Nystagmus physiology: Guyton and Hall flow chart

1. Normal purpose: stabilize the visual image on retina

Head rotates suddenly
        ↓
Semicircular ducts detect angular acceleration
        ↓
Vestibular nerve firing changes
        ↓
Vestibular nuclei in pons-medulla
        ↓
Medial longitudinal fasciculus (MLF)
        ↓
Oculomotor nuclei (III, IV, VI)
        ↓
Extraocular muscles contract
        ↓
Eyes move equal and opposite to head rotation
        ↓
Image remains steady on the retina
        ↓
Vestibulo-ocular reflex (VOR)

2. How vestibular nystagmus develops

Continuous rotation of head/body to one side
        ↓
Endolymph initially lags behind because of inertia
        ↓
Cupula in semicircular duct bends
        ↓
Unequal vestibular input from right and left labyrinths
        ↓
Vestibular nuclei interpret this as head rotation
        ↓
VOR produces slow eye movement
        ↓
Eyes drift slowly opposite to direction of head rotation
        ↓
Eyes reach eccentric position
        ↓
Cerebral/brainstem saccadic mechanism rapidly resets eyes
        ↓
Fast movement occurs in direction of rotation
        ↓
Repeated slow drift + rapid resetting movement
        ↓
NYSTAGMUS

Definition

Nystagmus = rhythmic oscillation of the eyes
           = slow phase + fast phase

• Slow phase: vestibular reflex phase. It is the physiologically important corrective eye movement.
• Fast phase: rapid resetting saccade.
• Nystagmus is named according to the fast phase.

Head turns RIGHT
        ↓
Slow eye movement LEFT
        ↓
Fast resetting movement RIGHT
        ↓
Right-beating nystagmus

3. Example: rotation to the right

Person rotates to the RIGHT
        ↓
Right horizontal semicircular duct is stimulated more
Left horizontal semicircular duct is relatively inhibited
        ↓
Vestibular nuclei generate VOR
        ↓
Eyes move slowly to the LEFT
        ↓
When eyes approach the left orbital limit
        ↓
Fast corrective saccade moves eyes to the RIGHT
        ↓
Cycle repeats
        ↓
RIGHT-BEATING NYSTAGMUS

4. What happens if rotation continues?

Constant rotation continues
        ↓
Endolymph gradually catches up with the rotating semicircular ducts
        ↓
Cupula returns toward its resting position
        ↓
Vestibular nerve asymmetry declines
        ↓
Slow phase declines
        ↓
Nystagmus progressively fades

5. Post-rotatory nystagmus

Rotation suddenly stops
        ↓
Semicircular ducts stop first
        ↓
Endolymph continues moving briefly because of inertia
        ↓
Cupula bends in the opposite direction
        ↓
Vestibular input now signals rotation in the opposite direction
        ↓
Slow phase reverses
        ↓
Fast phase reverses
        ↓
Post-rotatory nystagmus occurs in the opposite direction

Example

Rotate RIGHT, then stop suddenly
        ↓
Endolymph continues moving relative to the canal
        ↓
Brain perceives LEFTWARD rotation
        ↓
Slow eye movement RIGHT
        ↓
Fast eye movement LEFT
        ↓
LEFT-BEATING post-rotatory nystagmus

6. Role of cerebellum

Vestibular signals
        ↓
Vestibular nuclei + flocculonodular cerebellum
        ↓
Calibration and damping of vestibular eye responses
        ↓
Stable gaze during head movement

Cerebellar lesion

Flocculonodular lobe dysfunction
        ↓
Inadequate damping/calibration of eye movements
        ↓
Inability to hold steady eccentric gaze
        ↓
Rapid tremulous eye movements
        ↓
Cerebellar (gaze-evoked) nystagmus

One-line exam summary

Nystagmus is a rhythmic eye movement produced when asymmetric vestibular input causes a slow vestibulo-ocular drift of the eyes, followed by a fast resetting saccade; its direction is named by the fast phase.

Explanation of the waveform in your slide

Y-axis: Eye position
X-axis: Time

Eye gradually moves in one direction
        ↓
This is the SLOW PHASE
        ↓
Caused by vestibular drive through the vestibulo-ocular reflex
        ↓
The eye drifts away from its fixation target
        ↓
Eye reaches an eccentric position
        ↓
Central saccadic system produces a rapid corrective movement
        ↓
This is the FAST PHASE
        ↓
Eye is reset toward its starting/central position
        ↓
The same cycle repeats
        ↓
JERK NYSTAGMUS

Reading the graph shown

Gradual upward sloping line
        ↓
Slow upward drift of the eye
        ↓
Vestibular slow phase
        ↓
Sudden sharp downward line
        ↓
Fast downward reset of the eye
        ↓
Cycle repeats

Therefore, in this particular drawing

Slow phase: upward
Fast phase: downward
        ↓
Called: down-beating jerk nystagmus

Fast phase to right  → right-beating nystagmus
Fast phase to left   → left-beating nystagmus
Fast phase upward    → up-beating nystagmus
Fast phase downward  → down-beating nystagmus

Physiological sequence behind it

Head rotation / unequal vestibular input
        ↓
Vestibular nuclei are activated
        ↓
Signals pass via medial longitudinal fasciculus
        ↓
Extraocular muscles produce slow compensatory eye movement
        ↓
Eye slowly drifts in one direction
        ↓
Rapid central reset brings eye back
        ↓
Recurrent drift + reset
        ↓
Jerk nystagmus

Easy memory line

Slow phase = vestibular abnormal drift
Fast phase = central corrective reset
Name nystagmus by the fast phase

Caloric test: flow chart

Purpose

Caloric test
        ↓
Tests vestibulo-ocular reflex (VOR)
        ↓
Assesses mainly the horizontal semicircular canal
and superior vestibular nerve of each ear separately
        ↓
Produces vestibular nystagmus

Position of patient

Patient lies supine
        ↓
Head elevated by 30°
        ↓
Horizontal semicircular canal becomes vertical
        ↓
Thermal convection currents can act on endolymph

1. Warm-water caloric test

Warm water introduced into RIGHT external auditory canal
        ↓
Endolymph in right horizontal semicircular canal becomes warmer and lighter
        ↓
Endolymph rises
        ↓
Ampullopetal flow
        ↓
Excitation of right horizontal semicircular canal
        ↓
Brain interprets this as head turning to the RIGHT
        ↓
Vestibulo-ocular reflex causes slow eye movement to LEFT
        ↓
Central saccadic reset causes fast eye movement to RIGHT
        ↓
RIGHT-BEATING NYSTAGMUS

Warm water → fast phase toward SAME side

Example

Warm water in right ear
        ↓
Fast phase to right
        ↓
Right-beating nystagmus

2. Cold-water caloric test

Cold water introduced into RIGHT external auditory canal
        ↓
Endolymph in right horizontal semicircular canal becomes colder and denser
        ↓
Endolymph sinks
        ↓
Ampullofugal flow
        ↓
Inhibition of right horizontal semicircular canal
        ↓
Brain interprets this as head turning to the LEFT
        ↓
Vestibulo-ocular reflex causes slow eye movement to RIGHT
        ↓
Central saccadic reset causes fast eye movement to LEFT
        ↓
LEFT-BEATING NYSTAGMUS

Cold water → fast phase to OPPOSITE side

Example

Cold water in right ear
        ↓
Fast phase to left
        ↓
Left-beating nystagmus

COWS mnemonic

C = Cold
O = Opposite side fast phase

W = Warm
S = Same side fast phase

COWS names the fast phase, because nystagmus is conventionally named by its fast phase.

In a comatose patient

Cold water in one ear
        ↓
Vestibular apparatus and brainstem intact
        ↓
Slow tonic eye deviation toward the irrigated ear
        ↓
No fast corrective phase
        ↓
Because the fast phase requires intact cerebral cortical function

No tonic eye deviation after cold caloric irrigation
        ↓
Suggests severe brainstem dysfunction
or absent vestibular function

One-line viva answer

Caloric testing uses warm or cold irrigation of the external auditory canal to induce endolymph convection in the horizontal semicircular canal, causing nystagmus: Cold Opposite, Warm Same, referring to the fast phase.