Timothy C. Hain, MD. Page last modified: January 13, 2013
Saccades are used to bring the eye rapidly from one point of regard to another. As one does not see during a saccade, it is best to get them over as quickly as possible. Accordingly, saccades typically move at speeds between 200 and 600 degrees/sec -- for 300 deg/sec, to move gaze 90 degrees, it takes 1/3 second (which still seems like a rather long time not to see).
Things that happen very quickly often have a tendency to get out of control. With saccades, most disorders consist of either instability (oscillation, flutter and opsoclonus), or inability to inhibit saccades (square wave jerks, saccadic intrusions).
Although one might think that recording of a very fast eye movement would be very easy, as it is easy to spot with the naked eye, as of 2005, eye movement recording systems often fail to register saccadic nystagmus. Practically, we have failed to register obvious saccadic nystagmus with infrared reflection recording systems (one from Microguide, with a nominal 100 hz bandwidth), as well as on numerous occasions with an infrared VENG system from Micromedical Technologies. Rather than seeing back-back saccades, saccadic nystagmus looks like square wave jerks (see below).
This failure of many contemporary ENG clinical systems to work for this disorder is related to a combination of factors:
1. Saccadic nystagmus may exceed the bandwidth of the recording system. A 20 hz recording will not be able to capture a 10 hz. event.
2. Blink rejection algorithms that consider saccadic nystagmus as noise. Rapid back-back saccades appear similar to noise and may be replaced by a flat line by some EOG systems.
3. Saccadic nystagmus may be too small to resolve (as well as too fast). Microsaccadic oscillation may only be 0.2 deg in amplitude -- impossible to resolve with EOG or most IR systems.
4. Many systems use tiny eye-images, shown at low resolution and infrequently updated on a computer monitor. This just doesn't work at all.
Our advice is to either directly record the nystagmus with an analog VCR, a direct to digital method such as a DV recorder, or use a high-performance recording system such as a scleral eye coil. In our experience, infrared recording methods including commercial video ENG systems will not reproduce this nystagmus because of their limited spatial and temporal resolution, but DV recorders combined with optics that produces an iris image that nearly fills the screen work acceptably.
Do not rely upon commercial VENG systems to capture saccadic nystagmus -- you have to look yourself.
- Microsaccadic oscillation (MuSO)
- Macrosaccadic oscillation (MSO)
- Ocular flutter
- Square Wave Jerks (SWJ)
- Saccadic Intrusions
|Microsaccadic oscillation (image courtesy of D. Zee, M.D., recording method -- scleral search coil)|
Microsaccadic oscillation denotes a tiny (0.2 deg) back-back saccadic oscillation. It is generally benign although it can obscure vision. Microsaccadic oscillation cannot be recorded with EOG or VNG because it is too small and too fast. It is best seen with the ophthalmoscope in the hands of an experienced clinician, or a high-resolution eye movement recording device such as a scleral eye coil (see figure above).
MSO is a disorder where the eye makes saccades back and forth about a target. This is usually a sign of a serious cerebellar disorder.
|Ocular flutter in a normal person (sawtooth waveform).|
Ocular flutter denotes an instability of the eye where after the main saccade, the eye makes another saccade in the opposite direction. Ocular flutter is generally caused by cerebellar disorders or brainstem disorders (Bergenius, 1986; Schon, 2001). The recording above was made with an eye-coil. This accounts for it's ability to resolve this rapid and tiny waveform. Recordings made with infrared and EOG are usually unsuccessful.
The most common clinical syndrome is the "benign-encephalitis" syndrome with ocular flutter. These persons are typified by ataxia and shimmering eye movements. They can be later recognized because their eyes shimmer under closed eyelids (one can see the eye movement under the lids).
A single patient have been reported with anti-GQ1b antibodies (Zaro-Weber et al, 2008), and there are also pediatric case series suggesting an autoimmune disorder.
Although in theory drugs that slow down saccades should be helpful, at this writing little to no treatment is available for ocular flutter. Recently it has been reported that Levatactam was helpful in a single case (Eggenberger and Cherian, 2006). The author has tried many other medications -- including gabapentin and benzodiazepines -- with modest success.
As ocular flutter may be an early variant of a much more dangerous syndrome called opsoclonus (see following), some caution must be exercised, if the syndrome is recent in onset.
Ocular flutter due to disease is rare but about 8% of the normal population can voluntarily produce ocular flutter, usually during convergence. This can be spotted because the pupil constricts during the convergence effort.
When voluntarily induced, ocular flutter it instead is called "voluntary nystagmus". Most often ocular flutter is a variant of voluntary nystagmus as cerebellar disorders are much rarer.
Some patients who are dizzy use voluntary nystagmus to suppress their sensation of dizziness. This can cause a confusing picture of ocular flutter with an underlying nystagmus.
Supplemental material on the site DVD: Video of voluntary nystagmus.
Supplemental material on the site DVD: Another video of voluntary nystagmus. (Courtesy of Dr. Dario Yacovino).
Note how the pupil constricts during the rapid horizontal shimmering.
Video of opsoclonus in young woman, developed after the West Nile outbreak in Chicago. See the site DVD page for a list of more movies like this one.
Opsoclonus denotes chaotic back-back saccadic eye movements. It is a dramatic syndrome, usually indicating either cancer or a brainstem encephalitis. Opsoclonus essentially differs from flutter in that opsoclonus is oriented in any direction (horizontal, vertical, torsion), while flutter is generally always purely horizontal. It may look like a "shimmer" on direct observation. Opsoclonus may be difficult to record. It occurs so quickly -- with a frequency of up to 20 hz -- that it cannot be captured by low-bandwith devices such as clinical EOG or VNG systems. Videotaping may also have trouble capturing this rapid movement as the frame rate may be only 30 fps. Opsoclonus is generally best visualized by using a video-frenzel goggle system, having a large screen.
Opsoclonus is nearly always caused by a neural tumor (a neuroblastoma) or a paraneoplastic syndrome (a tumor elsewhere in the body). These diagnoses are very rare and accordingly opsoclonus is rarely seen and even more rarely recognized. A full-scale workup for neoplasm is generally indicated and productive in persons with opsoclonus. Lung cancer is the commonly found tumor, and thus a chest-Xray (rather than an MRI) is usually the most productive first test. Recently, PET scanning has been used to diagnose occult cancers in this situation (Bataller et al, 2003). Although antibodies such as anti-Hu, Yo, and Ri among others may occasionally be positive, commercial testing for antibodies is often of little diagnostic value. Recent research studies have implicated autoantibodies to so far unknown neuronal surface antigens (Blaes, Fuhlhuber et al. 2005)
Opsoclonus may also be caused by a viral infection of the brainstem or cerebellum, as well as autoimmune processes. A huge surge in opsoclonus appeared in Chicago, after the West Nile Virus outbreak of 2003. Nevertheless, this is usually a "wastebasket" type diagnosis, arrived at after screening for cancer has been unproductive.
Opsoclonus is classically attributed to malfunction of the pause-cells in the midline brainstem. However, functional MRI in two patients suggest increased activation in the deep cerebellar nuclei in opsoclonus (Helmchen et al, 2003). The fastigial oculomotor region projects to the burst neurons, omnipause neurons, and the local feedback loop of the brainstem saccade generator. The fastigial oculomotor region is inibited by the vermis. Thus the fundamental underlying problem in opsoclonus may relate to decrease activation in the posterior vermal lobule VII.
SQUARE WAVE JERKS (SWJ)
|Square wave jerks|
Square-Wave Jerks (SWJ) are inappropriate saccades that take the eye off the target, followed by a nearly normal intersaccadic interval (approximately 200 msec), and then a corrective saccade that brings the eye back to the target (Leigh and Zee, 1983).
Follow the link for more information on SWJ.
Saccadic intrusions are unintended saccades, not necessarily followed by a return movement as is the case for square wave jerks. Saccadic intrusions are very common and have very little diagnostic significance. Horizontal saccadic intrusions are most commonly attributed to psychiatric disease such as schizophrenia. In the vertical and torsional planes, saccadic intrusions are instead attributed to neurological disorders. Vertical or torsional saccadic intrusions may arise from irritibility of burst neurons in the midbrain (Bentley et al, 1998).
Timothy C. Hain, MD
Please read our disclaimer Return to Index. Page last modified: January 13, 2013
|© Copyright January 13, 2013 , Timothy C. Hain, M.D. All rights reserved. Last saved on January 13, 2013|