What is Nystagmus?
Nystagmus is a term used to describe a "bouncing" eye motion that is displayed in two ways: (1) pendular nystagmus, where the eye oscillates equally in two directions, and (2) jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a "saccadic" or fast movement.
Horizontal Gaze Nystagmus HGN
is a type of jerk nystagmus with the saccadic movement toward the direction of the gaze. An eye normally moves smoothly like a marble rolling over a glass plane, whereas an eye with jerk nystagmus (termed lack of smooth pursuit in reporting results of a Horizontal Gaze Nystagmus eye examination in a driving under the influence investigation) moves like a marble rolling across sandpaper. Most types of nystagmus, including HGN, are involuntary motions, meaning the person exhibiting the nystagmus cannot control it. In fact, the subject exhibiting the nystagmus is unaware that it is happening because the bouncing of the eye does not affect the subject's vision.
Alcohol and Nystagmus
There are several types of nystagmus. Alcohol causes two types: alcohol gaze nystagmus, which includes Horizontal Gaze Nystagmus (HGN), and 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
. Defendants sometimes claim or attempt to confuse matters by arguing that the nystagmus the officer saw was actually positional alcohol nystagmus and not alcohol gaze nystagmus.
Nystagmus Unrelated to the Consumption of Alcohol
Nystagmus is a naturally occurring condition in approximately four (4) percent of the population. It may result from fatigue, trauma, or other natural causes. The California Court of Appeal has discussed naturally occurring nystagmus in
Williams (1992) 3 Cal.App.4th 1326
In fact, there exist several non-alcohol related types of nystagmus
caused by neural or muscle activity. These other types are due to a variety of causes, such as other vestibular system (inner ear) and nervous system disturbances and pathological disorders. Attorneys representing individuals accused of driving under the influence must be preapred to introduce evidence and argument that the nystagmus the law enforcement officer saw was actually caused by something other than alcohol, medication or drugs.