Physical exercise and balance compensation after total ablation of vestibular organs

Vestibular compensation: Neural mechanisms and clinical implications for the treatment of vertigo

After unilateral peripheral vestibular lesions, the neural activity of neurons in the ipsi-lesional medial vestibular nucleus (ipsi-MVe) are markedly decreased, resulting in static and dynamic asymmetries of the vestibulo-ocular and vestibulo-spinal reflexes. Consequently, static vestibular symptoms such as spontaneous nystagmus and postural deviation and dynamic vestibular symptoms such as oscillopsia and swaying gait are induced. However, these behavioral asymmetries gradually recover after the lesion. Progressive balance restoration is termed vestibular compensation, which is divided into two phases: static and dynamic. Static vestibular compensation is further divided into initial and late processes. In the initial process of static vestibular compensation after unilateral labyrinthectomy (UL) in rats, plastic changes in the cerebello-vestibular and vestibular commissural inhibitory pathways suppress neurons in the contra-lesional MVe (contra-MVe), resulting in the restoration of symmetrical resting activity of MVe neurons on both sides at low levels. The declining frequency of spontaneous nystagmus after UL is an index of the initial process, and short-term administration of diazepam, a GABAA receptor agonist, has been shown to accelerate the initial process in rats. Accordingly, short-term administration of diazepam is recommended for the treatment of acute vertigo in patients with unilateral vestibular dysfunction. In the late process of static vestibular compensation after UL in rats, the resting activity of ipsi-MVe neurons gradually recovers due to changes in cell membrane properties, resulting in the reinforcement of balanced intervestibular nuclear activities to nearly normal levels without the suppression of contra-MVe neurons. The declining number of MK801-induced Fos-positive neurons in contra-MVe after UL is an index of the late process, and long-term administration of betahistine, a histamine H3 receptor antagonist, has been shown to accelerate the late process in rats. Accordingly, long-term administration of betahistine is recommended for the treatment of subacute vertigo in patients who were not compensated for unilateral vestibular dysfunction. In the process of dynamic vestibular compensation after UL, the sensitivity of ipsi-MVe neurons to head velocity and acceleration is restored due to synaptic changes such as long-term potentiation and sprouting of commissures, resulting in the restoration of the dynamic vestibulo-ocular and vestibulo-spinal reflexes. To facilitate dynamic vestibular compensation, early ambulation and subsequent vestibular rehabilitation exercise are recommended for the treatment of chronic vertigo in patients with uncompensated unilateral vestibular dysfunction. Although vestibular compensation after bilateral vestibular loss is not expected, vestibular rehabilitation with a sensory-substitution strategy can improve imbalance in patients with bilateral vestibular lesions.

Effects of Vestibular Rehabilitation on Dizziness and Imbalance

Vestibular rehabilitation is a specific approach to physical therapy aimed at reducing dizziness and imbalance by facilitating central nervous system compensation for peripheral vestibular dysfunction. This article reports preliminary results of studies concerning the relative effectiveness of vestibular rehabilitation, general conditioning exercises, and vestibular suppressant medication on dizziness and imbalance in patients with chronic vestibular symptoms of at least 6 months duration. Patients with positional and/or movement-related dizziness and abnormal posturography were randomly assigned to the three treatment groups. Preliminary results suggest that although all three treatment approaches reduce dizziness, only vestibular rehabilitation also improves balance. This study takes the first step toward determining the efficacy of a specific exercise approach for reducing dizziness and imbalance in patients with chronic peripheral vestibular disorders.

Effects of Vestibulo-ocular Reflex Exercises on Vestibular Compensation after Vestibular Schwannoma Surgery

OBJECTIVE

To assess vestibular function in a large group of vestibular schwannoma patients so that we could determine whether simple vestibular exercises speed vestibular dysfunction recovery after tumor removal surgery.

STUDY DESIGN

A prospective investigation of the vestibular dysfunction experienced by patients in the first 12 weeks after surgery.

SETTING

Vestibular investigation unit at a tertiary referral institution.

PATIENTS

Sixty-five patients with identified vestibular schwannoma referred for preoperative vestibular investigations. Thirty-two men and 33 women, with a mean age 51 years (range, 24-77 yr).

INTERVENTIONS

There were 27 control patients, 30 exercise patients, and 8 patients that had balance physiotherapy. Exercise patients began simple vestibulo-ocular reflex gaze stabilization exercises 3 days after surgery.

MAIN OUTCOME MEASURES

Postoperative vestibular function testing was performed at 2 to 3, 6 to 7, and 10 to 12 weeks after surgery. Objective measurements of vestibular compensation status were as follows: spontaneous nystagmus and sinusoidal harmonic acceleration asymmetry and gain values. Dizziness Handicap Inventory questionnaires were used to assess subjective perceptions.

RESULTS

The main findings were reduced dispersion in vestibulo-ocular reflex asymmetry at 2 to 3 weeks, reduced mean in asymmetry at 6 to 7 weeks, less dizziness/imbalance according to the Dizziness Handicap Inventory questionnaire, and that preoperative caloric tests did not predict postoperative severity of vestibular systems.

CONCLUSION

This large study provided unique evidence that a program of simple vestibular exercises and education can speed the rate of compensation after vestibular schwannoma surgery.

Comparative review of the human bony labyrinth

The bony labyrinth inside the petrous part of the temporal bone houses the organs of hearing and balance. Being functionally linked with sensory control of body movements and located in a part of the basicranium that is closely associated with the brain, this structure is of great interest in the study of human evolutionary history. However, few aspects of its morphology have been studied in nonhuman primates. This review compares the bony labyrinth of humans with that of the great apes and 37 other primate species based on data newly acquired with computed tomography combined with previous descriptions. With body mass taken into account, consistent differences are found between the size of the semicircular canals in humans, the great apes, and other primates. In particular, the arcs of the anterior and posterior canals are larger in humans than in the African apes. The functional implications of semicircular canal dimensions for registering angular head motion are evaluated in relation to locomotor behavior. Biophysical models, comparative studies, and some neurophysiological experiments all support a link between semicircular canal size and agility, or more specifically the frequency contents of natural head movements, but the evidence on the exact nature of this link is ambiguous. It is concluded that any link between the characteristic dimensions of the human canals and locomotion will be more complex than a simple association with the broad categories of quadrupedal vs. bipedal behavior. The functionally important planar orientations of the semicircular canals are similar in humans and the African apes as well as in many other species. In contrast, other aspects of the human labyrinth differ markedly in shape, following a pattern that seems to reflect the characteristic architecture of the human basicranium. Indeed, it is found that labyrinthine and basicranial shape are interspecifically correlated in the sample, and in most respects the human morphology is consistent with the general trend among primate species. Differences in brain growth and development are proposed as the predominant factor underlying both the unique shape of the human labyrinth as well as the interspecific labyrintho-basicranial correlations.

Programmatic vestibular rehabilitation

Although the use of vestibular exercises for patients with persistent vertigo and dysequilibrium has received some attention for many years, organized vestibular rehabilitation therapy programs have only recently been introduced. The benefits of such programs are becoming widely accepted. They typically involve a three-pronged approach: habituation exercises designed to facilitate central nervous system compensation by extinguishing pathologic responses to head motion, postural control exercises, and general conditioning activities. This article will describe the programmatic approach to vestibular rehabilitation, emphasizing selection criteria and reviewing results that may be anticipated when this treatment modality is used in a variety of patient populations.

Comparative study on gait abnormality in patients with vestibular neuronitis and patients with large acoustic neuroma

A comparative study on gait analysis comprising 11 patients with vestibular neuronitis and 10 patients with large acoustic neuroma was undertaken by the use of foot switches and electromyography. Central disorders in the neuroma group were evidenced by neurotological examinations. Fourteen healthy adults served as controls. Variables employed for the analysis were time from heel strike to forefoot strike (HA-I), time from heel off to forefoot off (HA-II) as well as duration of stance, swing and double supports (DS), location of maximum contraction of the gastrocnemius during stance, and location of the first and second peak of muscle contraction of the tibialis anterior from early swing phase (TA-off) to early stance phase (TA-on). The occurrence rate of abnormality of HA-I was the highest in both groups and most of the variables showed a higher rate in the large acoustic neuroma cases. In addition, a significantly higher CV value was obtained in HA-II of the lesion side's foot in the AN group. These changes could reflect the pathophysiological difference in the gait control systems between the two groups.

Medial vestibular neurons are endogenous pacemakers whose discharge is modulated by neurotransmitters

1. Neurons in the medial vestibular nucleus (MVN), recorded in a rat brain slice preparation, exhibit a highly regular, high-frequency (5- to 35-Hz) spontaneous discharge. The rhythmic firing rate was constant (< 5% variation) and sustained for a long time (maximum observation, 4 hr). 2. The rhythmic firing was evident even in neurons (n = 15) completely isolated from exogenous input fibers, suggesting that it is due to an endogenous pacemaker property. When recorded intracellularly, the discharge was found to be associated with a smooth, concave pacemaker prepotential, and the rate of firing was reduced in proportion to applied hyperpolarizing current, indicating that these are pacemaker discharges. 3. This conclusion is supported by the observation that perfusion with a low-calcium/high-magnesium Krebs-Ringer solution, which completely and reversibly blocks all synaptic transmission, did not abolish the spontaneous discharge. The low-calcium/high-magnesium solution increased spontaneous firing in some neurons and decreased in others, suggesting that the firing is synaptically modulated and the synaptic influence is tonically active. 4. Application of kynurenate (10 mM), an antagonist of the excitatory amino acid receptors, gradually reduced neuronal discharges in most neurons (22 of 25), while the addition of 10 mM sucrose as an osmotic control had no effect. Depression of neuronal discharges reached its minimum (an average of 60% of the control level) and was maintained at that level until gradually washed out. The rhythmic firing pattern persisted in all neurons even after the excitatory receptors were blocked. 5. When the GABAA receptor antagonist, bicuculline (20 microM), was applied, elevation of neuronal discharges was evident in most neurons (30 of 32) tested. The firing increased gradually, with a final control level of 130% (121-160%). In contrast, the GABAB receptor antagonist, phaclofen (20 microM and 100 microM), had no effect in most neurons (19 of 23) tested. Further, the excitatory and inhibitory action could be detected on the same neuron when bicuculline and kynurenate were both evaluated (n = 10). 6. These results indicate that the spontaneous discharge of MVN neurons is due to an endogenous pacemaker under the tonic influence of both inhibitory and excitatory transmitter actions. The bicuculline-sensitive GABAA receptors and the kynurenate-sensitive glutamate receptors both mediate the tonic modulation.