2023 Volume 82 Issue 2 Pages 105-113
Electrophysiological measurement of the eye rotation axis using stimulation of a single semicircular canal nerve showed that the eye rotated around the axis perpendicular to the plane of the stimulated semicircular canal. Therefore, the affected semicircular canal can be identified by analyzing the eye rotation axis in cases of abnormal nystagmus. When the main component of the abnormal nystagmus is horizontal, the origin of the nystagmus is the lateral semicircular canal. When the main component of the abnormal nystagmus is torsional, the origin of the nystagmus is the anterior and/or posterior semicircular canal. The eye rotation axis in cases of excitatory nystagmus is quite the same as that in cases of inhibitory nystagmus, although the direction of eye rotation is opposite between cases of excitatory and inhibitory nystagmus. Vestibular neuritis mostly involves the superior vestibular nerve. The superior vestibular nerve transmits sensory information transmitted by from the vestibular hair cells located in the anterior and lateral semicircular canals. Therefore, patients with vestibular neuritis exhibit nystagmus with both horizontal and torsional components caused by inhibition of both the anterior and lateral semicircular canals. In patients with Ménière's disease, during a vertigo attack, excitatory nystagmus of anterior and/or posterior and/or lateral semicircular canal origin can be seen. Because the involving ratio of each contributing ratios of the three semicircular canals to nystagmus can vary, the ratio of the torsional component of the nystagmus to the horizontal component also varies. While nystagmus is purely horizontal in some cases, it is purely torsional in others. In the posterior canal type of BPPV, during the Dix-Hallpike maneuver, transient torsional nystagmus with the torsional component directed toward the affected side can be seen. In the lateral canal type of BPPV (canalolithiasis), geotropic positional nystagmus can be seen when the patient is supine. In the lateral canal type of BPPV (cupulolithiasis), apogeotropic positional nystagmus can be seen when the patient is supine.
