Nystagmus Definition
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Nystagmus is a type of ocular condition that is characterized by involuntary movements of the eye. It can be acquired during infancy or at the later stages in life, and may lead to limited or reduced vision. It can be caused by toxicity, congenital factors, acquired or CNS disorders or from consumption of alcohol or pharmaceutical drugs.
The condition is also occasionally linked with vertigo.
Nystagmus Epidemiology
It is a common clinical condition that affects one in every 2000 to 3000 individuals. A survey in Oxfordshire, UK revealed that one in every 670 kids manifest nystagmus by 2 years of age. Another study revealed that it is more prevalent amongst the white Europeans as compared to Asians.
Nystagmus Types
Two main forms of nystagmus exist. These are:
Pathological Nystagmus
It involves a biphasic ocular oscillation that alternates a smooth pursuit or slow eye movement on one direction with a saccadic or fast eye movement on the other direction. The fast phase eye velocity or FPEV and slow phase eye velocity or SPEV are interrelated and can be taken as measurement of efficiency of system stimulus/response.
Nystagmus is considered to be pathologic when it occurs without accomplishing its normal functions. It thus deviates from healthy or normal ocular condition and is a result of critical damage to one or more vestibular system components (vestibular nystagmus), including the otolith organs, the semicircular canals, as well as the vestibulocerebellum. The pathological form generally causes impairment of vision to some degree, although its severity can vary greatly. Blind people might also have nystagmus in some cases, which is one of the reasons why some choose to wear dark glasses.
Pathological Nystagmus may have the following variations:
Central nystagmus
It is caused by normal/abnormal processes that are not related to vestibular organ. These might include lesions in the midbrain or the cerebellum, which might lead to upbeat and downbeat nystagmus.
Peripheral nystagmus
It might develop due to normal/diseased functional states of vestibular system and combine one rotational component with horizontal or vertical eye movements. It can either be evoked, positional or spontaneous. It is classified into the following four types:
Positional nystagmus
It develops when an individual’s head is placed in a definite position. Benign paroxysmal positional vertigo or BPPV is a condition in which this might occur.
Gaze Induced nystagmus
It is also known as Gaze Evoked Nystagmus. It might occur because of changing the gaze away from or towards a particular side that has an affected vestibular component.
Spontaneous nystagmus
It occurs randomly, without any regard to the placement of patient’s head.
Post rotational nystagmus
It is caused when stimulation of vestibular system by a rapid shaking or a rotational movement of the head leads to an imbalance between a diseased side and a normal side.
Physiological Nystagmus
It is a type of involuntary eye movement involving the vestibulo-ocular reflex or VOR, and is characterized by an alternating smooth pursuit movement in 1 direction and saccadic movement in opposite direction.
Direction of the nystagmus is determined by direction of the quick phase. For example, a left-beating nystagmus is marked by a left-moving quick phase and a right-beating nystagmus is marked by a right-moving quick phase. Oscillations might occur in horizontal, vertical or torsional planes as well as in other combinations. Thus the resulting nystagmus can be named as an overall description of the eye movement, such as:
- Upbeat nystagmus
- Downbeat nystagmus
- Periodic alternating nystagmus
- Jerk nystagmus
- Pendular nystagmus
- Seesaw nystagmus
- Mixed nystagmus
The two principal variants of physiological nystagmus are described below:
Opticokinetic nystagmus
It is induced when the patient looks at a moving visual stimulus, for e.g., moving vertical or horizontal lines, and/or stripes.
Post-rotatory nystagmus
It occurs if an individual spins continuously in a chair and then stops suddenly. The fast phase of this form is experienced in opposite direction of the rotational movement. The slow phase is felt in the direction of the rotation.
Nystagmus Causes
Pathological nystagmus can be caused by idiopathic or congenital factors. It can also occur as a condition that is secondary to a preexisting neurological disorder. Other factors, such as disorientation or usage of certain drugs may also lead to nystagmus.
Early-onset Nystagmus
This form occurs more often than acquired nystagmus. The condition can be either insular or follow other disorders such as Down syndrome or micro-ophthalmic anomalies. Early-onset nystagmus is generally non-progressive and mild. The affected individuals themselves are often not aware of having spontaneous eye movements. However, their vision can be seriously hampered due to such movements.
Early-onset nystagmus can occur due to the following disorders:
- Noonan syndrome
- Latent nystagmus
- Nystagmus blockage syndrome
- Infantile nystagmus, which is characterized into:
- Aniridia
- Albinism
- Idiopathic factors
- Rod monochromatism
- Bilateral congenital cataract
- Leber’s congenital amaurosis
- Bilateral optic nerve hypoplasia
- Optic nerve or a macular disease
- Persistent form of tunica vasculosa lentis
- Visual-motor syndrome of functional monophthalmos
X-linked infantile nystagmus is believed to be caused by mutations of gene FRMD7, located on X chromosome. The infantile form of nystagmus is also linked with 2 X-linked eye diseases referred to as complete and incomplete congenital stationary night blindness (CSNB and iCSNB or CSNB-2). Mutations in NYX or nyctalopin are believed to cause CSNB, whereas CSNB-2 is caused by mutations of the CACNA1F. It is a voltage-gated calcium channel that does not conduct ions when mutated.
Acquired Nystagmus
Certain diseases may present nystagmus that is acquired as a pathological symptom. These may include:
- Stroke
- Aniridia
- Head trauma
- Multiple sclerosis
- Whipple’s disease
- Tullio phenomenon
- Optic nerve hypoplasia
- Lateral medullary syndrome
- Pelizaeus-Merzbacher disease
- Wernicke-Korsakoff syndrome
- Superior canal dehiscence syndrome
- Benign Paroxysmal Positional Vertigo
- Ménière’s disease as well as other balance disorders
- Brain tumors, such as Astrocytoma, Medulloblastoma, or other tumor formations in posterior fossa
Nystagmus can be caused by the following toxic or metabolic reactions:
- SSRIs
- Lithium
- Ketamine
- Salicylates
- Barbiturates
- Amphetamines
- Benzodiazepines
- Phencyclidine (PCP)
- Thiamine deficiency
- Phenytoin (Dilantin)
- Alcohol intoxications
- Wernicke’s encephalopathy
- Lysergic acid diethylamide (LSD)
- Other sedatives or anticonvulsants
- Methylenedioxyamphetamine (MDA)
- Methylenedioxymethamphetamine (MDMA)
When in association with central nervous system disorders, the nystagmus can develop in any direction, including being horizontal (horizontal gaze nystagmus). Pure form of vertical nystagmus is commonly central in origin. However, it can also be caused by high phenytoin toxicity. The general causes include:
- Stroke
- Tumor
- Trauma
- Cerebellar ataxia
- Multiple sclerosis
- Chiari Malformation
- Thalamic hemorrhage
Other factors may include non-physiological causes, vitamin B12 or thiamine deficiency, malfunctioning of the trochlear nerve as well as vestibular pathology, such as Labyrinthitis, Ménière’s disease, superior canal dehiscence syndrome or SCDS and benign paroxysmal positional vertigo (BPPV).
Nystagmus Symptoms
Patients of nystagmus experience repeated involuntary oscillations of 1 or both the eyes. The disorder frequently affects the nerves that are behind the eye instead of the eye itself. The movements may be vertical (vertical nystagmus), horizontal (horizontal nystagmus), and circular (circular nystagmus) or combine various speeds and motions. The condition may manifest itself in varying degrees and ways. Although most affected individuals view the objects as being stationery since the brain is believed to make necessary adjustments, many people often experience reduced acuity due to the challenge of maintaining a fixed focus. Individuals having this condition often see objects in a lower contrast, with many also having issues with depth perception which affects balance and coordination. Many people having this disorder will tilt heads or nod frequently to compensate for impairments caused by it.
Many individuals having nystagmus are believed to be partially sighted, or mostly blind, and are generally unable to drive. A faster oscillation leads to lower visual acuity and a slower oscillation results in a higher visual acuity. However, it is necessary to note that visual impairment may also be caused by other underlying conditions. Most nystagmus patients have got a null point, i.e. a definite viewing angle when the eye movements get reduced and the vision is improved. Affected individuals often take on a certain body posture or head posture that facilitates best view, such as sitting sideways or moving the television screen.
Nystagmus Diagnosis
Nystagmus is often very noticeable but hardly recognized. It can be investigated by using several non-invasive standard tests. An example of these is the caloric reflex test where an external auditory meatus is flooded with cold or warm water or air. This temperature gradient provokes stimulation of horizontal semicircular canal which helps in detecting nystagmus. The resulting eye movements can be recorded and studied by electrooculography methods like Electronystagmograph (ENG) or video-oculography (VOG) procedures like Videonystagmograph (VNG) that is supervised by an audiologist. A form of special swinging chairs having electrical controls might be used for inducing rotatory nystagmus. Electrooculography or optokinetic drum can also be used to study eye movements. Other tests associated with the diagnosis of this eye disorder include:
- CT scans
- Ear exam
- MRI scans
- Vision testing
- Neurological exam
- Examination of the inner parts of the eye using an ophthalmoscope
Nystagmus Differential Diagnosis
A number of conditions show signs and symptoms which are typical of nystagmus. Hence, while determining the diagnosis of Nystagmus, it should be distinctly identified from such similar disorders. The conditions that make up the differential diagnoses of Nystagmus include:
- Opsocionus
- Ocular flutter
- Ocular dysmetria
- Opsoclonus myoclonus
- Macrosquare wave jerks
- Macrosaccadic oscillations
- Superior oblique myokymia
Nystagmus Treatment
Although Congenital nystagmus has been traditionally viewed as being non-treatable, medications and drugs have been identified in recent years that can be used to cure this condition. A drug known as Baclofen could be used to remedy periodic alternating nystagmus. The anticonvulsant drug gabapentin was proven effective in almost 50% of all cases. Other drugs that can be used to manage nystagmus include:
- Levetiracetam
- Memantine
- 4-aminopyridine
- 3,4-diaminopyridine
- Acetazolamide
Various therapeutic measures, such as drugs, contact lenses, surgery, as well as low vision rehabilitation to help the patients have been proposed as well.
The clinical trials of the surgical procedure known as Tenotomy were concluded in 2001. This procedure has been proven effective in reducing the oscillations of the eye, which eventually leads to the improvement of visual acuity.
The symptoms of Nystagmus have also been effectively reduced by acupuncture in some cases. Benefits have been noticed in cases where the acupuncture points of neck were utilized, especially the points on sternocleidomastoid muscle. Acupuncture has been shown to reduce the frequency of eye movements as well as a decrease in the slow phase velocities that led to increase in the foveation duration periods during and after the treatment.
Nystagmus Prognosis
Congenital nystagmus generally persists throughout a person’s life whereas the forms that develop from viral infections or drug toxicity commonly resolve once the underlying problem is managed. It is normally not painful and does not lead to complete blindness. The overall quality of life might get affected by reduced depth perception, a decreased reading speed, as well as periods of Oscillopsia. Most people having Nystagmus can lead productive, independent lives.
Nystagmus Videos
Here are some videos that show the movement of the eyes that are affected by nystagmus.
References:
https://en.wikipedia.org/wiki/Nystagmus
http://www.nlm.nih.gov/medlineplus/ency/article/003037.htm
http://www.allaboutvision.com/conditions/nystagmus.htm
http://www.whereincity.com/medical/topic/eye-and-vision/diseases/nystagmus-139.htm
http://www.thirdage.com/hc/c/nystagmus
