Part 2: How does a DWI (driving while intoxicated) investigation in Austin, Texas work? The HGN

HGN DWI nystagmus eye test dui drunk driving

Tagged with: Dwi Hgn Nystagmus Eye Test

Posted on: July 5, 2016

Part 2:  How does a DWI (driving while intoxicated) investigation in Austin, Texas work?  The HGN Test


If you decide to do the Field Sobriety Tests (FSTs), again, to be clear I recommend that you do not, there are three standardized field sobriety tests that are used by Austin Police Officers in trying to determine whether a driver is impaired for purposes of driving.  These standardized tests are almost universally given though they may be supplemented with additional tests or in some cases different tests may be chosen because the subject is not a good candidate for the standardized field sobriety tests (SFTs)


The three standardized tests are:  the Horizontal Gaze Nystagmus (HGN) test, the Walk and Turn test, and the One Leg Stand.  These tests are referred to as standardized because there have been studies done to validate the accuracy of these tests for purposes of determining whether the subject is impaired for purposes of driving.  This is not to say that these studies are not flawed for a variety of reasons.  It is important that you retain an Austin DWI Lawyer who is well versed in how these studies by the National Highway Traffic Safety Administration (NHTSA) were conducted and what the results of these studies actually were.  Police officers and prosecutors alike tend to try and argue that the studies show that if certain clues are observed there is a percentage attached to the likelihood that a person is impaired for purposes of driving.   If you read the studies carefully, this is simply not true. 

In this Blog, we will talk about the Horizontal Gaze Nystagmus (HGN) test since it typically is the first test given.  Most folks know this test as the eye test.  In addition, most folks have no idea what it is an officer is looking for in this test.  In a nutshell, the HGN test is where the officer uses a stimulus, typically a pen, light pen, or a finger and asks you to track it with your eyes without moving your head.  The officer is looking for three clues in each eye on this test.  These clues include:  lack of smooth pursuit, distinct and sustained nystagmus at maximum deviation, and an angle of onset of nystagmus before 45 degrees.  Unfortunately, due to the nature of this test, the actual results seen in the eye are generally not seen on the patrol video so it is difficult to dispute the officer’s findings.  There is also no way for the subject to get better at the test.  The test is designed to look for involuntary reactions in the eye caused by the introduction of alcohol or other depressants.


In order for the test to be properly conducted the officer must follow set guidelines in how to perform the test.


First, the officer must be aware of any conditions that may interfere with the subject’s performance.  This includes wind, dust, or anything else that may irritate the subject’s eyes.  The officer should also look for any visual or other distractions.  Namely, you want to face the subject away from rotating lights, strobe lights, and traffic passing in close proximity.  Failure to do so can lead to optokinetic nystagmus, which looks very similar to nystagmus caused by alcohol (Alcohol Gaze Nystagmus [AGN]


Next the officer should go over some administrative procedures with the subject.  If the subject is wearing eye glasses, the officer should ask the subject to remove the glasses.  The officer should then proceed with the instruction phase of the test.


The instructions should be given as follows:


                1.  “I am going to check your eyes.”

                2.  “Keep your head still and follow this stimulus with your eyes only.”

                3.  “Keep following the stimulus with your eyes until I instruct you to stop.”


The officer should then position the stimulus 12 to 15 inches from the subject’s nose and raise the stimulus slightly above eye level.  The officer should then make a few passes with the stimulus to make sure the subject is a good candidate for the test.  First, the officer should check for equal tracking.  This is done by moving the stimulus smoothly across the subject’s entire field of vision. If the eyes do not track together, meaning one eye lags behind the other, this is possibly a sign of a medical condition, injury, or blindness.  Next the officer should check for equal pupil size.  If the pupils are different sizes this is a possible indication of a head injury.


I am going to go through the HGN process meticulously and explain exactly what an officer should do because sometimes out of laziness, lack of experience, or just years on the job doing field sobriety tests an officer may cut corners.  In order for the findings of the test to be valid, the officer needs to do the test the proper way.  For instance, I often notice on DWI videos that officers say they checked for equal tracking and equal pupil size, but you can see clearly from the video that they did not.  This may be minor, but in a close case, it can mean all the difference in the world.


Now, we get to the test itself. The first thing the officer does is check for lack of smooth pursuit.  This is generally done at a fairly rapid speed.  Lack of smooth pursuit is difficult to explain, but I will do my best to come up with an analogy.  Imagine two marbles rolling across a table at a constant speed and then suddenly one or both of the marbles started traveling at a slower speed or stopped and then almost immediately caught back up with the original speed.  This is the best way I can explain lack of smooth pursuit. 


The nose is supposed to be used as the starting point on this portion of the test.  The test giver first checks the left eye, then the right eye.   The stimulus must be moved steadily in an even plane.  Each eye check should be 2 seconds out and 2 seconds back.  The tester must conduct at least 2 passes for each eye.  This should take a minimum of 16 seconds total if done correctly.  If either or both eyes cannot follow the moving stimulus smoothly, it should be counted as a clue.  There is a possible of two clues on this portion of the test, one clue for each eye. 


During the second phase of the test, the officer is looking for distinct and sustained nystagmus at maximum deviation.  This begs the question, what is nystagmus?  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 jerking when the eye gazes to the side.  In the impaired driving context, alcohol consumption or consumption of other central nervous system depressants, inhalants or phencyclidine, hinders the ability of the brain to correctly control eye muscles, therefore causing the jerk or bounce associated with HGN.  As the degree of impairment becomes greater, the jerking or bouncing, i.e. the nystagmus should become more pronounced and the angle of onset of nystagmus should occur sooner.  Lack of smooth pursuit, while a portion of the HGN test, is not nystagmus.  Officers often confuse these two very different concepts.     


The test for distinct and sustained nystagmus at maximum deviation is done at a slower speed than the check for lack of smooth pursuit.  Again, the nose is the starting point.  The officer is supposed to check the left eye first, then the right eye.  The officer does this by moving the stimulus across an even plane until the subject’s eye has gone as far as possible to the side.  (No white should be showing in the corner of the subject’s eye.)  The officer is to hold the stimulus at this extreme position (max deviation) for at least 4 seconds.  The officer must make at least 2 passes for each eye.  The minimum total amount of time to perform this portion of the test is 20 seconds.  Again, the jerkiness of the eye must be distinct and sustained, meaning lasting four seconds or more, for this to count as a clue.  There is a possible of two clues on this portion of the test, one clue for each eye. 


The final thing the officer looks for on the HGN portion of the test is an angle of onset of nystagmus before 45 degrees.  Again, the nose is the starting point.  The officer checks the left eye, then the right eye.  It should take the officer 4 seconds to move the stimulus across an even plane from the subject’s nose to shoulder.  On average, a person’s soldier is approximately 45 degrees.  Once the officer observes the beginning of the jerking, he or she should stop the stimulus and verify that the jerking continues.  It is important to note that there should be some white of the eye still showing closest to the ear when the jerking is observed.  If not the officer has probably gone beyond 45 degrees.  The officer should conduct at least 2 passes on each eye.  The minimum period of time this portion of the test should take is 20 seconds.  If an angle of onset is observed before 45 degrees, it is marked as a clue.  Again, it would be one clue for each eye.


There are a total of 6 clues on this test.  3 clues for each eye.  4 or more clues are considered a significant result.  Officers will sometime try to say that if they observed 4 or more clues there is somewhere between a 91% and 95% chance that the subject has a blood alcohol level that is greater than 0.08%.  This is an inaccurate reading, understanding, and interpretation of the various studies that have been done. 


Officers will often check for vertical nystagmus upon completion of the HGN.  The vertical nystagmus test is used to detect impairment due to drugs like PCP and CNS depressants.  The way this test is done is the officer positions the stimulus horizontally (parallel to the ground) about 12 to 15 inches in front of the subject’s nose.   The officer instructs the subject to hold his head still, and follow the stimulus only with his or her eyes.  The officer then raises the stimulus until the subject’s eyes are elevated as far possible.  The officer then holds the stimulus for approximately 4 seconds and looks for evidence of the eyes jerking.


There are a variety of types of nystagmus in addition to what has been described above.


There is alcohol gaze nystagmus (AGN) which is gaze nystagmus caused by the effects of alcohol on the nervous system.


There is caloric nystagmus.  This is a form of a vestibular system nystagmus caused by different temperature between the ears, e.g. one ear is irrigated with with warm water and the other is irrigated with cold water. 


There is epileptic nystagmus, which is nystagmus that is evident during an epileptic seizure.


There is jerk nystagmus, which is where the eye drifts slowly away from the point of focus and then quickly corrects itself with a fast movement of the eye back to the point of focus.


There is natural nystagmus.  This occurs without any apparent physiological, vestibular, or neurological disturbance.  Natural nystagmus occurs in approximately 2%-4% of the population.


There is neurological nystagmus, which is caused by some disturbance in the nervous system. 


There is optokinectic nystagmus, which is a nystagmus that is evident when an object that the eye fixates upon moves quickly out of sight or passes quickly through the field of vision, such as occurs when a subject watches utility poles pass by while in a moving car.  Optokinectic nystagmus is also caused by watching alternating moving images such as black and white spokes on a spinning wheel.


There is pendular nystagmus, which is nystagmus where the eye oscillates or swings equally in two different directions. 


There is physiological nystagmus.  This is a nystagmus that occurs so the light entering the eye will continually fall on non-fatigued cells in the retina.  Physiological nystagmus is allegedly so slight that it cannot be detected without the aid of instruments and occurs in everyone. 


There is positional alcohol nystagmus (PAN).  Positional nystagmus occurs when the foreign fluid is alcohol.  PAN I means the alcohol concentration is higher in the blood than in the vestibular system.


PAN 2 means that the alcohol concentration is lower in the blood than in the vestibular system.


There is positional nystagmus that occurs when a foreign fluid is in unequal concentrations between the blood and the fluid in the semi-circular canals of the vestibular system.


There is post-rotational nystagmus, which is nystagmus caused by disturbances in the vestibular system fluid when a person spins around.  Post-rotational nystagmus should only last for a few seconds after the person stops spinning. 


There is resting nystagmus, which occurs as the eyes are looking straight ahead.


There is rotational nystagmus, which is caused by disturbances in the vestibular system fluid when a person spins.  Rotational nystagmus only occurs only when the person is spinning.


Incidentally, the vestibular system is the system of fluid-filled canals located in the inner ear that assist in balance, coordination, and orientation.  


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