Vestibular Function and Dysfunction

1. Vestibular Function and Dysfunction Melanie Giesler

3. Vestibular Dysfunction Prevalence • Four percent of patients18-65 yo visit PCP with complaint of “dizziness” • Three percent consider it “Severely incapacitating” • Third most common complaint in elderly

4. Vestibular System • System of balance • Membranous and bony labyrinth embedded in petrous bone • 5 distinct end organs • 3 semicircular canals: superior, lateral, posterior • 2 otolith organs: utricle and saccule

5. Vestibular System • Semicircular canals sense angular acceleration • Otolithic organs (utricle and saccule) sense linear acceleration

7. Vestibular System • Semicircular canals are orthogonal to each other • Lateral canal inclined to 30 degrees • Superior/postereor canals 45 degrees off of sagittal plane

8. Vestibular Anatomy

9. Vestibular Blood Supply • 45% from AICA • 24% superior cerebellar artery • 16% basilar • Two divisions: anterior vestibular and common cochlear artery

10. Vestibular Nerve Supply • Superior vestibular nerve: superior canal, lateral canal, utricle • Inferior vestibular nerve: posterior canal and saccule

11. Vestibular Fluid • Membranous labyrinth is surrounded by perilymph • Endolymph fills the vestibular end organs along with the cochlea

12. Vestibular Fluid • Perilymph • Similar to extracellular fluid • K+=10mEQ, Na+=140mEq/L • Unclear whether this is ultrafiltrate of CSF or blood • Drains via venules and middle ear mucosa

13. Vestibular Fluid • Endolymph • Similar to intracellular fluid • K+=144mEq/L, Na+=5mEq/L • Produced by marginal cells in stria vascularis from perilymph at the cochlea and from dark cells in the cristae and maculae • Absorbed in endolymphatic sac which connected by endolymphatic, utricular and saccular ducts

14. Sensory Structures • Ampulla of the semicircular canals • Dilated end of canal • Contains sensory neuroepithelium, cupula, supporting cells

16. In the Ampulla… • Cupula is gelatinous mass extending across at right angle • Extends completely across, not responsive to gravity • Crista ampullaris is made up of sensory hair cells and supporting cells

17. In the Ampulla… Hair cells have 50-100 stereocilia and a single kinocilium.

18. In the Ampulla… stereocilia are not true cilia, they are graded in height with tallest nearest the kinocilium.

19. In the Ampulla… • Each afferent neuron has a baseline firing rate • Deflection of stereocilia toward kinocilium results in an increase in the firing rate of the afferent neuron • Deflection away causes a decrease in the firing rate

20. Angular acceleration acts on the hair cells in the Ampulla

21. Angular acceleration acts on the hair cells in the Ampulla • Kinocilia are located closest to utricle in lateral canals and are on canalicular side in other canals • Ampullopetal flow (toward the ampulla) excitatory in lateral canals, inhibitory in superior/posterior canals • Ampullofugal flow (away from the ampulla) has opposite effect

23. Physiology Semicircular Canals • Semicircular canals are paired • Horizontal canals • Right superior/left posterior • Left superior/right posterior • Allow redundant reception of movement • Explains compensation after unilateral vestibular loss

24. Otolithic Organs • Utricle and saccule sense linear acceleration • Cilia from hair cells are embedded in gelatinous layer • Otoliths or otoconia are on upper surface

25. Otolithic Organs • Calcium carbonate or calcite • 0.5-30um • Specific gravity of otolithic membrane is 2.71-2.94 • Central region of otolithic membrane is called the striola

26. Vestibular Physiology • Senses and controls motion • Information is combined with that from visual system and proprioceptive system • Maintains balance and compensates for effects of head motion

28. Vestibulo-ocular reflex • Vestibulo-ocular reflex • Membranous labyrinth moves with head motion • Endolymph does not causing relative motion • Cupula on right canal deflected towards utricle causing increase in firing rate, left deflects away causing a decrease in firing rate. • Reflex causes movement of eyes to the left with saccades to right • Stabilizes visual image

29. Nystagmus • If acceleration stops, and spin to right is at constant velocity, sensation of motion stops after 14-20 seconds as does nystagmus • Cupula only takes 8-10 seconds to return to equilibrium position • Vestibular integrator is the term for the prolongation and is mediated by the vestibular nuclei and cerebellum

30. Vestibulospinal Reflex • Senses head movement and head relative to gravity • Projects to antigravity muscles via 3 major pathways: • Lateral vestibulospinal tract • Medial vestibulospinal tract • Reticulospinal tract

32. Office Examination of the Dizzy Patient • Dix-Hallpike Maneuver • Used to provoke nystagmus and vertigo commonly associated with BPPV • Head turned 45 degrees to maximally stimulate posterior semicircular canal • Head supported and rapidly placed into head hanging position • Frenzel glasses eliminate visual fixation suppression of response

33. Dix-Hallpike Manuveur

34. Dix-Hallpike Manuveur • Positive test • Up-beating nystagmus • Nystagmus to the stimulated side • Rotary component to the affected ear • Lasts 15-45 seconds • Latency of 2-15 seconds • Fatigues easily

35. Pneumatic Otoscopy • Positive and negative pressure applied to middle ear • Hennebert’s sign/symptom – nystagmus and vertigo with pressure, alternates with positive and negative pressure • Can be present in patients with perilymphatic fistula, syphilis, Meninere’s disease, SCC dehiscence syndrome

36. Head Shake Nystagmus • Evaluates unilateral vestibular weakness • Head tilted back 30 degrees • Shake back and forth for 30 seconds as quickly as possible • Unilateral vestibular deficit causes slow phase nystagmus to the side of lesion • Low sensitivity (27%) • Good specificity (85%)

37. Head Thrust Test • Inhibitory response not as robust as the stimulatory response to stimulate VOR • Movements that overcome the inhibitory response of vestibule will result in VOR lag • Head tilted 30 degrees • Rapid head movements to either side with focus on examiner’s nose • Patients have catch-up saccade when rotated to side of weakness • Sensitivity 75%, Specificity of 85%

38. Dynamic Visual Acuity • Used for bilateral vestibular weakness • Visual acuity checked on Snellen chart • Rechecked while rotating head back and forth at 1-2 Hz. • Loss of 2-3 lines considered abnormal – usually b/l loss due to ototoxicity or aging.

39. Romberg Test • Patient asked to stand with feet together and eyes closed • Fall or step is positive test • Equal sway with eyes open and closed suggests proprioceptive or cerebellar site • More sway with eyes closed suggests vestibular weakness

40. Fukuda Stepping Test • Originally described by Fukuda using 100 steps on a marked floor. • Patients are asked to step with eyes closed and hands out in front • Rotation by more than 45 degrees is abnormal • Rotation usually occurs to the side of the lesion • Rotation often found in asymptomatic patients

41. Dysdiadochokinesia Testing • Most commonly tested with the hand slapping test • Abnormalities seen in patients with cerebellar dysfunction • Poor sensitivity and specificity

42. Tandem Gait Test • Patients are asked to walk heal to toe in a straight line or in a circle • Complex function evaluates many aspects of balance • Poor performance seen in cerebellar lesions, but can be seen in many disorders • Poor sensitivity and specificity

43. Orthostatic Hypotension • Most often in patients on BP meds with “light headedness” on sitting to standing • Defined as drop of SBP 20mm HG or DPB 10mm HG within 3 minutes of standing • Tilt exams offer objective measurements with well established norms • Patients with no symptoms will often “Tilt”

44. Voluntary Hyperventilation • Patients asked to over breathe for 90 seconds to 3 minutes (elevate pH – vasoconstriction) • Hyperventilation causes symptoms in some anxiety disorders • May provoke symptoms in normal • Poor test for vestibular diagnosis but can elevate pH, increase 8th nerve firing - lesions in petrous apex, acoustic schwannoma, 8th nerve demylination

45. Electronystagmography (ENG) • Divided into oculomotor tests, positional and positioning tests, and caloric tests • Only vestibular test with the ability to test individual labyrinths separately • Relies on the vestibulo-ocular reflex (VOR) to test the peripheral vestibular function • Mostly a test of HSCC function

46. Electronystagmography (ENG) • Oculomotor tests • All test eye movements that originate in the cerebellum • Saccadic tracking • Smooth pursuit tracking • Optokinetic testing

47. Oculomotor Tests • Saccadic tracking • Patients concentrates on a randomly moving target • Latency – difference in time between movement of object and eye (150-250 ms) • Velocity – speed of saccade 200-400 degrees/second low end of normal • Accuracy – amount of undershoot/overshoot of target (75-120%)

48. Smooth Pursuit Test • Tests ability to accurately and smoothly pursue a target • Gain of eyes compared to movement of target • Saccade movements eliminated from calculations • Asymmetrical pursuit highly suggestive of central disorders

49. Optokinetic Tests • Vestibular system and optokinetic nystagmus allow steady focus on objects • Target is rapidly passed in front of subject in one direction, then the other • Eye movements are recorded and compared in each direction • Asymmetry suggestive of CNS lesion • High rate of false positive results

50. Caloric Testing • Established and widely accepted method of vestibular testing • Most sensitive test of unilateral vestibular weakness • Patient positioned 30 degrees from prone (HSCC vertical allowing max stim) • Cold and warm water/air flushed into EAC