Saccadic intrusions are spontaneously generated involuntary eye movements which disrupt steady fixation. Patients typically complain of shimmering-jiggling or wavy vision, representing oscillopsia, an illusion of motion in the stationary environment.  Dizziness and disequilibrium are also described. 

Saccadic intrusions are initiated by fast saccadic movements that drive the eye away from central fixation. As a group, they are often provoked by shifts of gaze (ie, having the patient look eccentrically and then back to center will often induce a saccadic intrusion). They should be differentiated from saccadic dysmetria, in which the eyes overshoot or undershoot a target following an intended saccade¹.

Saccadic intrusions are divided into two broad categories: those with an intersaccadic interval between subsequent saccades and those without such an interval¹:

• Saccadic intrusions with an intersaccadic interval include square-wave jerks, macro–square-wave jerks, and macrosaccadic oscillations. 
• Saccadic intrusions without an intersaccadic interval include ocular flutter and opsoclonus.

With saccadic intrusions, the initial eye movement away from the target is a saccade. By contrast, with nystagmus, the initial eye movement is generally a slow phase. More importantly, unlike nystagmus, saccadic intrusions have no slow phase drift². 

Voluntary nystagmus also falls into the category of a saccadic intrusion, as it is made up only of fast saccadic eye movements, without any initial slow drift of the eyes and is, therefore, not truly nystagmus¹.

Ocular flutter and opsoclonus are important causes of oscillopsia, and may occur with very tiny amplitudes, making them invisible to the naked eye. Any patient with new-onset oscillopsia that is unexplained on examination should have a careful assessment for small oscillations on ophthalmoscopy.

Pathogenesis

For steady gaze fixation, pause cells, residing between the rootlets of the abducens nucleus, tonically inhibit burst neurons and thereby prevent the occurrence of unwanted saccadic pulses. 
The presence of an intersaccadic interval signifies the integrity of both pause cells that stop saccades and the neural integrator that sustains eye position between saccades³.

See also:  Physiology of Saccades