Nystagmus is the involuntary rhythmic movement of the eyes.
The common form is called jerk nystagmus which consists of two movements:
* a slow drift of the eyes
* a rapid corrective movement back to the point of fixation
The second form is called pendular nystagmus which consists of movement which occur at roughly the same velocity in both directions.
Nystagmus may be caused by:
* lesions in:
o labyrintine-vestibular system
* congenital abnormalities
The two types of nystagmus are:
* jerk nystagmus:
o a slow drift of the eyes
o a rapid corrective movement back to the point of fixation
o this is the most common form of nystagmus
* pendular nystagmus:
o eye movements are sinusoidal, that is both phases of movement have the same velocity
o the nystagmus is always in one plane, for example horizontal pendular nystagmus remains in the horizontal plane even on upward gaze
o pendular nystagmus may be converted to jerk nystagmus at the extremems of gaze
aetiology of jerk nystagmus
Jerk nystagmus is physiological in the following situations:
* when following a moving object, for example looking out of a train window - this phenomenon is termed optokinetic nystagmus
* during caloric stimulation of the ear
ertain patterns of jerk nystagmus have a limited differential diagnosis:
* vertical nystagmus:
o upbeat vertical nystagmus
o downbeat vertical nystamus
* convergence nystagmus
The causes of other pattern of nystagmus are discussed.
assessment of nystagmus
The eyes are assessed at rest:
grading jerk nystagmus
Jerk nystagmus may be graded as follows:
* first degree nystagmus is present only when looking in the direction of the quick component
* nystagmus is second degree if it is also present when looking straight ahead
* third degree nystagmus is where it is present when looking in the direction of the quick component, when looking straight ahead and when looking in the direction of the slow component
The eye movements in the horizontal and vertical plane are assessed by asking the patient to follow a finger which is moved smoothly at a distance of about 50 cm.
* jerk nystagmus may be elicited in a particular direction of gaze
* nystagmus in an abducting eye with failure of adduction of the other eye suggests an internuclear ophthalmoplegia
Saccadic eye movements to the left and right may reveal nystagmus-like movements including:
* subtle internuclear ophthalmoplegias
* ocular dysmetria
* ocular flutter
Nystagmus may be provoked by specific positions of the head - this is termed positional nystagmus.
Positional nystagmus occurs with certain positions of the head and is often accompanied by true rotatory vertigo.
Positional nystagmus associated with a peripheral pathology is usually short-lived and fatiguable unlike that associated with a central pathology which may change in direction and does not fatigue.
Positional nystagmus is seen in:
* benign paroxysmal positional nystagmus
* central - malignant - positional nystagmus
benign paroxysmal positional nystagmus
Benign paroxysmal positional vertigo (BPPV) is a disorder characterized by brief recurrent attacks of vertigo provoked by certain changes in head position with respect to gravity (1).
* it is one of the most common causes of vertigo
* also it is the number one vestibular disorder accounting for a 20-30% of referrals to vertigo clinics (2)
* the most common provocative movements are:
o rolling over in bed
o bending over
o looking upward (3)
Involvement of all three semi circular canals can be seen in this disease with the posterior (60-90%) and horizontal (5-30%) canals being the most commonly affected ones (2,4). However the prevalence of horizontal canal BPPV is more than what was previously thought (4).
Because benign positional vertigo is treatable it is an important diagnosis to make.
central (malignant) positional nystagmus
In central positional nystagmus, a coarse, variable and non-fatiguable nystagmus is set up when the head is placed in a certain position. There is no latent period before the nystagmus begins.
The condition occurs with:
* tumours of the posterior cranial fossa or midbrain
* multiple sclerosis
* vascular lesions
oscillopsia is a symptom of jumbling eye movements, manifested as blurred vision when walking or running, and is caused by poor stabilization of the retinal image during head movement
* occurs primarily from loss of the vestibular ocular reflex (VOR) or ocular oscillations (1,2,3)
o the former is a conjugate eye movement that stabilizes the vision in the opposite direction of head motion
* in contrast, ocular oscillation or nystagmus caused by certain brain abnormalities (e.g., cancer metastasis or radiation necrosis), may also provoke oscillopsia (4)
o radiation-induced oscillopsia in nasopharyngeal carcinoma patients is attributed to bilateral VOR loss, possibly as a result of higher radiation doses (5)
aetiology of pendular nystagmus
The defect is thought to lie in the gaze-holding smooth pursuit mechanisms.
Conditions which result in loss of central vision early in life are the commonest cause of pendular nystagmus:
* B12 deficiency
* multiple sclerosis
Pendular nystagmus may be congenital.
Tests for nystagmus
1.caloric tests for nystagmus
Caloric testing involves the irrigation of the external auditory meatus with hot (44øC) or cold (30øC) water with the head raised 30ø from the supine position. It produces a temporary nystagmus secondary to convection currents set up in the lateral semicircular canal.
Each ear can be tested separately and is irrigated for 40 seconds with 5 minutes being allowed between irrigations. The time elapsed between the commencement of irrigation and the cessation of nystagmus is recorded.
Ice water ensures a maximal stimulus and is used for testing for brain stem death or head injury prognostication.
results of caloric tests
Cold water reduces the vestibular output from one side, creating an imbalance and producing eye drift towards the irrigated ear - nystagmus results from rapid corrective movements.
Hot water increases the vestibular output, reverses the correction current and changes the direction of the nystagmus.
Remember COWS - Cold Opposite Warm Same.
Abnormalities on caloric testing encompass:
* canal paresis - responses to hot and cold on one side are diminished or absent
* directional preponderance - nystagmus to one side is easier to induce than nystagmus to the other side, irrespective of which ear is irrigated
2. optokinetic response
The optokinetic response examines the pursuit and saccadic systems separately, together with their interconnecting pathway, by asking the patient to look at a revolving drum with images - white lines on a black surface - held a short distance from the eyes.
The moving targets should not be too fast - not greater than 30/sec - as pursuit must be allowed to occur.
Optokinetic nystagmus is an involuntary reflex and cannot be voluntarily damped unless the patient can look past the testing drum.
A downgoing optokinetic stimulus is the best way of inducing convergence retraction nystagmus in Parinaud's syndrome.
Electronystagmography is a technique which allows the recording of nystagmus on a paper trace. Eye movements are detected by means of electrodes placed at each outer canthus.
Full electronystagmography involves multiple tests including eye movements, caloric testing, head position and eyes open and closed.
Full electronystagmography can answer three questions:
* whether there is spontaneous nystagmus
* whether the cause is central or peripheral
* which side a peripheral lesion is affecting.
4.rotation tests for nystagmus
This test for nystagmus involves accelerating and decelerating the patient in a rotating chair.
As the chair is accelerated a nystagmus is set up with the quick component in the direction of rotation. Once steady speed is achieved a post-rotation nystagmus is apparent with the fast component in the opposite direction. Upon deceleration, nystagmus is set up in the direction opposite to that of rotation and after the chair has stopped a second post-rotatory nystagmus is evident for a few seconds.
The vestibular threshold can be assessed by alteration of the angular velocity of the chair. The chair can be rotated in both directions.
Vestibular neuronitis and other unilateral peripheral lesions can produce asymmetrical results and congenital deafness or drug-induced ototoxicity cause decreased or absent responses.
Vertical nystagmus is caused by damage to the ventral portion of the brainstem in the pontomedullary and pontomesencephalic regions.
Upbeat and downbeat nystagmus have different aetiological implications.
The causes of upbeat nystagmus include:
* demyelinating disease
* cerebrovascular disease
* brainstem tumours
* Wernicke encephalopathy
Downbeat nystagmus is always caused by a central lesions.
The characteristic causes are:
* Chiari malformation
Other causes include:
* Wernicke's encephalopathy
* cerebellar disease
Convergence nystagmus consists of slow abduction of the eyes, followed by rapid adduction.
There is often an associated rhythmical retraction of the eyes termed nystagmus retractorius.
The cause is a lesion in the ventral midbrain, usually a tumour or infarct, and is usually associated with other features of Parinaud's syndrome.
other patterns of nystagmus
As a general rule:
* peripheral lesions e.g. vestibular damage cause nystagmus away from the side of the lesion
* a central lesion e.g. unilateral cerebellar damage causes nystagmus towards the side of the lesion
Brainstem lesions may cause:
* gaze-dependent horizontal nystagmus
* gaze-dependent vertical jerk nystagmus
Cerebellar disease causes:
* gaze-dependent horizontal nystagmus
* downbeat nystagmus
* related eye movement disorders:
o opsoclonus( Opsoclonus is a form of abnormal eye movement which occurs in all directions.The eye movements in opsoclonus show fast movements in both phases unlike nystagmus which consists of a slow and fast phase.)
o ocular flutter
o ocular dysmetria
Labyrinthine-vestibular lesions cause:
* horizontal nystagmus
* vertical nystagmus
* oblique nystagmus
* torsional nystagmus is common in labyrintine disease
* tinnitus, deafness, vertigo and nausea may be present
Nystagmus of peripheral origin is mixed. And unidirectional.
Central nystagmus is not lessened by visual fixation and is more prominent and persistent.
Bruns Nystagmus seen in Acoustic neuromas
Upbeat nystagmus; meningitis, organophosphorous poinsoning, Wernicke’s encephalopathy
Pendular nystagmus is seen in cerbellar lesions.
Horizontal nystagmus is a well-recognized finding in patients with a unilateral disease of the cerebral hemispheres, especially with large, posterior lesions. It often is of low amplitude. Such patients show a constant velocity drift of the eyes toward the intact hemisphere with fast saccade directed toward the side of the lesion.
Patients with horizontal nystagmus with unilateral hemispheric lesions, especially parietal or parietal-occipital lesions, show impaired optokinetic nystagmus when the drum is rotated toward the side of the lesion.
* Oculocephalic reflex (doll's head phenomenon)
o The oculocephalic reflex develops within the first week of life and essentially represents a vestibulo-ocular reflex normally suppressed in a conscious individual that attempts to turn the head to fixate on an object.
o This test consists of the rapid rotation of the patient's head in a horizontal or vertical direction. With intact vestibular nuclei and medial longitudinal fasciculi, the eyes move conjugately in the opposite direction of the head turn. Alternatively, the test may be performed by having the patient extend the arm out in front of the body and fixate on the outstretched thumb. Patients should be instructed to rotate their torso back and forth about their longitudinal axis such that the thumb remains in front of the body at all times.
o Patients with the ability to suppress the oculocephalic reflex should be able to maintain fixation on their thumb while rotating. An abnormal test result would show the patient continuously losing fixation of the thumb.
o Inability to suppress the oculocephalic reflex is common in patients with vestibular imbalance.
* Caloric testing
o Instilling cold or warm water into the external auditory canal can reproduce the same movement of endolymph in the semicircular canals produced by rotations of the head. Instillation of water into the external auditory canal causes endolymph convection currents that in turn induce nystagmus.
o While sitting erect, the patient tilts the head back 60°. While in supine, the patient elevates the head 30°; this brings the horizontal semicircular canals into the vertical plane.
o The external auditory canal is irrigated with cold or hot water. Cold water instilled into the right ear causes the endolymph in the right semicircular canal to cool and sink. This movement of endolymph is the same movement induced by a rotation of the head to the left, inducing a horizontal nystagmus directed to the left (ie, to the opposite side the water was placed). Warm water in the same ear produces the opposite effect (ie, a horizontal nystagmus directed to the right or toward the same side the water was placed); ie, cold-opposite, warm-same (COWS).
* Note whether the character of the nystagmus changes with otolithic stimulation. Failure to respond to otolithic stimuli implies peripheral vestibular disease.
Grading of nystagmus:
First degree: present when gaze is at the direction of the fast component
Second degree: present when gaze is straight ahead
Third degree: present in all gaze directions
* Seesaw nystagmus
o Rostral midbrain lesions
o Parasellar lesions (eg, pituitary tumors)
o Visual loss secondary to retinitis pigmentosa
* Downbeat nystagmus
o Lesions of the vestibulocerebellum and underlying medulla, including the following:
+ Arnold-Chiari malformation
+ Demyelination (eg, multiple sclerosis)
+ Microvascular disease with vertebrobasilar insufficiency
+ Brain stem encephalitis
+ Tumors at the foramen magnum (eg, meningioma, cerebellar hemangioma)
+ Drugs (eg, alcohol, lithium, antiseizure medications)
+ Nutritional (eg, Wernicke encephalopathy, parenteral feeding, magnesium deficiency)
o Heat stroke
o Approximately 50% have no identifiable cause.
* Upbeat nystagmus
o Medullary lesions, including perihypoglossal nuclei, the adjacent medial vestibular nucleus, and the nucleus intercalatus (structures important in gaze holding)
o Lesions of the anterior vermis of the cerebellum
o Benign paroxysmal positional vertigo
* Periodic alternating nystagmus
o Arnold-Chiari malformation
o Demyelinating disease
o Spinocerebellar degeneration
o Lesions of the vestibular nuclei
o Head trauma
o Posterior fossa tumors
o Binocular visual deprivation (eg, ocular media opacities)
* Pendular nystagmus
o Demyelinating disease
o Monocular or binocular visual deprivation
o Oculopalatal myoclonus
o Internuclear ophthalmoplegia
o Brain stem or cerebellar dysfunction
* Spasmus nutans
o Usually occurs in otherwise healthy children
o Chiasmal, suprachiasmal, or third ventricle gliomas may cause a condition that mimics spasmus nutans.
* Torsional - Lateral medullary syndrome (Wallenberg syndrome)
* Abducting nystagmus of internuclear ophthalmoplegia
o Demyelinating disease
o Brain stem stroke
* Gaze evoked
o Drugs - Anticonvulsants (eg, phenobarbital, phenytoin, carbamazepine) at therapeutic dosages