1999).
The abnormalities detected by the VEMPs
Acknowledgements
The authors would like to thank Mr Roberto Alloni, Mrs Maria Laura Delnevo and Miss Alessandra Spiritelli for their skilfull technical assistance.
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