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Anatomy and Physiology of the Vestibular System

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Anatomy and Physiology of the Vestibular System 1/28/2013 Overview Anatomy and Physiology of the Vestibular System J. Douglas Green, Jr, MD Input Central Processing Output Anatomy Vestibular Anatomy • Angular movements stimulate semicircular canals and are primarily important for visual stabilization • Semicircular canals paired with a contralateral semicircular canal (i.e. R Lat SCC with L Lat SCC) • Movement in any plane stimulates on SCC and inhibits corresponding SCC on opposite side • Otolithic labyrinth (utricle and saccule) respond to linear movement and primarily orient with regard to gravity • Static tilt results in continued stimulation 1 1/28/2013 Vestibular Hair Cells Anatomy Semicircular Canals • Stereocilia • Orientation • Kinocilium • Ampulla • Resting firing rate of 90 spikes/sec • Increased firing rate when deflection towards kinocilium Physiology Central Vestibular System Semicircular Canals • Push pull interaction • Sup SCC, Lat SCC, utricle and ant/sup portion • VOR of saccule innervated by Superior Vestibular Nerve • Post SCC and majority of saccule innervated by Inferior Vestibular Nerve • VIII consists of bipolar neurons with cell bodies in Scarpa’s Ganglion in IAC • Vestibular Nuclei in brainstem 2 1/28/2013 Vestibulo‐Ocular Reflex Physiology Otolithic Organs • Stabilizes gaze during active head movements • Otoconia • Absent VOR results in bouncing environment • Resting neural discharge while walking, riding in car or other activities • Linear acceleration (oscillopsia) • Superior Vestibular Nerve synapses in medial and superior vestibular nuclei then on to MLF and III, IV, and VI • Second VOR pathway is multisynaptic Anatomy Central Vestibular System Otolithic Organs • Orientation • Otolithic neural signals synapse in medial and lateral vestibular nuclei • Macula • Second order neural signals extend to anterior horn cells of cervical cord (medial vestibulospinal tract) and entire spinal cord (lateral vestibulospinal tract) • Innervate antigravity muscles of neck, thorax and lower limbs • Also supplies EOM’s to produce rotational and vertical ocular adjustments during head tilt • Cerebellum produces inhibitory influence through Purkinje cell activity which fine tunes response and assures proper interaction with visual and proprioceptive cues • Ascending vestibulocortical pathways provide conscious awareness of body orientation 3 1/28/2013 Ocular Motor Control Ocular Motor Control Saccadic Eye Movements Smooth Pursuit Eye Movements • Reflexive eye movements that occur during • Generates smooth conjugate eye movements vestibular or optokinetic stimulation to track small, discrete, slowly moving objects • Returns eye to neutral position • Visual information (via visual cortex) • Characteristics compared to vestibular information in – Conjugate deviation vestibular nuclei then on to oculomotor nuclei – High velocity (200 – 600 degrees/sec) • Declines with age, changes in visual acuity, – Accuracy target speed, sedation, inattention and CNS – Short Latency (<250 msec) pathology Ocular Motor Control Visual Vestibular Interaction Optokinetic Eye Movements • VOR keeps object stable with head is moving • Visual tracking systems (saccadic, pursuit and • Driven by full field peripheral retinal OPK) follow object in motion when head is stimulation stable • Same neural pathway as • When head is moving and object is moving smooth pursuit both systems must work together • Deficits parallel smooth • With vestibular injury, smooth pursuit and pursuit dysfunction OPK make up partially for loss of vestibular cues and VOR 4 1/28/2013 Unilateral Vestibular Loss Unilateral Vestibular Loss • Damage to one labyrinth results in asymmetric input from the vestibular system • Results in slow deviation toward damaged ear and saccadic reset towards intact ear • Results in sensation of self movement (vertigo) towards intact ear • CNS suppression of nystagmus (cerebellar clamp effect) within hours to days occurs • Over weeks, rebalancing of dynamic asymmetry occurs to create a more meaningful eye movement during head rotation Bilateral Vestibular Loss Somatosensory Input • May occur from ototoxicity, trauma or autoimmune • Pressure receptors on soles of feet combined disorders with stretch receptors from cervical, thoracic and • Peripheral loss may not be asymmetric with lack of vertigo lumbar musculature and spontaneous nystagmus • Oscillopsia results from absent VOR and retinal slippage of • Both monosynaptic (knee jerk) and multisynaptic image (long motor arc reflex) • Oculomotor systems (smooth pursuit and OPK) can partially • Impaired in peripheral neuropathy substitute for VOR at slower head movements (<100 – Sensory degrees/sec) – Motor • With training, saccadic systems can replace deficient VOR at higher velocities (100 – 200 degrees/sec) • With vestibular loss, lack or somatosensory input • Nothing can replace VOR at high velocities (>200 causes more significant problems degrees/sec) resulting in oscillopsia 5 1/28/2013 Dynamic Equilibrium Maintenance of Posture Determination Choice of • Visual and somatosensory input compared of Body Position Body Movement with vestibular cues to keep COG over feet Compare, Select Select and Adjust • Eccentric COG results in corrective motor and Combine Senses Muscle Contractile Pattern response – Ankle response Vestibular Somato‐ Ankle Thigh Trunk Neck Vision System sensation Muscles Muscles Muscles Muscles – Hip response – Step response Environmental Generation of Interaction Body Movement Summary 1. SCC sense angular acceleration to aid in gaze Vestibular Function Testing stabilization during head movement (VOR) 2. Otolith organs (utricle and saccule) sense linear acceleration and gravity changes to promote compensatory postural and oculomotor responses William J. Eblin, Jr., Au.D. 3. Cerebellum controls both systems to assure appropriate motor activity and sensory integration 4. Vestibular nuclei are critical for sensory integration 5. Thalamus and cerebral cortex play an important role in motion perception 6. Normal balance and oculomotor control occur at a subconscious level 6 1/28/2013 Assessment of the dizzy patient Goals of Testing • Completely assess vestibular system • Site and extent of lesion History • Functional performance/objectify complaints • Determine status of compensation • Physiologic Compensation • Functional Compensation Exam Testing • Help determine who may be appropriate for rehab Anatomy Overview Key Concepts • Semicircular Canals • Nystagmus – Sensitive to rotation – Slow phase (“drift”) vestibular system • Otoliths – Fast Phase (“beat”) brainstem – Sensitive to linear accelerations • Acute labyrinthine loss… • Utricle – Eyes drift toward the side of lesion, “beat” toward better ear • Saccule – Example: left ear hypofunction • Because left ear “pulls” eyes to the right, eyes drift left • Snap back (beat) to the right (mediated by the brainstem) 7 1/28/2013 Key Concepts: Key Concepts: Spontaneous Nystagmus Peripheral vs. Central Spontaneous Nystagmus • Nystagmus always beats toward the more • Fixation active neural side – Peripheral: decreases – Central: no change or increases – Acute right vestibular hypofunction: leftbeating • Direction spontaneous nystagmus – Peripheral: most often mixed direction; • Alexander’s Law: nystagmus intensifies when torsional and horizontal – Central: usually single plane; vertical or looking toward quick phase of nystagmus horizontal • Exception: irritative lesions • Gaze effects – Peripheral: increases with gaze toward quick – e.g, Meniere’s Disease phase • Diseased labyrinth has increased neural activity – Central no change or reverses direction Central Nystagmus Signs Contemporary Vestibular Assessment • Downbeat nystagmus • Audiometrics • Computerized Dynamic Posturography • Upbeat nystagmus • Rotational Testing • VNG/ENG • VEMP • Periodic alternating nystagmus – Cervical – Ocular • Subjective Visual Vertical 8 1/28/2013 Additional Tests So…what are we assessing? • Postural Tests • Horizontal SCC / Superior Vestibular Nerve – Modified Clinical Test of Sensory Interaction on – Caloric Testing Balance (mCTSIB) – Rotational Testing – Limits of Stability (LOS) – Dynamic Visual Acuity • Dynamic Visual Acuity (DVA) • Saccule / Inferior Vestibular Nerve / Central Pathways – cVEMP • Gaze Stabilization Test (GST) • Utricle / Central Pathways • Unilateral Centrifugation – oVEMP – w/ SVV – SVV Tests that are less specific… What are we assessing? VNG/ENG Rotational SVV cVEMP oVEMP • Do not specifically measure labyrinth or Testing vestibular nerve: Semicircular (Dehiscence) – Oculomotor testing Canals XX – Computerized Dynamic Posturography Utricle XX Saccule X Superior XX Vestibular Nerve Inferior X Vestibular Nerve 9 1/28/2013 Contemporary Vestibular Assessment Audiometric Evaluation • Audiometrics • Provides information about the auditory system • Computerized Dynamic Posturography • Helpful in the identification of retrocochlear pathology • Health of EAC and middle ear before testing • Rotational Testing • i.e., intact TM prior to caloric testing • VNG/ENG • Conductive/Mixed HL prior to VEMP testing • Useful to identify patterns associated with vestibular • VEMP and balance disorders – Cervical • Vestibular labyrinthitis – Ocular • Meniere’s • Acoustic neuroma • Subjective Visual Vertical • Etc. Audiometric Evaluation Contemporary Vestibular Assessment • Audiogram • Audiometrics • Computerized Dynamic Posturography • Tymps / Reflexes (ipsi and contra) • Rotational Testing • VNG/ENG • OAE’s • VEMP – • cochlear vs. neural pathology Cervical – Ocular • Subjective Visual Vertical
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