BB2 Vestibular System 2014


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BB2 Vestibular System 2014

  1. 1. The Vestibular System Gavin Giovannoni Barts and The London
  2. 2. Objectives 1. 2. 3. 4. 5. 6. 7. Describe the anatomy and embryology of the vestibular system List the functional roles of the vestibular system Describe how the vestibular system detects angular and linear acceleration Describe the vestibulo-ocular reflex and explain how nystagmus can occur Describe the central pathways of the vestibular system List the clinical signs of vestibular system dysfunction Describe tests for balance disorders
  3. 3. Anatomy & embryology
  4. 4. Vestibular apparatus P Semicircular Canals Utricle (Macula) H A Ampullae (Crista) Vestibular Ganglion Saccule (Macula)
  5. 5. MRI of the vestibular system
  6. 6. Blood supply
  7. 7. Summary of embryological development
  8. 8. Embryology: formation of the otic vesicles from thickened otic placodes
  9. 9. Embryonic development of the human inner ear A, At 28 days. B, At 33 days. C, At 38 days. D, At 41 days. E, At 50 days. F, At 56 days, lateral view. G, At 56 days, medial view. H, Central reference drawing at 56 days. (From Carlson B: Patten’s foundations of embryology, ed 6, New York, 1996, McGraw-Hill.)
  10. 10. Function of the Vestibular System
  11. 11. Form & Function Anatomy & Physiology
  12. 12. A reductionist approach What is a reflex? Detector (sense organ) Peripheral processing Central processing - level 1 Response 2 Central processing - level 2 Response 3 Central processing - level 3 Response n Central processing - level n +/- Percept EFFERENT Response 1 Relay (nerve) AFFERENT Sensory Input
  13. 13. Vestibular functions 1. Detection & conscious perception of head position, movement & gravity 2. Compensatory eye movements during head movement (image stabilization & tracking) 3. Postural reflex adjustments following head movements To do this it must connect to the spinal cord, cerebellum and occulomotor nuclei
  14. 14. Central connections of the vestibular system
  15. 15. Functional connections of the vestibular apparatus Visual tracking functions Occulomotor nuclei ICP Cristae (rotational changes) Vestibulo-cerebellum MLF S M Medial VST Vestibular nuclei I Reticular formation (gaze centres) (head stabilization) L Lateral VST Cervical Spinal cord LMNs Lumbar (regulates anti-gravity muscles) ML Thalamus & Cortex Maculae (linear changes) (conscious appreciation of balance & head position) VST = vestibulospinal tract ML = medial lemniscus Balance & posture reflex functions MLF = medial longitudinal fasciculus ICP = inferior cerebellar peduncle
  16. 16. Motion detection & vestibular receptors • Angular (rotational) acceleration – Semicircular canals – Vertigo (rotational sensation) • Linear acceleration/deceleration – Utricle & saccule – Dysequilibrium (rocking ship sensation)
  17. 17. Anatomy of semicircular canals Detect rotational acceleration/deceleration Deflection of the stereocilia TOWARD the kinocilium results in an INCREASE in the firing rate of the vestibular fiber associated with the hair cell, while deflection AWAY from the kinocilium results in a DECREASE in the firing rate of the vestibular fiber. Endolymph Ampulla Cupula Hair cells } (have 1 large kinocilium and several small sterocilia) Crista
  18. 18. Function of semicircular canals Detect rotational acceleration/deceleration R R L L R L •Canals on either side of head act in a push-pull rhythm •Excitation is towards side of rotation •Push-push rhythm causes nausea & vertigo •Nerve damage causes vertigo & nystagmus due to afferent imbalance.
  19. 19. Vestibulo-ocular reflex, VOR (doll’s eye reflex) MLF - Medial longitudinal fasciculus Connects vestibular nuclei to ocular motor nuclei (III, IV, VI)
  20. 20. Anatomy of a utricle/saccule Macula [ (Sensory epithelium) hair cell & support cell) Hair cells of the maculae are excited (or inhibited) by bending of the stereocilia toward (or away from) the kinocilium by opening or closing potassium channels
  21. 21. Anatomy of a macula striola striola • Utricles - hair cells are polarized (excited) towards the striola which divides each macula into medial and lateral halves • Saccules - hair cells are polarized away from the striola (divides each macula into anterior and posterior halves) • The hair cells of the utricles and saccule work together to provide for a three dimensional representation of the direction of linear force.
  22. 22. Maculae detect linear acceleration, deceleration, tilt & effects of gravity Saccule (vertical)
  23. 23. Maculae detect linear acceleration, deceleration, tilt & effects of gravity Utricle (horizontal)
  24. 24. Biochemistry (1) Perilymph is a typical extracellular fluid (~ plasma or CSF). The compositions of ST perilymph and SV perilymph are not the same; SV perilymph has higher K+ and lower Na+ levels. Endolymph is a unique extracellular fluid, with an ion composition unlike that that found anywhere else in the body. ST = scala tympani SV = Scala vestibuli
  25. 25. Biochemistry (2)
  26. 26. Train travel direction Nystagmus • Is an uncontrolled oscillation of the eyes • Has a slow phase and a fast phase • Direction of nystagmus is specified by direction of fast phase • Slow phase is VOR (for image tracking) • Fast phase due to cortically derived signal (to allow another object to be tracked) • Optikokinetic (fixation) nystagmus – normal • Spontaneous nystagmus – abnormal due to damage to vestibular apparatus, brainstem or cerebellum.
  27. 27. Head-righting reflex 1. Vestibular neurons receive signals of the changing head position (downwards) relative to gravity. These come from both the utricle and the saccule, and signals on forward rotational acceleration from the vertical semicircular canals. 2. The MVST neurons process this information and transmit inhibitory signals to the neck flexor muscles. 3. At the same time, excitatory signals are sent to the neck extensor muscles. The result is a neck movement upward, opposite to the falling motion, to protect the head from impact.
  28. 28. Rotational nystagmus Left rotation of head & body: Acceleration in inner ear horizontal canals During rotation there is left-beating nystagmus Nystagmus Fast component Slow component Eye mov’t in VOR Eye mov’t in VOR On abrupt cessation there is a post rotational (slow) nystagmus as the SSC endolymph does not stop as Reset reset quickly and so nystagmus appears in the opposite (fast) direction which lasts a few seconds (<15s)
  29. 29. Clinical manifestations Symptoms and Signs • Vertigo (or dizziness) • Syncope (light-headedness or woozy-ness) • Dysequilibrium (rocking ship sensation) • Oscillopsia (visual jumping or blurring) • Nystagmus • Nausea & vomiting • Ataxia (unsteadiness of gait) • Associated cochlear symptoms – Hearing loss – Tinnitus Causes • Nerve/inner ear infections • Tumours • Vascular insufficiency • Trauma • Endolymph fistulae Clinical examples 1. BPPV 2. Meniere’s disease 3. Drug toxicity (quinine & aminoglycoside antibiotics) 4. Usher’s syndrome
  30. 30. Tests for vestibular disorders Clinical Examination Vestibular Testing Vestibular Imaging Eye movements Electronystography (ENG) Caloric Testing Hallpike Manoeuvre Posturography Bárany chair MRI
  31. 31. Caloric test Water changes temperature between middle and inner ear causing convection currents to occur in SSCs to elicit the VOR; tests integrity of pons Normal “COWS” Cold water decreases ipsilateral ampulla firing; warm water increases firing Unconscious (brainstem intact, cortex nonfunctional) No F phase due to lack of input from cortical gaze centres Bilateral MLF lesion Irrigation produces lateral deviation of eye only on less active side
  32. 32. Hallpike manoeuvre (a test for BPPV) Benign Paroxysmal Positional Vertigo (BPPV) Vertigo due to damaged otoconia from the utricle being displaced into the semicircular canals within the inner ear. Otoconia from the saccule are not able to migrate into the canals. The utricle can be damaged by head injury, infection, or other disorder of the inner ear, or may have degenerated because of advanced age. 1. Lower head to the table and turn to one side. 2. Watch eyes for nystagmus. 3. If patient gets dizzy & exhibits nystagmus, the ear pointed to the floor is the affected ear. 4. If nothing happens, repeat test on the other side to check the other ear. 5. The person is then moved back to the upright position.
  33. 33. Figure 1. Instruction for the modified Epley’s procedure (for benign paroxysmal positional vertigo of the posterior semicircular canal of the right ear). Radtke A et al. Neurology 1999;53:1358
  34. 34. Additional Reading • • The nervous system, Chapter 8 Web resources – ular.html – –