Timothy C. Hain, MD • Page last modified: August 2, 2020
Nystagmus is defined as involuntary movement of the eyes. Most frequently it is composed of a mixture of slow and fast movements of the eyes. Nystagmus can occur normally, such as when tracking a visual pattern. Nystagmus may also be abnormal, usually in situations where one would want the eyes to be still, but they are in motion. Vertigo (a sensation of spinning), is often accompanied by nystagmus. The following material attempts to illustrate and describe nystagmus subtypes.
|Typical appearance of vestibular type spontaneous nystagmus.|
Spontaneous nystagmus denotes movement of the eyes without a cognitive, visual or vestibular stimulus. Most commonly spontaneous nystagmus is caused by a vestibular imbalance. Normally, both vestibular nerves fire at a tonic rate. The two nerves input is subtracted centrally. When the head is still, this results in a signal of 0 spikes/sec, and no nystagmus. When one vestibular nerve has decreased firing relative to the other, this provides an offset which produces a constant nystagmus. Vestibular nystagmus is "jerk" nystagmus" -- the eye moves slowly towards the side of decreased firing, and then "jerks" back towards the center.
Vestibular nystagmus is typically inhibited by visual fixation and also typically follows Alexander's law (it becomes greater on gaze in direction of the fast phases). If the nystagmus does not follow these characteristics, it is likely not peripheral vestibular (for example it may be due to congenital nystagmus), or there may also be other problems superimposed with the oculomotor system or central pathways. Processes that increase gaze-evoked nystagmus, such as ingestion of sedating medications, increase the effects of Alexander's law. In very high velocity vestibular nystagmus, or in persons with poor vision, fixation may be ineffective also.
Normal individuals are able to null out spontaneous nystagmus from minor fluctuations in vestibular tone over a period of days-weeks through a combination of low level automatic processes perhaps related to denervation hypersensitivity, and peripheral and central adaptation. This nulling out process is rapid, with the great majority being done in a few weeks. For complete unilateral lesions, a small residual nystagmus may persist for years.
With this in mind, spontaneous nystagmus is abnormal, but its significance may not always be apparent (Kumar, 1982). A spontaneous nystagmus today might be related to a recent, relatively minor vestibular imbalance or an ancient, complete unilateral vestibular loss. Spontaneous nystagmus must also be considered in the context of the recording method that one has available. Using highly sensitive recording methods such as video-ENG, or infrared, small amounts (e.g. 2 deg/sec) may be significant. Using the more common electronystagmographic recording, which is prone to drift, the upper limit of normal is 5 deg/sec.
In Meniere's Disease, vestibular imbalance type nystagmus is typically seen during an acute attack. Attacks typically last 2 hours only, but usually the next day or two there will be some nystagmus also. In about 85% of the cases, the nystagmus is horizontal with the fast component directed towards the healthy hearing ear, suggesting a vestibular paresis on the side to which the slow phases are directed. The nystagmus slow-phase velocity can reach as high as 40 deg/s. Occasionally nystagmus is also seen directed in the opposite direction. This may occur early on, reflecting a temporary excitation, or later, reflecting a recovery nystagmus (known as Bechterew's phenomena).
Following unilateral vestibular lesions (such as vestibular neuritis, transtympanic gentamicin, acoustic neuroma surgery, or vestibular nerve section), spontaneous nystagmus gradually fades away over months.
This type of nystagmus is classically due to a dorsal midbrain lesion.
This type of nystagmus is rare, and is classically due to the Arnold Chiari malformation.
This rare oculomotor disorder may occur congenitally, or be acquired as a result of injuries to the cerebellar nodulus. It can sometimes be effectively treated with the drug baclofen. It is typified by a nystagmus that first goes one way, then reverses, and then repeats. A typical period is 200 seconds, or about 3 to 4 minutes.