58.2.1 Vestibular Physiology

In order to understand how loss of vestibular function and how the treatment of vestibular rehabilitation assists with functional recovery, it will be helpful to briefly review the normal physiology of the vestibular system.s. Literatur There are five end organs housed within the inner ear: the utricle, saccule, horizontal (lateral), superior (anterior), and posterior semicircular canals. The otolith organs (utricle and saccule) sense linear acceleration in the horizontal and vertical planes, respectively, and head tilt (utricle). The semicircular canals sense angular head acceleration. The utricle, superior canal, and horizontal canal are innervated by the superior vestibular nerve. The inferior vestibular nerve innervates the saccule and posterior canal ampulla. One primary function of the vestibular system is maintaining objects of interest steady on the fovea during head movements via the vestibulo-ocular reflex (VOR). As this reflex must be quick and sensitive to keep the visual world in focus during demanding tasks (e.g., running on uneven ground), the system maintains a fast and constant neural firing rate, which is then increased or decreased depending on the direction of head acceleration. This cardinal feature of the system allows the VOR to function on the order of 7 ms.

The non-zero resting firing rate of the vestibular nerve has implications for the expected outcome from nerve dysfunction. A unilateral sudden loss of function will be transmitted to the brain as an asymmetry in peripheral firing rates. This asymmetry is then interpreted as an intense and sustained head acceleration toward the healthy ear. We will use the horizontal canal as an example, and assume that the normal firing rate of the vestibular nerve is 100 spikes per second, which increases up to 400 spikes per second for an ipsilateral head turn, and decreases to 0 spikes per second for a contralateral head turn. Therefore, if one horizontal canal ceases firing, the resting firing rate of the contralateral side, 100 spikes per second, will appear to the brain as a head turn toward the functioning side. This will cause the VOR to drive the eyes toward the nonfunctioning side, and nystagmus will result, with the fast phase (central repositioning of the eye due to a physical limit on rotation of the globe) directed toward the functioning side. As nystagmus is named for the fast phase, the nystagmus driven by the horizontal canal will beat toward the healthy ear. The effects of a sudden loss of vestibular function are summarized in Table 58‑1 .

Loss of unilateral utricular function results in peripheral asymmetry in perception of head tilt. This is because the utricle, which is horizontally oriented, encodes information about the tilt of the head with respect to gravity. Loss of functionality causes a characteristic triad of behavioral responses called the “ocular tilt reaction”: head tilt (to the ipsilateral side), ocular tilt (superior poles of the eyes rotate toward the ipsilateral side), and skew deviation (ipsilateral eye depresses, and contralateral eye elevates). This response occurs because of perceived tilt of the head to the contralateral side. This functionally can result in vertical diplopia.s. Literatur

Bilateral loss of vestibular function, which can occur as a result of bilateral VS in neurofibromatosis type 2, will not cause the sensation of rapid head acceleration. As the peripheral input is symmetrically diminished or absent, there is no percept of vertigo. Instead, bilateral loss manifests with loss of the normal functionality of the vestibular system. Loss of the VOR causes movement of visual objects of interest with head movement; this bobbing of the visual world is termed “oscillopsia.”

Loss of the vestibulospinal reflex results in difficulty in maintaining posture, and the effect is magnified with decreased visual and lower extremity proprioceptive input. Postural instability and ataxia can occur, and the subject typically falls toward the side of the lesion. However, it should be noted that the vestibular system projects to multiple areas of the central nervous system, including the brainstem, cerebellum, reticular formation, thalamus, and cortex; therefore, the effects of vestibular dysfunction can affect not just gaze and posture but also spatial orientation, reasoning, alertness, autonomic function, and mood.

58.2.2 Vestibular Compensation after Unilateral Loss of Function

After a unilateral loss of function, central mechanisms initiate a series of steps aimed at adaptation.s. Literatur The resting asymmetry in firing rates between the vestibular nerves is addressed through the process of static compensation. First, cerebellar clamping occurs, which decreases the resting firing rate on the contralateral (normal) side. This occurs within a few hours of tonic firing of the vestibular nerve, and reduces the asymmetry in firing rates. In order to eliminate the asymmetry, a second stage of compensation occurs in which the ipsilateral firing rate increases (thought to be from commissural fibers from the intact side), and the clamping is further increased on the contralateral side, until the resting asymmetry is either gone or reduced to a subclinical threshold. This ipsilateral increase in firing accounts for “recovery nystagmus,” which is sometimes observed as a peripheral nystagmus with the fast phase directed toward the damaged side. Over time, the cerebellar clamping is tapered off, and the resting firing rate of the ipsilateral side is restored to prelesion levels. Once static compensation is achieved, spontaneous nystagmus is absent, and the percept of resting head rotation is eliminated.

Dynamic compensation is aimed at improving the vestibular response to rapid head movements. VOR gain (ratio of eye movement to head movement) is decreased after unilateral loss of function. This is because only the intact labyrinth can respond to head movement, and with a resting firing rate around 100 spikes per second, there is a floor in the reduction in firing rate that can occur with ipsilateral head turns. Improvements in gaze stability can be achieved through improvements in VOR gain on the ipsilateral side, and with compensatory saccades. Vestibular physical therapy plays an important role in promoting dynamic compensation.