Structure and Function of the Semicircular Canals

Vestibular System

• The brain detects head movement and direction through comparison of inputs between the two vestibular systems.
• The Semi-Circular Canals (SCCs) work in coplanar fashion, with increased firing rate on one side and decreased firing rate on the other side during head rotation.

Push-Pull Mechanism

• The brain recognizes the difference in firing rates between the two SCCs and interprets movement.
• A faulty interpretation can lead to difficulties with gaze stabilization, postural stability, and motion perception.

Inhibitory Cutoff

• When the head is rotated to the ipsilateral side, simultaneous hyperpolarization of the opposite labyrinth occurs.
• The inhibition of the hair cells in the opposite labyrinth can only reduce the firing rate to zero, at which point the inhibition is cut off.

Velocity Storage System

• The signal generated by movement of the cupula is brief, lasting only as long as the cupula is deflected.
• This reflex is helpful to maintain corrective eye position during any change in head position and to correct eye movement rapidly.

Head Impulse Test

• The test is performed by having the patient fixate on a near target and then manually rotating their head in an unpredictable direction using a small-amplitude, moderate-velocity, and high-acceleration angular impulse.
• When the VOR is functioning normally, the eyes move in the direction opposite to the head movement and gaze remains on the target.

Head-Shaking Induced Nystagmus Test

• This test is useful in the diagnosis of a unilateral peripheral vestibular defect.
• The patient is instructed to close their eyes, and the head is shaken in a side-to-side motion.

Hair Cells

• The kinocilia and stereocilia are mechanosensing cilia and organelles.
• The direction of deflection of the hair cells tells the brain how the head is moving.
• Deflection of the stereocilia toward the kinocilia leads to excitation (depolarization), and deflection away from the kinocilia leads to inhibition (hyperpolarization).

Otolith Organs

• The Utricle is responsible for horizontal translation of the head and head tilt.
• The Saccule is responsible for vertical translation of the head.
• Together, they sense linear acceleration and static tilt of the head with respect to the gravitational axis.

Oscillopsia

• Oscillopsia is the subjective experience of motion of objects in the visual environment that are known to be stationary.
• It can occur with head movements in patients with vestibular hypofunction since the vestibular system is not generating an adequate compensatory eye velocity during the head motion.

Examination of Eye Movements

• The examination of eye movements is critical for defining and localizing vestibular pathology.
• Key tests include observation for nystagmus, Head Impulse Test, Head-Shaking Induced Nystagmus test, positional testing, and Dynamic Visual Acuity (DVA) test.

Observation for Nystagmus

• Nystagmus is the primary diagnostic indicator used in identifying most peripheral and central vestibular lesions.
• An involuntary eye movement, nystagmus due to a peripheral vestibular lesion is composed of both slow and fast components.
• The direction of the nystagmus is named by the direction of the fast component.