Vestibular Pathophysiology Flashcards
what are the two labyrinth organs
1) semicircular canals
2) otolith organs
what stimulates the semicircular canals?
stimulated by angular acceleration (dynamic equilibrium)
- gives signal of approx angular velocity
what stimulates the otolith organs?
stimulated by linear acceleration and gravity force
gives signal of head acceleration and tilt
what are 5 uses of the signals produced by the labyrinth organs?
- control balance
- provide spatial reference for other motor actions
- provide compensatory reflexes
- provide proprioception
- tune CVS for re-orientations
what is the result of dysfunctional control of balance and spatial reference ?
ataxia
“instability of gait and posture”
what is the result of a loss of compensatory reflex production?
total loss and unilateral loss
a total loss leads to oscillopsia (objects oscillate)
unilateral loss leads to nystagmus (involuntary eye movement)
what is the result of proprioception dysfunction”
dizziness
what is the result of not tuning the CVS to re-orientations?
hypotension
3 types of vestibular disorders?
structural- destructive or irritative disease
functional- misinterpretation of sensory input, maladaptation, loss of rules of correspondence
both- structural disorder provoking chronic dysfunction
6 types of disruption to the vestibular system
1) vertigo
2) vestibular ataxia: instability of gait and posture
3) vestibular nystagmus
4) oscillopsia
5) motion sickness
6) acute phase of vestibular loss
what is vertigo?
time scales of vertigo
false perception of movement in space.
- Can last anywhere between seconds and minutes
- most common possibly due to vertebrobasilar insufficiency (migraine)
- Seconds due to BPPV
- Hours of vertigo caused by Meniere’s syndrome.
how would you differentiate the causes of vestibular nystagmus and oscillopsia?
VN: caused by unilateral vestibular lesions
Osc: caused by Bilateral vestibular lesions
what is motion sickness?
loss of co-ordination on directional reorientation, oversensitivity to visual motion in the environment.
what is acute phase of vestibular loss?
slight impairment of orthostatic control –> severe nausea and vomiting.
what are the two components of the otolith organs?
utricle (horizontal)
saccule (vertical)
omnidirectional organs
used in static equilibrium i.e. linear acceleration
what is the function of the utricle
senses movement in the horizontal plane
what is the function of the saccule
senses movement in the vertical plane
what features of the otolith leads to the pulling of hair cells?
otoconia (layer of calcium carbonate) sits on a gelatinous layer
this is heavy so movements of the head displace the otoconia and therefore pulls hair
linear acceleration all moves the heavy otoconia layer
movement in reaction to kinocillium
movements towards it leads to stimulation of the hair cells (there is one per hair cell)
when there is movement away from it, the cell is not stimulated
what do the 3 semi-circular canals link to?
what does this structure contain?
they connect to an ampulla
the ampulla contains a capulla full of hair bundles
what needs to happen to the ampulla to fire off APs in from the hair cells?
endolymph needs to be displaced in the canals
this moves the ampulla and fires of the APs
what is the effect of linear acceleration on the ampulla in terms of net movement?
there is an equal force on each side of the ampulla
so there is no net movement
how does depolarisation occur in the ear in the direction which the head turns into?
what happens in the opposite ear?
with a left turn, the movement is towards the kinocilium in the left ear, but it is away from the kinocilium in the right ear.
There will be depolarisation in the left ear as hair cells are stimulated.
There will be hyperpolarisation (inhibition) in the right ear
process of AP firing at rest
constant tonic firing at rest
pathway of vestibular projections
via the thalamus into tempera-parietal cortex
what is the role of the vestibular projections
regulate perception of motion in space
what happens to signalling in an unilateral lesion?
the tonus of the intact canal gives a signal as if the head is rotating to the intact side
the other side has lost its intrinsic tone
Benign, Paroxysmal Positional Vertigo (BPPV)
Debris causing false stimulations:
Accounts for a 1/3rd of all incidences of vertigo and is caused by otoconial debris in the canals.
BPPV is TRANSIENT and provoked by head movement.
Debris floating in the canals stimulates the ampulla causing false signals of head rotation – vertigo.
–> Treated by vigorous head rotation to flush debris.
what is the mechanism in nystagmus
Acute UNILATERAL vestibular disorder:
- the unopposed tone of the intact canal causes the eyes to be driven to the lesioned side (vestibular-ocular reflex).
- The movement is detected by the brainstem which then corrects eye position with fast movements.
- effect minimised by visual suppression mechanisms
pathway of the vestibular-ocular reflex
Superior and medial vestibular neurons project to motor nuclei supplying the extraocular muscles:
1) Axons ascend in the MLF (Medial Longitudinal Fasciculus)
2) excite the contralateral abducens (CN VI) nucleus
3) excite ipsilateral oculomotor (CN III) nucleus
4) compensatory eye movement against the direction of movement
abducens nucleus in the pons
oculomotor nucleus in the midbrain
what is the purpose of the vestibular-ocular reflex
acts to maintain gaze on a selected target
when the head rotates left, the eyes rotate right
therefore continuous head rotation causes a physiological vestibular nystagmus
what is the mechanism of oscillopsia
Loss of vestibular function impairs eye stabilisation during rapid head movements as the VOR is the only mechanism to drive fast compensatory reflex.
how is oscillopsia diagnosed?
head shaking test- so while the head oscillates, the eyes are monitored
2 types of vestibular function loss in oscillopsia
attempt to correct with catch-up cascades
1) bilateral loss- eyes taken off target by head swing, multiple catch-up saccades made to regain target
2) unilateral loss- eyes remain on target on intact side, while multiple saccades occur in rotation towards lesioned side
2 types of vestibular ataxia
1) Bilateral – mild gait ataxia that is worse at speed, when negotiating rough ground or when vision is reduced.
2) Unilateral – tendency for the body and head to lean and fall to the lesioned side which becomes pronounced in difficult balancing situations.
lateral vestibulo-spinal tract pathway
descends ipsilaterally in ventral funiculus of spinal cord. Axons terminate in lateral ventral horn and influence motor neurons to the limbs.
describe the medial vestibule-spinal tract pathway
descends bilaterally in medial longitudinal fasciculus to cervical and upper thoracic spinal cord.
Axons terminate in medial ventral horn and influence motor neurons to neck and back.
The lateral VS tract reaches lower parts of the spinal cord
what happens in calorics?
Thermal stimulation of individual horizontal canals of the labyrinth.
Temperature differences between the liquid inserted and the temperature of the endolymph causes convection currents that stimulate the ampullary hair cells.
Warm water stimulates that side
what are the caloric results?
o 30C (colder than BT)
- stimulation of opposite ear
- tonus of opposite canal drives eyes ipsilateral initially (slow phase)
- nystagmus to opposite side (fast phase)
o 44C (warmer than BT)
- stimulation of ipsilateral canals excited
- eyes driven contralateral initially (slow phase opposite to direction of stimulation)
- nystagmus to the ipsilateral side (fast phase towards stimulation)
calorics measures
nystagmus duration, peak velocity and subjective sensation.
how is calorics interpreted?
o Small amplitude, short duration to both hot and cold ipsilateral – canal paresis.
o Bilateral small short responses – bilateral hypofunction.
o Asymmetry of hot/cold responses – of little significance/normal.
symptoms of acute vestibular lesions
Ipsilateral tilt
attempted compensation and/or an ipsilateral head tilt and skew
treatment of acute vestibular lesions
- Reassurance of patient
- drugs (steroids, anti-virals etc)
- treatment of associated anxiety e.g. cognitive behaviour therapy with desensitisation and physiotherapy, behavioural anxiolytic tactics
- minimise risk factors
primary symptoms of motion sickness
pallor
sweating
nausea
vomiting.
associated symptoms of motion sickness
headache dizziness nystagmus instability restlessness drowsiness feeling of eye strain.
treatment of motion sickness
Cognitive behaviour therapy with desensitisation in SMALL doses
behavioural anti-emetic
anxiolytic (reduce anxiety) exercises.
