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Horizontal Gaze Nystagmus (HGN)

Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. Horizontal gaze nystagmus (HGN) refers to a lateral or horizontal (side to side) jerking when the eye moves and gazes along the horizontal plane. In the driving under the influence (DUI) context, alcohol consumption hinders the ability of the brain to correctly control eye muscles, causing alcohol induced nystagmus which is the jerk or bounce associated with Horizontal Gaze Nystagmus. As the degree of impairment becomes greater, the jerking or bouncing ( nystagmus) becomes more pronounced and the angle of onset decreases which is assessed in the Horizontal Gaze Nystagmus eye examination. Nystagmus can also occur naturally.

The Horizontal Gaze Nystagmus eye examination is one of three field sobriety tests that comprise the standardized field sobriety test (SFST) battery (the other two tests are the one leg stand test and the walk and turn test). Evidence compiled by scientists conducting studies for the Federal Government published studies reporting that Horizontal Gaze Nystagmus eye examination is a reliable roadside measure of a person's impairment due to alcohol when properly administered. The obvious limitation is that eye examinations conducted by law enforcement officers on public streets, highways and freeways at night while conducting criminal investigations are not comparable conditions to examinations conducted under laboratory conditions by scientists. Even when conducted under ideal conditions, Horizontal Gaze Nystagmus was determined to be an accurate indication of alcohol influence only 77 percent of the time. Proof beyond a reasonable doubt requires a higher degree of correlation, although it would seem that some judges are lax of their application of this bedrock legal principle.

California Highway Patrol officers and some officers of other law enforcement agencies enforcing California's DUI laws receive training in administering and interpreting the results of a Horizontal Gaze Nystagmus (HGN) eye examination. The officer is trained to conduct the examination in a way that ensures that the subject's eyes can be seen clearly, i.e., in a well lit area or by use of a flashlight to illuminate the subject's face. Additionally, the examination should not be performed while the subject is facing in the direction of a police car's blinking lights or passing cars, because this light source can interfere in the examination by causing optokinetic nystagmus.

The law enforcement officer informs the subject "I am now going to check your eyes." The officer is not "testing" the subject's vision, as an ophthalmologist or optometrist would, but instead, the officer is "checking" the eyes for the physical manifestation of HGN.

Before checking the subject's eyes, the officer is trained to ask the subject to remove eyeglasses and ask whether the subject is wearing hard or soft contact lenses. While the removal of the eyeglasses makes it easier for the officer to observe eye movement, glasses do not effect the HGN test results. Early concerns that contact lenses, especially hard contact lenses, may affect the HGN test result led some to provide for the subject to remove the lenses. However, contact lenses, hard or soft, do not affect the test in any way. While hard contact lenses may pop out when the eye moves as far to the side as it will go, officers are not taught to have subjects remove contact lenses. However, officers are trained to note whether the subject is wearing contacts and which type.

The officer also asks the subject whether he or she has any medical impairment that would either prohibit taking the test or may affect the test results. The officer should note any condition which prohibits the taking of the test and then move on to the remaining SFSTs. If the subject claims to have a natural nystagmus or any other condition that may affect the test result, but does not prohibit the taking of the HGN test, the officer should note the condition but still perform the test.

The HGN test requires the subject to follow an object (stimulus) with their eyes. The stimulus may be a pen, the tip of a penlight or the officer's finger. The officer is trained to place the stimulus approximately twelve to fifteen inches from the subject's face and slightly higher than eye level. Placing the object above eye level opens the subject's eyes further and makes their movement easier to observe. The subject does not have to be standing but can be sitting down, which is the proper method for administering the examination when the subject is taller than the officer.

The subject does not have to see the stimulus clearly to perform the HGN test. The subject just has to see the stimulus well enough to be able to follow it with his eyes. Blurry vision is not a medical condition making the driver under investigation an improper candidate for a Horizontal Gaze Nystagmus examination.
The officer instructs the subject to follow the object with the eyes and the eyes only ­ the head should remain still.

After positioning the stimulus, but before conducting the test, the officer checks for signs of medical impairment. First, the officer checks for equal tracking of the eyes by moving the stimulus quickly across the subject's entire field of vision to see whether the eyes follow the object simultaneously. The officer then checks for equal pupil size. Lack of equal tracking or equal pupil size may indicate blindness in one eye, a glass eye, a medical disorder or an injury. If the subject exhibits these characteristics, the officer should discontinue the HGN test and may need to seek medical assistance for the individual if a medical disorder or injury appears to exist.

Evaluation of Horizontal Gaze Nystagmus

While conducting the test, the officer looks for six "clues," three in each eye, which indicate impairment. The officer should record the clues on the HGN Guide. The left eye is checked for the clue, and then the right eye.

These clues are:

Lack of Smooth Pursuit - The officer moves the object slowly but steadily from the center of the subject's face towards the left ear. The left eye should smoothly follow the object, but if the eye exhibits nystagmus, the officer notes the clue. The officer then checks the right eye.

Distinct Nystagmus at Maximum Deviation - Starting again from the center of the suspect's face, the officer moves the object toward the left ear, bringing the eye as far over as possible, and holds the object there for four seconds. The officer notes the clue if there is a distinct and sustained nystagmus at this point. The officer holds the object at maximum deviation for at least four seconds to ensure that quick movement of the object did not possibly cause the nystagmus. The officer then checks the right eye. This is also referred to as "end-point" nystagmus.

Onset of Nystagmus Prior to Forty-Five Degrees - The officer moves the object at a speed that would take about four seconds for the object to reach the edge of the suspect's left shoulder. The officer notes this clue if the point or angle at which the eye begins to display nystagmus is before the object reaches forty-five degrees from the center of the suspect's face. The officer then moves the object towards the suspect's right shoulder. For safety reasons, law enforcement officers usually use no apparatus to estimate the forty-five degree angle. Generally, forty-five degrees from center is at the point where the object is in front of the tip of the subject's shoulder.

The officer also checks for vertical nystagmus. The officer checks for vertical nystagmus by raising the object several inches above the subject's eyes. Vertical nystagmus is not one of the HGN clues nor is it a part of the SFST battery. However, vertical nystagmus is a good indicator of high doses of alcohol, other central nervous system (CNS) depressants or inhalants, and the consumption of the drug phencyclidine (PCP). The officer should note the result and take precautions if vertical nystagmus is evident.

If all six clues are present, the officer has been trained to opine that the driver was under the influence.

The Law Relating to Horizontal Gaze Nystagmus (HGN)

Evidence of the presence of Horizontal Gaze Nystagmus finds its way into the courtroom in driving under the influence cases as one of the Standardized Field Sobriety Tests. Proponents of HGN evidence contend it provides the best evidence of impairment behind chemical blood and breath tests. However, most judges do not understand the limitations of the Horizontal Gaze Nystagmus examination.

Prosecutors will sometimes attempt to introduce the law enforcement officer's observations relating to Horizontal Gaze Nystagmus not only to establish the presence of alcohol, but also as evidence of a driver's blood alcohol level (BAC). This is in violation of established legal precedent and attorneys must be prepared to object based upon controlling authority or waive the issue. In People v. Loomis (1984) 156 Cal.App.3d Supp 1 , decided that Horizontal Gaze Nystagmus evidence fails the test announced by the California Supreme Court in People v. Kelly (1976) 17 Cal.3d 24.

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