Nystagmus is an involuntary, rhythmic oscillation of the eyes. The prevalence of nystagmus in preschool children in the United States is estimated to be 0.35%. Nystagmus can be due to a motor defect that is compatible with relatively good vision, an ocular abnormality that impairs vision or fusion, or a neurologic abnormality. Distinguishing between these causes can be challenging. See BCSC Section 5, Neuro-Ophthalmology, for additional discussion of nystagmus.
The plane of nystagmus can be horizontal, vertical, or torsional, or a combination of these. The condition is often characterized as either jerknystagmus, which has a slow and a fast component, or pendular nystagmus, in which the eyes oscillate with equal velocity in each direction. By convention, jerk nystagmus is described by the direction of its fast-phase component; for example, a right jerk nystagmus consists of a slow movement to the left, followed by a fast movement (jerk) to the right. Nystagmus is conjugate (as opposed to dysconjugate) when its direction, frequency (number of oscillations per unit of time), and amplitude (magnitude of the eye movement) are the same in both eyes.
Nystagmus characteristics may change with gaze direction. Pendular nystagmus can become jerk nystagmus on side gaze. Jerk nystagmus can have a null point or null zone (gaze position in which the intensity [frequency × amplitude] is diminished and the vision improves), or it can decrease in intensity with gaze in the direction opposite that of the fast-phase component (analogous to Alexander’s law for vestibular nystagmus). The abnormal head position that patients assume in order to reduce nystagmus can be the most prominent manifestation of their condition.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.