Invited reviewVestibular evoked myogenic potentials: Past, present and future
Introduction
An important aim of vestibular research is to develop techniques for assessing vestibular function in clinical settings. Methods based upon short-latency reflexes and evoked responses have been applied to the vestibular system only relatively recently, with the development of the VEMP (vestibular evoked myogenic potential). The VEMP is a short-latency myogenic response which is evoked by brief pulses of air-conducted (AC) sound, bone-conducted (BC) vibration or electrical stimulation and recorded using surface electrodes placed over muscles. The VEMP as currently understood was first described by Colebatch and Halmagyi (1992) and Colebatch et al. (1994a), who measured electromyographic (EMG) activity from the sternocleidomastoid (SCM) muscles following vestibular stimulation with brief pulses of sound (clicks). It is a manifestation of the vestibulo-collic reflex, resulting from activation of the vestibular nerve and nucleus, the vestibulo-spinal tract, the accessory nucleus and nerve and the sternocleidomastoid muscle (Fig. 1). Since the initial report, VEMPs have become a standard clinical test of otolith (predominantly saccular) function, with documented diagnostic utility in varied vestibular and central nervous system disorders.
In more recent years, similar myogenic responses have been evoked in other muscle groups, such as the masseter (Deriu et al., 2005). In particular, recordings made from around the eyes have demonstrated that short latency potentials can also be recorded from the extraocular muscles as part of the vestibulo-ocular reflex (e.g. Rosengren et al., 2005, Todd et al., 2007), i.e. by activation of the vestibular nerve and nucleus, and transmitted possibly via the medial longitudinal fasciculus, the oculomotor nuclei and nerves and the extraocular muscles (Fig. 2). Due to the strength of the vestibulo-ocular reflex arc, the extraocular responses are easily recorded and have promising diagnostic potential. To distinguish them from the conventional VEMP, the extraocular responses are referred to as ocular VEMPs (oVEMPs or OVEMPs), while SCM responses are now sometimes qualified as cervical VEMPs (cVEMPs, CVEMPs or VEMPs). This review describes the historical development of both the cVEMP and oVEMP, provides practical information for recording VEMPs and discusses the clinical applications of the reflexes.
Section snippets
Historical aspects
The development of the digital averager (e.g. Dawson, 1954) led to the initial reports of click-evoked responses recorded from the scalp (Geisler et al., 1958), which Geisler and Rosenblith (1962) reported were largest over the occiput. Bickford et al. (1963) subsequently recorded widespread responses over the scalp to loud AC tone bursts, with the shortest latencies over the cervical muscles. They also made the crucial observations that these responses were myogenic in origin and that the
Ocular VEMPs
Ocular VEMPs are recorded from closely spaced surface electrodes located near the eyes and represent the synchronous extraocular EMG activity associated with the vestibulo-ocular reflex (VOR). The oVEMP was first proposed as a potential test of vestibular function only recently (Chihara et al., 2007, Iwasaki et al., 2007, Todd et al., 2007, Welgampola et al., 2008), but evidence suggesting that extraocular muscle activity can be recorded with surface electrodes can be traced to earlier
Vestibular stimulation
The vestibular organs are naturally activated by angular and linear head acceleration, however, imposed head accelerations can be difficult to standardise and are prone to causing electrical artefact. In contrast, other means of activating the vestibular system, such as sound, vibration and galvanic current can be delivered repeatedly and thus produce stereotyped short-latency vestibular reflexes. Though effective, they are typically non-physiological, as they do not reproduce natural
Clinical applications
Although the important factors affecting cVEMPs are now relatively well-established, such as the effects of age on amplitude, many clinical studies were conducted before this information was available. The absence of dedicated VEMP recording systems also means that many clinicians rely on modified evoked potential systems with limited methods for recording background EMG. As such, many published studies would not meet the recording standards recommended above. In the following section on
Conclusion
The ability to test all five vestibular end-organs is the ultimate aim of diagnostic neuro-otology. Sound- and vibration-evoked VEMPs, combined with subjective visual vertical testing and head impulses in the three canal planes, fulfil this aim. Routine use of the cVEMP with caloric testing provides a more complete initial assessment of the dizzy patient than either alone. VEMPs, when compared with caloric testing, are better tolerated and technically simpler to record but are not as widely
Acknowledgements
The authors are very grateful to Prof. G. Michael Halmagyi for his contribution to the initial version of the review. The authors were supported by the National Health and Medical Research Council of Australia and the Garnett Passe and Rodney Williams Memorial Foundation.
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