Info

I 2.67s

Well-compensated VS3

Platform — AP head 0.1 Hz eyes open

Well-compensated VS3

I 10s 0.1 Hz eyes open

Poorly compensated VS6

Poorly compensated VS6

I 10s 1.25 Hz eyes open

Control

Control

Well-compensated VS1

Poorly compensated VS5

Poorly compensated VS5

I 2.67s

Figure 20.1. Time series of an exemplar trial of head (solid line) and platform (dashed line) AP displacement with vision available (eyes open). (a) Age-matched control at 0.1 Hz; (b) well-compensated subject at 0.1 Hz; (c) poorly compensated subject at 0.1 Hz; (d) control subject at 1.25 Hz; (e) well-compensated subject at 1.25 Hz (f) poorly compensated subject at 0.75 Hz. VS: vestibular subject. (Reprinted from Buchanan and Horak , 2001-2002, with permission from IOS Press.)

Figure 20.1. Time series of an exemplar trial of head (solid line) and platform (dashed line) AP displacement with vision available (eyes open). (a) Age-matched control at 0.1 Hz; (b) well-compensated subject at 0.1 Hz; (c) poorly compensated subject at 0.1 Hz; (d) control subject at 1.25 Hz; (e) well-compensated subject at 1.25 Hz (f) poorly compensated subject at 0.75 Hz. VS: vestibular subject. (Reprinted from Buchanan and Horak , 2001-2002, with permission from IOS Press.)

Stabilization of head and trunk

Vestibular information is important in stabilizing head and trunk motion in space (Buchanan and Horak, 2001-2002; Creath et al., 2002). When the support surface is moved sinusoidally in pitch at low frequencies, healthy subjects maintain their balance by moving their head and center of mass in phase with the platform movement (Buchanan and Horak, 2001-2002). At higher frequencies, healthy subjects switch to a "head-fixed-in-space" strategy with the result that there is little head displacement. Unlike healthy subjects, however, subjects with BVH have greater head and trunk pitch displacement and center of mass movement is coupled to platform movement regardless of the frequency of platform movement. These findings support the hypothesis that sensory information is re-weighted according to frequency of movement: at low frequencies, somatosensory information is weighted more heavily and at high frequencies of movement, vestibular information is weighted more heavily (Creath et al., 2002). Interestingly, three BVH subjects who were able to maintain balance at all frequencies were identified as being well compensated. These subjects demonstrated the same strategies as the healthy subjects at both low and high frequency of support surface rotation suggesting that some other mechanism has compensated for the lost vestibular function (Fig. 20.1; Buchanan and Horak, 2001-2002).

The role of the vestibular system in stabilizing head and trunk motion in space becomes more obvious during activities such as walking and hopping. Healthy subjects have similar gait speed and vertical amplitude of the head in the light as in the dark (Pozzo et al., 1990; Pozzo et al., 1991). In contrast, subjects with BVH modify their gait in the dark by decreasing walking speed, shortening stride length, decreasing arm swing, widening their base of support and increasing stance phase. These subjects also hold their head stiffly relative to the trunk and have a downward tilt of the head. Healthy subjects demonstrate an out-of-phase relationship between vertical head amplitude and head angular displacement. During hopping, when the head translates upward, it simultaneously rotates downward and vice versa. This acts to stabilize gaze during gait activities. Subjects with BVH do not show a clear phase relationship between head rotation and translation and the majority of BVH report oscillop-sia during hopping.

20.5 The role of the vestibulo-ocular reflex in balance

It is well known that certain eye movements affect balance. For the most part, it has been assumed that the interaction between eye movements and postural stability is related to the manner in which eye movements affect the visual cues used for postural stability (Iwase et al., 1979; White et al., 1980; Oblak et al., 1983). When small saccadic eye movements are made, during which visual cues that would disturb balance are suppressed, there is little or no change in stability (Iwase et al., 1979; White et al., 1980; Oblak et al., 1983). Pursuit eye movements against a stationary background, which results in the perception of movement of the visual scene, cause an increase in sway (Strupp et al., 2003). More recent studies suggest that efference copy of the oculomotor signal may also affect postural stability (Jahn et al., 2002; Strupp et al., 2003). For example, smooth pursuit eye movements in the dark produce significant increases in postural sway (Strupp et al., 2003). Although it is well known that vestibular loss affects postural stability, it is less clear whether the loss of the vestibulo-ocular reflex (VOR) itself contributes to balance deficits in people with vestibular hypofunction. Logically it seems that it should - without the VOR, gaze stabilization during head movement is poor. The use of visual cues to help maintain postural stability would not be particularly useful in patients with bilateral vestibular loss because without the VOR, the eyes are not stable during head movement and visual acuity is degraded. Even at a static visual acuity of 20/40, postural stability is decreased. The issue of the degradation of visual cues during head movement coupled with our awareness that people with bilateral vestibular loss have an increased incidence and risk for falls led us to explore the possible relationship between gaze stabilization and fall risk in people with vestibular loss.

Measurement of gaze stability during head movement

One of the primary roles of the vestibular system is to stabilize gaze during head movement. When the eyes are stable in space, the image of the target of visual interest falls on the fovea of the retina and the person sees clearly while the head is moving. Patients with vestibular loss frequently complain of oscillopsia - a visual blurring or jumping of the environment during head movement (Gresty et al., 1977; Chambers et al., 1985; Bhansali et al., 1993). It is a serious problem that can result in decreased activity levels and avoidance of driving with resultant diminished independence, limited social interactions and increased isolation. Studies of people with vestibular loss typically focus on the problems of postural stability and do not address the functional problems associated with poor gaze stability. Recent methodology, a computerized test of visual acuity during head movement (dynamic visual acuity or DVA), controls for periods in which head movement slows and fixation or pursuit eye movements, rather than the VOR, could be used to enable target identification. In the computerized DVA test, a single optotype (the letter E) is displayed on the computer monitor only when horizontal head velocity is between 120 and 180 degree/s (as measured by a Watson rate sensor). The subject is required to identity the orientation of the "E". The reliability of the computerized DVA test has been determined for both normal subjects (intraclass correlation coefficient (ICC) r = 0.87) and for patients with vestibular deficits (ICC r = 0.83; Herdman et al., 1998). Measurement of visual acuity during head movement, therefore, provides the clinician with a way of assessing the functional impact of vestibular loss.

Patients with vestibular hypofunction have a significantly greater decrement in vision during head movements than do age-matched healthy subjects (Fig. 20.2). The computerized DVA test distinguishes between both normal subjects and patients with unilateral vestibular loss and normal subjects and patients with bilateral vestibular loss. There is

Figure 20.2. The relationship between age, vestibular hypofunction and DVA scores (Herdman, unpublished data). Data for age (mean + 1 SD) are plotted on the x-axis and DVA in logarithm of the minimum angle of resolution (log MAR) on the y-axis. DVA scores based on: healthy subjects, n = 83; UVH (affected side), n = 150; BVH, n = 56.

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