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The HGN Test in DUI Cases

The HGN Test in DUI Cases

Procedures of Horizontal Gaze Nystagmus Testing: The Three Clues

The Test Itself - HGN

  • Nystagmus= involuntary bi-phasic movement of the eyes seen as a slow(er) drifting movement of the eyes away from the point of visual fixation followed by a quick(er) movement returning to the point of visual fixation.
  • many different types of nystagmus:
    1. the type involved in field sobriety testing is HGN
      1. it represents the status of the central nervous system (CNS).
      2. At a threshold level for the individual, alcohol can cause HGN to be present.
  • Vertical nystagmus, it should be noted, is a different phenomenon with different causes.
    • The relevance of vertical nystagmus is primarily to indicate the presence of a neurologic crisis in the mid brain portion of the central nervous system.
    1. To administer the HGN test, the officer instructs the suspects (test subject), “keep your head still, head straight ahead and follow the target object with your eyes.”
    2. officer then moves a finger, pencil, pen or other suitable target from the center of the head steadily toward one side then the other side in the prescribed test sequence.
    3. target is held 12-15” directly in front of the test subject.
      1. 4 components to the HGN test:
        • check for equal tracking;
        • check for smooth pursuit;
        • distinctnystagmus at maximum gaze and
        • onset of nystagmus at or before 45 degrees.
      2. Evaluation of the eyes creates confusing eye movements that can stimulate the clues sought during the HGN test.
      3. PASS DEFINED - A pass consists of moving the target, in a straight line, from the midline (center) to either the left or right as far as the end of visual gaze and back to the midline.
  • Equal tracking of the eyes and simultaneous evaluation for equal pupil size are the preliminary first 2 passes of the HGN test's 14 passes
    1. to alert the officer to obvious presence of neurological symptoms that may require immediate medical attention.
  • EXCEPT FOR EQUAL TRACKING - rest of the passes are done in duplicate.
  • Checking for lack of smooth pursuit is done by repeating this sequence of the passes:
    1. midline to right back to midline then to left and back to midline.
    2. Each clue of the six clues in the HGN test has two passes plus the two passes of the preliminary check for equal tracking resulting in the total of 14 passes for a HGN test.
  • The presence or lack of smooth pursuit is the first set of clues in the HGN matrix of three clues per eye for a total of six clues for each HGN test result.
    1. Speed of the target is critically in this phase of the HGN test.
    2. Lack of smooth pursuit can be caused by a target moved too fast.
    3. Smooth pursuit, or lack thereof, is seen when a target is moved between 20-30 degrees per second. This means that covering the distance from mid line to the end of visual gaze (approximately 80 degrees) should take 3-4 seconds in each direction. That is 6-8 seconds for each pass.
      1. Lack of smooth pursuit is seen as a horizontal jerking motion of the eyes as the eyes jump from one point of visualization to another. This jumping motion is known as saccadic movement.
  • Next clue checked is distinct nystagmus at maximum gaze.
    1. target is held at the point of gaze where the eyes no longer move laterally from the midline;
    2. held at this location for a minimum of four seconds before scoring the clue.
    3. After 4 seconds, if there is nystagmus present a clue is scored for that eye when confirmed by the second pass (observation).
  • last clue in the HGN test is nystagmus at or before 45 degrees.
    1. The target is slowly moved from the mid line literally until nystagmus is observed.
    2. At the point of observation the target is stopped to make sure that there really is nystagmus present.
    3. The current location of the 45 degrees line is defined by the diagonal line created by a square formed when the distance of the target from the subject's face is projected laterally.
    4. The eyes of a person under the influence of alcohol will show nystagmusonset at a lower angle as their blood-alcohol concentration (BAC) increases.
      1. Attempts have been made to approximate the BAC from the measured angle of onset. This is referred to as Tharp's equation where the
        • angle of onset is subtracted from 50 and the result is the approximate BAC expressed as a decimal and a percentage.
        • Although it lacks scientific validity it is not uncommon to have an officer to attempt to testify to the BAC level using this formula. Accordingly the defense counsel may want the nystagmus evidence admitted if the nystagmus began at 45 degrees indicating a BAC of only 0.05%.
  1. The Lack of Smooth Pursuit (Clue #1) - The eyes can be observed to jerk or “bounce” as they follow a smoothly moving stimulus, such as a pencil or penlight.
    1. The eyes of an unimpaired person will follow smoothly, i.e., a marble rolling across a smooth pane of glass, or windshield wipers moving across a wet windshield.
  2. Distinct and Sustained Nystagmus At Maximum Deviation - (Clue #2) Distinct and sustained nystagmus will be evident when the eye is held at maximum deviation for a minimum of 4 seconds.
    1. People exhibit slight jerking of the eye at maximum deviation, even when unimpaired, but this will not be evident or sustained for more than a few seconds. When impaired by alcohol, the jerking will be larger, more pronounced, sustained for more than four seconds, and easily observable
  3. Onset of Nystagmus Prior To 45 Degrees (Clue #3) - The point at which the eye is first seen jerking. If the jerking begins prior to 45 degrees it is evident that the person has a BAC above 0.08, as shown by recent research.

                  The higher the degree of impairment, the sooner the nystagmus will be observable.


Estimating a 45-Degree Angle


  1. How far you position the stimulus from the suspect's nose is a critical factor in estimating a 45-degree angle, (i.e., If the stimulus is held 12" in front of the suspect's nose, it should be moved 12" to the side to reach 45 degrees. Likewise, if the stimulus is held 15" in front of the suspect's nose, it should be moved 15" to the side to reach 45 degrees.)


  • "I am going to check your eyes."
  • "Keep your head still and follow this stimulus with your eyes only."
  • "Keep following the stimulus with your eyes until I tell you to stop."
    1. Move the stimulus smoothly, at a speed that requires approximately two seconds to bring the suspect's eyesfar to the side as it can go.
    2. While moving the stimulus, look at the suspect's eye and determine whether it is able to pursue smoothly.
  • Movement of the stimulus should take approximately two seconds out and two seconds back for each eye.
  1. Repeat the procedure.
  1. for distinct and sustained nystagmus at maximum deviation - beginning with the suspect's left eye.
    1. Simply move the object to the suspect's left side until the eye has gone as far to the side as possible. Usually, no white will be showing in the corner of the eye at maximum deviation.
    2. Hold the eye atthat position for a minimum of four seconds, and observe the eye for distinct and sustained nystagmus.
  • Repeat the procedure.
    • Fatigue Nystagmus. This type of nystagmus may begin if a subject's eyes are held at maximum deviation for more than 30 seconds.
  1. onset of nystagmus prior to 45 degrees –
    1. Start moving the stimulus towards the right (suspect's left eye) at a speed that would take approximately four seconds for the stimulus to reach the edge of the suspect's shoulder.
    2. Watch the eye carefully for any sign of jerking.
  • When you see it, stop and verify that the jerking continues.
  1. Now, move the stimulus to the left (suspect's right eye) at a speed that would take approximately four seconds for the stimulus to reach the edge of the suspect's shoulder.
  2. Repeat the procedure.
    • It is important to usethe full four seconds when checking for onset of nystagmus. If you move the stimulus too fast, you may go past the point of onset or miss it altogether.
    • If the suspect's eyes start jerking before they reach 45 degrees, check to see that some white of the eye is still showing on the side closest to the ear. If no white of the eye is showing, you either have taken the eye too far to the side (that is more than 45 degrees) or the person has unusual eyes that will not deviate very far to the side.
  3. Nystagmus may be due to causes other than alcohol. These other causes include seizure medications and some other drugs. A large disparity between the performance of the right and left eye may indicate a medical condition.
  4. Based on the original research, if you observe four or more clues it is likely that the suspect's BAC is above 0.10. Using this criterion you will be able to classify about 77% of your suspects accurately. This was determined during laboratory and field testing and helps you weigh the various field sobriety tests in this battery as you make your arrest decision.
  • Only validated to indicate for a specific BAC
  • Caused by DID drugs

  • Medical Impairment

    • Resting Gaze Nystagmus, Equal Pupil Size, Equal Tracking

  • Officers are reminded to ask questions about the subject's eyes and general health conditions prior to administering the HGN test.


Study was done in reference to defense arguments


Stimulus Speed

1 second and 2 seconds Stimulus Height

0 inches, 2 inches, and 4 inches Stimulus Distance

10 inches, 12 inches, 20 inches


When administered CORRECTLY!!!

67% < 0.08 had 4/6 clues or more
65% < 0.05 had 4/6 clues or more
85% (6/7) < 0.03 had 4/6 clues or more 6/6 observed at BAC of 0.029

Stimulus Too High

91% < 0.08 had 4/6 clues or more 90% < 0.05 had 4/6 clues or more

Robustness 20study 20  20hgn 20too 20high 20  20false 20positives


HGN is a robust procedure

4 clues can be 0.03 BAC and above 6 clues can be 0.06 BAC and above

SAN DIEGO (1998)

The False Positives

37% < 0.08 had 4/6 clues or more on HGN 52% < 0.08 had 2/8 clues or more on WAT

41% < 0.08 had 2/4 clues or more on OLS 28% < 0.08 were estimated to be over

The Court of Special Appeals of Maryland has judicially recognized 38 non-alcohol-related causes of Horizontal Gaze Nystagmus. They include:

  1. Problems with the inner ear labyrinth;
  2. Irrigating the ears with warm or cold water under peculiar weather conditions;
  3. Influenza;
  4. Streptococcus infection;
  5. Vertigo;
  6. Measles;
  7. Syphilis;
  8. Arteriosclerosis;
  9. Muscular dystrophy;
  10. Multiple sclerosis;
  11. Korsakoff's syndrome;
  12. Brain hemorrhage;
  13. Epilepsy;
  14. Hypertension;
  15. Motion sickness;
  16. Sunstroke;
  17. Eyestrain;
  18. Eye muscle fatigue;
  19. Glaucoma;
  20. Changes in atmospheric pressure;
  21. Consumption of excessive amounts of caffeine;
  22. Excessive exposure to nicotine;
  23. Aspirin;
  24. Circadian rhythms;
  25. Acute trauma to the head;
  26. Chronic trauma to the head;
  27. Some prescription drugs, tranquilizers, pain medications, anti­convulsants;
  28. Barbiturates;
  29. Disorders of the vestibular apparatus and brain stem;
  30. Cerebellum dysfunction;
  31. Heredity;
  32. Diet;
  33. Toxins;
  34. Exposure to solvents, PCBs, dry-cleaning fumes, carbon monoxide;
  35. Extreme chilling;
  36. Eye muscle imbalance;
  37. Lesions;
  38. Continuous movement of the visual field past the eyes, i.e., looking from a moving train; and
  39. Antihistamine use.

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