Vestibulo-ocular Reflex and Saccades

• The documents consists of study notes titled "Table of Contents"

The Basics

• Vestibulo-ocular Reflex (VOR) purpose is to hold images steady on the retina
• VOR achieves this by producing compensatory eye movements when the head briefly rotates or translates
• VOR is created by a three-neuron arc

Saccades

• Saccades are rapid, brief, conjugate eye movement that shifts the line of sight to bring target images onto the fovea.
• Saccades are mediated centrally (cerebellum)
• Types of saccades: volitional, reflexive, predictive and memory guided

Smooth Pursuit

• Smooth Pursuit main purpose is to maintain the image of a small, slowly moving target
• It does this on the fovea when the head is still
• Smooth pursuit is voluntary as its driven by visual objects and modulated by attention and motivation
• Smooth Pursuit can suppress vestibular and optokinetic responses during head and eye tracking

Optokinetic Response

• Its purpose is to stabilize retinal images during head movements
• Optokinetic Response uses visual inputs to infer direction and speed of head motion
• It occurs rather than responding directly to head velocity signals
• The response is necessary during prolonged head movement
• The slow phase is like VOR or smooth pursuit, fast phase like saccade

Nystagmus

• Nystagmus is named after the fast phase
• The degree and severity is based on the slow phase
• The two phases are a slow Peripheral phase and a fast Cortical phase
• The fast Cortical phase resets eyes during prolonged rotation and direct gaze toward oncoming visual scene
• Degrees of Nystagmus include:
• 1st degree which is only present when gazing towards the fast phase direction
• 2nd degree which is present when gazing toward fast phase and ahead
• 3rd degree which is present when looking in all directions (usually acute)
• Can be linear or torsional
• 6° is significant

Peripheral Gaze Evoked Nystagmus

• Peripheral Gaze Evoked suppresses with fixation
• However this cant happen if its very acute phase
• Follows Alexander's law
• Fast phase toward the healthy ear
• Direction is fixed
• Only for gaze-evoked nystagmus
• Linear slow phase

Central Gaze Evoked Nystagmus

• It involves Pure vertical movements
• The Direction can be fixed or change direction
• Can persist well beyond the acute stage
• Is enhanced/unaffected by fixation
• Nonlinear (decreasing) slow phase velocity

Other Nystagmus

• Rebound: a few beats of nystagmus (opposite fast phase) following eccentric gaze for extended period
• Endpoint: at extremes of gaze
• Congenital: direction-changing gaze-evoked nystagmus that is not affected by fixation
• Congenital has been present since childhood
• It is typically, horizontal
• Has a null point
• Non-linear, and increases in slow phase velocity

ENG

• Uses the corneorentinal potential of the eye
• Front of eye (cornea) is +
• Back of eye (retina) is –
• Sensitive to changes in light Pros:
• Less expensive in short-term
• Useful in situations where goggles can't be uses Cons:
• Can't record torsional eye movement
• Eye blink artifact
• Set-up time

VNG

• Reflects light towards imaging device Pros:
• Video recording available
• Torsional views Cons:
• Upfront investment required
• Glasses don't fit everyone
• Can cause Ptosis (droopy eyelids)

VNG Testing

• Saccades
• Evaluate ability to maintain stable gaze and look for nystagmus
• Tests in primary gaze (spontaneous nystagmus) and eccentric gaze (gaze evoked nystagmus)
• Assesses how far over/under target (%)
• Hypometric: undershoot target before correcting
• Rule out visual impairments, alertness, fatigue, medications, and blinking
• Accuracy: how far over/under target (%)
• Hypermetric: eyes overshoot and correct to target
• It can rule out visual impairments, blinking, improper calibration
• Tests Velocity (how fast eyes move to the target in degrees/second)
• Internuclear ophthalmoplegia: normal abduction (out), slowed adduction
• Measures Latency (time between presentation of the target and start of eye movement (ms))
• Latency: time between presentation of the target and start of eye movement (ms)
• Abnormal latency can be due to Parkinson's or other central pathology or fatigue

Smooth Pursuit tests

• The target is a dot during continuous movement
• It is Influenced by motivation
• Measures include Gain (how closely eyes follow target)
• Abnormal result is indicative of central pathology
• Medication, inattention, fatigue, and head movement can complicate reading
• a Bilaterally impaired smooth pursuit suggests saccadic overlay to smooth pursuit aka cogwheeling

Optokinetic Tests

• It stabilizes objects while head/full field is moving (at least 90% of the visual field must be filled with stimuli to stimulate)
• Combines smooth pursuit and saccade systems
• Test at 20°/s and 40%s velocity for 20 seconds Measure with:
• Velocity gain is how closely eye movement matches stimulus
• Bilateral reduction is a disorder in smooth pursuit of saccadic systems
• A Asymmetry is greater than 25% an often caused by overlaying spontaneous nystagmus

Nystagmus

• Placement of head/body into static positions to look for “static positional” nystagmus
• Vision is denied, if there is nystagmus, do a visual test to assess visual suppression
• Doctor will Ask alerting questions
• Measures need to assess strength, duration, direction, and visual suppression of the nystagmus

Positioning Nystagmus

• Before testing: Vertebral artery screen
• Patient movess head left or right with head looking upward for about 30 seconds to Identifies problems with blood flow from the vertebral arteries to the brain
• A ask patient about dizziness, lightheadedness, nausea, blurred vision, double vision
• Don't do the Dix-Hallpike if there is a positive response
• Head Shake Test will have Vision denied with head 30 degrees to allow proper allignment with HSCC
• After aligning, Examiner shakes head for 20 seconds at 2 Hz with a 30° left/right displacement
• Analyise
• In normal or bilateral loss= indicates no nystagmus
• Assymetry indicates Dynamic imbalance which indicates that it is nystagmus beating toward the better ear, which should decay in about 30 seconds
• The Dix-Hallpike tests will have head rotated patients head 45° left or right, have patient quickly lay back, have 30° cervical extension and watch for nystagmus
• Interperetation Beats toward the affected ear _ PSCC up beating with torsional movement _ASCC down beating with torsional movement

Roll Test

• This is For horizontal BPPV
• Test must be done supine
• Head lifted 30°, turn head quickly to one side looking for nystagmus, then to the other side
• Interpretation: Geotropic (toward the ground) nystagmus: Canalithiasis
• The more intense response is the affected ear
• Caused by free-floating otoconiae
• Should fatigue in 15-30 seconds
• Slightly delayed nystagmus onse

Ageotropic (away from the ground) nystagmus: Cupulolithiasis

• Side with less intense response is affected ear
• Caused by displaced otoconia that adhered to the cupula
• Persistent nystagmus when in the position
• Immediate nystagmus onse Canal jam: otoconia become stuck in the narrow point of the canal Constant nystagmus

Positional Alcohol Nystagmus

• This is a Direction-changing peripheral nystagmus
• PAN I:
  • move into position (supine with right or left head turn)
  • Alcohol reaches cupula before endolymph and cupula floats= geotropic nystagmus -PAN II:
  • move out of position
  • Alcohol leaves cupula earlier than endolymph and cupula sinks= ageotropic nystagmus

Caloric Test

• You need to Assess responsiveness and symmetry of HSCC and superior vestibular nerve
• Caloric heat/cold changes endolymph density, therefore changing the balance of force across the cupula
• Test at 30° supine
  • Air: Warm (50°C) cool (24°C) flow rate (800 mL)
• Water: Warm (44°C) cool (30°C) Flow rate (250 mL)
• The excitory phase is usually: warm because the cupula deflects ampullopetally, which causes, well... excitation Cool deflects ampullofugally
• COWS: cool will result in a response to the opposite side & warm will cause activity on the same side
• limitations include only tests HSCC & SVN
• must ensure equal stimulation bilaterally
• not always well tolerated.
• keep middle ear contents in mind
• low frequency is stimulation relative to optimal frequency of stimulation of system

Analysis

-POD/butterfly: first calculate TRE then TLE

• TRE = peak RC - peak RW ( add the absolute values)
• TLE peak LW - Peak LC
• next, calculate for unilateral weakness: UW = ( TRE - TLE) / (TRE + TLE)

Interpretation of UW score

• if positive the left side is weaker
• if negative the right side is weaker.
• Bilateral HYPOFUNCTION
• indicates that TRE is less than 12 and TLE is less than 12 ( both test results)
• confirm it is not a result of medications, patient not alert, or inadequate heat transfer check with HIT, ice water, and or rotary chair test if suspecting hypofunction

Unilateral Weakness

-25% indicates either 30-40% of haircells may be damaged of attached portions of nerve fibers are

• Hyperactivity requires us to rule out calibration or technology issues but is defined by both TRE and TLE being GREATER THAN 140
• possible Cerebellar lessions
• Fixation supression- check if FIR = over 0.6. if yes then suspect degeneration

Ice water Caloric

• is when a provider fails to get a response from typical caloric tests and TRE is LESS THAN 6

VEMPS tests are split primarily for the following

• transient decreases in flexor muscle electromyographic activity of sternocleidomastiods after sounds
• Saccule activation by loud sounds tests saccule & inferior vesticular nerve -ipsilateral response from SCM
• Biphasic waveform: p13 & n23

Afferent Limbs of a VEMP

• Sacule - Scarpa's- Inferior Vestibular Nerve - medical/lateral vestibular nuclei

Efferent limb

• VN - CN XI - SCM
• Parameter
• Low freq tone (500 hz) & rate needs to be 5hz
• Best seen @ high levels around 95-100db, absent at or below 75 db. * Gating/duration: blackman window & 7msec
• Measurement Variables
• EMG activity- noise added if patient cant maintain muscle tone

C-VEMP

• Age- smaller amplitude w/age
• CHL is affected, not SNHL
• Vestibulotoxic meds can hurt C-VEMP if hair cell damage
• Analysis:
• is it present?
• amplitude symmetry b/n ears
• Age 18-40: mean +2sd = 42.84%
• age+40 mean+2sd 47.868
• OVEMP tests utricle SVN and its pathway includes all CN implicated to those points in brain -Motor neurons = ipsalteral inter oblique and contralteral Sup rectus.
• Analysis requires an all or nothing response above DB levels where you suspect them to engage
• low freq and tone at 500hz is large and high amplitude and present waveform at 400 HZ is indication of Canal

Bedside Test

Ocular Stabilty Test a test of slew test which evaluates for vertical ocular drft or central test where Pt Fixes on small object as examiner alternates covering one ye after the other testing for compensatory movement of either that just was uncovered in positive test one drifts up the other does -tests of VOr for side specific canal by having client follow your nose pt looking for complete VOR necessitating catch up saccades -head shake to test for unliateral pheripheral disfunction where pateint is tested wiht head tipped forward 30 degs adn tested for 1thz for 15 ticks and lookin for countralesional nystagmus

PT tests

-Rommberg- stand arms at side of body of crossed with eyes cloased scoring for balance Tandem gait- straightline heeltoe arms at side rating scale

• Fukuda Test- frankeshtins monster with eyes closed for 50- 100 test looking at body toration

VHIT testing

• Tests VOR Tests how head rotates to eyes ability to move in repsonse to the VOR
• looks at 6 SCC all
• looks for tracing overt catch up Saccades
• assesses Gain -normalis 79 1-20 Asymetrical testing means incomplete VOR Why you see cathcups whealthy excitation alone needs matching velocital

VHIT Vs calorics

average head rotation 2 hz and 5.8 v calorics is .003/hz and v is at 5 hzs often are aligned and what can be issues

• normals + abnormal = sit of partial dis abnorm + normal document for recovery some structure change

Posturography

• Computerized dynamic Posture cdp overall objective assesment of sensory motor helps w/non physic

Sensory organ test subtest

Eyes ope, eyes closed sway, eyes sway surface system Pattern for result 5-6 +6 and or visuals and

• and somto sensory abm 367
• Motocontrol test ability for moto Adjust relative adjust Vsrr long loop neural pathway medilagl planning and
• symmetry strength adaptation test min swaying

gfor fall

Rotary Chair

Helps evaluate for evaluationperipheral beyond low frz Patients don't like

Test to test

-Bilateral

• Tine intev pt forward degree rotation. and goggles eye movement name phase disregardless and
• Sha is when at different frez note to more adiquet

Explore the vestibulo-ocular reflex (VOR) and saccades and how they help maintain stable vision during head movements. Learn about compensatory eye movements generated by VOR and different types of saccades, including reflexive and predictive saccades. Understand the neural pathways involved and testing methods like rotational chair testing.