The vestibular system transduces and processes angular and linear acceleration and deceleration of the head, enabling postural balance, locomotor control, and gaze stabilization, particularly during head movement 1-3. This system functions like an inertial guidance system, integrated into a complex multi-sensory interplay between the cortex, cerebellum, brain stem, spinal cord, eye, inner ear and somatosensory inputs 4. It is a bilateral system, with organs on each side of the head and optimal interpretation of stimuli dependant upon input from both the left and right sides. Furthermore, the vestibular system includes central and peripheral components.
The peripheral components include the vestibular labyrinth within the inner ear, containing the three semicircular canals, two otolith organs (e.g., the utricle and saccule), and the vestibular sensory afferent nerve branches. Each of these end organs has a neuroepithelium made of hair cells and supporting cells. The hair cells synapse on vestibular sensory afferent nerve branches, which have cell bodies in the vestibular ganglion. Contributions of the semicircular canals to gaze stabilization and balance are often distinguishable from the contributions of the otolith organs 5-7. Furthermore, different tests are used to distinguish deficits of the canals versus the utricle and saccule end organs 5-8. The fluid dynamics of the semicircular canals detect angular (rotational) accelerations and velocities in 3 dimensions. This enables contribution to the control of head position on the body and the angular vestibulo-ocular reflex (VOR), particularly during high-frequency, high-acceleration rotatory head movements, thus providing compensatory eye and head movements for gaze stability or acuity. The otolith organs are pouches with crystals of calcium carbonate attached to gel membranes overlying the sensory hair cells. These inertial masses aid in detecting gravity and linear accelerations in all planes, and so contribute to the descending vestibulospinal and reticulospinal pathways involved in maintaining postural set 9, 10 and responses to maintain upright head and body postures 11, 12. The otolith organs also contribute to gaze stabilization (linear VOR) and head orientation relative to gravity in static conditions, and during other linear head movements (e.g., side-to-side, up and down). The vestibular membranous labyrinth is filled with endolymphatic fluid, which is responsible for the overall maintenance of vestibular homeostasis and function 3, 13, 14.
The central components of the vestibular system include the vestibular nuclei in the medulla and the cerebellum, which receive 75% and 25%, respectively, of the primary vestibular inputs. Secondary vestibular afferent inputs are distributed ipsi- and contralaterally via projection pathways from the nuclei and cerebellum to the brainstem, oculomotor nuclei, spinal cord and higher central sites. There also is an efferent innervation to the vestibular system, from brainstem neurons. Unlike other special sensory systems (vision, olfaction, hearing, etc.), the information processed by the vestibular system is, for the most part, concerned with automatic, subcortical control. Thus, contribution to conscious perception is limited primarily to perception of verticality and changes in the speed and direction of head movement.
The peripheral and central nervous system components function together to convert and regularly update head position and movement information for processing by the central nervous system for the coordination and execution of basic motor reflexes and/or complex movements of the eyes, head, limbs and trunk 13.
Due to the distinct contribution of the vestibular end organs to two distinct functions, two separate assessments of vestibular system function are proposed. One recommended assessment will focus on performance dependent upon the angular vestibulo-ocular reflex (e.g., gaze stabilization), and the second will focus on performance dependent upon vestibulospinal output (e.g., postural control output, or balance ability). A third assessment will test the integrity of central vestibular system pathways, which is needed for appropriate interpretation of tests of peripheral vestibular function, and to identify a central lesion.
However, it must be noted that vision, somatosensation and motor system integrity, in addition to vestibular system function, are determinants of gaze stability and balance. Furthermore, development of appropriate and successful interventions for these functional deficits is dependant upon accurate identification of the related impairment (e.g. vestibular, motor or other sensory). Therefore, it is important that the tests selected are valid and sensitive to the identification of vestibular gaze stability and vestibular balance.
The imbalance and disequilibrium impairments are of particular note since: 1) in individuals 65 years of age and older, falls are the leading cause of death from injury (National Center for Injury Prevention and Control, Division of Unintentional Injury Prevention, March 2000) 15, and 2) children with vestibular deficits since birth or early childhood, which is common in children with sensorineural hearing loss or chronic otitis media with effusion, have a progressive deficit in motor and balance development 16-25. Although rehabilitation has been shown to facilitate recovery of vestibular related impairments of balance, its success is dependent upon appropriate identification of the vestibular or other involvement.
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