3 Nystagmus

Athiya Agarwal, Amar Agarwal

DEFINITION

Nystagmus is a rhythmic to and fro oscillation of the eyes.

GENERAL CONSIDERATIONS

The specific neurophysiologic mechanism of nystagmus is not well understood. Like all eye movements, nystagmus involves all or more of the five known supranuclear pathways1,2 namely:

•Saccadic system pathway

•Pursuit system pathway

•Vergence system pathway

•Non-optic reflex system pathway

•Position maintenance system pathway.

In nystagmus, generally the movement in slow phase is in one direction and the fast phase in the opposite direction. The fast phase of nystagmus is mediated by the saccadic system under all conditions. One or more of the other systems mediates the slow phase. It is important to remember that nystagmus is given its direction based on the fast phase. This means that if we say a nystagmus is to the right, it means that the fast phase of the nystagmus is to the right. But actually, the important point of nystagmus is the slow phase. So actually,

nystagmus should be given its direction depending on the slow phase—but this is not done. An abnormality in the slow phase is more significant. But, alas, convention makes us talk only of the fast phase.

The eye position at any given moment results from all the impulses fed into the III, IV and VI cranial nerve nuclei, from the supranuclear mechanism, the gaze systems and the gaze centers. Normally the input is balanced and the eye movements are smoothly coordinated. Nystagmus develops when the normal balance is interrupted by a

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change of stimulus in a gaze system, frequently the vestibular system. Thus, in jerk nystagmus, a defect in one system results in eye deviation (slow phase) and repetitive attempts at correction of that deviation (by fast phases).

In many kinds of nystagmus, the patient has the subjective experience that the world is moving or oscillopsia. Oscillopsia or perception of motion of the visual field associated with nystagmus seems to be present primarily during the slow phase of nystagmus, during which time the environment appears to move in the direction of the fast phase. You can demonstrate this for yourself by following your finger slowly back and forth horizontally in front of you. Notice that the background appears to move in a direction opposite to your slow eye movement (if the slow phase of nystagmus is to the left, the field appears to move to the right).

During saccades (the fast phases of the nystagmus) the background does not appear to move. Try making saccades by looking rapidly from one corner of the room to the other. The background will not be perceived because of an elevated visual threshold. This elevation of visual threshold actually occurs prior to the start of the saccades. Some investigators believe that a discharge associated with the oculomotor activity of the saccade causes an increase of threshold in the visual afferent system. Other evidence suggests that the elevation of visual threshold occurs in the retina as a response to the forms and contour in the visual environment.

Environmental motion perceived during nystagmus occurs predominantly during the slow phase, but in a direction that happens to coincide with the direction of the fast phase. Consequently, a patient with a large amplitude right-beating nystagmus (fast phase to the right) might state that the room appears to be moving to the right. During the fast phase of the nystagmus and during all saccades, visual perception is suppressed.

TERMINOLOGY

Before we proceed further we should understand what certain terms mean in nystagmus.

Pendular Nystagmus

In this there is an undulatory movement of equal speed and amplitude in both directions.

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Jerky Nystagmus

Jerky nystagmus demonstrates a biphasic rhythm wherein a slow movement in one direction is followed by a rapid saccadic return to the original position.

Micronystagmus

Micronystagmus is a term applied to a nystagmus, which is subclinical, so that it is incapable of being detected with ordinary clinical tests because of its extremely small amplitude. The diagnosis is apparent by the fixation pattern, which shows a regular jerky type of nystagmus with fast and slow phases of extremely small amplitude within the parafoveal areas so that it may be revealed only by a careful examination with the visuoscope or direct ophthalmoscope.

Null Zone

The field of gaze in which the intensity of nystagmus is minimal is termed the null zone.

Neutral Zone

It is that eye position in which a reversal of direction of jerky nystagmus occurs and in which any of several bidirectional waveforms, pendular nystagmus or no nystagmus may be present.

Alexander’s Law

Jerky nystagmus usually increases in amplitude with gaze in the direction of the fast component. This is called Alexander’s law.

GRADES

Nystagmus is divided into three grades.

Grade I Jerky nystagmus is evident only in the direction of the fast phase, i.e. on conjugate deviation to one side.

Grade II When in addition, it is evident in the primary position. Grade III When it is evident in all positions of the eyes.

EXAMINATION OF A CASE OF NYSTAGMUS

There are certain points one should check when one is examining a case of nystagmus. They are:

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•Is the nystagmus pendular or jerky?

•The fast phase of the nystagmus is on which side?

•Grade of nystagmus

•Symptoms of nystagmus

•Is squint present or not and if present, the type of squint?

•Is the nystagmus affected by heads position?

•Is the nystagmus worse with the eyes open or with them closed?

•Is the nystagmus affected by convergence?

•How wide are the ocular excursions?

CLASSIFICATION

Nystagmus can be divided into various groups (Flow chart 3.1).

•Ocular nystagmus

•Vestibular nystagmus

•Cerebellar nystagmus

•Central nystagmus

•Miscellaneous.

OCULAR NYSTAGMUS

Ocular nystagmus is due to a defect or embarrassment of central vision, which renders fixation difficult or impossible. It can in turn be either physiological or pathological. The physiological nystagmus can in turn be either deviational nystagmus or optokinetic nystagmus.

Flow chart 3.1: Types of nystagmus

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Deviational Nystagmus

Deviational nystagmus is also called end-point nystagmus. It is a jerky nystagmoid movement of a physiological type when the fixation of the axes are deviated beyond the limits of the field of binocular fixation and an effort is made to keep them there. It would generally happen if a person looks in the extreme lateral gaze. The fast phase is in the direction of deviation. It would also occur if a person is tired or if there is a paresis of a muscle.

Optokinetic Nystagmus

Introduction

If a target moving in one direction is shown to a person, then the eyes move in the direction of the target and when the target goes out of the limit of gaze, the eyes rapidly comes back to the center to refixate a new target. This is optokinetic nystagmus.

Clinical Test

A simple way to perform the optokinetic nystagmus (OKN) test is to hold a tailor’s tape in both our hands. One should stand one meter away from the patient. Keep one hand stationary and with the other hand move the tape. The patient looks at the tape. As the tape moves in one direction, the patient follows the movement of the tape by a slow eye movement (pursuit). Then there is a fast eye movement (corrective saccade) to bring back the eyes to refixate on the tape.

Kestenbaum’s Newspaper Method

The same result can be done with a sheet of a large newspaper moved slowly in front of the eyes in a direction perpendicular to the lines of the newspaper.

Barrie’s Ruler Test

One can use a ruler about 12 inches long to perform the OKN test. The ruler is held with its long edge horizontal and with its short edge vertical and moved to the right and left of the eye.

Optokinetic Nystagmus Drum

The best method to test OKN is to use the OKN drum. This is a special drum, which rotates. The drum has black and white stripes painted on it. As the drum rotates the patient fixates on the stripes.

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occipital areas mediate horizontal pursuit movements to the ipsilateral side. In other words, the right occipital lobe mediates horizontal pursuit movements to the right.

From the occipital lobe, impulses go to the same side pontine gaze center. In this case, the impulses from the right occipital lobe go to the right pontine gaze center. From here impulses go to the right VI nerve nucleus and the left III nerve nucleus. Till here is the supranuclear pathway. From the right VI nerve nucleus and the left III nerve nucleus impulses go via the infranuclear pathway to the lateral rectus and the medial rectus. Thus, the patient’s eyes move in the direction of the target.

When the moving target goes away from the field of vision the eyes which were moving slowly to that side have to come back to their original position. A fast eye movement does this, in other words a saccade. This is the corrective saccade. If a stream of cars are going in front of our vision, then we keep on following one car and when it goes out of the field of vision our eyes would come and fixate back to the car in the center of our field of vision. This would be done by the corrective saccade. As the impulses from the target moving to the right reaches the occipital lobe (area 19) and the object is going out of the field of vision, the occipital lobe sends impulses to the ipsilateral frontal lobe to perform the corrective saccade. In this case the right occipital lobe (Fig. 3.2) sends impulses to the right frontal lobe (area 8). This means there has to be a communication between the occipital lobe and the frontal lobe. From the right occipital lobe impulses pass to the frontal lobe via the parietal lobe.

From the right frontal lobe, impulses then pass to the left pontine gaze center which in turn sends impulses to the left VI nerve nucleus and the right III nerve nucleus. This is the supranuclear pathway. Then, the infranuclear pathway takes over and impulses got to the respective lateral and medial recti and the eyes move to the left as a fast eye movement. This is the corrective saccade.

One can illustrate this with an optokinetic drum, which is a drum with black and white stripes. The drum is rotated and the eyes fixate on it. When the stripes go away from the field of vision, the corrective saccade occurs. This leads to a type of nystagmus known as optoki-

netic nystagmus.

Parietal Lobe Lesion

If the person has a parietal lobe lesion, then there is a problem (Fig. 3.3). When the corrective saccade has to work the impulse would not

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pass beyond the parietal lobe. Thus, this would lead to a deficit in the corrective saccade. So a deep parietal lobe lesion causes loss or decrease of the fast phase of the OKN, when movement of the drum is towards the side of the lesion.

Type of OKN Abnormalities

There are four types of OKN abnormalities from type I to type IV (Table 3.1). The problems in OKN could occur if the lesion is in the pursuit system starting from the retina, optic nerve to the occipital lobe or in the saccadic system. There also could be a combination of both systems involved.

Inverse OKN

An inverse OKN, wherein in horizontal movements the more rapid excursion occurs in the direction of the moving object, can be seen in cases of congenital nystagmus of ocular origin or in amblyopic

Table 3.1: Types of optokinetic abnormalities

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nystagmus. The inversion is due to the fact that a pre-existing nystagmus takes precedence over the optokinetic phenomenon and may thus augment it or interfere with it.

Pathological Ocular Nystagmus

Amaurotic Nystagmus

Nystagmus of pendular or rarely jerky type may occur in those who have been blind for a long time. The nystagmus is sometimes constant and at other times it appears only when the attention is aroused.

Amblyopic Nystagmus

This is due to a defect in central vision in both eyes, which precludes the normal development of the fixation reflex.

Spasmus Nutans

In this the nystagmus occurs with head nodding. It is also called Dunkel nystagmus. It generally occurs within the first year of life. The cause appears to be difficulty in maintaining fixation, which is frequently associated with inadequate light. There is also insufficient control due to instability of the motor cortical centers in early life.

Miner’s Nystagmus

This is an acquired occupational disease of the nervous system with special manifestations in the ocular motor apparatus, occurring in workers in coalmines (Fig. 3.4). Basically it is due to lack of illumination. In the early stages which is the latent stage slight nystagmus starts. Then in the acute stage trembling of the head and hands occurs with marked nystagmus and a pathognomic attitude of the head being thrown back. Then the psychopathic stage starts in which there are cramps, tremors, headaches and insomnia. The nystagmus is generally pendular in type in the primary position but frequently changes to the jerky type on lateral gaze. The treatment of this condition is to give the patient surface work and improve the general health.

Latent Nystagmus

In this condition, nystagmus is not normally present when both eyes are open but is elicited on covering either eye. In the classical case the nystagmus appears on closing one eye. Bilateral jerky nystagmus is

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Fig. 3.4: Miner’s nystagmus

seen with the fast phase towards the uncovered eye. Another condition is called manifest latent nystagmus, which occurs in patients with amblyopia or strabismus who although viewing with both eyes open are fixing monocularly. Again the fast phase is towards the direction of the intended viewing eye. The phenomenon of latent nystagmus is particularly evident when the visual acuity of the two eyes are unequal. Sometimes if one eye has a very poor vision on covering the better eye instead of nystagmus, a conjugate deviation of both eyes occurs towards the side of the closed eye. This is called—the latent deviation of Kestenbaum. One is not sure of the reason for latent nystagmus. It could be due to lack of coordination of the supranuclear centers. It could also be due to the fact that the nystagmus was latent but kept in check by convergence so that abolition of the impulse to binocular convergence allowed it to become manifest.

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Fig. 3.10: Left directional preponderance. Both the two stimuli necessary to produce nystagmus to the right fail, that means cold in the left ear and hot in the right ear. This proves that the nerve endings are normal but the brainstem mechanism for gaze to the left is defective

There are some situations in which both canal paresis and directional preponderance may be combined. This is often encountered when an acoustic nerve tumor or other posterior fossa lesion is displacing the brainstem.

Rotational Nystagmus

Rotation of the head can also produce a rotational nystagmus. If a lateral semicircular canal is stimulated, the vestibular system starts to work. If the head is rotated to the left (Fig. 3.11), the left lateral semicircular canal is stimulated. If we tilt our head to the left, the eyes should generally keep looking straight ahead (the ultimate aim of the whole process). For the eyes to look straight ahead when we have tilted our head to the left the eyes will move to the right. Try this on yourself by tilting your head to the left. You will note your eyes move to the right so that you keep on looking straight ahead.

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Fig. 3.11: Rotational nystagmus

LRLateral rectus; MRMedial rectus; LELeft eye; RERight eye; IIIIII nerve nuclei; VIVI nerve nuclei; VN – Vestibular nuclei; PGCPontine gaze center; MLFMedial longitudinal fasciculus

When the semicircular canal is stimulated, impulse goes to the same side (in this case left side) vestibular nucleus. From the left vestibular nucleus, impulses go to the opposite side pontine gaze center which in turn sends impulses to the right VI nerve nuclei and left III nerve nucleus. This is the supranuclear pathway. Then the infranuclear pathway takes over to the right lateral rectus and left medial rectus and the eyes turn towards the right. This constitutes the vestibular influence on eye movements.

On cessation of the rotation the right semicircular canal takes over producing a deviation of the eyes to the left. Thus, the slow phase is to the left and this is the postrotational nystagmus (Fig. 3.12). Remember when we use the direction of nystagmus, it is always said according to its fast phase.

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Fig. 3.12: Postrotational nystagmus

LRLateral rectus; MRMedial rectus; LELeft eye; RERight eye; IIIIII nerve nuclei; VIVI nerve nuclei; VN – Vestibular nuclei; PGCPontine gaze center; MLFMedial longitudinal fasciculus

Doll’s Head Phenomenon

The rotational nystagmus provides a simple method for testing vestibular responses in an infant or comatose patient. If an infant or comatose patient is held at arm’s length and the head tilted slightly towards the examiner and rotated to the patient’s left, the infant will develop a slow tonic deviation to the right and a corrective saccade to the left. This is the doll’s head phenomenon. This is extremely helpful in confirming the diagnosis of congenital oculomotor apraxia in infants. In this disorder, the saccadic mechanism is defective. During rotation of the head, the child will have the slow phase movement of the eyes opposite the direction of rotation of the head, but the eyes will not have a fast phase saccadic return.

If you are not able to perform the caloric test in your office, you could ask the adult patient to spin around several times while you

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note the postrotational nystagmus when the spinning is stopped. The normal patient who is spun to the left will develop postrotational nystagmus to the right once the spinning is stopped. In other words the slow phase will be towards the left and the fast phase towards the right. The environment will appear to move in the direction of the fast phase.

CEREBELLAR NYSTAGMUS

The exact mechanism of cerebellar nystagmus is not known. When nystagmus occurs it is opposite that found in a vestibular lesion. In a right-sided vestibular lesion, the slow phase of the nystagmus is to the right and the fast phase to the left. This means the nystagmus is to the left, in other words opposite the side of the lesion. In cerebellar disease, the fast phase of the nystagmus is on the same side of the lesion. So, if there is a right sided-cerebellar lesion, the fast phase of the nystagmus is towards the right side. This could be due to the flocculo-oculomotor pathway, which works in the reverse of the vestibular pathway. The left vestibular pathway pushes the eyes to the right whereas the left flocculo-oculomotor pathway from the left cerebellum pushes the eyes to the left.

CENTRAL NYSTAGMUS

In central nystagmus, the nystagmus is of the jerky type. It is occasionally present when the eyes are at rest, but usually develops only when they are deviated to one or the other direction. The nystagmus is symmetrical. This means that the movement starts at the same angle of eccentricity and has approximately the same excursion whether the gaze is directed to one or the other side.

MISCELLANEOUS

Voluntary Nystagmus

Voluntary nystagmus is habit learned and retained. The movements are pendular and minute and in a horizontal direction.

Hysterical Nystagmus

Hysterical nystagmus is just like voluntary nystagmus but the oscillations are quicker.

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Idiopathic Congenital Nystagmus

In this there is congenital nystagmus without any known cause. This includes all forms of nystagmus noted at birth or within the prenatal period. It is usually horizontal but may be vertical, circular or elliptical. It may be pendular or jerky. Certain important points about congenital nystagmus are:

•It is binocular and there is similar amplitude in both eyes

•There is no oscillopsia and it is abolished in sleep

•It is dampened by convergence and increased by a fixation effort. As it is dampened by convergence the child usually has good near acuity and can do well in school

•It is uniplanar. This is the hallmark of congenital nystagmus. Plane of the nystagmus, usually horizontal remains unchanged in all positions of gaze including the vertical gaze. This phenomenon is seen only in three entities—congenital nystagmus, peripheral vestibular nystagmus and periodic alternating nystagmus

•One can frequently identify a null zone of gaze in which the nystagmus is least marked and the visual acuity is the best

•The patient may manifest a head turn to keep the eyes in the null zone or alternatively have a muscle surgery to create the same effect

•High astigmatism is frequently found which can be treated by contact lenses.

Nystagmus Blockage Syndrome

In this there is reduction of nystagmus or blockage of the nystagmus in some particular gaze. The nystagmus diminishes or disappears with the willed act of forced convergence while fixating a distant target. This should not be confused with the dampening of congenital nystagmus during convergence on a near target.

SYMPTOMS

The various symptoms of nystagmus are oscillopsia, diplopia, tilting of the head or head nodding.

TREATMENT

The treatment can be treating the cause, use of prisms or surgery in which Faden’s operation is done. The methods to treat nystagmus are shown in flow chart 3.2. The treatment can be general treatment where the cause is treated or specific treatment, which can be medical

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Flow chart 3.2: Treatment of nystagmus

or surgical. In medical treatment one can improve the visual acuity by using prisms base out to simulate fusional convergence. One can use prisms to eliminate anomalous head postures also. For a head turn to the left, the neutral zone is in dextroversion and a prism base out before the right eye and base in before the left eye will shift the eyes conjugately along with the neutral zone towards the primary position. One can also use occlusion in which partial occlusion of the sound eye with a neutral density filter to decrease visual acuity in the fixating eye to a level below that of the amblyopic eye but not dark enough to elicit the nystagmus is used. Surgically one can perform Faden’s operation in which the required muscle creating the nystagmus is sutured to the sclera at the equator.

REFERENCES

1.Sunita Agarwal, Athiya Agarwal, et al. Textbook of Ophthalmology 4th vol; Jaypee, India 2003.

2.Amar Agarwal. Handbook of Ophthalmology; Slack USA 2005.