FIXATION INSTABILITY (SACCADIC INTRUSIONS)

INDEX

Ocular flutter

Ocular flutter is a saccadic intrusion characterised by very-high-frequency (10 to 25 cycles per second), high-velocity, back-to-back horizontal saccades that oscillate about the midline and have no intersaccadic intervals between subsequent saccades.

Microflutter and micro-opsoclonus mayoccur and may carry the same concerns regarding underlying paraneoplastic conditions1.

Pathogenesis
Flutter and opsoclonus are associated with pathology affecting either the pons,where burst neurons that control horizontal saccades are located, or the cerebellum. It is likely that increased GABA-A receptor sensitivity is present in a circuit involving the cerebellum, inferior olives, and brainstem saccadic premotor burst neurons. There is likely cerebellar/brainstem dysfunction that inhibits omnipause neurons (located in the pons) and activates burst neurons (located in the paramedian pontine reticular formation, and in the case of opsoclonus, rostral interstitial nucleus of the medial longitudinal fasciculus), resulting in uncontrolled, involuntary saccadic intrusions.

Inherent properties of the saccadic burst neurons themselves predispose them to oscillating, and ion channel dysfunction of burst neurons is also proposed as a contributory mechanism.

Causes

  1. Paraneoplastic conditions: associated with the following antibodies: amphiphysin, anti–neuronal nuclear antibody type 2 (ANNA-2, or anti-Ri), Hu (ANNA-1), Yo, Ma, P/Q-type calcium channel, N-methyl-D-aspartate (NMDA) receptor, GAD, and GQ1b
  2. Brainstem encephalitis (including HIV)
  3. Multiple Sclerosis

     
Figure 1. Ocular flutter. Note back-to-back saccade without fixation between saccades.


 

 

Videos 1 and 2. Ocular Flutter

 

(vv)Ocular Flutter.mp4(tt)

From: Zaro-Weber O, Galldiks N, Dohmen C, Fink GR, Nowak DA. Ocular flutter, generalized myoclonus, and trunk ataxia associated with anti-GQ1b antibodies. Arch Neurol. 2008;65(5):659-661. doi:10.1001/archneur.65.5.659

(vv)Flutter.mp4(tt)

From: Rucker JC. Nystagmus and Saccadic Intrusions. Continuum (Minneap Minn). 2019;25(5):1376-1400. doi:10.1212/CON.0000000000000772

 

 

Video 3. Ocular Flutter in Serotonin Syndrome

 

(vv)Serotonin.mp4(tt)

From: Sim SS, Sun JT. Ocular Flutter in the Serotonin Syndrome. N Engl J Med. 2016;375(18):e38. doi:10.1056/NEJMicm1506066
 

 

References

  1. Rucker JC. Nystagmus and Saccadic Intrusions. Continuum (Minneap Minn). 2019;25(5):1376-1400. doi:10.1212/CON.0000000000000772