1.2 Vestibular system Flashcards
The peripheral vestibular system lies deep in the temporal bone, and consists of the three semicircular canals and the __________________ on each side:
• Involved in transduction of head movement (detects head _______________ → sends signals to brainstem via vestibular nerve (CN VIII))
• Information is passed to different parts of the CNS, and promotes reflex functions (e.g. vestibulo-ocular reflex, vestibulo-spinal reflex) and perception of sensations (e.g. updating position in a dark room → especially for turning when walking)
• Linked to autonomic pathways (cardiorespiratory drive/motion sickness)
otolith apparatus (saccule & utricle);
acceleration and deceleration;
The 3 semicircular canals (superior, horizontal, posterior) are arranged perpendicular to each other (covers the 3 principle axes of rotation), and detect ____________________
• Contains endolymph and an enlargement (ampulla) at one end → ampulla contains vestibular hair cells (on the ____________) covered with a gelatinous mass (___________)
• Cupula has the same specific gravity as the endolymph → displaced during angular acceleration of the head to cause excitation or inhibition of hair cells
- Superior: Along _____________
- Horizontal: Along _____________
- Posterior: Along ________________
angular acceleration;
crista;
cupula;
sagittal plane (as in nodding) ;
transverse plane (as in shaking head);
coronal plane (as in touching ears to shoulders)
The vestibular hair cells differ from the auditory hair cells as they possess a _________________ and a cluster of stereocilia:
• Kinocilium is the longest cilium located on the hair cell next to a cluster of 40 – 70 stereocilia → determines direction of stereocilia displacement in order for axon depolarisation to occur
• Uses a similar mechanoelectrical transduction mechanism to the cochlea
large kinocilium
When the head is rotated, the semicircular canals and their ___________ also rotate:
• Cupula (anchored to the crista) moves before the endolymph (“lags” due to inertia) → dragged through the endolymph → deflection of the stereocilia on the hair cells
• Endolymph only begins to move as ________________ → eventually catches up (no change in axonal firing)
There is always a tonic signal in the vestibular nerve even at rest → deflections of stereocilia only cause _____________________
- Towards kinocilium: Increase (___________ of hair cells)
- Away from kinocilium: Decrease (_______________ of hair cells)
attached cristae;
rotation continues to occur;
changes in the nervous impulses:;
depolarisation;
hyperpolarisation
When the head stops moving, the endolymph continues to move (due to _____________), and the events which occur during acceleration occur in reverse:
• Continued movement of endolymph pushes the _______________ in the opposite direction → reversal of polarisation state of hair cells and axonal firing
• When the head stops rotating left, the ________ is inhibited, and ____________ is excited
inertia;
cupula and kinocilia;
left canal;
right canal
The otolith organs (sacculus and utricle) detect ___________________ (pure accelerometers which detect net acceleration):
• Otolith masses (composed of ______________________) overlie the vestibular hair cells in the sacculus and utricle
• Gravitational forces (present all the time) act on the otolith mass and moves it across the vestibular hair cells, causing movement of stereocilia
- Towards kinocilium: Increase (depolarisation of hair cells)
- Away from kinocilium: Decrease (hyperpolarisation of hair cells)
tilt and linear acceleration;
mucopolysaccharides and calcium carbonate crystals
When the head is upright, the _________________ is oriented horizontally, and that of the __________ is oriented vertically:
• Bilateral arrangement of otolith organs allows every possible orientation of the head to be encoded by excitation/inhibition of the vestibular hair cells
• Each position of the head has a unique pattern of activity from the afferent nerves innervating the organs → detailed information about the position of head in space
Utricle
- _____________ of head → excitation of ipsilateral utricle
- ________________ tilting of head → inhibition of ipsilateral utricle
Saccule
- Responds to movements in all directions:
• “Pitch” (forward/backward) and “roll” (lateral/medial) as well as up/down movements of the head
macula of the utricle;
saccule;
Forward/lateral tilting;
Backward/medial;
When acceleration is quick (e.g. in a jet aircraft), the otoliths summate the vertical and horizontal vectors as ___________________________
• Only measure net acceleration (aeroplane → net acceleration = diagonal acceleration)
• Brain must analyse whether the body is tilting or undergoing rapid horizontal acceleration by receiving visual cues and cues from the semicircular canals
• Analysis is usually accurate → brain may become confused when different cues do not agree (e.g. sitting confined in an aircraft cabin that does not seem to be moving)
one diagonal acceleration (just like a tilt)
The vestibulo-ocular reflex (VOR) helps to assess the integrity of the vestibular apparatus, vestibular nerve, and its connections in the brain:
• Turning of the head to the right causes increased ___________________ and decreased _________________ → brain detects and signals right head motion
• VOR test: slow head movement (doll’s eye movement) / fast head movement (head impulse test). In normal VOR, the patient’s eyes _______________________ (VOR moves eyes in opposite direction → stabilise images on the retinas).
right vestibular nerve firing;
left vestibular nerve firing;
remain focused on the examiner’s nose during slow and fast movements
The VOR is mediated by a 3-neurone arc (very fast) with the vestibular nerve signalling head movement and synapses in the vestibular nucleus:
• To look right, the right lateral rectus (by _____) and left medial rectus (by _______) must be activated
• Nerve fibres split into two after synapsing to travel to the _____________ (in the pons) and ________________ (in the midbrain)
• Fibres can move up and down in the medial longitudinal fasciculus (MLF)
- Rotational: ______________
- Translational: ________________
CN VI;
CN III;
abducens nucleus;
oculomotor nucleus;
Semicircular canals ;
Otolith organs (saccule, utricle)
The horizontal rotational VOR (the one tested in the VOR test) is detected by the semicircular canals → signals sent via the vestibular nerve → vestibular ganglion → vestibular nucleus:
• Fibres cross to the left abducens nucleus to synapse with 2 pathways:
1. Projects directly to the left lateral rectus muscle via _________________
2. Projects via the _________________ to right oculomotor nucleus
• Fibres from right oculomotor nucleus then project to right medial rectus muscle via CN III
• The left vestibular apparatus gives rise to the inhibitory part of the reflex (inhibits the other muscles) → results in eye movement to left
CN VI (excitatory) ;
MLF (connects abducens to oculomotor nucleus) and para-pontine reticular formation (PPRF)
Deficient VOR (patient’s eyes do not remain fixed on the target as head moves) indicates a\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (inner ear/vestibular nerve) on that side: • Visual world moves as gaze stability is not maintained (\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_) → to-and-fro illusion of environmental motion • Bilateral vestibular hypofunction only occurs when the head is moving (e.g. walking or riding in a car)
peripheral vestibular lesion;
oscillopsia
Dizziness
= ________________: sensation of motion of self/environment (may/may not have nystagmus)
= Seeing environmental motion: oscillopsia (implies nystagmus if patient still)
Vestibular-motion perception
Nystagmus (brainstem reflex): Alternating phases of a slow drift in one direction with a corrective quick “jerk” in the opposite direction (direction named is direction of fast phase):
• Normally: right and left horizontal canals balance each other out
• Lesions of the right horizontal canal → loss of ________ → unopposed rightward “push” from intact left horizontal canal → slow ___________(slow phase)
• Counteracted by a leftward fast phase (corrective brainstem action)
• Loss of right horizontal canal creates left-beating nystagmus
tonic leftward vestibular input;
rightward drift of the eyes
All the vestibular nuclei project to the ____________
• Thalamic nuclei then project to two cortical areas (2V and 3a) which are part of the head region of the primary somatosensory cortex
• Projects to __________ involved in spatial orientation
• These cortical projections may account for feelings of dizziness (vertigo) during certain kinds of vestibular stimulation
ventral posterior and ventral lateral thalamic nuclei:;
superior parietal cortex (vestibular cortex)
Perception (dizziness) is mediated by higher brain regions (e.g. cortex) → bilateral projections from either peripheral vestibular apparatus to the cortical areas:
• Predominantly in the _____________________ regions
Vertigo is a symptom (sensation of spinning of self/environment) which may occur without any activation of peripheral vestibular or brainstem structures:
• May occur due to problems in the ___________________ (rare; vestibular epilepsy)
parietal, posterior temporal, temporoparietal;
ear, vestibular nerve, brainstem or cortex
The duration of the illness modifies the symptoms and signs, so patients in emergency situations tend to have acute vestibular syndrome while patients appearing to clinic tend to have chronic vestibular syndrome:
Acute: Patients with loss of function in the left ear would suffer from ______________→ brought out by covering right eye and flashing a bright light in the left eye to reduce visual fixation:
*Strong perceptuo-reflex coupling:
• Presentation: strongly correlated nystagmus (reflex) and dizziness (perception) → when nystagmus goes away, dizziness goes away as well
Chronic: Patients have similar problems as acute vestibular syndrome but for many years:
• No nystagmus at rest but a __________________
• Due to brainstem adaptation → brain uses all its motion detection capacity from the functioning side (to compensate for loss of peripheral vestibular function on the other side) → brain plasticity
*Uncoupling between perception and reflex:
• Presentation: significant dizziness (but measurement of brainstem function or peripheral vestibular function show no significant findings
right-beating nystagmus (at rest);
deficient head impulse test to one side (VOR)
People who are trained and exposed to dizzy stimuli (e.g. ballet dancers who do pirouettes) are more resistant to dizziness than those who are not (e.g. rowers):
• Vestibular responses in dancers are typically briefer → smaller vestibulocerebellum
• Inverse relationship between grey matter density and nystagmic response in dancers
With training, the relationship between perception and reflex can be changed and thus uncouple how dizziness is felt with what is measured:
• Rehabilitation: ____________________ (form of training) for chronic dizziness
vestibular rehabilitation
The gaze test checks for the presence of any ________________s in any direction (patient looks at object to centre, left, right, up, down):
• Expose the ____ to better identify direction of nystagmus (always in one direction)
• Typical for vestibular nystagmus
• Magnitude of nystagmus: greatest when looking in the direction of ____________ → less when looking straight ahead → least when looking in ___________________
spontaneous nystagmu;
sclera;
fast phase;
opposite direction of fast phase
The _____________________ checks for paroxysmal vertigo and nystagmus (e.g. BPPV) if posterior semicircular canal dysfunction is present in the lower
• Patients are lowered quickly from the upright sitting position to the supine position (with their eyes open) then neck extended 30° upwards → observe for 1 minute
• Record the following: latency, duration, fatiguability, direction, and intensity of vertigo and nystagmus (if observed) → see table below
• Deviations may occur in rare forms of peripheral positional vertigo or central lesions ear:
Dix-Hallpike test (Nylan-Barany test)
Peripheral positional nystagmus
- latent period
- duration of nystagmus
- fatigability
- direction of nystagmus
- intensity of vertigo
- 2- 20 seconds
- <1min
- fatiguing with repetition
- only one type (may change direction with gaze)
- severe
Centrl positional nystagmus
- latent period
- duration of nystagmus
- fatigability
- direction of nystagmus
- intensity of vertigo
- no
- > 1 minute
- non fatiguing
- May change direction with a given head position
- severe (sometimes none)
The Epley manoeuvre* (and the Huxley-Simmons manoeuvre) can be used to treat BPPV by removing the ________________ in the semicircular canals:
• Allows otoliths from the affected canal to be relocated (using gravity) back into the utricle → cannot stimulate the cupula of the canals anymore
• Patients may experience some dizziness during every step of the procedure (due to the otoliths moving around)
debris (canalithiasis)
Fundoscopy looks at the retina (in the back of the eye) and checks for spontaneous nystagmus:
• Ensures that patients with acute vertigo do not have ___________ (optic disc swelling secondary to elevated ICP → e.g. in ______________)
• Picks up subtle nystagmus of low intensities (by looking at the back of the eye):
papilloedema;
posterior cranial fossa tumours
BPPV is a disorder characterised by repeated brief periods of vertigo with movement, and is one of the most common neurological causes of vertigo:
• Benign as it does not involve any lesions or neuritis
- Dislodgement of otoliths from _____________ → enters semicircular canals when head is tilted (gravity moves them in) → __________ displacement → abnormal stimulation → vertigo
- Adherence of otoliths to ___________ in semicircular canals → makes cupula heavier than surrounding endolymph → sustained immediate excitation of canal afferents → vertigo
Signs & symptoms
• Usually brief (acute repeated episodes lasting seconds)
• Unwell feeling for hours (associated prolonged malaise) • Nystagmus (see features below)
• Dizziness when turning over in bed (distinguishing feature from postural hypotension → no postural challenge)
utricle;
endolymph;
cupula
What are the features of BPPV from Hallpike test?
- Latency: 5 – 10 seconds (before onset of nystagmus)
- Adaptability: eventually goes away on its own
- Fatiguability: intensity wanes after several times
What are the red flags for BPPV?
• Headaches • Atypical nystagmus (e.g. vertical nystagmus) → usually central nystagmus and not peripheral BPPV nystagmus
- Typical nystagmus: counter-clockwise for right post. canal BPPV, clockwise for left
- Atypical nystagmus has no latency (immediate onset), no adaptability (does not go away on its own), no fatiguability (does not wane)
Vestibular neuritis occurs due to inflammation of one of the vestibular nerves causing a loss of vestibular function on that side:
• May be due to viral infections (most common reason) or other causes (may result in ______________ syndrome if it is VZV shingles)
Pathophysiology
• Dizziness experience due to loss of tonic vestibular nerve input on one side → asymmetry between vestibular nerves (normally signifies head movements and brain interprets it as such)
• _____________ suppresses nystagmus and sensation of dizziness → resolves it in 90% of cases within 2 – 3 days (not due to regeneration of vestibular nerve function)
Ramsey Hunt;
Massive brainstem plasticity
what are the signs and symptom of vestibular neuritis?
• Continuous vertigo (not affected by postural changes, rapid onset, gait is unaffected) • Obvious vestibular nystagmus • Positive head impulse test (VOR)
Red flags of vestibular neuritis/ migrainous vertigo
• Headache (40%; ____________ stroke)
• Gait ataxia: may be only non-vertiginous manifestation of a __________
• Hyperacute onset: suggests _________ origin
• Vertigo + hearing loss: ______ stroke (supplies cochlea and vestibular labyrinth) or urgent ENT problem
• Prolonged symptoms (> 4 days): ______________ problem
posterior circulation;
cerebellar stroke;
vascular;
AICA;
floor of 4th ventricle
what are the signs and symptoms of migrainous vertigo?
• Acute vertigo with/without headache • Isolated vertigo and/or abnormal eye movement examination
How to diagnose migrainous vertigo?
• Differentials to consider: cerebellar stroke • Diagnosis of exclusion → must do CT brain scan then lumbar puncture 12 hours after headache to exclude differentials
What are the signs and symptoms of a cerebellar stroke?
- Thunderclap-onset vertigo (embolic stroke) → often in the setting of a Valsalva manoeuvre (e.g. straining, lifting heavy objects, bending down)
- Difficulty walking
- Headache
Meniere’s disease is very rare (< 1%) and typically presents with auditory symptoms (unlikely without auditory prodrome):
• Auditory prodrome (fullness in the ear) → tinnitus on one side → acute vertigo (lasting for hours) → settles down
• ____________ is typical of Meniere’s disease (not found in the other diagnoses above)
Auditory aura
The Ramsey Hunt syndrome (herpes zoster oticus) is essentially shingles affecting _____________________:
• Characteristic signs: painful red vesicular rash (on one ear), facial weakness and paralysis (on side of unaffected ear)
• Associated signs: ear pain, hearing loss, tinnitus, difficulty closing one eye, vertigo, change in taste perception/loss of taste (damage to chorda tympani nerve), dry mouth and eyes (less of dry mouth due to intact parotid glands)
o Due to proximity of CN VIII to the ____________ (where VZV is reactivated)
CN VII (results in facial motor abnormalities);
geniculate ganglion
