FIXATION INSTABILITY (SACCADIC INTRUSIONS)

INDEX

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I ANATOMY                

II PHYSIOLOGY                

III EYE MOVEMENTS & NYSTAGMUS

V SUPRANUCLEAR to NUCLEAR   

VI VESTIBULAR SYSTEM

 VII CEREBELLAR EYE   MOVEMENTS

VIII CN PALSIES, VISUAL FIELDS, PUPIL & THE EYE

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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:

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

 

References
  1. Rucker JC. Nystagmus and Saccadic Intrusions. Continuum (Minneap Minn). 2019;25(5):1376-1400. doi:10.1212/CON.0000000000000772
  2. Eggers SDZ, Bisdorff A, von Brevern M, et al. Classification of vestibular signs and examination techniques: Nystagmus and nystagmus-like movements. J Vestib Res. 2019;29(2-3):57-87. doi:10.3233/VES-190658
  3. Wong, A. M. (2008). Eye movement disorders. Oxford: Oxford University Press.