2025 Volume 42 Issue 2 Pages 173-187
Nystagmus is an abnormal, rhythmic, to-and-fro eye movement characterized by alternating slow and fast phases. During the slow phase, the eyes pathologically shift, followed by a fast phase that returns them to their original position. Nystagmus can be categorized into four types: spontaneous, positional, positioning, and gaze-evoked.
Spontaneous nystagmus is sustained and unidirectional, resulting from an imbalance in the semicircular canal-vestibular system. This includes the anterior semicircular canal reflex, causing contralateral rotation and upward deviation of the eye; the lateral semicircular canal reflex, resulting in contralateral deviation of the eye; and the posterior semicircular canal reflex, leading to contralateral rotation and downward deviation of the eye.
Positional nystagmus is induced by specific head positions and is sustained; however, it may change direction or disappear, when the head position is altered. This is an abnormal response of eye’s position to gravity and is caused by abnormalities in the otolithic vestibular system or by cupulolithiasis.
Positioning nystagmus is a transient condition triggered by specific head movements and is caused by abnormal endolymph flow. This abnormal flow is typically due to free-floating otoconia moving within one of the semicircular canals, a condition known as canalolithiasis.
Finally, gaze-evoked nystagmus is triggered by gazing at a target in the peripheral visual field. It is caused by centripetal deviation of the eyes due to a disorder in the gaze-holding mechanism.
