Vestibular System
The vestibular system is concerned
with balance, postural reflexes and
eye movements, and is one of the oldest systems of the brain. It consists of a
peripheral transducer component which projects to the brainstem (including the
oculomotor nuclei), and from there to the thalamus and sensory cortex as well
as to the cerebellum and spinal cord. Disruption to the system (e.g., vestibular
neuronitis/labyrinthitis) results in the symptoms of dizziness,
vertigo, nausea with or without blurred vision with signs of eye movement
abnormalities (typically nystagmus; see Chapter 56) and unsteadiness. In the
comatose patient, clinical testing of the vestibular system can provide useful
information on the integrity of the brainstem, as it is associated with a
number of primitive brainstem reflexes (see Chapter 44).
Vestibular transduction
The peripheral transducer component
consists of the: labyrinth, which is made up of two otolith organs (the
utricle and the sacculus) together with the ampullae located
in the three semicircular canals. The otolith organs are primarily
concerned with static head position and linear acceleration while the
semicircular canals are more concerned with rotational (angular) acceleration
of the head. Hair cells are found in both the otolith organs and the
ampullae and are similar in structure to those found in the cochlea (see
Chapters 23 and 27). As in the cochlea, deflection of the stereocilia towards
the kinocilium depolarizes the cell and allows transmitter to be released from
the hair cell, leading to activation of the associated afferent fibre. The
converse is true if the stereocilia are deflected in the opposite direction.
Movement of the cilia is associated
with rotational movement of the head (ampullae receptors in the semicircular
canals) and acceleration or tilting of the head (otolith organs in utricle), as
although head movement causes the endolymph bathing the hair cells to
move, it ‘lags behind’ and so distorts the stereocilia.
Spontaneous activity in the
afferent fibres is high, reflecting the spontaneous leakage of transmitter from
the cell at the synapse. Hyperpolarization of the hair cell therefore results
in a reduced afferent discharge, while depolarization is associated with an
increase in firing. Efferent fibres from the brainstem terminating on the hair
cells can change the sensitivity of the receptor end-organ.
Peripheral disorders of the
vestibular system
Damage to the peripheral vestibular
system is not uncommon. Examples include:
•
Benign
paroxysmal positional vertigo (BPPV) commonly occurs after trauma or infection of the
vestibular apparatus with the deposition of debris (e.g. otolith crystals or
otoconia) typically in the posterior semicircular canal. This condition, which
is characterized by paroxysms of vertigo, nausea and ataxia induced by turning
the head into certain positions (such as lying down or rolling over in bed), is
therefore the consequence of distortion of endolymph flow in this canal secondary
to the debris. It is diagnosed using Hallpike’s manoeuvre, which seeks to
manipulate the head in such a way as to provoke the episode of vertigo.
Treatment and cure can be effective by undertaking a series of head manoeuvres
(classically Epley’s manoeuvre), which allows the debris to fall out of the semicircular canal and into the
ampullae.
•
Viral
infections of the vestibular apparatus are common (laby- rinthitis)
and can be severely disabling with profound dizziness and vomiting without any
head movement. Such infections are usually self-limiting.
•
Bilateral
failure of the vestibular apparatus can result in oscillopsia, a
symptom describing an inability to visually fixate on objects especially with
head movements (see Chapter 56). In contrast, powerful excitation of the
vestibular system, such as that encountered during motion sickness produces
dizziness, vomiting, sweating and tachycardia, caused by discrepancies between vestibular
and visual information.
Vestibular function can be tested
by introducing water into the external meatus (caloric testing).
When warm water is applied to a
seated subject whose head is tilted back by about 60°, nystagmus towards the
treated side is observed.
Cold water produces nystagmus
towards the opposite side. These effects reflect the changes in the temperature
of the endolymph and an effect resembling head rotation away from the irrigated
side.
Central vestibular system and
vestibular reflexes
Afferent vestibular fibres in the
eighth cranial nerve have their cell bodies in the vestibular (Scarpa’s)
ganglion and terminate in one of the four vestibular nuclei in
the medulla, which also receive inputs from neck muscle receptors and the
visual system.
The vestibular nuclei project to:
•
The Spinal
Cord (See Chapters 9, 37 And 40);
•
The
Contralateral Vestibular Nuclei;
•
The
Cerebellum;
•
The
Oculomotor Nuclei;
•
And The
Ipsilateral And Contralateral Thalamus.
Some of these structures are
important in reflex eye movements, such as the ability to maintain visual
fixation while moving the head – the vestibulo-ocular reflex (VOR; see Chapters
40, 49 and 56). Other projections of the vestibular nuclei are important in
maintaining posture and gait. The cortical termination of the vestibular input
to the CNS is the primary somatosensory cortex (SmI) and the posterior
parietal cortex (see Chapter 34). Very rarely, epileptic seizures
can originate in this area and give symptoms of vestibular disturbance.
Disorders of central vestibular
pathways Caloric testing of
the vestibular system examines the integrity of the vestibular apparatus and
its brainstem connections. Therefore, it can be useful in comatosed patients
when the degree of brainstem function needs to be ascertained. Less severe
central damage to the vestibular apparatus can occur in a number of conditions
including multiple sclerosis (see Chapter 62) and vascular
insults (see Chapter 64). In most cases other structures are involved and
so there are other symptoms and signs on
examination.
