How many clues are in the DUI eye test and what do they mean?

Problems with the "Eye Test" in DUI arrests:

The "Eye Test" that is given by police officers during DUI investigations is called the Horizontal-Gaze Nystagmus Test. In this test officers are looking for three clues which include:

1. Lack of smooth pursuit 
2. Distinct and sustained nystagmus at maximum deviation
3. And, onset of nystagmus prior to a 45 degree angle.

The test is administered by the officer placing a stimulus (usually a pen or his index finger) in front of the person's face and asking the person to follow the stimulus with his eyes and his eyes only without moving his head as he moves it back and forth in front of the person's face.

What is Nystagmus?
Nystagmus is an involuntary rapid jerking of the eye. Nystagmus can be caused by many different things. One thing that can cause Nystagmus is alcohol use. The problem is that nystagmus can be caused by so many conditions that it is hard for the officer on the side of the road to determine what is causing any nystagmus that the officer may see. To read further about nystagmus and its causes, see this page from the American Academy of Ophthalmology. www.aao.org/eye-health/diseases/what-is-nystagmus

What the officer is looking for in each step:

1.  Lack of Smooth Pursuit: In this first phase of the test, the officer is looking to see if the eye tracks smoothly from right to left and left to right as he moves his finger or the pen back and forth from side to side. The best way to think about this clue is to imagine windshield wipers on a windshield. If everything goes perfectly and there are no other factors affecting the test, then the person's eyes, like windshield wipers (that work great and are on a smooth, wet, windshield) would glide very smoothly back and forth. The clue the officer is actually looking for is for the eye to jerk as it moves back and forth from left to right.
2. Distinct and Sustained Nystagmus: This is the second clue in the HGN Test. The officer moves the stimulus all the way out to the right and left and holds the stimulus still (all the way out to the side)for a minimum of 4 seconds. The officer is watching for the eye to continue to jerk while it is focusing on the stimulus while the stimulus is held still. 
3. Onset of Nystagmus prior to a 45 degree angle: Essentially there is approximately a 90 degree angle from straight in front of your face to your outside of your shoulder if facing straight forward. Approximately half way between straight forward and outside of your shoulder would be 45 degrees. Presumably if your eye starts to jerk prior to the stimulus reaching the 45 degree angle, that should indicate more alcohol use/impairment level.

Issues with the HGN Test: 
One of the most problematic issues with the HGN Test is that nystagmus can be caused by many types of medical or neurological issues that a person may or may not know that they have. Here is a quick list of causes of nystagmus:

• Developmental problems with your brain or eye.
• Retina or optic nerve disorders.
• Inner ear disorders, such as benign paroxysmal positional vertigo (BPPV) and Ménière’s disease.
• Stroke.
• Brain tumor.
• Eye or head trauma (injury).
• Alcohol or drug use.
• Albinism (lack of pigmentation in the skin).
• Vision problems, including nearsightedness or astigmatism.
• Certain medications, such as antiseizure drugs.
• Diseases affecting your central nervous system, like multiple sclerosis (MS).
• Eye problems in babies, including strabismus (crossed eyes), focusing issues and cataracts.

If a perso n has a medical issue that causes nystagmus, the officer on the side of the road has very little ability to make a medical diagnosis as to the cause for nystagmus. The Cleveland Clinic has a list and explanation of some natural causes of nystagmus and the related symptoms.  Nystagmus: Symptoms and Causes. The problem is that if nystagmus is observed then it is likely attributed to drug or alcohol use instead of any medical issue. Frequently, the HGN test is given immediately after car accidents when the person has been hit in the head with an airbag or had a severe hit to a steering windshield, windshield, or anything else in the car. 
In Summary: It is always important to have an experienced DUI lawyer, review the evidence in your case, any prior medical or injury history, and THEN make a determination as to whether any alleged nystagmus observed by an officer was related to alcohol or drug use OR some other cause.