Timothy C. Hain, MD. Page last modified: August 2, 2020
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 2014, 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 (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 has been reported with anti-GQ1b antibodies (Zaro-Weber et al, 2008), and there are also pediatric case series suggesting an autoimmune disorder. As a general comment, antibodies to various neuronal antigens are always being reported in neurological syndromes - - one should not make much of any individual report.
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 Levataceteram was helpful in a single case (Eggenberger and Cherian, 2006). The author has tried many other medications -- including gabapentin and benzodiazepines -- with modest success. Gabapentin is the most useful.
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.
Opsoclonus denotes chaotic back-back saccadic eye movements. It is a dramatic syndrome, sometimes due to cancer or a brainstem encephalitis such as West-Nile or Dengue. See this link for more information about opsoclonus.
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).