Horizontal nystagmus is the involuntary movement of the eye towards either the right or the left side of the occipital orbit. It is often seen in patients with large brain lesions in one hemisphere. The eye turns towards the healthy side of the brain and quickly moves back towards the side of the head with the lesions. A person with nystagmus usually has trouble seeing things in the distance, but should be able to read up close without any difficulty.
The involuntary eye movements of horizontal nystagmus may indicate a neurological problem, or possibly the ingestion of alcohol or an inhaled substance. Some inner ear disorders can cause the symptoms of nystagmus. Congenital nystagmus usually develops by the age of three months, and may be accompanied by other problems. Some cases are caused by poor eyesight, or a strain on the eye muscles.
Eyes normally move in a smooth controlled manner in the direction the person is looking. A person under the influence of a central nervous system depressant may have the involuntary jerky eye movements that are typical of people with horizontal nystagmus. The brain is unable to operate the ocular system properly when under the influence of such a substance.
Nystagmus sufferers exhibit eye movements in two different ways. Persons affected by pendular nystagmus have involuntary eye movements in both directions at the same speed. A person with jerk nystagmus will move his eyes in a quick choppy motion to either side of his head, depending on the direction the person is looking at the time, and the second return movement is slower than the first.
Treatment for horizontal nystagmus begins with eyesight correction. Prescription glasses or contact lenses may be enough to stop the symptoms. Some patients may require surgery to alter the length of the muscles that control eye movements to correct the nystagmus.
Police officers may conduct a horizontal gaze nystagmus (HGN) test to determine if a person is driving while intoxicated. A positive test can result in the arrest of the person for driving while impaired. The test is done as part of a complete field sobriety test (FST).
During the HGN test, a bright light will be held in front of a person’s eyes. The person will then be asked to hold his head still and follow the light with only the movement of his eyes. Each movement of the light across the person’s field of vision will take about four seconds. Any jerky movements of the eye will be an indication that the officer should proceed with further field sobriety tests.