Postural responses to vibration of neck muscles in patients with uni- and bilateral vestibular loss
Introduction
Recordings of muscle spindle afferents during the vibration of muscle in both animals [1]and in humans 2, 3have shown that vibration selectively activates muscle stretch receptors; particularly Ia spindle afferents giving rise to a false signal of stretch. In human subjects, the responses to such `artificial stretch' signals are plastic. Vibration applied to a relaxed muscle gives rise to a tonic muscular contraction (tonic vibration reflex; [4]) and an illusory motion about the respective joint 5, 6when the actual movement is frustrated by a mechanical restraint. For example, vibration applied to Achilles tendons in a seated subject induces a tonic contraction of the ankle extensors or illusory dorsiflexion of the feet. Whereas, in a standing subject, Achilles tendon vibration gives a whole body sway backwards 7, 8and illusory, whole-body tilt forwards when the torso is restrained 5, 8. These findings show that central processing of proprioceptive afference depends on the postural context; specifically what possible reference (e.g. feet, trunk, etc.) is taken to be stationary. The choice of a particular reference is of crucial importance for the central interpretation of proprioceptive signals as they carry information about relative motions of the body segments rather than their trajectories in external space.
In the example of the Achilles tendon vibration the likely reference is the ground (i.e. the foot support). Afferent signals from the apparent lengthening of the extensor tendon are interpreted by the brain as the whole body deviation forward with respect to stationary feet, thus giving rise to either illusory forward tilt and/or backwards body sway as a compensating postural adjustment.
A backwards sway deviation can also be observed when other postural muscles of the posterior body surface are stimulated by vibration; the only exception being that vibration of the neck region results in a body sway forwards 9, 10, 11. It has been hypothesised that this difference is due to the fact that proprioceptive signals from the neck are processed jointly with vestibular input with the head taken as a reference [12]. Since, with the neck vibration, there is no concomitant signal from the labyrinth of head tilt, the brain estimates the signal as a tilt of the body beneath the space-fixed head with the neck as axis of rotation. This `illusory' tilt gives rise to a forwards sway response which is directed to returning the body to the upright.
On this hypothesis, one could expect that vestibular dysfunction would lead to a misinterpretation of proprioceptive signals from the neck. This has been tested in the present study which examines sway responses to vibration of the neck muscles in patients with peripheral vestibular lesions. Bilateral vibration of the dorsal neck muscles has been reported to increase sway in patients with central vestibular lesions whereas patients with unilateral peripheral lesions were unaffected by vibration [13]. Alternatively, other authors [14]have reported a lateral component (directed to the lesioned side) in the body sway evoked by vibration of dorsal neck muscles in compensated patients with unilateral vestibular dysfunction. Our experiments differ in that the vibration was applied unilaterally, thus uncovering asymmetries of postural response in patients with unilateral vestibular lesions (ULDs).
Section snippets
Apparatus
Subjects stood on a 50 cm square sway platform 20 cm high above the ground. The centres of the heels were approximately 10 cm apart and the feet splayed out at approximately 35°. Subjects were allowed to wear their everyday shoes and fortunately none of the women tested had fashion heels.
An electromechanical vibrator, 6.0 cm long, 2.5 cm wide with a flat 6×3.5 cm rectangular contact surface was positioned over the medial and superior aspect of the right (or left) trapezius muscle and held in
General characteristics of responses
In normal subjects, vibration of the neck muscles resulted in a whole body sway forward which was usually visible and attained up to 10° of tilt of the long axis at maximum. The response recorded by the sway platform consisted of a small backwards torque, seen better on average, followed by a forward deviation (Fig. 1, Fig. 2). At the offset of vibration the body returned to the upright.
In BLD patients, neck vibration resulted in neither sway deviation nor a postural instability but it was
Mechanisms of postural responses to neck vibration
A quiet stance is usually not impaired in patients with compensated vestibular disorders 16, 17but may become unstable when other sensory inputs (e.g. visual, proprioceptive, somatosensory) are manipulated 18, 19, 20, when adopting unusual attitudes 17, 21or during movement of the head 22, 23or body [24]. The causes of this instability may be twofold; the more basic being the impairment of vestibulo-spinal reflexes 25, 26. The second, as hypothesised by Black et al. [18], is the instability
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