Variables in Vestibular Compensation
We have been discussing the process of vestibular compensation where the brain responds (or adapts) to a change in labyrinthine function and works to resolve the conflict between the two labyrinths and to restore visual clarity while moving. This describes only the impact on the Vestibular Ocular Reflex (VOR), but after a labyrinthine injury other parts of the body are also responding. Let’s revisit the definition of vestibular compensation outlined in the first post of the series:
Vestibular Compensation consists of the entire repertoire of strategies used by a patient to reduce symptoms. These strategies may include adaptation, substitution, alternative predictive, cognitive or avoidance strategies.
The first variable to consider (and it’s the big one) is whether the patient has normal cerebellar function. If the cerebellum is compromised by age, injury, or medication, the recovery process will be compromised. If there is a lesion in the neural pathway between the two vestibular nuclei, adaptation and recovery of VOR function may be impossible.
When the brain recognizes a change or weakness in the VOR, it may decide to use inputs other than the ears as the primary reference for balance. It is not unusual for people with chronic inner ear weakness to become more dependent on their tactile sense and/or visual feedback. This is described as substitution, and relies on intact visual and somatosensory feedback. Depending on the health status of the patient prior to a vestibular injury, this might not work. For example, someone with neuropathy of the lower extremities who suffers a vestibular insult may become almost entirely dependent on their vision for balance, while at the same time their vision is compromised when they move around.
When a vestibular injury such as that suffered with a bout of vestibular neuritis occurs, it usually does not completely wipe out the function on the affected side, and the unaffected side is typically functioning normally. It is not unusual for the patient to require therapy not only to enhance recovery of the VOR, but also to learn to rely again on the vestibular systems as a contributor to overall balance.
Lastly, it is important to remember that the brain is working overtime to make up for the weakened vestibular response. When the brain is well rested and functioning efficiently, it will manage this effectively. Anything that disrupts the brain’s efficiency can result in return of vestibular type symptoms, even though nothing has changed in the ear. So, a poor night’s sleep, a head cold, some temporary stress or anxiety, or a new medication (particularly one that affects the central nervous system) can all trigger a period of DE-compensation, with a return of symptoms.
