Acute vestibular system loss, whether unilateral or bilateral, has profound effects on control of balance and gait. However, this acute loss does not necessarily indicate a lifelong disability for all patients. In fact, some total vestibular loss patients do not need any type of assistive device, even when navigating complex terrain.1 Recent findings indicate that following vestibular system loss, the central nervous system can develop the ability to compensate for the missing information by relying upon other sensory systems.1 While vestibular system information is continuously being processed by health individuals, somatosensory and visual systems provide similar information that can be used when vestibular information is lost. Some studies even indicate that the vestibular system plays a minor role in balance corrections unless somatosensory and visual information is unavailable.2 Situations where somatosensory and visual system information is unavailable include standing in the dark or on an unstable surface or even if leg ischemia is present.1,2 This paper aims to synthesize two separate studies on the role of the vestibular system. The first study by Horak1 summarizes the role of the vestibular system in control of posture. The study focuses on identifying the mechanisms behind the posture control, the natural compensation that occurs following vestibular loss, the biofeedback options for sensory substitution and the implications for rehabilitation. The second study by Runge et al2 investigates the role of vestibular information for initiation of rapid postural responses. This paper observes the effects of head accelerations, presence of stepping responses, EMG responses , kinematic responses, and presence of a hip torque.
In the article by Horak1, patients with unilateral and bilateral vestibular system loss were exposed to various surface perturbations. The automatic postural response to a particular perturbation was recorded. Other studies discussed looked into the effects of using visual, audio, and vibrotactile-biofeedback for posture and gait control of patients with vestibular system loss.1 In comparison to the article by Runge et al2, seven patients with adulthood acquired bilateral vestibular loss were matched with seven corresponding controls. In addition, two individuals with infancy acquired bilateral vestibular loss participated in an identical study but results were kept separate. The experiment was performed in a single two-hour session where subjects stood on a support surface that was capable of translation in the anterior and posterior direction using various speeds. Responses were recorded for posterior translations only however. Perturbations were presented in 5 blocks, each containing velocity specific perturbations in a random order. Center-of-pressure readings were also recorded to monitor for anticipatory learning. Motion capture was used to observe body position in the sagittal plane. EMG activity was recorded for the medial gastrocnemius, tibialis anterior, rectus femoris, biceps femoris, rectus abdominis (at level of umbilicus), and erector spinae at the location of the iliac crest. Force transducers observed the vertical reacion force and horizontal shear force. Net joint torques were calculated. Subject-independent motor patters were identified.2
While postural response to achieve equilibrium is often associated with the vestibular system, Horak states that it is actually somatosensory loss that is responsible for response latencies.1 When compared to controls, vestibular patients had normal postural response latencies in the ankle muscles, hamstrings and quadriceps, and abdominal muscles.1 Patients with loss of dorsal root afferents from limbs however had profound (or absent) postural responses.1 Vestibular patients did however have trouble standing on a rotating surface at specific velocities (interference of somatosoensory information). Horak found that the most unstable velocity was 4 deg/sec and most stable velocity (showing an almost normal pattern) was slower than 1 deg/sec or faster than 32 deg/sec.1 It was proposed that graviceptive sensory information was used to control sway at slow speeds and primary and secondary endings of velocity-sensitive muscle spindles was used for faster speeds.1 It is also proposed that vestibular patients that are well compensated can reweight somatosensory information to visual information when the rotation amplitude is high1. However, Horak does note that the vestibulospinal system may help activate neck muscle responses to head perturbations.1
Runge et al noted a similar finding of vestibular system involvement when observing head accelerations and later in the paper had to reject their hypotheses that the vestibular system may also be involved in automatic postural responses when platforms are perturbed in a linear horizontal fashion (non-rotational).
Horak notes that patients with vestibular loss show excessive postural sway when standing on an inclined surface with eyes closed, despite the fact that the surface is stationary. Based on this finding, it is proposed that somatosoensory information alone does not provide enough information on surface orientation for vestibular patients.1 In healthy individuals, the nervous system compares somatosensory information with vestibular information. When both systems agree, somatosensory information is relied upon.1 In vestibular loss patients, the vestibular system is inactive and thus, somatosensory information is always used. Surfaces that require a vestibular patient to stand in a narrow stance (ie, balance beam) also prove to be difficult for balance maintenance using somatosensory information alone. The proposed postural control strategy is possibly due to quick trunk and hip torques known as the “hip strategy.” When a hip torque strategy is used, the body is divided into trunk and legs as a double inverted pendulum .2 The hip toque will initiate the forward movement of the trunk and then gravity will provide the downward force.2 According to Horak, vestibular patients are unable to use this strategy. Instead, vestibular patients use an “ankle strategy” for wide stance, and will use it even when hip strategy is required for stability.1
Runge et al found that during posterior translations of the support surface, vestibular patients exhibited ankle torques at the slowest speeds and knee torques at moderate speeds.2 This finding is consistent with those found by Horak. At the highest speed of translation however, Runge et al found that four of the six vestibular patients that could maintain balance exhibited a hip torque pattern.2 This hip torque (in addition to knee and ankle) was similar to that of controls up to a speed of 400ms. Following this finding, Runge et al supports the hypothesis that the somatosensory information alone may sufficiently generate postural responses.2 It is important to note however that Runge et al also looked for hip strategy in two vestibular patients that had infant-onset vestibular-loss. Not only did these patients not lose balance for all speeds tested, they never employed the hip strategy while their controls did at the higher speeds.2 It was proposed that the hip strategy may require vestibular information during development to be used as a solution to surface translation.2 Potentially, the hip strategy may not surface until after the age of 6 or perhaps a higher velocity of translation is needed for the strategy during early childhood.2 Further research will be necessary.
The vestibular system does play a big role in controlling the orientation of the head and trunk in space.1 In vestibular loss patients, this information can come from somatosensory systems or from vision. Interpretation of somatosensory information within muscles, joints, and skin allows for the understanding if motion comes from the head or from the body alone.1 However when the ground is oscillating in continuous motion, the stable vertical reference is unavailable (unless the oscillation is small enough).1 If the eyes are closed, vestibular patients will slowly drift their upper bodies until equilibrium is lost.1 This loss of vertical perception can also be seen when vestibular patients are unable to orient a hand held rod with gravity when eyes are closed on an oscillating surface.1
Following vestibular system loss, the central nervous system will gradually adapt to the missing information by relying upon other available sensory systems. This compensation is dependent upon an individual’s age, neural plasticity, environment, and the function of remaining systems.1 When vestibular system loss is slow, compensation will be most complete, unlike sudden loss where an individual will experience vertigo, nystagmus, nausea, and tilting towards the side of the lesion.1 This tiling towards the side of the lesion can in part be due to. However, even with slow compensation, individuals with bilateral vestibular loss will not be able to navigate unstable terrain with eyes closed, stand on one foot or navigate a straight line with eyes closed or during head rotation.1 In these scenarios, sensory information from other systems is unavailable to compensate for the loss. In addition, visual disturbances during head movements due to the loss of the vestibulo-ocular reflex may remain challenging. For vestibular patients that have remaining vestibular function, a focus on reweighting function to the other side will be best compensated.1 Horak found a linear relationship between the ability of a patient to reweight vestibular function and improvement on Coen’s Vestibular ADL scale.1 The use of vision can also be substituted for some vestibular information, as discussed previously. For vision to be used effectively, a patient will need to learn predictive control of vision to control stability.1 It is important to note that vestibular patients can become too resilient upon vision and can destabilize themselves if the visual reference does not align to gravity. Horak also found that light touch provided an effective reference for posture.1 This touch provides a patient with a reference to vertical. Functionally, a cane might be a good option for someone with vestibular issues to supply this link. Biofeedback in the form of visual, audio or vibrotectile can also be an effective strategy for vestibular patients. Vibrotactile biofeedback in particular shows great abilities to reduce sensory noise and improve postural stability during gait.1 The most effective rehabilitation interventions should focus on utilizing remaining vestibular function, using surface somatosensory information as primary sense, using a light touch device such as a cane, learn to use visual references appropriately, and to participate in balance training therapy.1 Balance training should focus on diverse skills and environments that gradually encourage the vestibular patient to develop effective movement strategies.1 Functional goals should be developed and cognitive strategies practiced.
The effects of acute vestibular loss can be well compensated for many patients. Somatosensory and visual information are the most useful for maintaining posture, with or without perturbations. Surfaces with oscillations, rotations, or give are vestibular directed in healthy individuals but are navigable for vestibular patients with additional sensory information. While debate exists on the use of the hip strategy by vestibular patients, it is currently speculated that individuals with late onset vestibular loss are able to use the strategy to maintain balance while individuals with infant onset vestibular loss do not need it. Rehabilitation strategies should focus on using the remaining vestibular function, using somatosensroy information, using stable visual references and developing movement strategies for a variety of conditions.
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