Head or body movement or head tilt triggers a signal in the semicircular canals and otoliths
Signal goes from the inner ear via the vestibular nerve to the medullary vestibular nuclei
To generate the horizontal vestibulo-ocular reflex, the signal goes directly from the vestibular nucleus to the sixth nerve nucleus and from there via the MLF to the medial rectus nucleus
and eventually to the lateral and medial rectus muscles to move the eyes horizontally
To generate the vertical vestibulo-ocular reflex, the signal travels from the vestibular nuclei on both sides to the midbrain and from there to appropriate extraocular muscles to produce vertical shifts in the eyes
To generate the torsional vestibulo-ocular reflex, the signal also goes to the midbrain and from there to appropriate extraocular muscles to produce conjugate ocular torsional movements
Unilateral lesion of the semicircular canals, otolith, or vestibular nerve
Produces jerk nystagmus with its fast component in a direction opposite to the side of the lesion
Produces small horizontal contraversive conjugate eye movements in the head impulse test when the head is moved rapidly to the side of the lesion
Common causes: vestibular neuritis, labyrinthitis
Bilateral lesions of the semicircular canals, otolith, or vestibular nerves
Produces horizontal contraversive binocular saccades in the head impulse test when the head is moved rapidly back and forth
Produces impaired visual acuity in the dynamic visual acuity test when the head is moved passively back and forth
Common cause: ototoxicity from aminoglycoside medication
Tip:
bilaterally symmetric dysfunction of the semicircular canals or otoliths does not cause nystagmus!
Lesion of the pons or medulla
Produces an abnormal static ocular tilt reaction, which causes conjugate ocular torsion and vertical misalignment of the eyes (“skew deviation”)
Tip:
the patient does not notice the conjugate ocular torsion, but does notice the vertical misalignment as diplopia or blurred vision; the vertical misalignment, often small, can usually be detected with the cover test
May produce downbeat nystagmus by damaging the downgaze vestibulo-ocular reflex more than the upgaze vestibulo-ocular reflex
May produce upbeat nystagmus by damaging the upgaze vestibulo-ocular reflex more than the downgaze vestibulo-ocular reflex
Lesion of the thalamus or midbrain
May produce a static ocular tilt reaction, which causes persistent skew deviation
May produce an oscillating ocular tilt reaction, which causes seesaw nystagmus
May produce intermittent (“paroxysmal”) skew deviation