GAZE EVOKED NYSTAGMUS
Timothy C. Hain, MD
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An important point of information to be gained from the fixation test is the adequacy of gaze holding, as impaired gaze holding may indicate the presence of a central (cerebellar or brainstem) lesion.
Gaze-evoked nystagmus (GEN) is a drift of the eye which is only present for certain directions of gaze away from straight ahead. It is the most common form of nystagmus encountered in clinical practice. When using EOG recordings, any persistent nystagmus for ocular displacements of 30 degrees or less is considered abnormal. When using infrared recordings, small amounts of weak (0.5 deg to 3.0 deg/sec) gaze-evoked nystagmus can be recorded in normal subjects (Abel et al, 1978).
"End point nystagmus" is a variant of GEN. It is basically GEN in persons who are otherwise normal. It is more frequently seen with prolonged gaze holding and also with large eccentricities. It is often somewhat torsional. The judgement as to whether nystagmus is called GEN or "end point", is presently one made by the experience of the examiner. We think it best to avoid the term "end point nystagmus" entirely.
Causes of gaze-evoked nystagmus are listed in the table above. There are several distinct patterns which can be identified by close scrutiny of the eye position trace. (Note that we didn't say that you can get this by reading the computerized ENG report - - commercial ENG computers don't understand GEN).
Ordinary gaze-evoked nystagmus, acquired from an ancient ENG system. There is drift towards the center when the patient looks to either side.
Supplemental material on the site DVD: Video of gaze-evoked nystagmus (patient with cerebellar subarachnoid cyst, courtesy of Dr. Dario Yacovino).
The most common variety of GEN consists of a drift towards the center of the orbit, interspersed by corrective outgoing saccades attempting to acquire a target which has drifted off the fovea. In this situation, the initial rate at which the eye drifts is directly proportional to how far the eye is from center, because elastic restoring forces are proportional to displacement. Accordingly, as the eye approaches center, the rate of drift decreases, accounting for the characteristic decreasing exponential trajectory of ocular drift. The decreasing exponential pattern may be difficult to appreciate if the patient makes frequent saccades to the target, and one must look for a slow phase in which the patient allowed his eye to drift close to the center. Gaze-evoked nystagmus on lateral gaze and upward gaze is common while gaze-evoked nystagmus on downward gaze is infrequent.
Certain patients with congenital nystagmus or with acquired central nystagmus varieties have increasing exponential velocity patterns. More about this is below.
Rarely, GEN is due to weak eye muscles, such as in myasthenia gravis. As myasthenia can affect eye muscles to different extents at different times, the variations are many. A nystagmus that comes and goes, and that is asymmetrical between the eyes might be myasthenia. This type of nystagmus can change dramatically after injection of a medication that temporarily improves muscle strength (such as edrophonium). Any change is counted as a positive "tensilon test".
Amount of GEN
There are several factors which contribute to the amount of GEN. The first relates to the patterns of neural firing associated with maintenance of eye position against elastic restoring forces. Central disorders, particularly those involving the cerebellum, can disrupt the neural "step" of firing, and cause centripetal drift.
A second consideration relates to how proficiently the patient can use visual tracking mechanisms such as pursuit or optokinetic responses to offset and eliminate drift, even though it is self-generated. A person can have GEN in the dark, but not in the light, if they have good tracking. On ENG's, we don't think it is necessary to record GEN in the light, because this information is obtained in the saccade test. However, we do think it is prudent to record GEN (and rebound) in the dark.
A third factor relates to the frequency with which the patient develops corrective saccades. Because these factors are normally not controlled for, the judgment that a patient has an abnormal amount of GEN is usually a qualitative one, based on the observation that the patient has nystagmus on gaze in one direction, but none in the other, or nystagmus for unusually small displacements from center.
Medication and GEN
GEN is an extremely common consequence of medication, especially sedatives or anticonvulsants. Phenytoin, for example, can be monitored through watching for GEN. No GEN generally means that the patient is not taking his/her medication. All seditives produce GEN. Alcohol and recreational drugs commonly cause GEN - -Ketamine being an extreme example.
Technical Issues regarding GEN
There are several major problems in interpreting GEN, mainly deriving from technical error.
If your equipment doesn't record the eyes looking far to one side (i.e. saturate), you will think there is no GEN. This is mainly an issue with infrared recording systems, which are rarely used in present day ENG equipment (because they saturate)
Another common problem is that the person testing for GEN makes one of several common errors -- they may not have the person look far enough, or not encourage them to hold gaze for 10 full seconds. They may allow the person to move their head and eyes. They may allow the person to close their eyes -- no video system can track through closed eyes. All of these errors can be difficult to catch when one is presented with a recording loaded with strange findings. The best way to catch these things is to compare the clinical exam with the recording, and in-service the person doing testing when there are discrepencies.
Asymmetric gaze-evoked nystagmus: Alexander's law -- the gaze-evoked nystagmus seen in vestibular disorders.
Supplemental material on the site DVD: Video of Alexander's Law
Gaze-evoked nystagmus which is of greater when looking in one direction than the other occurs in several situations. In vestibular disorders, when gaze-evoked nystagmus is combined with a spontaneous nystagmus, they add when gazing towards the fast phase of the spontaneous nystagmus and subtract in the opposite direction. This often results in the pattern of a greater overall nystagmus when gazing towards the fast-phase direction of the spontaneous nystagmus. This common clinical pattern is called "Alexander's law" (Robinson et al, 1984), and occurs in patients with peripheral and in some patients with central vestibular imbalance.
Brun's nystagmus, which occurs in patients with cerebellar lesions, refers to asymmetrical nystagmus in which there is little or no spontaneous nystagmus in primary position, but an asymmetry exists at the extremes of lateral gaze. Baier and Dieterich recently reported a study of Brun's (see end of this page).
Patients with internuclear ophthalmoplegia (INO) often exhibit a discongugate gaze-evoked nystagmus in which the abducting eye exhibits a more prominent nystagmus than the adducting eye.
Gaze-evoked nystagmus seen in CN (Congenital Nystagmus)
Certain patients with congenital nystagmus or with rare acquired central nystagmus varieties often have "increasing exponential" velocity patterns. Often the eyes "jump the wrong way" -- in other words, one might see right-beating nystagmus in left gaze.
These waveforms are usually easy to spot because the nystagmus is so vigorous. Persons with CN also generally have less nystagmus in the dark as well as nystagmus in central gaze. Another clue is the history of nystagmus from infancy.
There are people with mild variants of CN who are misdiagnosed as something else -- such as multiple sclerosis, or a cerebellar degeneration. This can be easily figured out with some thought -- these people don't have any neurological findings other than CN, and they don't get worse over time.
Gaze-evoked nystagmus seen in INO
Patients with a saccadic disorder called internuclear ophthalmoplegia (INO) often exhibit a discongugate GEN in which the abducting eye exhibits a more prominent amplitude nystagmus than the adducting eye. This is easily detected through the saccade test, which shows slowing of adducting saccades.
Gaze-evoked nystagmus seen in cerebellar disorders.
Patients with cerebellar disturbances often have gaze-evoked nystagmus. The common syndromes are:
- Too much gaze-evoked nystagmus for medication (see discussion above)
- Downbeating nystagmus on lateral gaze
- Rebound nystagmus
These are all dealt with elsewhere except for downbeating nystagmus on lateral gaze. An example of this is shown below.
Downbeating nystagmus on lateral gaze. Recorded using an Micromedical Technology IR tracking system at Chicago Dizziness and Hearing.
This record was obtained in a young woman from a family with a familial cerebellar degeneration. The velocity of downbeating nystagmus increases to an astounding 33 deg/second on right lateral gaze. This pattern of DBN increased by lateral gaze suggests a cerebellar disorder, with the main suspects being paraneoplastic cerebellar degeneration, a Chiari Malformation, or other cerebellar disorder (such as was the case here). This patient also has a left-beating spontanous nystagmus.
The movie below, from a patient with a paraneoplastic cerebellar degeneration, shows what this looks like using the video frenzel goggles.
Movie of downbeating nystagmus in lateral gaze in case of paraneoplastic cerebellar degeneration (7 meg). If you would like to easily access all of the movies on this site, see the site DVD.
According to Baier and Dieterich (2011), about a third of patients with cerebellar strokes have "unidirectional" GEN. This presumably is the same nystagmus previously called "Brun's" nystagmus. This study is very difficult to interpret as the criteria for identifying GEN were omitted from the article, and there was no data concerning the prevalence of spontaneous nystagmus, which might reasonably convert a bilateral GEN into a unilateral. In our experience, "unilateral" GEN is generally associated with an inner ear disturbance.