FIXATION INSTABILITY (SACCADIC INTRUSIONS)

INDEX

Square wave jerks (SWJs) are eye movements of 1-5° away and back from the neutral position, with an intersaccadic interval of about 200 msec.  Up to 15 per minute may be seen in normal individuals, more usually about 5/minute1,2.  If required, the patient may be asked to fixate on a stationary target for up to 30 seconds.

SWJs are seen with:

  1. Cerebellar disease, notably Friedreich's ataxia.
  2. Parkinson's disease (PD), and Parkinsonian syndromes, such as progressive supranuclear palsy (PSP), multiple system atrophy and corticobasal degeneration. SWJs tend to be larger in PSP than in PD.
Figure 1. Square wave jerks. Fixation of approximately 200 ms between two saccades


Figure 2. Frequent square wave jerks are characteristic findings of progressive supranuclear palsy syndrome16.

From: Jung I, Kim JS. Abnormal Eye Movements in Parkinsonism and Movement Disorders. J Mov Disord. 2019;12(1):1-13.

Pathogenesis
Omnipause neurons (OPNs) located within the pontine raphe interpositus nucleus (RIP) exert a tonic inhibition on burst neurons during fixation and smooth pursuit. Analsysis of the RIP in patients with PSP showed a significant correlation between oculomotor abnormalities and loss of OPNs in this area.
SWJs may also be caused by lesions of the projections from the frontal eye field and the rostral pole of the superior colliculus, 

Treatment
SWJ may be treated with methylphenidate, diazepam, or phenobarbital.
 


 

Video 1. Intermittent Square Wave Jerks

 

(vv)digre.mp4(tt)

From: Digre K.  Intermittent Square wave jerks. Moran-NOVEL Collection. Retrieved from: https://morancore.utah.edu/section-05-neuro-ophthalmology/intermittent-square-wave-jerks/
 


 

Video 2. Square Wave Jerks

 

(vv)SWJ.mp4(tt)

From: Digre K, Jacobson D.  Square wave jerks. Moran-NOVEL Collection. Retrieved from: https://morancore.utah.edu/section-05-neuro-ophthalmology/square-wave-jerks/
 


 

Video 3. The video shows alternating horizontal saccades that intrude on steady fixation. The eyes immediately drift back after each saccade, which indicates a saccadic pulse without a step signal (pulse-step mismatch). These eye movements are consistent with single saccadic pulses.

 

(vv)758433.mp4(tt)

From: Lee E-S, Choi J-Y, Kim J-S. Alternating horizontal single saccadic pulses in progressive supranuclear palsy. Teaching Video NeuroImages: Neurology Jan 2017, 88 (3) e32-e33; DOI: 10.1212/WNL.0000000000003520
 



 

References

  1. Danchaivijitr C, Kennard C. Diplopia and eye movement disorders. J Neurol Neurosurg Psychiatry. 2004;75 Suppl 4:iv24-31.
  2. Wong, A. M. (2008). Eye movement disorders. Oxford: Oxford University Press.