Session 12 - Hearing and Vestibular

Describe the vestibular system

A set of CNS neurons that: contribute to conscious orientation in space and mediate reflex adjustments for the maintenance of equilibrium and visual acuity during head movement

What are the 3 key functions of the vestibular system?

Motion detection, Maintenance of Balance/Equilibrium, Gaze stability

What are otolith organs made up of?

Utricle and saccule maculae

What does the otoloith organ detect?

Linear motion

What weighs down the otolithic membrane?

Crystals of calcium carbonate called otoconia

The saccular macula has a _____ orientation and responds to linear acceleration such as _______

Vertical; riding in an elevator

The utricular macula has a ______ orientation and responds to linear acceleration such as ______

Horizontal; riding in a car

What type of motion does the semicircular canal detect?

Rotational motion

In the semicircular canals, what deflects the hair cells to cause sensation?

Movement of endolymph

Where do cell bodies of the vestibular system reside?

Vestibular ganglia, or ganglia of Scarpa

How many vestibular nuclei are there?

4 pairs located in the lateral pontomedullary junction

Describe the Commissural Inhibitory System

Vestibular nuclei of each side reciprocally interact by way of commissural fiber system; activity in one side of the nuclear complex results in inhibition of the opposite side

Describe the projections of the vestibular system

From vestibular nuclei, fibers ascend to thalamic nuclei and on to the cerebral cortex to the bilateral parietal and insular areas

What is Subjective Visual Vertical

A diagnostic test of the inner ear to assess a patient’s perception of verticality and detect if there are signs of an abnormal tilt that can cause dizziness or vertigo

Describe the vestibular motor system

Lateral and Medial vestibular nuclei give rise to 2 upper motor neuron descending tracts

What are the 2 vestibular motor tracts

Lateral vestibulospinal tract and Medial vestibulospinal tract

What does the lateral vestibulospinal tract do?

Excitatory to ipsilateral spinal cord nuclei; facilitates and maintains extensor tone in limbs and trunk

What does the medial vestibulospinal tract do?

Descend to cervical and upper thoracic spinal cord to stabilize and regulate head movements for fixation of gaze

Describe the vestibular system’s projections to the reticular formation

All 4 pairs of vestibular nuclei send bilateral projections to pontomedullary areas of brainstem RF, 

Describe the vestibular system’s projections to the cerebellum

Primary vestibular afferents terminate in the flocculonodular lobe of the cerebellum, all 4 nuclei send projections to the vestibulocerebellum

Vestibulocollic reflex example:

Chicken; maintenance of head position while body is moving

Describe vestibular system’s projections to extraocular motor neurons

Projections from the superior and medial vestibular nuclei project to nuclei of CN 3,4,6 to assist in regulation of extraocular eye muscle contractions

Vestibulo-occular reflex

Equal and opposite head and eye movement

What is vestibular nystagmus

Rhythmic, conjugate deviations of the eyes consisting of a slow phase on one direction and fast phase in the other; induced by deflection of the capula when rotation starts or stops after adaptation has occurred

Nystagmus beats toward which direction

Toward the healthy ear

What is caloric testing?

Vestibular system is activated through warm or cold air/water to elicit nystagmus

What is the cardinal sign of vestibular dysfunction?

Vertigo; any subjective or objective illusions of motion or position

Autonomic signs of vestibular dysfunction

Nausea, vomiting, pallor, perspiration, blood pressure drop, tachycardia

Peripheral vestibular disease would affect what

Pathology of the labyrinth (inner ear) or vestibular nerve

Central vestibular disease would affect what

Pathology of the vestibular nuclei (caudal pons), their projections or central sites of termination

How do we answer if the vestibular disease is central or peripheral?

H.I.N.T.S + examination: 

Head impulse test

Nystagmus observation

Test of skew

+ sudden onset of unilateral hearing loss

What is Benign Paroxysmal Positional Vertigo?

Common cause of vertigo due to macular debris being displaced into a semicircular canal, with rapid changes in head position, symptoms occur

How do you diagnose Benign Paroxysmal Positional Vertifo

Dix-hallpike (anterior/posterior canals) or Supine roll test (horizontal canal)

How do you treat Benign Paroxysmal Positional Vertigo

A series of specific head movements called canalith repositioning maneuvers (CRM)

Describe vestibular neuritis

Unilateral peripheral vestibular disorder due to inflammation of vestibular nerve; diminished or absent response to caloric testing of horizontal canal on affected side

With vestibular neuritis what would a Head Impulse Test look like?

Positive Head Impulse Test towards the side of the lesion

What is meniere’s disease

Episodic syndrome that affectes hearing and equilibrium, thought to be caused by changes in endolymph in the inner ear

Describe attacks of Meniere’s disease

Abrupt and characterized by sensorineural hearing loss, tinnitus, aural fullness, nausea, and vomiting

What are central causes of vertigo, nystagmus, and disequilibrium

Ischemic stroke of brainstem or inner ear

Cerebellar lesions may directly or indirectly apply pressure on the brainstem and the vestibular nuclei