Nystagmus (Abnormal Eye Movements) Types, Video, Causes
Our eyeballs can be moved in various directions by the actions of the extraocular muscles. This allows us to focus in the direction of our choice and it is a voluntary action. However, there are instances where this movement of the eyeball is involuntary and repetitive in a way that compromises visual acuity. Nystagmus is one such case.
What is nystagmus?
Nsytagmus is the abnormal repetitive movements of the eyeball that is involuntary and usually rapid in motion. This appears as if the eyes are ‘swinging’ or ‘ticking’ (oscillations) like a pendulum. This movement may be side-to-side (horizontally), up and down (vertically) or round-and-round (circular/rotation). It is usually bilateral (affecting both eyes) but in some cases may appear unilateral (one-sided) although in these instances the rhythmic movements are just more pronounced on one side than on the other.
Although the back and forth movements can hamper normal accommodation and therefore vision, many people with nystagmus do not experience any vision distortion. Nystagmus can be acquired or congenital. The acquired form may develop at any point in life depending on the underlying cause. The congenital type rarely is evident at birth and should be termed infantile nystagmus. Spasmus nystagmus becomes apparent with 3 to 15 months of age and then disappears around 3 to 5 years old. In some rare cases, this form of nystagmus may continue till 5 to 6 years of age.
How do the eyes move?
There are six muscles that control movement of the eye. These muscles are known as the extra-ocular muscles. The human eye has to be able to move in order to view a certain part of the landscape clearly, since the most visually acute part (the fovea) is very small. Control of eye movement is voluntary although nystagmus is an involuntary action. These muscles are controlled by three cranial nerves – the oculomotor nerve that controls most of the muscles, the abducens nerve and the trochlear nerve.
Types of Nystagmus
Nystagmus can be classified according to the different characteristics of the oscillations. It is important that the type of nystagmus is determined by a medical professional as it can be confusing. Broadly the nystagmus may be pendular where the oscillations are of the same extent and speed, or it may be jerking where the oscillations first have a slow initiating phase and then a fast corrective phase.
These acquired types include:
- Downbeat nystagmus
- Upbeat nystagmus
- Horizontal nystagmus
- Torsional nystagmus
- Pendular nystagmus
- Seesaw nystagmus
- Vestibular nystagmus
- Gaze-evoked nystagmus
- Periodic alternating nystagmus
- Abducting nystagmus
- Spasmus nutans
The congenital types include:
- Afferent nystagmus
- Efferent nystagmus
It has been proposed that spasmus nutans be classified as a subtype of congenital nystagmus.
Causes of Nystagmus
Nystagmus is caused by a host of different conditions, mainly those that affect the central nervous system and specifically the brain. This may include a variety of diseases, head injuries and substances that are known to act on the central nervous system, like alcohol, illicit substances and certain prescription medication. The possible causes of some of the types of nystagmus is listed below.
Downbeat Nystagmus
Possible causes include any disorder, diseases or damage to the vestibulocerebellum and medulla, such as:
- Arnold-Chiari malformation
- Demyelination (multiple sclerosis)
- Microvascular disease with vertebrobasilar insufficiency
- Brain stem encephalitis
- Tumors at the foramen magnum (like a meningioma or cerebellar hemangioma)
- Trauma
Drugs (such as lithium, antiseizure medications), alcohol and nutritional disorders (such as Wernicke encephalopathy and magnesium deficiency) may also be responsible. Heat stroke is another possible cause. However, almost half of the cases is unknown (idiopathic).
Upbeat Nystagmus
Possible causes include damage, disease or disorders of the:
- Medulla
- Cerebellum (specifically of the anterior vermis)
This type of nystagmus may also occur with benign paroxysmal positional vertigo.
Seesaw Nystagmus
Possible causes include ostral midbrain lesions, parasellar lesions such as pituitary tumors and loss of vision that may arise with conditions like retinitis pigmentosa.
Pendular Nystagmus
Possible causes include:
- Demyelinating disease
- Oculopalatal myoclonus
- Internuclear ophthalmoplegia
- Brain stem or cerebellar dysfunction
Blocking one or both eyes can also lead to nystagmus.
Gaze-Evoked Nystagmus
Possible causes include:
- Drugs like anti-convulsants.
- Alcohol
Periodic alternating nystagmus
Possible causes include:
- Arnold-Chiari malformation
- Demyelinating disease
- Spinocerebellar degeneration
- Head trauma
- Encephalitis
- Syphilis
- Posterior fossa tumors
- Binocular visual deprivation
Diagnosis of Nystagmus
The treatment of nystagmus varies greatly due to the diversity of causes. In the absence of a clearly identifiable cause, further diagnostic investigations will be necessary. This should be preceded with a thorough case history, opthalmic examination and neurological assessment. Neuroimaging will then be conducted and this involves an MRI (magnetic resonance imaging) scan.
In most instances the oscillating movements of nystagmus can be visibly assessed. However, in some instances it may be difficult to discern and other tests may be needed. An electronystagmographs monitors the contraction of the extra-ocular muscles that move the eyeball. An electroretinography allows for the assessment of the inner layer of the eyeball, the retina, and is done when retinal disease is suspected as a cause.
Treatment of Nystagmus
The treatment of nystagmus depends on the underlying cause. Changes to medication and lifestyle changes may be necessary when these factors are responsible. There is no drug for nystagmus specifically although medication may be needed to treat the underlying condition. If effectively treated then the nystagmus may resolve. Certain drugs like muscle relaxants, neuromuscular blockers and anticonvulsants can be useful in reducing the severity of nystagmus to some extent.
Botulinum toxin injections may be administered to reduce the nystagmus. Its effects are short-lived and needs to be administered again once it wears off. However, it may also impair voluntary eye movements and can lead to drooping eyelids (ptosis) as well as double vision (diplopia). Injecting only one eye can sometimes worsen the nystagmus in the other eye.
Surgery may be considered in some cases, especially for congenital nystagmus to move the eyeball back to a ‘normal’ alignment. The usefulness of surgery has to be assessed on a case by case basis and the benefits must outweigh the possible complications that could arise.
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