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Vestibular system: continued

Vestibular system: continued. Nystagmus. Occurs when rotation continues for a period of time or when there is an imbalance in either the canals or their projections to the brain Named by the direction of the fast phases Water irrigation of the ear canal stimulates Caloric Nystagmus.

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Vestibular system: continued

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  1. Vestibular system: continued

  2. Nystagmus • Occurs when rotation continues for a period of time or when there is an imbalance in either the canals or their projections to the brain • Named by the direction of the fast phases • Water irrigation of the ear canal stimulates Caloric Nystagmus

  3. Caloric nystagmus • Mnemonic for fast phases is COWS • cold-opposite • warm-same

  4. Geometry of canals

  5. Whenever fluid moves toward the ampulla in one canal, fluid will move away from the ampulla in the opposite canal • Rotation to the left not only excites the LHC but also inhibits the RHC

  6. Rotation to left • Excites LHC • More contraction of LMR and RLR • Inhibits the RHC • Less contraction their antagonists, RMR and LLR!!!

  7. Vertical canals • Downward movements stimulate the anterior canals and inhibit the posterior canals • More contraction of both SR and IO • Less contraction of both IR and SO • Upward movements stimulate the posterior canals and inhibit the anterior canals.

  8. Unilateral loss • Disease of a single canal • nystagmus in the plane of that canal • independent of the orientation of the eye in the orbit • Loss of all semicircular canals or of the vestibular nerve on one side • mixed horizontal-torsional nystagmus • slow phase is toward the side of the lesion

  9. Pairs of vertical canals • RAC paired with LPC • Effective stimulation of RAC, inhibition of LPC: move downward at 45° angle to the right • LAC paired with RPC • Effective stimulation of LAC, inhibition of RPC: move downward at 45° angle to the left

  10. +RAC, -LPC • Excite RAC • excitatory connections (through the medial and lateral vestibular nuclei) with RSR, LIO • both eyes move up in the orbit, with some torsion • Inhibit LPC • Relax antagonists, RIR and LSO

  11. Removing the fast phases

  12. Removing the fast phases

  13. Inhibitory connections between the semicircular canals and the muscles

  14. +RAC, -LPC • Excite RAC • inhibitory connections (through the medial and lateral vestibular nuclei) with RIR, LSO • Aids in relaxing anatagonists • Inhibit LPC • Removes normal inhibitory connections to RSR and LIO

  15. Quantifying vestibular function • Need to know how fully the eyes compensate for head movements • Because canals adapt quickly to sustained rotation in the same direction and with same speed, oscillate patients with continuously changing speed and direction • Quantify by gain of VOR = eye magnitude/head magnitude

  16. Sinusoidal rotation is better

  17. Gain • 0 means no vestibular function (like JC) • 1 means perfect function • Is gain always 1? • No--having a mental task is important • Ask patient to imagine a target on the wall and keep eyes on it • Particularly important when testing in dark

  18. Suppression of VOR • We should not be prisoners of the VOR • If a target moves with us and we want to look at it (e.g., reading in a car), we should be able to suppress the VOR • Suppression task is normally part of clinical vestibular evaluation

  19. Plasticity of VOR • If visual input changes by magnification, minification or even complete inversion, should VOR remain constant? • If the VOR stayed constant, eye movements would not match the head movements and thus the world would move, across the retina with each head movement.

  20. When do images get magnified or minified? • With spectacles! • Corrections that magnify or minify images can cause transient dizziness because the eyes move too much or too little • If patients wear new lenses, they adapt

  21. Bielschowski's head tilt test • tilting the head to the shoulder • otolith-ocular reflex • Activate SO and SR on the side of the tilt • IO and IR on the contralateral side • with a muscle palsy, see elevation or depression • SO : head tilt to the paralytic side causes elevation ipsilateral eye • SR is acting unopposed.

  22. Effects of head tilt

  23. Parks 3-step method • Step 1: right hypertropia? left hypertropia? • If a right hypertropia, the right eye is too high or the left eye is too low • Which muscles are involved?

  24. Parks 3-step method • Step 1: right hypertropia? left hypertropia? • If a right hypertropia, the right eye is too high or the left eye is too low • Which muscles are involved? • 4 vertical muscles can be paretic • RIR or RSO (depress OD) • LSR or the LIO (elevate OS)

  25. Parks 3-step method • Step 2: does deviation increase on left gaze or on right gaze? • If right, RSO and LSR are eliminated • RSO is less active in abduction than is RIR • LSR is less active in adduction than is LIO • Implicates either RIR or the LIO

  26. Parks 3-step method • Step 3: does deviation increases on right head tilt or on left head tilt? • If increase on head tilt to the left • problem in muscles producing intorsion OS • or in muscles producing extorsion OD • RIR causes extorsion OD • LIO does not cause intorsion OS • RIR is isolated as the paretic muscle.

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