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1. Why are habituation exercises not recommended for bilateral vestibular hypofunction? Would they be recommended if there is still some residual function?
2. Regarding the RCT on slides 133-135 in the lecture, the study showed significant improvement in DVA after vestibular rehab for pts with bilateral hypofunction. Why does it say on the next slide that these improvements may not be due to vestibular function improvement? Is vestibular improvement not expected with bilateral vestibular hypofunction which is why substitution exercises are recommended?
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1. Habituation exercises are inappropriate for patients with bilateral vestibular loss, because they are designed to decrease unwanted responses to vestibular signals rather than to improve gaze or postural stability (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3259492/). Adaptation exercises, on the other hand, may be beneficial if there is some residual vestibular function. Adaptation exercises aim to improve gaze stability while habituation exercises aim to desensitize someone to provocative motions.
2. In this study, the 7 of 8 subjects with bilateral vestibular hypofunction had improvements in their dynamic visual acuity and 5 of 8 returned to their age-referenced range for DVA. However, none of them had improvements in their VOR gain as measured by the rotary chair test. From this, we can conclude that adaptation exercises do not change the vestibular system itself and changes are likely due to central programming/neuroplastic changes.
The authors phrased it like this: “We do not think that this improvement in DVA reflected a change in vestibular function. We found no evidence of a change in VOR gain as measured by rotary chair testing…A second mechanism for improvement in DVA may be the use of central preprogramming of other types of eye movements to improve gaze stability.19-21 We have previously demonstrated that subjects with unilateral vestibular hypofunction have better visual acuity during self-generated head rotation than during unpredictable head movements.14 This suggests that central programming of eye movements may contribute to gaze stability during predictable head movements. Centrally programmed eye movements have been described in patients with peripheral vestibular hypofunction and include preprogrammed saccades that occur during the head movement as well as high-velocity, slow-phase eye movements (velocities of 80°/s to 120°/s)”
This is similar to stroke recovery. Following stroke, there are no changes to the core ischemic zone. Improvements happen because other areas of the central nervous system adapt to take over the lost function.
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