Nystagmus

Nystagmus can be defined as a repetitive, involuntary, to-and-fro oscillation of the eyes.¹ It may be physiological or pathological and may be congenital or acquired. It can be described according to:

• The direction of movement: this may be horizontal, vertical, torsional or non-specific.
• Amplitude - how far the eyes move: this can be fine or coarse.
• Frequency - how often the eyes oscillate: this is said to be high, moderate or low.

Nystagmus is said to be one of three forms:

• Jerk nystagmus: this is characterised by a slow drifting movement followed by a fast corrective jerking movement. The direction of nystagmus is described according to the fast component.
• Pendular nystagmus: the drifting and corrective movements occur slowly.
• Mixed nystagmus: there is a pendular movement in the primary position of gaze (i.e. looking ahead) but a jerk nystagmus on lateral gaze.

Furthermore, nystagmus is said to be symmetrical, asymmetrical, bilateral or unilateral (this is rare and is usually actually asymmetrical but more evident on one side²). It may be conjugate (both eyes move together) or disconjugate (the eyes appear to move independently of each other). The exact incidence and prevelence of nystagmus is not known but it is thought to occur in about 1 in 1000 people.

The degree to which these patients can be assessed depends on the patient's age and their ability to co-operate with instructions. A history and some degree of examination should be possible in most patients - even very young babies should look with interest at brightly coloured objects or the light of a pen torch.

History

• Ask about onset: the age will help determine which type of nystagmus this is and hence point to a possible diagnosis.
• It is helpful to know when it occurs and when (if at all) it ceases - accommodation and sleep are two occasions to specifically enquire about.
• Ask about related visual symptoms. Oscillopsia is the term used to describe the symptom of continual movement of the visual environment described by some patients with nystagmus. This usually occurs in patients who developed nystagmus later on in life but careful questioning of patients who have had infantile nystagmus may reveal a history of headaches, tearing, avoidance of near tasks and blurry vision.³
• Related systemic symptoms - particularly with regards to the nervous system - are highly relevant.
• Use of prescribed or non-prescribed drugs is important, particularly anticonvulsants.
• Enquire about a family history.

Examination

• Nystagmus is described as above, with a note made on which position of gaze it occurs in.
  • Primary position - looking straight ahead.
  • Secondary positions - looking straight up / down (also known as the midline positions), straight right or left.
  • Tertiary positions - these are the four oblique positions: up and right, down and right, up and left, down and left.
  • Cardinal positions - these include the right and left (secondary) positions and all the tertiary positions.
• Examine the patient sitting facing you and observe the nystagmus in the primary position.
• Using your finger or a small fixation target, observe the nystagmus in all positions of gaze.
• Ask the patient to comment on any visual symptoms as the eyes move (e.g. it has gone blurry towards the left, I can see two fingers looking up and right).
• Enquire about the 'null' point: this is an angle which some patients find limits their visual impairment - it often results in abnormal head positioning.
  

  Oculocephalic reflex (doll's head phenomenon)2 The oculocephalic reflex develops within the first week of life and represents a vestibulo-ocular reflex normally suppressed in a conscious individual that attempts to turn the head to fixate on an object. This test consists of the rapid rotation of the patient's head in a horizontal or vertical direction. Normally, 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 such that the thumb remains in front of the body at all times. 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. Inability to suppress the oculocephalic reflex suggests vestibular imbalance.

• Other tests of the vestibular system include Romberg's test and caloric testing (see end of the article: vestibular nystagmus).
• Carry out a full neurological examination.
• Complement with any other examination depending on findings.

Associated problems⁴

• Patients with nystagmus can tire easily from the extra effort it takes to look at things. They may also complain of balance problems as their depth of perception may be impaired and uneven surfaces or stairs may be difficult to negotiate. This may be perceived as clumsiness by others.
• There may be associated stress and nervousness at being in unfamiliar surroundings. Confidence may also be a problem in these patients who often have poor vision and have difficulty in making and maintaining eye contact.
• School children and students may need extra time for reading and sitting exams - there are issues here surrounding the education of carers and teachers. Small print can usually be read with aids but children will find it hard to share books.
• Patients need to inform the DVLA - many will not be authorized to drive.

Management

• It is necessary to refer patients with nystagmus for further investigation. Ophthalmologists are a good first port of call but abnormal neurological findings warrant a referral directly to the neurologists.
• Subsequent management will depend on the underlying condition but may be conservative, medical (e.g. gabapentin and baclofen) or surgical. The latter is unusual and involves altering the insertion of the extraocular muscles involved but neurosurgery may also be performed where there is an underlying resectable lesion. Severe, disabling nystagmus can be treated with retrobulbar injections of botulinum toxin.⁵
• Many patients will have some degree of decreased visual acuity requiring spectacles; some will be so severely affected as to need registration as partially sighted or blind.
• Associated physical and psychosocial factors need to be explored and addressed where necessary (see associated problems above).

This nystagmus occurs after 6 months of age. It is worth noting that ~5% of the normal population can voluntarily induce a predominantly horizontal, high-frequency, low-amplitude rhythmic oscillation of the eyes - this phenomenon may be associated with behavioural tics.⁶

End-point nystagmus

This is the nystagmus associated with extreme positions of gaze. It is a fine jerk nystagmus with the fast phase being in the direction of the gaze.

Optokinetic nystagmus

This describes the nystagmus that occurs when following a moving object (such as looking out of a train window). This is a jerk nystagmus: the slow phase follows the target and the fast phase fixates onto the next target. An optokinetic drum is an instrument consisting of a handle on which is mounted a cylinder which can rotate. The cylinder is printed with thick, regularly spaced vertical black and white stripes and as it rotates, optokinetic nystagmus is induced. It is a helpful instrument in assessing the visual acuity of very young infants and also to detect patients feigning blindness.

Nystagmus at this age may be:

• Idiopathic
• Neurological in origin
• As a result of sensory deprivation.

Babies presenting with nystagmus all need referring (preferably to a paediatric ophthalmologist) for investigation.

Congenital idiopathic nystagmus (CIN)

• Overview - infants with a nystagmus in all positions of gaze but with clinically normal eyes and normal developmental milestones are said to have CIN. This may be X-linked or autosomal dominant. This diagnosis is only made when neurological and ocular abnormalities have been excluded.
• Presentation - these infants present within the first two months of life. The nystagmus is characterised by a horizontal nystagmus in all positions of gaze which may be pendular or jerk, which dampens with convergence / accommodation and which disappears during sleep. The visual acuity is usually fairly good (of the order of 6/9-6/12). The child may eventually adopt an abnormal compensatory head position.
• Associated diseases - none by definition.
• Management - the continual movement of the eye may reduce the visual acuity (depending on the speed of movement, whether there are periods of rest from the movement and whether the nystagmus is reduced by accommodation) and visual correction with spectacles may be necessary. There should not be any progression of the severity of the nystagmus beyond that established in the first few months of life.

Neurological nystagmus

• Overview - neurological disease can present with many forms of nystagmus and this possibility must always be considered in the infant presenting with nystagmus. A history of failure to thrive, developmental abnormalities or any other evidence of neurological abnormalities should prompt investigation for a neurological cause.³
• Presentation - these patients tend to present before two months of age.
• Associated diseases - this form of nystagmus is associated with space occupying lesions, metabolic diseases, neurodegenerative disorders and trauma.
• Management - this depends on the underlying cause, as does the prognosis.

Sensory deprivation nystagmus (SDN)

• Overview - this occurs as a result of an abnormality at some point in the visual pathway leading to sensory deprivation. It accounts for 80-90% of childhood nystagmus.
• Presentation - these children often present within the first two to three months of life with a bilateral, conjugate (eyes move together) nystagmus. The movements are horizontal and disappear during sleep.³ There is often a family history of sensory deprivation and examination reveals poor vision, photophobia, abnormal pupillary reactions and optic neuropathy. More detailed assessments will reveal the presence of a high refractive error and retinopathy.
• Associated diseases - any abnormalities of the eyes causing abnormal vision including corneal opacities, aniridia (absence of the iris), cataracts, albinism, retinopathy of prematurity and the rod/cone dystrophies. Other culprits include chorioretinal abnormalities, Leber's congenital amaurosis and optic nerve abnormalities.
• Management - this will depend on the underlying cause of the sensory deficit, as does the prognosis.

This group of conditions can be usefully subdivided into symmetrical and asymmetrical nystagmus. The symmetrical conditions can be further classified according to the direction of movement of the eyes. The management and outcome depends on the associated diseases / causative factors. Patients - children or adults - presenting with late-onset nystagmus need to be referred for investigation; their management and outcome will depend on the causative factor.

Symmetrical vertical nystagmus

Upbeat nystagmus

• Description - this is a jerk nystagmus with the fast phase going upwards. It is apparent in the primary position of gaze and increases on looking up.
• Associated diseases - it is most commonly seen as a side effect of anticonvulsants but it may also occur in cerebellar and ponto-medullary abnormalities. It may also be a sign of Wernicke encephalopathy.¹ Occasionally, it seen with benign paroxysmal positional vertigo or it can be a manifestation of an atypical familial CIN.

Downbeat nystagmus

• Description - this jerk nystagmus has a fast downward phase which is present in the primary position but worse on looking down.
• Associated diseases¹ - there are a variety of causes including any abnormality at the craniocervical junction (e.g. Arnold-Chiari malformation), cerebellar degeneration and drug intoxication (particularly with lithium, phenytoin, carbamazepine and barbituates). It also occurs in Wernicke encephalopathy, demyelination, brain stem encephalitis, tumours at the foramen magnum and hydrocephalus.

Symmetrical horizontal nystagmus

Periodic alternation nystagmus (PAN)

• Description - this is a horizontal jerk nystagmus, the direction of which usually reverses every 2-3 minutes. There are cyclical phases where the nystagmus amplifies and then decreases. There is a quiet interlude of ~10-20 seconds before the reversal occurs.
• Associated diseases¹ - cerebellar and brainstem abnormalities, demyelination, Louis-Bar syndrome, drug intoxication (especially phenytoin) and atypical CIN. It is also seen following head trauma, with encephalitis and syphilis. Binocular visual deprivation may produce a PAN.

Symmetrical mixed vertical / horizontal nystagmus

Gaze paretic nystagmus

• Description - this is a jerk nystagmus in the direction of eccentric gaze (e.g. when you look right, the nystagmus is to the right). When it is unilateral, its direction is toward the side of the lesion.
• Associated diseases - this may be associated with vestibular, cerebellar and brainstem disease as well as with drug intoxication.

Rebound nystagmus

• Description - this is a horizontal jerk nystagmus which changes direction after several seconds of eccentric gaze and then reverts back to its original pattern when the eyes are returned to their primary position.
• Associated diseases - this occurs in posterior fossa lesions and in cerebellar disease.

Acquired pendicular nystagmus

• Description - this is a high-frequency, low-amplitude pendular nystagmus in all directions of gaze.
• Associated diseases - causes include demyelinating disease, oculopalatal myoclonus and drug intoxication.

Asymmetrical nystagmus

Spasmus mutans

• Description - this rare self-limiting condition is characterised by an acquired monocular (or asymmetrical binocular) fine, rapid nystagmus usually occurring within the first year of life (range: 3-18 months). The nystagmus may have vertical and torsional components and is frequently increased in amplitude on abduction.¹ It usually resolves by about the fourth year of life. It is often accompanied by head nodding and torticollis. All these signs disappear during sleep.
• Associated diseases - usually this is a benign condition but space occupying lesions (especially gliomas of the anterior visual pathway) may present in a similar way and therefore these children need investigating to rule this out.

Latent nystagmus

• Description - this jerk horizontal nystagmus only occurs if one eye is occluded (or has a reduction of light input by using a filter). The nystagmus is bilateral with the fast phase towards the uncovered eye.
• Associated diseases - infantile esotropia.

See-saw nystagmus (of Maddox)

• Description - this pendular nystagmus is characterised by one eye rising and intorting as the other simultaneously lowers and extorts.
• Associated diseases - this is classically seen with space occupying lesions in the suprasellar region (when there will also often be a bitemporal hemianopia) but it is also associated with optic nerve hypoplasia and brainstem disease. It may be seen in patients with visual loss secondary to retinitis pigmentosa.

Ataxic nystagmus

• Description - this is a rhythmic oscillation of the eye on abduction.
• Associated diseases - internuclear ophthalmoplegia.

Vestibular nystagmus²

• This jerk nystagmus arises from an altered input from the vestibular nuclei to the horizontal gaze centres.
• It may be elicited by caloric stimulation which is an attempt to discover the degree to which the vestibular system is responsive and also how symmetric the responses are (i.e. between left and right):
  • When cold water is poured into the right ear, the patient develops a left jerk nystagmus (fast phase to the left).
  • When warm water is poured into the right ear, the patient develops a right jerk nystagmus (fast phase to the right).
  • When cold water is poured into both ears simultaneously, there is a fast upward phase and warm water produces a fast downward phase.

Convergence-retraction nystagmus¹

• Description - this is caused by co-contraction of the extra-ocular muscles (especially the medial recti) resulting in a jerk nystagmus induced by optokinetic stimulation downwards (see physiological nystagmus above). Upward refixation brings the two eyes towards each other (convergence) and there may be associated globe retraction.
• Associated diseases - this is caused by lesions of the pretectal area such as pinealomas and vascular accidents.