Field Sobriety: Horizontal Gaze Nystagmus

What is Horizontal Gaze Nystagmus?

The HGN test evaluates abnormal eye movement caused by the influence of an intoxicant such as alcohol or drugs. Nystagmus is an involuntary jerking movement of the eye.[1] There are several forms of nystagmus or jerking of the eyes. Of the forty-plus types of medically recognized nystagmus, officers are trained on only three categories; vestibular, neural, and pathological disorders and diseases. Furthermore gaze nystagmus is only one of three types of neural nystagmus. Because there are so many types of nystagmus, it is easy for an officer to confuse the nystagmus they believe is caused by alcohol with another type of nystagmus not caused by alcohol but some other condition. Such other conditions include, but are not limited to: mental disorders, vertigo, inner ear fluid imbalance, head trauma, stroke, multiple sclerosis, diabetes, and stroke. Therefore, it is incredibly important the test be properly administered as to rule out other types of nystagmus not caused by an intoxicant which may mimic the nystagmus caused by intoxicants.


Measurement of nystagmus is accomplished through three different methods. (These are known as the “six clues” recognized by NHTSA as valid indicators of HGN; that is, three clues for each eye). The first measuring technique is to look for lack of “smooth pursuit,” i.e., rather than following a moving object smoothly, the eye jumps or tugs. This technique is not a description of nystagmus. It is a condition that can result from many factors, such as the flashing blue lights of the officer’s vehicle or the passing lights of other motorists (optokinetic nystagmus).

The second method of measurement is to determine whether the nystagmus becomes more “distinct” when the eye is moved to the lateral extreme (so no white is apparent between pupil and outer edge of eye). Again, many people can have nystagmus or nystagmoid-like eye movement at this point of extreme lateral gaze and not be under the influence of alcohol or drugs.

The third method is to measure the angle of onset of nystagmus. By measuring the angle at which the eyes begin jerking. Nystagmus as a result of an intoxicant should begin at or before the 45 degree angle from the straight ahead gaze. There should be a fast component in the direction of gaze, with a slow recovery phase back towards center.

Pre-Test Checks

HGN testing should be preceded by a series of questions designed to ensure the subject is not medically disqualified from taking the test. “Officers are reminded to ask questions about the subject’s eye and general health conditions prior to administering the HGN test.”[1] Additionally, “if there are any abnormal findings on the pre‐test checks, the officer may choose not to continue with the testing. If HGN testing is continued, officers are reminded this does not follow the standardized protocol and should acknowledge such in any report.”[2] After asking these preliminary medical clearance questions, the officer is ready to proceed with administering the test.

Test Procedures

To properly administer the HGN test, the officer must:

  1. have the suspect remove his eye glasses;
  2. properly instruct the suspect that the officer is going to check his eyes and that his is to hold his head still and follow the stimulus with his eyes only and to keep following it until he is told to stop;
  3. hold the stimulus 12-15 inches in front of the subject’s face;
  4. check to determine if both eyes track the movement together (equal tracking), check for resting nystagmus (caused by certain medical conditions unrelated to alcohol consumption) and to ensure the pupils are of the same size (this step rules out other potential causes of nystagmus unrelated to alcohol consumption);
  5. keep the tip of the stimulus slightly above the subject’s eyes;
  6. always move the stimulus smoothly across the subject’s entire field of vision;
  7. always check for all 3 clues in both eyes, starting with the left eye;
  8. check the clues in this sequence: lack of smooth pursuit; distinct and sustained nystagmus at maximum deviation; onset of nystagmus prior to 45 degrees;
  9. always check for each clue at least twice in each eye;
  10. the typical time for each pass is two seconds out and two seconds back;
  11. when checking for distinct nystagmus at maximum deviation, the officer is to hold the subject’s eyes in the extreme position for at least four seconds;
  12. when checking for the onset of nystagmus prior to 45 degrees, it should take the officer 4 seconds to move the stimulus from the suspect’s nose to the angle—once jerking of the eye is first observed the officer is to stop moving the stimulus to ensure the jerking continues (validate nystagmus);
  13. total the clues (need to observe at least four out of six clues to indicate impairment); and
  14. check for vertical gaze nystagmus (separate test to determine if subject has taken a high dose for that particular subject).

Is HGN Accurate?

 In September, 2007, NHTSA conducted another study, “The Robustness of the Horizontal Gaze Nystagmus Test.” This study examined HGN in depth and, amongst other experiments, tested the false positive rates associated with improper administration of HGN. A false positive indicates a subject whose BAC was below 0.08 but the examiner nonetheless observed four out of six clues. The results were surprising. Evenwhen properly administered, 36.1% of test subjects falsely exhibited a positive result. Subjects whose stimulus was held too low (at eye level) exhibited a false positive rate of 52.7%.Subjects whose stimulus was held too high (four inches above eye level) exhibited a false positive rate of 61.1%. This is just one example of how one small error in administering the HGN test can produce a false result.


Attorneys need to be familiar with the instructor and participant NHTSA manuals, new case law, and the facts of their case to ensure the HGN test is properly administered and interpreted. As we saw, even the slightest deviation can compromise the validity of the test. For non-lawyers, it is important to know the HGN test, along with the other FSTs (discussed in Part III and IV), are entirely voluntary. Therefore, you should never consent to the participation of FSTs.           

  If you have been arrested for driving under the influence, contact the law firm of W. Scott Smith at 404.581.0999 today for a free case evaluation. You’ll find a local Atlanta DUI attorney ready to aggressively fight on your behalf. You can also find out more detailed information about Atlanta laws here.

by Casey Cleaver

[1] Session 8, page 29 of 95.

[2] Session 8, page 29 of 95.

[1] The NHTSA manual defines Horizontal Gaze Nystagmus (HGN) as, “involuntary jerking of the eyes, occurring as the eyes gaze side to side. In addition to being voluntary, [the] person is usually unaware it is happening [and] the person is powerless to stop it or control it.” The Manual also states that, “alcohol and certain other drugs cause HGN.” Session 8, page 18 of 95.