The peripheral vestibular system connects directly with the central nervous system for control of posture and eye movements.
Any neuroanatomic disruption to the peripheral and central components will manifest in clinical signs of vestibular dysfunction.
Peripheral pathways: (Labyrinths)
The peripheral components of the vestibular system are enclosed within the vestibule of the inner ear, hence the name. Receptors
for the vestibulocochlear system are contained within a membranous labyrinth that is encased within a bony labyrinth (semicircular
canal, vestibule and cochlea). The membranous labyrinth is composed of the cochlear duct, three semicircular ducts, and the
otolithic organs (utricle and saccule). The cochlear duct contains the major sensory area for hearing, the organ of Corti.
The semicircular ducts, utricle and saccule contain sensory areas for controlling equilibrium. Perilymph, similar in constituency
to cerebrospinal fluid (CSF), separates the boney and membranous labyrinths. The membranous labyrinth contains the fluid endolymph.
Endolymph provides a medium by which the receptors within the semicircular ducts and otolithic organs can detect changes in
Receptors detecting angular acceleration and head rotation are located within the crista ampullaris contained in the ampulla
of each semicircular duct. The vestibular nerve transmits the appropriate signal for rate and direction of rotation to the
central nervous system. Receptors detecting static head orientation and linear acceleration lie within the sensory area (maculae)
of the utricle and saccule. The macula of the utricle lies in the horizontal plane and is important for detecting equilibrium
changes in the upright position. The macula of the saccule lies in the vertical plane and detects equilibrium changes in the
recumbent position. Differences in hair cell patterns within the macula allow for appropriate signal transduction concerning
head orientation with respect to gravity and linear acceleration (primarily the utricle).
Central pathways: (Brainstem)
Vestibular neurons are bipolar with cell bodies located in the internal acoustic meatus of the petrosal bone. The vestibular
nerve enters the brainstem at the level of the rostral medulla and caudal cerebellar peduncle in proximity to CN V and CN
VII and primarily synapses with the vestibular nuclei. The central or brainstem portion of the vestibular system bilaterally
consists of four vestibular nuclei (rostral, caudal, medial and lateral) that also receive input from the spinal cord and
cerebellum. These nuclei have UMN projections to the spinal cord, reticular formation of the brainstem, CN nuclei III, IV,
and VI, and cerebellum. The cerebellum functions closely with the vestibular system to control equilibrium during rapid changes
Neurologic examination findings
An animal with vestibular disease manifests abnormalities in eye position/movement, in head and body position and voluntary
motor movements. The purpose of the neurologic examination is to determine the neuroanatomic localization of the vestibular
disease i.e. peripheral vs. central disease. This is important because central involvement more commonly has a guarded prognosis
and the diagnostic approach will differ. The abnormal neurologic examination findings are summarized below:
- Peripheral: usually alert, disoriented
- Central: Usually abnormal - disoriented, depressed/obtunded
Presence of a head tilt obviously indicates vestibular dysfunction and an asymmetric disease process. The head tilt may vary
in severity and may be a residual deficit with resolution of the disease process. In subtle cases, blindfolding the animal
may accentuate the tilt.
- Peripheral: head tilt ipsilateral to lesion
- Central: head tilt ipsilateral or contralateral to lesion
A head turn to the side of the lesion indicates unilateral disease. If associated with a head tilt, the yaw is vestibular
in origin. (a turn without a head tilt the lesion may be forebrain - aversive syndrome)