GAZE EVOKED NYSTAGMUS

 

Gaze-evoked nystagmus

Causes of Gaze-evoked nystagmus

Gaze evoked nystagmusA 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 lesion. Gaze-evoked nystagmus is a drift of the eye which is only present for certain directions of gaze. 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)

Causes of gaze-evoked nystagmus are listed in the table above. There are several distinct patterns which can be identified by scrutiny of the eye position trace. The most common variety 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.

There are two factors which contribute to the amount of gaze-evoked nystagmus found in individual patients. 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, which holds the eye in place against elastic forces, 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.

Abel LA, Parker L, Daroff RB, et al. End-point nystagmus. Invest Ophthalmol Vis Sci 17:539-544, 1978

Asymmetric gaze-evoked nystagmus: Alexander's law -- the gaze-evoked nystagmus seen in vestibular disorders.

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. 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.

Robinson DA, Zee DS, Hain T, Holmes AM, Rosenberg LF (1984) Alexander's law -- its behavior and origin in the human vestibulo-ocular reflex. Ann. Neurology., 16:714-722.

Gaze-evoked nystagmus seen in CN

Certain patients with congenital nystagmus or with rare acquired central nystagmus varieties have "increasing exponential" velocity patterns. These waveforms are generally easy to spot because are so vigorous. 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 distrupt 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 third factor relates to the frequency with which the patient develops corrective saccades. Because these factors are normally not controlled for, the judgement of 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. GEN is an extremely common consequence of medication, especially sedatives or anticonvulsants. Asymmetrical GEN occurs in several situations. In vestibular disorders, when GEN 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). Brun's nystagmus, which occurs in patients with cerebellar lesions, is probably due to the same general mechanism.

Gaze-evoked nystagmus seen in INO

Patients with internuclear opthalmoplegia (INO) often exhibit a discongugate GEN in which the abducting eye exhibits a more prominant amplitude nystagmus than the adducting eye. Rebound nystagmus is a primary position nystagmus which is provoked by prolonged eccentric gaze holding. An abnormal amount of rebound consists of at least 3 beats of clear nystagmus, with the slow-phase direction oriented towards the previous position of gaze. Rebound after gaze holding for periods more prolonged than 30 sec, or for eccentricities larger than about 45 deg is of uncertain significance as normal subjects may exhibit rebound under such circumstances (Gordon et al, 1989). Vertical rebound is extremely rare compared to horizontal rebound. Rebound is nearly always pathological, and related to brainstem or cerebellar disease. Accordingly, if an unsually large gaze-evoked nystagmus is observed, one should automatically look for rebound nystagmus. On the other hand, a large GEN without rebound is usually of little significance (usually medication related or a form of CN). Rebound is nearly always associated with poor pursuit.

Rebound Nystagmus

Rebound nystagmus is a primary position nystagmus which is provoked by prolonged eccentric gaze holding. It appears after the eyes are returned to primary position. An abnormal amount of rebound consists of at least 3 beats of clear nystagmus, with the slow-phases directed towards the previous position of gaze. Rebound after gaze holding for periods more prolonged than 30 sec, or for eccentricities larger than about 45 deg is of uncertain significance as normal subjects may exhibit rebound under such circumstances. Vertical rebound is rare.

Rebound is always pathological, and is related to brainstem or cerebellar disease. Accordingly, if an unusually large gaze-evoked nystagmus is observed, one should automatically look for rebound nystagmus. On the other hand, gaze-evoked nystagmus without rebound is usually of little significance. Rebound is always associated with poor pursuit.

Clinical situations in which rebound is commonly encountered include MS involving the pons, typically a lesion of the middle cerebellar peduncle, and ischemic pontine hypertensive lesions. Rebound nystagmus is also a feature of EA2 (Episodic ataxia, type 2).

Gordon SE, Hain TC, Zee, DS and Fetter M. (1986) Rebound nystagmus. Soc. Neurosci. Abstr., 12:1091.