Scoring Horizontal Gaze Nystagmus Test

//Scoring Horizontal Gaze Nystagmus Test
Scoring Horizontal Gaze Nystagmus Test 2017-11-13T15:30:13+00:00

In order to score this part of the test, the police officer has to move the stimulus to a 45-degree angle. This allows the eye to match this angle. The police officer is looking for the eye to jerk during this movement. If nystagmus is observed, the officer will stop the stimulus. They will make note of the continued jerkiness. If the jerkiness does continue, the police officer must observe if there is still white showing in the corner of the eye. The officer will then note the angle as prior to 45 degrees. If there isn’t any jerkiness, the stimulus has to continue until one of two things occurs. Either the jerking occurs or the 45-degree angle is reached. If there isn’t any white of the eye showing, the eye has probably gone too far to the right. This would indicate maximum deviation.

The other option as to why this occurred is because the person has unusual eyes. These eyes will not deviate very far to one side. The criteria of onset before 45 degrees can be used only if some white of the eye can be seen at the outside of the eye. Unfortunately, police officers often incorrectly estimate the angle. Or, they score with no white showing in the corner of the eye.

A question to ask is whether or not a police officer can hypothetically pick a 45-degree angle without having actual lines to use as a reference point. The angle of onset is deemed the most reliable in determining whether or not probable cause exists to believe that someone is under the influence of alcohol. Also, the angle of onset shouldn’t be administered if the driver is lying down. But, it can be given to a person who is sitting or standing.

HGN Clues

The maximum number of clues in the HGN test in one eye is three. The total number for any driver is six (three for each eye). There is also the vertical gaze nystagmus test used to determine if a driver is under the influence of drugs. Vertical gaze nystagmus is the involuntary jerking of the eyes up and down which occur when the eyes look up at maximum elevation. The VGN test is sometimes given when it is presumed that the driver has ingested large amounts of alcohol or certain drugs.

Types of Nystagmus

There are three categories of nystagmus. Rotational nystagmus occurs when a person is spun around or rotated quickly. This causes the fluid in the inner ear to become disturbed. If the eyes of a person rotating were to be observed, they would be seen to jerk noticeably. Post-rotational nystagmus occurs when a person stops spinning. This happens because the fluid in the inner ear remains disturbed for a period of time. When this happens the eyes continue to jerk. Caloric nystagmus occurs when fluid motion in the canals of the vestibular system is stimulated by temperature. For example, putting warm water in one ear and cold in the other.

Conditions that Affect the Test

There are a few conditions that can affect a gaze nystagmus test. A person who has a glass eye or vision in only one eye cannot be given this test for evaluation of just one eye with a doubled score. This assumes that the other eye would render the same results. If the person taking the test has what is known as a lazy eye, the officer is trained to test one eye while the person’s other hand covers the other eye. A person with color blindness cannot take this test because nystagmus is common for the condition.

A natural nystagmus may also present due to some form of neurological disorder, brain damage, epilepsy or pathological disorder. The narrowness of certain individuals’ eyes can also make determination of nystagmus more difficult. A large disparity between the right and left eye is another indicator of a possible problem. If there is an accident, and the driver sustains a concussion, this can bring on pathological nystagmus which invalidates the test.

Administering the Test

Police officers are trained to administer the HGN test. They must do this with the driver facing a quiet, still background. The driver must turn away from police cruisers and any oncoming traffic. The reason for this is to avoid the possibility of inducing a condition known as optokinetic nystagmus. This develops when a person focuses on several objects at one time or on any objects that are moving away from the driver. Optokinetic nystagmus is a defense mechanism of the human body to prevent the eyes from tiring. There are numerous visual or other distractions that can also impede the results of the HGN test. Certain environmental factors such as wind, dust, rain, etc. can interfere with the performance of the nystagmus test. The law enforcement community considers the HGN test to be about 77 percent accurate when performed alone.

Some of the questions that need to be answered in relation to the HGN test are:

  • Did the officer ask you to perform a test in which you were requested to follow a finger, pen, or some other object?
  • What type of object did the officer use?
  • Was there an explanation given to you was about the test?
  • What type of motion did the officer use to move the object?
  • Did anything distract you while taking this test?
  • How long did the test take?
  • Did the officer comment on your performance?
  • Do you have any medical issues with your eyes that could have impacted your performance on this test? (i.e. astigmatism).

For more information, contact our office. We can assist you through this process.