Nystagmus is involuntary back and forth movement of the eye. It is due to problems with eye muscles rather then the eye itself, but the reason this occurs is unknown. The movements may be in any number of directions or patterns. The frequency and duration of occurrence varies, but may be triggered by eye positioning or viewing properties.
Pendular nystagmus is when the speed, frequency, and duration is equal in each direction of movement. Jerky nystagmus has slower movement in one direction with faster movement in another. Congenital pendular nystagmus is often present with other congenital vision problems like cataract or aniridia, It may also be part of a neurological disorder or reaction as the result of drug interaction.
Consider the properties of normal vision for a moment. Although a person may have 90 degree visual field, most often the only part of the image we notice is a certain object of attention. For example, while reading the computer screen you are probably not paying attention to the items around the monitor which are still within your view. That doesn't mean you can't see them, it just means they aren't what you are concentrating on. The brain instructs our eyes to focus (fixate) on the object which we want to look at.
Nystagmus causes a loss of ability to fixate on an object. This sets up a battle between the brain and the eye muscles. The brain gets image data from the eye and then tells the eye muscles what to do in order to get the attention object to the center of the view. But during a nystagmus episode, the muscles are doing their own thing as well. It is important to note that the quality of the image is (usually) not affected, just the positioning of the object to be fixated upon. Thankfully, the brain does an outstanding job of compensation and the image that the person perceives is steady (excepting severe cases or instances). A person may experience dizziness or vertigo during times when the object of attention appears to be moving (severe cases mentioned above).
Not much can be done to cure nystagmus excepting the possible use of surgery to reposition eye muscles. Usually this only works when known positions trigger the movement and a new positioning angle will still enable good vision. The good news is that the brain does manage to compensate (as noted above) for most occurrences. The brain may also invoke an involuntary head tilting or nodding as an adjustment method. People (usually children) with nystagmus may experience difficulty reading while the words (or line) cannot be fixated upon. A typoscope is a card with a small viewing hole which shows one word or line only. This may help some people because there is less distracting objects around.
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