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Chapter 11
CEREBELLAR DISORDERS:
NYSTAGMUS AND SACCADIC ANOMALIES

Key Words: nystagmus, saccades, acquired, congenital, developmental

Outline
VIII. Cerebellum (contí): Disorders of the Cerebellum

Nystagmus anomalies:

  • Taxonomy of nystagmus
    Waveform pendular and jerk- (slow and fast phase)
    Quantify -- frequency, amplitude, direction, constancy, conjugacy, comitance and null point - Alexander's Law
     
  • Clinical categories: Gaze Stabilization (GS) and Holding (GH) anomalies
    Normal -- OKN, VOR, Physiological, Voluntary
    Congenital -- Congenital- pendular near null, jerk away from null; Spasmus Nutans- pendular w/ head nodding and tilt
    Developmental -- (GS) Latent and Manifest Latent- Infantile squint syndrome
    Acquired -- (GS) Vestibular - diagnose with caloric nystagmus (COWS)
                       (GH) gaze holding anomalies:  Gaze holding nystagmus, end point nystagmus, rebound nystagmus, periodic alternating nystagmus (PAN), up and down beat nystagmus, see-saw nystagmus (pendular)
     
  • Associated conditions to be noted by Optometrist
    albanism, head tilts, head oscillations, strabismus, amblyopia, oscillopsia, vertigo, history of trauma, disease or familial history of the anomaly

Saccadic anomalies:


Introduction
As discussed in Ch. 10, two cerebellar areas associated with eye movements are the vestibulocerebellum and the vermis.  Disorders of the vestibulocerebellum are characterized by nystagmus.  Disorders of the vermis are characterized by saccade disorders.

Three categories of eye movement disorders associated with the cerebellum are:

  1. gaze stabilization disorders
  2. gaze holding disorders
  3. saccadic disorders

Gaze stabilization is normally accomplished by OKN (in response to optic flow) or by the VOR (in response to mechanical sensing of head rotation).  These two reflexes are controlled by opponent paired nuclei and any lesion that creates an imbalance can cause the eyes to drift off target.  If the pursuit tracking system is normal, the errors resulting from imbalance can be suppressed with vision, however, they become manifest in darkness.  If pursuits cannot compensate for the nystagmus, then it becomes manifest in light.

Gaze holding refers to our normal ability to fixate steadily in any direction of gaze.  This ability results from tonic discharge of neurons in the brainstem.  The CNS changes eye position with velocity signals.  These velocity signals are transformed to tonic position signals at supranuclear sites (neural integrators) in order to hold the eye at the new position.  This transformation from velocity to position is described mathematically as integration and it requires some interaction with the cerebellum to get the correct amplitude of tonic innervation.  The cerebellum calibrates these areas of integration.  When the integrators fail, our eyes can't hold fixation in eccentric gaze and they drift toward primary position.  There are interactions with pursuits to compensate for these drifts just as there are in gaze stabilization problems.  Acquired and developmental nystagmus anomalies are classified as either anomalies of gaze stabilization or gaze holding (fixation).

Saccadic disorders, on the other hand, result from uncontrolled triggering of saccades, such as opsoclonus (dancing eyes) and flutter, or uncalibrated amplitudes of tracking saccades (saccade dysmetria).  Generally the vermis and PPRF are involved in triggering saccade pulses, and the flocculus is responsible for refining saccade step amplitude.  Technically these are not nystagmus problems because they involve only saccades and have no slow phase components.  The main saccadic disorder is dysmetria, which is an oversized saccade followed by ringing oscillations (ocular flutter) as subsequent saccades attempt to acquire the target.  This ringing may be a manifestation of an underdamped breaking response.   The dysmetria is usually horizontal but can involve vertical saccades.  The opsoclonus (or ìdancing eyes syndromeî) is thought to represent uncontrolled pause release activity in both the vertical and horizontal directions of gaze.  The back-to-back saccades of ocular flutter are mainly horizontal and intermittent.  Voluntary nystagmus is similar to ocular flutter, but it is associated with excessive convergence.


Taxonomy of nystagmus

Classification by waveform
Nystagmus is defined as a rhythmic oscillation of the two eyes. The name is derived from Greek and means ìhead noddingî.  It is classified by wave form, and quantified in terms of frequency and amplitude.  The two waveforms are jerk and pendular (and also mixed).

 

Fig 11.1
Classification of Nystagmus by waveform type.

Jerk nystagmus has a slow following phase in one direction and a fast saccadic phase that resets eye position in the opposite direction.  The slow phase results from the anomalous condition, but we classify direction of nystagmus by the fast phase because its easier to see clinically.  The slow phase of jerk nystagmus can be linear as in latent nystagmus, increasing exponential velocity as a result of too high a gain, and decreasing exponential velocity as a result of too low a gain (for cerebellar gain control associated with vestibular nuclei).

 

Fig 11.2
Jerk (A-C) and Pendular (D) nystagmus.

Types of Jerk Nystagmus slow phase:
A) Linear Velocity slow-phase (ex: Vestibular Nystagmus)
B) Decelerating slow-phase (ex: Gaze-Evoked Nystagmus)
C) Accelerating slow-phase (ex: Congenital Nystagmus)

Pendular nystagmus is a sinusoidal oscillation or equal slow movements in both directions with velocity under 100 deg/sec.  If the nystagmus is really back-to-back saccades then technically it isn't nystagmus or pendular and we call it saccadic oscillations.

There are also combinations of jerk and pendular nystagmus where the wave form changes from pendular in primary position to jerk in eccentric tertiary positions.  In mixed nystagmus, usually the fast phase is toward the desired eccentric fixation locus and the slow phase is toward primary position of gaze.
 
Classification by frequency, amplitude, etc.
Nystagmus is also described in terms of frequency, amplitude, orientation-direction, comitance, constancy and conjugacy.  Frequency is described as slow if it is less than 0.5 Hz, medium if between .5 and 2.0 Hz, and fast if above 2 Hz.  Amplitude is considered small if it is less than 2 deg, medium if between 2 and 10 degrees, and large if greater than 10 deg.  Direction is described by the fast phase of jerk and the meridian for pendular. 

 

Fig 11.3
Direction of nystagmus is described by the fast phase.

The nystagmus can be horizontal, vertical or cyclo-torsional.  Most forms of nystagmus are yoked or conjugate.  Yoked nystagmus may still be unequal in the two eyes even though it is moving the eyes in the same direction.  There is a rare form of retraction nystagmus in which the eyes over-converge and all muscles co-contract causing the eye to pull back into the orbit.  Nystagmus varies in its duration.  It can be constant or intermittent.  Usually the nystagmus is apparent when the patient attempts to fixate but is less apparent in periods of rest. 

Comitance of nystagmus is another descriptive variable.  Comitance refers to constancy of the magnitude of nystagmus in different directions of gaze.  Most nystagmus is non-comitant because its amplitude changes with fixation position.  Normally the nystagmus is calmest or smallest in one position called the null point.  As the eyes attempt to fixate farther from this point its amplitude increases.  In the case of jerk nystagmus, the direction of the fast phase can even reverse on either side of the null point.  The fast phase can be leftward when looking to the left and rightward when looking to the right of the null point.   Alexander's law states that the amplitude of the jerk nystagmus increases as the eyes fixate in fields of gaze in the direction of the fast saccadic phase.

Normally the nystagmus is calmest or smallest in one position called the null point marked by N in the figure below.  As the eyes attempt to fixate farther from this point its amplitude increases.  In the case of jerk nystagmus,  the fast phase is in the direction opposite to the null point. Thus the fast phase will be leftward when looking to the left and rightward when looking to the right of the null point.   The relationship between gaze direction, relative to the null point, with amplitude and direction of
fast phase of jerk nystagmus is described by Alexander's law  which states that the amplitude of the jerk nystagmus increases with eccentricity of gaze, away from the null point, in the direction of the fast saccadic phase.

 

Fig 11.4
Alexanderís Law:

Amplitude of the jerk nystagmus increases as the eyes fixate in the direction of the fast saccadic phase.



Clinical Categories of Nystagmus
Nystagmus is also categorized by age of onset as congenital, developmental, and acquired.  Congenital forms occur in the first 6 months of life.  Developmental nystagmus occurs within the first 6-8 years of life.  Acquired generally is found in older cases but it can occur at any age due to trauma, disease processes, or systemic illness.  It is important to classify nystagmus in these three categories of action.  Congenital nystagmus requires no immediate treatment because it is stable.  Developmental nystagmus is often symptomatic of other conditions such as strabismus or anisometropia that are interfering with normal binocular development.  Acquired nystagmus is often the result of serious trauma or a rapidly developing disease that requires immediate attention of a neurologist.  The following sections will review the types of nystagmus found in these three categories.

Congenital forms of nystagmus
There are two forms of congenital nystagmus, both of unknown cause or etiology.  The most common is simply called congenital nystagmus and it is present at birth.  It begins as a jerk nystagmus at birth and becomes pendular by one year of age.  It is highly variable in amplitude and wave form.  The wave form changes from pendular to jerk with a null point in the field of gaze where the nystagmus is minimal.  It is also dampened by convergence.  One treatment capitalizes on this and uses lateral and base out prisms to align the eyes at the proper version and vergence state that coincides with the null point.  Some patients come to this solution on their own and develop a convergent esotropia to block the nystagmus.  This is called nystagmus blocking syndrome.  It is accompanied by a head turn to place the eyes in the direction of the null point.  Very often congenital nystagmus is accompanied by albinism, reduced visual acuity, optic neuritis, congenital cataract, or anaridia.  It is very likely that you will see an albino patient in your practice with congenital nystagmus.

The other congenital anomaly is spasmus nutans.  It is a pendular form of nystagmus that occurs after birth in the first year between months 4 -12.  It is a high frequency (11 Hz) small amplitude (2 deg) horizontal nystagmus that usually disappears by age 2 years.  The nystagmus is said to resemble shimmering eyes. The nystagmus can be unequal in the two eyes and even disconjugate.  The nystagmus is part of a triad that also includes a 3 Hz horizontal head nod, and abnormal head position turned to one side.  The head nodding decreases the nystagmus.  There is little need for concern or treatment since the patients usually grow out of the problem.

Developmental nystagmus
Latent nystagmus is a jerk form of nystagmus that is not congenital but occurs in infancy due to disruption of normal development of binocular vision as a result of esotropia during the first 6 months of life.  The esotropia is accompanied by several asymmetric motor anomalies referred to as the infantile squint syndrome (ISS).  The components of the syndrome include latent nystagmus, asymmetric OKN, and dissociated vertical deviation (DVD). 

 

 

 

Figs 11.5 and 11.6
Latent Nystagmus

Notice that the nystagmus is only present when an eye is occluded.

 

Latent nystagmus is precipitated by occlusion of one eye.  Both eyes drift to the side of the occluded eye and then saccade to the side of the open eye.  The nystagmus reverses direction (i.e., left to right) when the occluder is placed over the other eye.  When there is a constant strabismus, only one eye is used for fixation and the other eye is suppressed.  In these cases the nystagmus responds as though the suppressed eye were occluded and is present when both eyes are open.  This is referred to as manifest latent nystagmus.

Latent nystagmus is classified as an anomaly of the stabilization system because its cause appears to be a disorder of the optokinetic pathways.  (See Chapter 13, Fig 13.12)  Normally the pretectal nucleus (called the nucleus of the optic tract, or NOT) receives input from both the retina of the contralateral eye and the cortex of the ipsilateral eye.  Normally, when one eye is occluded the open eye sends a balanced input to the contralateral NOT via a subcortical projection and to the ipsilateral NOT via the cortical projection.  When there is strabismus, the cortical input does not develop so that during monocular viewing only the contralateral input to the NOT from the open eye is stimulated.  This imbalanced input causes the eyes to drift to the side of the stimulated NOT and a nystagmus appears during monocular viewing.  When both eyes are open each eye stimulates its respective contralateral NOT and a balanced innervation keeps the eyes stationary.  The resulting latent nystagmus often disappears in adolescence when the patient learns to suppress it with pursuits.  However other asymmetries persist.

One result of the loss of cortical input is that during monocular viewing, OKN can only be stimulated by nasalward motion of the visual field.  Temporal motion is ineffective because that is normally sent via the cortex to the ipsilateral NOT.  This condition is known as asymmetric OKN and it constitutes the second component of ISS (infantile squint syndrome).
 
The third component of ISS is DVD (dissociated vertical deviation).  This is also known as the Bielchowsky darkening sign because when one eye is darkened by occlusion it deviates upward in the orbit.  When the other eye is occluded it also deviates upward.  While this is a common component of ISS its etiology is unknown.  It may be related to a torsional deviation of the occluded eye. When the occluder is removed the deviating eye moves to primary position with a single disconjugate vertical downward saccade that appears to violate Hering's law of conjugate ocular movements.  There does not appear to be any cure for the ISS, even when the strabismus is corrected by surgery or eye exercises.  However, the problematic nystagmus only occurs during monocular viewing or when the strabismus is present so that it is rarely manifest once normal binocular eye alignment is restored.  Furthermore like latent nystagmus, it tends to disappear during adolescence.

 

Fig 11.7

Incidence of strabismus during the first several years of age.

Acquired nystagmus
There is one gaze stabilization form of jerk nystagmus (vestibular) that results from anomalies of the vestibular equilibrium system.  Viral infections can irritate the vestibular canals causing a peripheral anomaly, or other disorders can disrupt the vestibular pathways or vestibulo-cerebellum causing a central disorder.  In either case, when the opponent components of the VOR are imbalanced, the eyes are driven by the stronger of the paired nuclei.  For example, if the right horizontal canal is impaired, the stronger response of the left horizontal canal causes the eyes to deviate slowly to the side of the impaired canal and a saccade is needed to realign the eyes back toward the side of the healthy horizontal canal. 

Similar responses can be induced in a normal person by profusing one ear with cold water which causes the two eyes to deviate to side of the cold ear and saccade to the opposite side.  The opposite effect occurs with warm water.  This is known as caloric nystagmus and the direction the eyes move can be remembered with the acronym COWS, cold opposite and warm same, that refers to the direction of the fast phase of the jerk nystagmus.  Vestibular anomalies are accompanied by nausea, vertigo, oscillopsia, no suppression of the VOR by pursuits, and sudden onset.  A common form of the illness is called Meniere's disease.  It can be quite distressful, requiring the patient wear a medicated patch over the back of the ear to numb the more sensitive vestibular system.  This permits the patient to regain a balance between paired nuclei.

There are five other types of acquired jerk nystagmus and all of these are classified as gaze holding anomalies.   The most common of these is gaze holding nystagmus which is also known as neuromuscular insufficiency.  When the eyes are in a non-primary position of gaze the eyes drift toward the primary position because there is not adequate tonic eye position signals to hold gaze in the tertiary position.  This problem is likely to result from a floccular anomaly as it interacts with the vestibular nuclei and supranuclear muscles responsible for integrating velocity signals for horizontal eye position (prepositus).   A normal version of this condition is known as endpoint nystagmus (physiologic nystagmus).  If you attempt to fixate near the limits of your gaze, your eyes will fatigue and drift toward primary position and then saccade back to the tertiary position.  This phenomenon is used by the California Highway Patrol to detect alcohol intoxicate drivers.  Another related condition is known as rebound nystagmus.  Sustained efforts to overcome endpoint nystagmus have an aftereffect called rebound nystagmus in which jerk nystagmus occurs in the opposite direction when the patient returns gaze to the primary position.  The eyes then drift toward the previous eccentric gaze position and saccade back toward primary position.  This is a secondary problem which is a symptom of an adaptive response of the oculomotor system by the cerebellum to overcome the end point nystagmus.  The gaze nystagmus is most evident in darkness where there is no pursuit stimulus to overcome the ocular drift resulting from an inadequate tonic innervation for fixation.

 

 

Fig 11.8
Endpoint and rebound nystagmus.  Look closely -- notice that the fast phase of rebound nystagmus occurs in the opposite direction as the endpoint nystagmus.

Another related condition is called periodic alternating jerk nystagmus or PAN.  The jerk nystagmus in PAN reverses direction every 1-2 minutes.  It is believed to involve anomalies of the nodulus component of the vestibulo-cerebellum.  The reversal of direction is thought to result from attempts of the cerebellum to repair the unstable jerk nystagmus.  Smooth pursuits and OKN are also impaired.  There is difficulty in all slow visually-evoked eye movements and this makes visual stabilization very difficult.

 

Fig 11.9
Periodic Alternating Nystagmus.

The direction of nystagmus changes every 1-2 minutes, and varies in amplitude.

There are two vertical forms of jerk nystagmus associated with problems of gaze holding.  These are upbeat and downbeat nystagmus where the name describes the fast phase direction of the vertical jerk nystagmus.  The frequency of the nystagmus is about 3 Hz and the amplitude is small (2 deg).  These are thought to result from anomalies of the vestibular nuclei and flocculus of the vestibulocerebellum.  The jerk nystagmus is present in primary position and increases in amplitude as the eyes move into the direction of the fast phase, thus obeying Alexander's law.  Vertical smooth pursuit and vertical VOR are also reduced in these patients.  The downbeat variety is more serious than the upbeat variety, which can be simply a benign disorder.
 
Another vertical form of nystagmus is see-saw nystagmus which has a pendular wave form.  This is a very large amplitude slow nystagmus of 1 Hz where the eyes move upward on one side and downward on the other and then the cycle reverses and the upper eye moves down while the lower eye moves up.  During the upward phase of the nystagmus the eye intorts while the downward eye extorts.  This is a versional cyclotorsion of clockwise and counterclockwise oscillations.  This is an anomaly of the vertical gaze holding system and probably involves both the otoliths and the vestibulo-cerebellum.  It can be associated with a bitemporal hemianopsia in which the temporal fields are blind.

 

Fig 11.10
See-saw Nystagmus
(Schematic representation)

Note only 1/2 cycle is shown.  (Next, the lower eye will move up and intort, and the upper eye will move down and extort.)


Saccadic Anomalies of the Cerebellum
 
Involvement of the floccular region of the cerebellum influences the corrective control of saccade amplitude that results from the step innervation of the saccade.  Without this control, saccade amplitudes become inaccurate in a condition known as saccadic dysmetria.  This is a gaze holding problem.  Saccades become uncalibrated and are either too large (hypermetric) or too small (hypometric).  Conditions such as Parkinson's disease manifest hypometric saccades, whereas Wollenberg's syndrome manifests hypermetric saccades.

Disorders of the cerebellum that effect calibration of the pulse component of the saccade usually involve the Vermis.  These are problems involving unintentional triggering of unwanted saccades.  Back to back saccades in opposite directions result in ocular flutter.  The saccades are very small (1-2 deg) and occur very rapidly (3-10 times/sec).  They are usually horizontal and are more prevalent in upward vertical gaze.  Sometimes the saccades occur uncontrollably in all directions in the condition called opsoclonus or dancing eyes.  Its as thought the pause neurons were all shut off and the bursters were firing randomly without any higher level supervision.


Fig 11.11
Saccadic disorders.

A) Dysmetria (inaccurate saccades)
B) Macrosaccadic oscillations (hypermetric about the position of target)
C) Square wave jerks (1-5 deg)
D) Macrosquare wave jerks (10-40 deg)
E) Ocular flutter (back-and-forth saccades without an intersaccadic interval)

Cogwheeling is the interruption of smooth pursuit movements by saccades, resulting in jerky, uneven eye movement.  It is seen in cerebellar lesions and probably represents the superinposition of saccadic disorders with normal pursuits.

 

Fig 11.12
Cogwheel pursuits


Encountering Disorders in Practice

There are many other forms of rare types of nystagmus in addition to the most common varieties presented in this chapter that you could encounter in private practice.  Some of these disorders are also known under various disease names, but are categorized them in a systematic approach that classifies nystagmus in two general categories, either by age of onset, or by gaze mechanism.  Most of the forms of nystagmus are jerk wave forms and conjugate.  The main thing to remember is to classify the condition as one that is benign or actively progressing i.e. congenital or acquired, and then to advise the patient so that they may make an informed decision about future courses of action.  It is likely that you will only encounter subtle forms of acquired nystagmus because the more obvious forms will result in self referrals to a neurologist.  Do not alarm patients, but be sure they take responsible action, particularly when a child or supervised patient is involved.  Make careful notes of your observations and when referring.  Do not try to diagnose the site of the lesion or even classify the syndrome by name.  Simply describe the waveform, frequency and amplitude, directions of gaze in which nystagmus occurs, conjugacy, and any latent component.  Include any associated conditions such as albinism, head tilts, head oscillations, strabismus, amblyopia, vertigo, history of trauma, disease or familial history of the anomaly.  The neurologist or neuro-ophthalmologist will keep you informed of the patient's disposition.  This will be a rare event but an important one in your career.

A final note: One normal form of nystagmus is called voluntary nystagmus.  It really isn't nystagmus because it is made up of back-to-back saccades that can be made 3 times/sec for short periods of time.  This is something you can learn to do, much like wiggling your ears. Sometimes a child uses voluntary nystagmus to alarm its parents and get out of school.  Because it is so tiring its hard to keep up for more than 30 seconds, it is distinguishable from true saccadic disorders.


Review Questions:

  1. Define Nystagmus
  2. Describe Alexander's Law
  3. List two forms of congenital nystagmus
  4. What are the components of the infantile squint syndrome?
  5. List 5 types of gaze holding acquired nystagmus.
  6. List 2 saccadic anomalies of the cerebellum.

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