EM guidemap - Nystagmus

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Introduction

Definitions and general principles

Diagnostic approach to pathologic nystagmus

Eye movement disorders that mimic nystagmus
 
Introduction

- this guidemap is a companion guidemap to the ataxia, incoordination and dysequilibrium guidemap and the vertigo guidemap, and this guidemap offers a reader much more detailed information on nystagmus

- this guidemap serves two purposes:-

(* note that this guidemap is very selective in its presentation of information on nystagmus, and that it does not deal with latent nystagmus or congenital nystagmus => this guidemap is primarily focused on providing basic problem-solving information on acquired nystagmus in adult patients and older children)
 
Definitions and general principles

What is nystagmus?

Nystagmus is a rhythmic oscillation of the eyes, which may be physiologic or pathologic

The ryhthmic oscillation may be symmetrical (speed of oscillation to one side is the same as the speed of oscillation to the other side) - labelled pendular nystagmus

Alternatively, the rhythmic oscillation of the eyes may be asymmetrical (speed of oscillation to one side is slower than the speed of oscillation to the other side) - labelled jerk nystagmus

The direction of a jerk nystagmus is defined by the direction of the rapid oscillatory phase

In jerk nystagmus, the slow phase is the first (pathologic) phase, and the rapid phase is the second (corrective) phase. Therefore, if the rhythmically oscillating eyes drift slowly to the left and then jerk back rapidly to the right => the jerk nystagmus is classified as a right horizontal jerk nystagmus

Nystagmus may occur with the eyes in primary forward gaze, and/or only in eccentric gaze (to the sides, up-or-down, or obliquely)

If the jerk nystagmus occurs spontaneously in primary forward gaze, then it is classified according to the direction of the nystagmus - and it may be classified as horizontal, vertical, torsional, oblique or mixed

Vertical nystagmus can be sub-classified, according to whether the rapid phase is upward (upbeat nystagmus) or downward (downbeat nystagmus)

Upbeat jerk nystagmus refers to vertical nystagmus where the primary slow drift is downwards, and the rapid corrective phase is upwards

Downbeat jerk nystagmus refers to vertical nystagmus where the primary slow drift is upwards, and the rapid corrective phase is downwards

Although vertical nystagmus is defined according to the direction of the fast phase, the amplitude of the slow phase and rapid corrective phases can vary according to the direction of gaze. For example, downbeat nystagmus, which is present in primary gaze, can be worse on downward or lateral gaze (eccentric eye position). Sometimes, downbeat nystagmus may not be present in forward gaze and only be present on downward gaze. In that situation, it is still defined as downbeat nystagmus - because of the downward direction of the rapid (corrective) phase of the nystagmus. However, if a jerk nystagmus only occurs in an eccentric eye position, and not the primary eye position, it is called a gaze-evoked nystagmus. Therefore, a downbeat nystagmus that only occurs on downward gaze, is classified as a gaze-evoked downbeat nystagmus

If jerk nystagmus only occurs with certain positions of the head (rather than varying positions of gaze) it is classified as a positional nystagmus.

(* see the vertigo guidemap for further information on how to differentiate peripheral positional nystagmus from central positional nystagmus)

What causes physiologic nystagmus?

Normally the eyes remain in the position of primary gaze, even when a person is not deliberatedly focusing on any visual target directly ahead, because of balanced input from the left-and-right vestibular systems, which are finely tuned to respond to any movement of the head relative to the body.

If a person turns his head to the side without making any deliberate attempt to fixate on any visual target, the eyes will remain centered in the neutral position of primary gaze as a result of the vestibulo-ocular reflex (a reflex mediated via the vestibular system), which ensures that the eyes respond promptly to any movement of the head relative to the body in order to keep the eyes centered in the socket. If that person spins around in a circle (like a ballerina), theoretically the eyes should follow the head at the same speed, so that an observer will notice that eyes remain centered in the head as the person swivels around. However, if that person spins too fast, vertigo will ensue if the movement of the eyes cannnot keep up with the speed of rotational movement of the head (due to the slow latency of response of the vestibulo-ocular reflex) => the vertigo is due to the disparity in eye movements relative to head movements. Physiologic nystagmus will occur if the conscious brain of the spinning person deliberatedly interferes with the smooth movement of the eyes during rotation of the body, by deliberately fixating on different visual targets during the spin. If the eyes fixate on a vusual target during a rotational spin, the eyes will obviously be lagging behind the movement of the body as it continues to rotate, and it would appear to an observer that the eyes are moving smoothly in the opposite direction of the body. Ballerinas use this technique to prevent getting dizzy during rapid pirouettes. When the ballerina reaches a certain point in her rotational spin (when the eyes are displaced to the near-extreme limits of its excursion in the socket), she re-fixates her eyes on another visual target, and the voluntary saccade causes a rapid eye movement to the intended re-fixation position. In other words, the pirouetting ballerina has self-induced physiologic nystagmus - a rhythmic oscillation of the eyes, with alternating fast phases and slow phases, and the fast phase occurs a result of the conscious brain's attempt to re-fixate a visual target.

What causes pathologic nystagmus?

- pathologic nystagmus occurs when the eyes drift slowly from their intended position (primary forward gaze or eccentric gaze) as a result of an abnormality of one of the slow eye movement controlling systems or the neuro-regulatory "gaze-holding" system controlling eccentric gaze.  In most cases, the drift is corrected by a rapid eye movement - an automatic phenomenon of a conscious brain - that brings the eye back to its intended position
 
Diagnostic approach to pathologic nystagmus

What are the symptoms and signs of nystagmus?

- nystagmus is rarely the main clinical sign, and many patients with pathologic nystagmus present with a symptom complex that varies with the underlying disease (eg. acute peripheral vestibular syndrome, or acute cerebellar syndrome) and the other symptoms dominate the clinical picture

- when nystagmus is the main clinical sign, the patient may complain of blurred vision or oscillopsia

- oscillopsia is an illusory sensation of enviromental movement, and the sensation can vary from a to-and-fro oscillation of the enviroment to various degrees of tilting of the enviroment

- nystagmus can be subtle, and use of a slit lamp or ophthalmoscope may help a physician detect a subtle torsional nystagmoid movement (remember that the direction of nystagmus is reversed when using an ophthalmoscope)

- when observing a patient with nystagmus, it is worthwhile making the following series of observations, because the correct classification of the nystagmus significantly narrows the number of differential diagnoses causing the nystagmus

- this guidemap delineates a particular system of classifying nystagmus, based on the above observations

Binocular symmetric oscillations of the conjugate type

- binocular symmetric nystagmus may include patients with symmetric eye movements in the same direction (conjugate nystagmus), or opposite directions (dysconjugate nystagmus)

- binocular conjugate nystagmus is divided into two types => jerk nystagmus and pendular nystagmus

- spontaneous jerk nystagmus is divided into two types => primary position jerk nystagmus and eccentric position jerk nystagmus (gaze-evoked jerk nystagmus)

- spontaneous primary position jerk nystagmus is divided into two groups => vertigo present, vertigo absent

Peripheral vestibular nystagmus

What causes peripheral vestibular nystagmus?

Normally the left and right vestibular organs are constantly responding to movements of the head in space, and there is continuous input into the reflex vestibulo-ocular system. Pathologic nystagmus occurs when there is an imbalance in the input from the different sides of the vestibular system. An example would be unilateral disease of one vestibular organ causing a loss of input signal from the one side. Then the only remaining signal inputting the reflex vestibulo-ocular system would be from the other vestibular organ, which drives the eyes slowly and tonically in the direction of the opposite side (towards the affected ear). The conscious brain cannot tolerate drift of the eyes from the primary forward gaze position and it instructs the eyes to make a rapid corrective saccade back to the central position. The direction of the fast corrective movement is arbitrarily defined as the direction of nystagmus, although the pathological eye movement is really the initial slow movement. Therefore, in unilateral vestibular disease involving the left ear, the eyes will drift slowly towards the left ear (because of the unopposed tonic input from the normal right vestibular organ) and then rapidly jerk back towards the opposite side (the side of the normal ear). Because the pathological imbalance in vestibular function is continuously present in the acute phase, there is a continuous tonic tendency for the eyes to slowly drift conjugatedly towards the affected side, and then jerk back to the normal side - and this produces a constantly recurring jerk nystagmus

(* note that a conscious brain is needed to produce the rapid corrective eye movement, and that peripheral nystagmus disappears during sleep).

Can peripheral vestibular nystagmus occur in the presence of bilateral vestibular disease?

If the vestibular disease affected both vestibular organs to the same degree, then the lack of tonic input signal from the two sides would cancel-out, and there would no tendency for the eyes to drift to one side, and there would no physiological need for a corrective rapid phase => nystagmus would not occur. Peripheral nystagmus only occurs if there is an imbalance in tonic signal from the opposite sides, because one side is affected more than the other side. Therefore, nystagmus only occurs if a particular vestibular disease affects both vestibular organs (eg. vestibular neuronitis), but there is a significant difference in the degree of disease-involvement between the left and right vestibular organs.

Why is peripheral vestibular nystagmus always horizontal or horizonto-torsional, and never vertical?

Each vestibular organ consists of three semicircular canals - anterior, posterior and horizontal. The right anterior canal works in tandem with the left posterior canal to detect any movement of the head in one plane, while the right posterior canal works together with the left anterior canal to detect movement in another plane at right angles to the first plane. The vestibular system uses these pairs of canals in a "push-pull" reciprocal manner, so that any angular movement of the head will maximally activate one semicircular canal and maximally inhibit its counterpart. Therefore, the tonic signals from the left anterior canal balances the tonic signals from the posterior canal of the opposite side (and vica versa), while the horizontal canal tonic signals cancel each other out.

If a disease caused loss of right anterior canal function, there would be a nystagmus that is a mix of upbeat and counterclockwise torsional fast phases; and if there was a loss of right posterior canal function, there would be a nystagmus that is a mix of downbeat and counterclockwise torsional fast phases. If all three right sided canals were affected by an unilateral vestibular disease, the upbeat and down beat componenets would cancel-out, but the counter-clockwise torsional effects would add together with the left beating nystagmus (due to disease of the right horizontal canal) to create a mixed torsional-horizontal nystagmus.

Vertical nystagmus doesn't occur in peripheral vestibular disease because the pathology would have to differentially affect the posterior canals of both sides, summating the resulting loss of upward slow drifts and cancelling the opposing slow torsional drifts - which is very unlikely.

Pure torsional nystagmus could theoretically occur if unilateral vestibular disease affected both the anterior and posterior canal on one side, sparing the horizontal canal. However, this is very unlikely to occur because the nerve and blood supply of the horizontal + anterior canals are separate from that of the posterior canal. So, even if the unilateral vestibular disease was partial, it would not affect only the anterior canal without affecting the horizontal canal. In fact, partial vestibular neuronitis either causes a dysfunction of the one horizontal canal, or both the horizontal and anterior canal of one side.

What are the typical characteristics of peripheral vestibular nystagmus?

- some physicians grade peripheral vestibular nystagmus Central vestibular nystagmus

- although vertigo is also present in central vestibular syndrome, it is not as prominent a complaint (compared to acute peripheral vestibular syndrome)

- posterior fossa neurological symptoms/signs are common

What are the typical characteristics of central vestibular nystagmus?

- a common cause of central nystagmus is lateral medullary syndrome (Wallenberg's syndrome) due to a vertebral artery occlusion or posterior inferior cerebral artery stroke

- signs of Wallenberg's syndrome may include:-

Periodic alternating nystagmus

- PAN is the only binocular conjugate nystagmus that occurs spontaneously in primary gaze (without any accompanying vertigo) that is horizontal in direction

- PAN is a jerk nystagmus with a cyclically moving or wandering null point, and the nystagmus may disappear for 10 seconds to be followed by ascending and decending intensity unilateral jerk nystagmus lasting about 90 seconds, followed by a 10 second null period of no/minimal nystagmus in a downbeating direction, and then a repeat pattern of crescendo horizontal nystagmus in the opposite direction for 90 seconds

- the patient may complain of episodic visual blurring or oscillopsia

- usually due to a lesion in the cervicomedullary area affecting the vestibular nucleus (eg, Arnold-Chiari malformation) => the patient requires diagnostic MRI neuro-imaging of the posterior fossa

- other causes of PAN include degenerative spincocerebellar disease, multiple sclerosis, Creutzfeldt-Jacob disease, ataxia-telangiectasia, brainstem infarcts, cerebellar mass lesions, neurosyphilis, hepatic encephalopathy, trauma, anticonvulsant drugs, or following visual loss

Upbeat nystagmus

- the nystagmus has an upbeating rapid phase in the primary position, and is often worse with upgaze

- convergence may enhance, suppress or reverse the direction of the nystagmus

- impaired smooth pursuit always occurs

- associated gaze-evoked or rebound nystagmus may also occur

- internuclear ophthalmoplegia and skew deviations are less common

- does not have much localizing value and is mainly seen in lesions of the medulla, cerebellar vermis, and midline brainstem

- when downgaze increases the nystagmus, the lesion is usually in the medulla

- causes of upbeat nystagmus include:-

Downbeat nystagmus

- the nystagmus has a downbeating rapid phase in the primary position

- the vertical nystagmus is often worse with downgaze or lateral gaze just below the horizontal, and less severe in upgaze

- there may be a mild horizontal component, so that the nystagmus can be oblique on lateral gaze

- the patient may present with blurred vision, oscillopsia or gait imbalance

- oscillopsia is often present during reading, because the nystagmus is often worse on downward gaze

- the nystagmus may be intermittent or constant

- a compressive problem (Arnold-Chiari malformation, hydrocephalus or osteophytes compressing the vertebral artery) should be suspected if oscillopsia is provoked by neck extension or rotation

- there may be diplopia due to associated skew deviations of the eyes

- other eye movement abnormalities are commonly associated with downbeat nystagmus, especially poor downward smooth pursuit

- the "floccular syndrome" consists of downbeat nystagmus, gaze-evoked nystagmus and rebound nystagmus, and abnormal horizontal smooth pursuit; and is due to a cerebellar flocculus lesion

-  the most common pathology causing downbeat nystagmus are cervico-medullary junctional area lesions

- causes of downbeat nystagmus include:-

Gaze-evoked (eccentric position) nystagmus

- this form of binocular conjugate nystagmus is provoked by moving the eyes into an eccentric gaze position, and is not present at rest in the primary forward gaze position

- gaze-evoked nystagmus is usually due to a defect in the central neural integrators controlling gaze-holding, and infrequently due to weakness of the extraocular muscles or their innervation (gaze-paretic nystagmus)

(* the central neural intergrators consist of neural connections between the vestibulocerebellum, the medulla and the interstitial nuclei of Cajal in the midbrain)

- diseases that cause central gaze-evoked nysgamus often also produce defects in visual fixation and smooth pursuit eye movements

- common types of gaze-evoked nystagmus include:-

Physiologic nystagmus

- also called end-point nystagmus, and occurs when a person looks laterally beyond 40 degrees for an extended period

- the nystagmus is a low-amplitude horizontal nystagmus with the rapid phase directed laterally

- the initial slow phase is due to the intrinsic elastic pull of the stretched eye muscles/tendons which are constantly pulling the eyes centripetally back towards the neutral position, and the rapid phase is a corrective movement back towards the eccentric position of desired gaze (secondary to input from the brain's central "gaze-holding" neural integrator)

- usually does not occur until the eyes have been held in an eccentric position for longer than a few seconds (often only after > 30 seconds)

- fatigue nystagmus may become increasingly torsional with prolonged deviation effort, and it may be greater in the adducting eye

- gaze-evoked nystagmus is not physiological if:- i) there is asymmetry of nystagmus in two directions of gaze, ii) the amplitude of the nystagmus exceeds 4 degrees, iii) the slow phase is not linear within a gaze angle of 40 degrees (slow phase has an exponentially decreasing velocity), or iv) if there are any associated eye movement defects eg. impaired smooth eye pursuit movements

Gaze-paretic nystagmus

- represents a failure to maintain an eccentric eye position due to relative weakness of certain eye muscles

- occurs during the recovery phase of central gaze palsy or other gaze pareses (eg. myasthenia gravis, or Miller-Fisher variant of Guillane Barre syndrome) when recovery is sufficient to make a conjugate movement, but recovery is not sufficient to maintain an eccentric gaze position

- following an initially adequate saccadic gaze movement, the eyes drift back towards the primary eye position, and a rapid corrective saccade is initiated taking the eyes back to the intended eccentric position of gaze

- the jerk nystagmus is often bi-directional in central causes of gaze-evoked nystagmus, because the rapid phase is always in the direction of gaze (and the slow phase is always the initial centripetal drift of the eyes, which is usually due to an insufficient step-signal from the central neural integrator responsible for maintaining gaze-holding of the eyes in an eccentric gaze position)

- if the amplitude of the nystagmus is equally great when looking eccentrically to the left and right, it suggests a central gaze palsy and is of no localizing value; if the gaze-evoked nystagmus is asymmetric, then it suggests a peripheral gaze palsy, or a central gaze palsy due to unilateral brainstem or cerebellar disease and the amplitude of the nystagmus is greatest when looking towards the side of the lesion

- gaze-evoked upbeat vertical nystagmus on upgaze almost always co-exists with symmetrical horizontal gaze-evoked nystagmus, and it has no CNS localizing value; gaze-evoked nystagmus on downgaze is very uncommon

Drug-induced nystagmus

- the jerk nystagmus is usually horizontal or horizontal-torsional, but can be upbeat vertical on upgaze

- due to anticonvulsant drugs, which are the most common cause of gaze-evoked nystagmus

Horizontal rotatory nystagmus

- a common non-specific finding in brainstem disease

- Brun's nystagmus is a combination of a gaze-evoked central nystagmus (from compression of the pons) and horizontal-rotatory peripheral nystagmus (from damage to the vestibular verve), secondary to lesions in the cerebellopontine angle eg. acoustic neuroma => a right sided lesion will produce a slow, large-amplitude right beating nystagmus on gaze to the right (gaze-evoked nystagmus) + a left beating nystagmus of medium amplitude in the primary position (peripheral vestibular nystagmus) + a left beating fine horizontal rotatory nystagmus on gaze to the left (peripheral vestibular nystagmus)

Rebound nystagmus

- type I rebound nystagmus occurs when alcoholic patients, who have developed vestibulo-cerebellum degeneration, move their eyes into an eccentric gaze direction => this provokes gaze-evoked jerk nystagmus, which then slowly wanes, to be subsequently followed by a rebound jerk nystagmus in the opposite direction (fast phase towards the primary position of gaze - called centripetal nystagmus) while the eyes are still in the same position of eccentric gaze

- type II rebound nystagmus develops in patients with cerebellar parenchymal disease, and occurs when the eyes return to the primary gaze position following prolonged eccentric gaze => the rapid phase of the rebound nystagmus is opposite the direction of sustained gaze

Downbeat nystagmus

- downbeat nystagmus may only occur on downgaze, and not spontaneously in the primary position of gaze

Pendular nystagmus

- there are two types of binocular conjugate nystagmus - jerk nystagmus (already described) and pendular nystagmus

- in pendular nystagmus, the to-and-fro oscillations are of equal amplitude, and there are no fast and slow phases; the slow pendular movements may be vertical, horizontal, torsional or any combination thereof

- a common form of pendular nystagmus is seen when blindness is acquired at any age, and it is called visual deprivation pendular nystagmus

- if the afferent visual system is normal, there are three common causes of pendular nystagmus

Motor nystagmus

- due to ischemia or multiple sclerosis of the brainstem

- the symmetric pendular nystagmus may be diagonal (oblique nystagmus) and the directional vectors can continuously change (windmill nystagmus)

Spasmus nutans

- a rare triad of pendular nystagmus, torticollis and head nodding

- usually presents between the ages of 3 months and 14 months

- usually benign, and often resolving within a few months or years

- the oscillations may be vertical, horizontal, torsional or mixed

- the pendular nystagmus may appear monocular, but a fine nystagmoid movement can often be seen in the other eye using a slit lamp or ophthalmoscope

- rarely due to optic nerve or optic chiasmal gliomas or third ventricular tumors => a MRI is always indicted to r/o rare afferent visual system tumors

Ocular mycoclonus

- continuous vertical pendular nystagmus, often occurring in association with tremulous movements of the face and palate (oculopalatal myoclonus)

- the ocular oscillations may sometimes be dysconjugate

- caused by damage to the red nucleus area in the brainstem

- the abnormal palatal tremors and pendular movements are often present during sleep

Binocular dysconjugate nystagmus

- this is a form of binocular nystagmus, where the eye movements are symmetric, but in opposite directions

- there are three common types of dysconjugate nystgamus, one involves vertical eye movements (seesaw nystagmus), and the other two involve convergence of the eyes (convergence-retraction nystagmus and convergence nystagmus)

Seesaw nystagmus

- seesaw nystagmus is the only vertical dysconjugate nystagmus

- characterized by the alternate elevation and depression of one eye accompanied by a similar movement in the other eye, but in the opposite direction (seesaw motion)

- in one half of the cycle, the rising eye also intorts and the falling eye extorts

- can occur as a congenital or acquired form

- acquired seesaw nystagmus is most commonly due to parasellar tumors compressing the optic chiasm and/or  optic nerve

- rarely due to brainstem infarcts, septo-optic dysplasia, multiple sclerosis, Arnold-Chiari malformation, syringobulbia, or head trauma

Convergence-retraction nystagmus

- convergence-retraction nystagmus with retraction of the eyeball into the socket occurs on attempted upgaze

- voluntary convergence can induce downbeat nystagmus, or convert an upbeat nystagmus into a downbeat nystagmus

- occurs as part of Parinaud's syndrome (dorsal midbrain syndrome), which is most often due to extrinsic lesions (eg. pinealoma) compressing the third ventricle area of the brainstem

- associated with near-light dissociation, failure of vertical upgaze and eyeball retraction on attempted upgaze

Convergence nystagmus

- a pendular nystagmus induced by convergence, and the nystagmus is usually convergent

- may be accompanied by eyelid nystagmus

- often accompanied by synchronous contraction of the muscles of mastication, or synchronous palate and mandibular muscle contractions (oculomasticatory myorhythmia); this oculomasticatory phenomenon is pathognomonic of Whipple's disease if it is associated with a supranuclear vertical gaze palsy

Monocular or asymmetric binocular nystagmus

- this particular sub-type of nystagmus either only involves one eye (monocular nystagmus), or involves both eyes with asymmetric involvement of the eyes (dissociated nystagmus)

- dissociated nystagmus is the term used to describe nystagmus which is different in both eyes

- the sub-classification of dissociated nystagmus is based on whether the nystagmus occurs spontaneously in the position of primary gaze, or whether it is gaze-evoked and only occurs in an eccentric position of gaze

There are 4 types of monocular nystamus/asymmetric binocular nystagmus that occur in the primary position of gaze

Spasmus nutans

- see previous discussion of spasmus nutans

Monocular visual deprivation

- adults or children who develop monocular blindness may rarely develop monocular vertical pendular nystagmus, that can range in amplitude from fine to coarse

- the nystagmus may develop years after the visual loss, and disappear if the visual loss is corrected

- adults who acquire severe monocular visual loss (eg. due to dense cataracts) can develop monocular vertical drifts and jerk nystagmus (Heimann-Bielschowsky phenomenon) that can also disappear with recovery of vision

Superior oblique myokymia

- a monocular, small-amplitude, high frequency torsional/oblique eye movement due to tremor of the superior oblique muscle fibres

- the patient complains of episodic tilting of the image of one eye, oscillopsia or shimmering, or an awareness of a strange ocular movement

- the episodes only last a few seconds, but are repetitive

- the episodes may be induced by looking in the direction of action of the superior oblique muscle (downwards and inwards) or by moving the eye from that position back to the primary gaze position

- not usually associated with any other neurological findings, and the source remains unknown; very rarely due to pontine tumors or multiple sclerosis of the brainstem

Motor nystagmus of multiple sclerosis

- a low frequency pendular nystagmus involving one eye, or both eyes asymmetrically

- the vector can be multivectorial, circular, elliptical or oblique ("windmill" oscillations)

- the patient may complain bitterly of oscillopsia

- the lesion is thought to be in the brainstem or cerebellum

There is one major type of monocular nystagmus that occurs in an eccentric gaze position

Internuclear ophthalmoplegia

- INO is the most common cause of a dissociated nystagmus

- INO is seen as a lack, or lag, of movement of the adducting eye + nystagmus of the abducting eye

- other eye abnormalities may be present eg. slow adducting saccades, hypermetric abducting saccades, skew deviation and upbeat nystagmus

- INO is indicative of intrinsic brainstem pathology

- the most common cause of INO in patients < 40 years is multiple sclerosis, and the most common cause in patients > 40 years is atherosclerotic vertebro-basilar artery disease

- restrictive diseases (eg. thyroid orbitopathy) or innervation diseases (eg. myasthenia gravis) that impair eye  adduction may mimic INO

Divergence nystagmus

- a rare divergent nystagmus where both eyes move asymmetrically away from the nose

- thought to be due to cerebellar disease
 
Eye movement disorders that mimic nystagmus

Saccadic oscillations

 - saccadic oscillations are back-to-back involuntary saccadic eye movements without any slow phases, and they occur due to an abnormality of saccadic eye movements (in contrast to nystagmus, which occurs due to an abnormality of slow eye movements) => abnormal saccades move the eyes away from the intended position of gaze, and corrective saccades carry the eyes back

- in saccadic intrusions, such as square wave jerks and macro-square wave jerks, brief pauses occur (intersaccadic intervals) between the opposing saccades; while no intersaccadic intervals occur in ocular flutter and opsoclonus

Ocular flutter

- consist of bursts of back-to-back horizontal saccades without any intersaccadic intervals

- the bursts last a few seconds, occur in primary forward gaze, and may be precipitated by re-fixation of gaze after an eye movement

- thought to be due to a disturbance of the pause cells in the pontine paramedian reticular formation subserving horizontal eye movements

- it is almost never present without ocular dysmetria, it may appear with opsoclonus, and it probably represents a further deterioration of brainstem-cerebellar function

Opsoclonus

- opsoclonus is also called saccodomania

- opsoclonus consists of chaotic back-to-back saccades in multiple directions including horizontal, vertical and torsional; and those unpredictable high amplitude eye movements produce the dramatic clinical picture of "dancing eyes"

- thought to be due to disruption of the cerebellar input to the paramedian pontine reticular formation

- in a young child with opsoclonus, myoclonus and cerebellar ataxia, consider an occult neuroblastoma => perform appropriate diagnostic testing; the opsoclonus may remit following successful surgery

- other causes of opsoclonus in adults include CNS tumors, parainfectious encephalitis, meningitis, CNS sarcoidosis, hydrocephalus, thalamic hemorrhage, nonketotic hyperosmolar coma, drug toxicity (lithium, haloperidol, tricyclics, phenytoin, toluene and diazepam) or multiple sclerosis

Ocular dysmetria

- manifests as a binocular conjugate saccadic undershooting or overshooting of the target during re-fixation, with a short series of oscillatory step-saccades as re-fixation occurs

- seen in cerebellar diseases, and is often associated with other cerebellar signs

Square wave jerks

- this is an ocular movement disorder manifested by an observable break in fixation because of an intrusion of a sudden unwanted saccadic movement in reflex response to an object which suddenly enters the patient's visual field, followed by a rapid return to refoveation after a nearly normal intersaccadic interval (the eye movement mimics a small amplitude horizontal nystagmus)

- secondary to neurodegenerative diseases, especially progressive supranuclear palsy and Parkinson's disease, and focal cerebral lesions

- square wave pulses (macro square waves) are large-amplitude square wave jerks seen in demyelinating diseases of the cerbellum

Macro-saccadic oscillation

- a saccadic intrusion that is really a type of saccadic dysmetria; a hypermetric saccade overshoots the target and is followed by a series of hypermetric, corrective saccades that straddle the target and gradually decrease in size until the target is fixated

- in contrast to square wave jerks, they do not occur in darkness

- secondary to cerebellar dysfunction

Ocular bobbing

- in ocular bobbing, the eyes repeatedly move briskly downward from the primary position, remain eccentric for a few seconds, and then slowly move back to the primary position

(* the opposite of vertical downbeat nystagmus - in which the slow upward movement precedes the rapid downward movement)

- horizontal eye movements are usually absent

- indicates severe brainstem dysfunction, and is seen infrequently in comatose patients

(see the coma guidemap for more details about the different patterns of ocular bobbing)

Voluntary nystagmus

- appears as a low amplitude, high frequency pendular nystagmus that occurs in short bursts

- the pendular nystagmus is voluntarily produced, but cannot be sustained, and it will disappear if the patient is instructed to continuously keep his eyes open

- the capacity for self-induced nystagmus apparently runs in families, and it's used as a parlor-trick

Disclaimer: My EM guidemaps reflect my personal approach to problem-solving/managing clinical cases in an ED setting and they should not be regarded as the standard of care. They merely represent the personal opinions of the author and they should only be used in clinical practice if the reader-user has substantial reason to believe that the clinical advice contained in the guidemaps is valid and accurate. The guidemaps are not meant to be "authoritative" and the reader-user should consult standard medical textbooks and expert opinion articles/guidelines for more authoritative advice. The reader-user should particularly confirm all drug doses, their indications and contra-indications, prior to their use.