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Vestibular System

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 System, Vestibular transduction, Peripheral disorders of the vestibular system, Central vestibular system and vestibular reflexes, Disorders of central vestibular pathways

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.