Vestibulocollic reflexes: normal values and the effect of age
Introduction
Several novel methods of assessing vestibular reflex function have been described recently. Click-, tap- and galvanic-evoked vestibular reflexes recorded from anterior neck muscles occur at short latencies and specific components of these reflexes have been shown to be vestibular-dependent (Colebatch et al., 1994, Halmagyi et al., 1995, Watson and Colebatch, 1998). Although the specific receptors stimulated in each of these reflexes have not as yet been unequivocally established, current evidence supports at least a contribution from the otoliths. Each of these techniques activates the vestibular afferents in different ways. Clicks are conducted through the middle ear and activate the saccular hair cells (McCue and Guinan, 1994, Murofushi et al., 1995); forehead taps produce a wave of vibration that bypasses the middle ear and directly activates the vestibular apparatuses on both sides simultaneously (Halmagyi et al., 1995) and transmastoid galvanic stimulation acts upon the distal part of the primary vestibular afferents (Goldberg et al., 1984, Watson and Colebatch, 1998). Cathodal stimulation results in an increase and anodal stimulation in a decrease in resting discharge level (Goldberg et al., 1984). These tests provide additional information to current methods of vestibular assessment. The aim of this study was to further investigate their potential clinical application by defining the range of normal values and the reproducibility of the individual values. In doing so we have also investigated the effects of ageing as they apply to these reflexes.
Age-related morphological changes affecting the vestibular system from the end organs to the central nuclei are well documented. The vestibular epithelium shows hair cell loss of 6% per decade between the ages of 40 and 90 years (Rosenhall, 1973). Bergström (1973) reported a decrease in the number of vestibular nerve fibres by 5.5% per decade over a similar age range. A decrease with age in the number of cell bodies in Scarpa's ganglion (Richter, 1980) and a loss of neurons in the vestibular nuclear complex (Lopez et al., 1997) have also been reported. Although morphological changes are marked, tests of vestibular function have been reported to show only modest changes with age. Caloric responses have been reported to increase up to the 5th or 6th decades, followed by a small decline thereafter (Mulch and Petermann, 1979). Peterka et al. (1990) reported only a small reduction in vestibulo-ocular reflex (VOR) gain in older age groups to sinusoidal stimulation, but a greater decrease has been reported in response to high amplitude or high acceleration rotational stimuli (Baloh et al., 1993, Tian et al., 2001). The dissociation between conventional vestibular function tests and morphological measures suggests relative preservation of vestibular function despite loss of end organ receptors and afferents with increasing age.
Section snippets
Subjects and methods
Seventy community-dwelling, healthy adults aged 25–85 years (34 females, 36 males) were studied after informed consent and local ethics committee approval. Vestibulocollic responses to high intensity clicks, forehead taps and short duration galvanic stimulation were measured. Subjects who had suffered any single episode of vertigo exceeding 30 min and those who had suffered recurrent episodes of vertigo were excluded. None of the subjects tested was taking vestibular suppressant medication.
Click-evoked responses
Sixty-four of the 70 subjects had measurable click-evoked responses bilaterally. Five of the remaining patients had unilaterally absent responses and one subject had bilaterally absent responses. One absent response and one attenuated response occurred in association with conductive hearing loss and were excluded from further analysis. All subjects below the age of 60 had click-evoked responses, with corrected reflex amplitudes (see Section 2) ranging from 0.5 to 3.0 (uncorrected amplitudes of
Discussion
Loud monaural clicks evoke vestibular-dependent, short-latency myogenic responses in the ipsilateral SCM muscle during its tonic activation (Colebatch et al., 1994). In experimental animals, clicks have been shown to activate primary vestibular afferents, principally those arising from the saccular otoliths (McCue and Guinan, 1994, Murofushi et al., 1995) which due to their proximity to the footplate of the stapes, may be mechanically stimulated by loud sound (von Békésy, 1935). Their short
Acknowledgements
This research was supported by a grant from the National Health and Medical Research Council of Australia. M.S.W. received a Medical Research Fellowship from the Garnett Passe and Rodney Williams Memorial Foundation during the period of this study. We thank Ms Jennifer Robins for assistance with data collection, Mr Dusan Hadzi-Pavlovic for helpful criticism and assistance with statistical analyses and Ms Sally Rosengren for editorial assistance.
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