Jerk nystagmus is a rhythmic eye oscillation characterized by a slow drift of the eyes in one direction that is repeatedly corrected by fast movements in the reverse direction. In most cases the slow drift is the problem, of which there are two main causes:
- A tonic imbalance in a "slow eye movement" system, such as the vestibular or pursuit systems. This is most often due to an acute asymmetry in vestibular activity, either peripheral or central. (See "Overview of nystagmus".)
- An inability to hold an eccentric eye position against the normal viscoelastic forces that tend to bring the eye back to midline ("gaze-holding").
In both types, corrective fast eye movements restore the eye to its desired position. These quick or fast phases are likely generated through the same brainstem structures that create saccades.
Jerk nystagmus is subdivided by trajectory and the conditions under which it occurs (table 1). Some forms are always present, even when the eyes are in the primary position. Nystagmus in the primary position is classified according to trajectory: downbeat; upbeat; horizontal; torsional; or mixed. The direction named is the direction of the fast phase. Other forms emerge only under specific conditions such as peripheral gaze (gaze-evoked) and certain head positions (positional).
PRIMARY POSITION JERK NYSTAGMUS
Nystagmus in the primary position is classified according to trajectory: downbeat, upbeat, horizontal, torsional, or mixed.