Nystagmus Caused by Non-Alcohol Related Disturbance of the Vestibular System

Rotational Nystagmus

Rotational nystagmus is caused by a disturbance in the inner ear fluid when a person spins
around. The nystagmus lasts only as long as the person is being spun. If an observer could see a person's eyes while that person was spinning, a distinct jerking of the eye would be evident. Post-rotational nystagmus occurs after the person stops spinning. The nystagmus lasts for several seconds and can easily be seen.

Caloric Nystagmus

Caloric nystagmus is caused by the movement of the inner ear fluid due to a difference in temperature of the fluid between the left and right ear. One way this can occur is if warm water is poured in one ear and cold water is poured in the other. Obviously this is an implausible scenario at roadside.

Nystagmus Caused by Neural Activity

Some types of nystagmus are caused by neural or muscle activity. Optokinetic nystagmus occurs when the eyes fixate on an object that moves quickly out of sight or passes quickly through the field of vision, such as occurs when a subject watches utility poles pass by while in a moving car. Optokinetic nystagmus also occurs when the eyes watch an object displaying contrasting moving images, such as black and white spokes on a spinning wheel. In either case, because the nystagmus is caused by the eye trying to catch up with the moving object, it lasts only as long as it takes for the object to stop moving, for the object to pass out of the field of vision, or for the eye to catch up to the object. Epileptic nystagmus is also a jerk nystagmus caused by neural activity that occurs primarily during epileptic or other types of seizures.

In addition, some people will exhibit a slight eye tremor when the eye moves to maximum deviation. This tremor is due mostly to eye strain rather than to any type of alcohol impairment or medical condition. When the HGN test is administered properly, a law enforcement officer cannot confuse this eye tremor with HGN due to alcohol impairment for several reasons. First, the eye tremor lasts only briefly and law enforcement officers are taught to hold the eye at maximum deviation for at least four seconds to ensure that the jerking is sustained. Second, the officer is looking for a distinct nystagmus, not a slight eye tremor. And finally, distinct nystagmus at maximum deviation is only one clue among the three the officer is looking for when checking for HGN.

Nystagmus Due to Pathological Disorders

Nystagmus may occur in people with brain damage, brain tumors or inner ear diseases. These disorders and others like them occur in a small number of the general population and even less often in drivers. Many of these alternative causes are so severe that it is unlikely that persons afflicted with the disorders would be driving, would not know they have the disorder or would be unaware of the effect the disorder has on their body. In addition, these types of nystagmus may be pendular rather than jerk nystagmus.

One claim of impaired drivers exhibiting HGN is that fatigue and not alcohol is the cause of their impairment. NHTSA studies show that fatigue has no significant effect on the manifestation of HGN.

Natural Nystagmus

The defense may argue that the nystagmus the law enforcement officer detected was actually a naturally occurring nystagmus rather than the result of alcohol impairment or any of the conditions listed above. As outlined below, the differences between any type of naturally occurring nystagmus and HGN are many and a properly trained officer will have no trouble distinguishing between the two at roadside.

Research indicates that a very small number of people exhibit a visible natural nystagmus. Those who have natural nystagmus generally know they have it and will most likely tell the officer before the test is administered. Visible natural nystagmus is evident only at particular angles of gaze, but not before or beyond that point.51 However, when administering the HGN test, the law enforcement officer is looking for not only nystagmus at a particular angle of gaze, but smooth pursuit and end-point nystagmus as well.

Furthermore, in making the ultimate decision of whether the subject is impaired, the law enforcement officer is continually taking into account other facts, such as the subject's performance on the other SFSTs, that suggest the subject is impaired by alcohol or other drugs. The law enforcement officer will never base an arrest decision solely on the results of the HGN test.

Physiological Nystagmus

Physiological nystagmus exists in every person's eye in order to keep the eye from tiring when fixated on one point. This nystagmus occurs so that light entering the eye will continually fall on non-fatigued cells of the retina. Physiological nystagmus cannot be seen with the naked eye and is controlled by a part of the brain system other than that affected by alcohol impairment. Because the officer can easily see HGN caused by alcohol with the naked eye, there is virtually no chance that a law enforcement officer could confuse physiological nystagmus with HGN.

The HGN test is designed to check the eyes for one type of nystagmus ­ horizontal gaze nystagmus. Its results are not invalidated by virtue of the fact that other types or causes of nystagmus exist. As shown above, the various types of nystagmus manifest themselves in different ways. Law enforcement officers will not confuse HGN with any other type of nystagmus if the HGN test is conducted correctly. Research shows that the HGN test is a valid and reliable indicator of alcohol impairment and is the most effective roadside test for impaired drivers.