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Alcohol Induced Nystagmus

There are two types of alcohol induced nystagmus. The first is alcohol gaze nystagmus, which includes Horizontal Gaze Nystagmus (HGN). The second is positional alcohol nystagmus. Although alcohol causes both, alcohol gaze nystagmus and positional alcohol nystagmus are very different and easily distinguishable.

Testing for positional alcohol nystagmus is not a part of the Standardized Field Sobriety Test battery. Sometimes the nystagmus the officer saw was actually positional alcohol nystagmus and not alcohol gaze nystagmus.

For purposes of clarification the characteristics of both are described below:

Alcohol Gaze Nystagmus (AGN)

Gaze nystagmus is a type of jerk nystagmus where the eye gazing upon or following an object begins to lag and has to correct itself with a saccadic movement toward the direction in which the eye is moving or gazing.7 Gaze nystagmus is due to disruptions within the nervous system. Alcohol gaze nystagmus (AGN) is gaze nystagmus caused by alcohol. AGN occurs as the eye moves from looking straight ahead (called resting nystagmus), to the side (called HGN), or up (called vertical nystagmus or VGN).

The effect of alcohol on eye movement has been described as follows:

Alcohol is a central nervous system depressant affecting many of the higher as well as lower motor control systems of the body. This results in poor motor coordination, sluggish reflexes, and emotional instability. The part of the nervous system that fine-tunes and controls hand movements and body posture also controls eye movements. When intoxicated, a person's nervous system will display a breakdown in the smooth and accurate control of eye movements. This breakdown in the smooth control of eye movement may result in the inability to hold the eyes steady, resulting in a number of observable changes of impaired oculomotor functioning.

Positional Alcohol Nystagmus (PAN)

Positional nystagmus occurs when a foreign fluid, such as alcohol, is in unequal concentrations in the blood and the fluid contained in semi-circular canals in the vestibular (inner ear) system. The vestibular system controls a person's balance, coordination and orientation. The eyes depend on the vestibular system to stabilize them against any head movements.9 Disruptions in the vestibular system will have an adverse effect on the messages sent to the eyes when the head moves. Positional nystagmus manifests itself as jerk nystagmus in which the direction of the saccadic movement depends on head movement.

Positional alcohol nystagmus (PAN) occurs when alcohol is the foreign fluid. There exist two types of PAN. In PAN I, the alcohol concentration is higher in the blood than in the vestibular system fluid and occurs when a person's blood alcohol content (BAC) is increasing. In PAN II, the alcohol concentration is lower in the blood than in the vestibular system fluid and occurs when a person's BAC is decreasing.

Nausea, dizziness, vertigo and vomiting accompany PAN I and PAN II, which indicate high doses of alcohol. High intensity PAN is evident when a subject's eyes are open, but open eyes block
lower intensity PAN. As a result, PAN is most easily recorded when the subject is lying down, head to the side with the eyes closed.

AGN and PAN Compared

In comparing AGN and PAN it is evident that both are caused by alcohol, yet their origins and manifestations are very different. AGN is a neurological nystagmus while PAN is a vestibular system nystagmus. Unlike AGN, PAN manifests itself only when the subject is lying down, with the head turned to the side and the eyes closed. At low intensities, PAN stops when the eyes are open. Furthermore, PAN changes direction depending on the position of the head while the direction of AGN depends on the direction of the gaze. Because of these differences, officers conducting the HGN test are not likely to confuse AGN and PAN indicators.

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