The NHTSA training manual states that the eye examinations that you can conduct to assess possible drug or medical impairment include:
- Resting nystagmus
- Tracking ability
- Pupil size
- Horizontal gaze nystagmus (HGN)
- Vertical gaze nystagmus (VGN)
Resting Nystagmus is referred to as jerking as the eyes look straight ahead. This condition is not frequently seen. Its presence usually indicates a pathological disorder or high doses of a Dissociative Anesthetic drug such as PCP.
Tracking ability will be affected by certain categories of drugs, and also by certain medical conditions or pathological disorders.
If the two eyes do not track together, the possibility of a medical condition or injury is present.
By passing a stimulus across both eyes, you can check to see if both eyes are tracking equally.
The ability of your eyes to track an object can be affected by certain categories of drugs, and also by certain medical conditions or pathological disorders. Officers are trained that if the two eyes do not track together, the possibility of a medical condition or injury is present. By passing a stimulus across both eyes, the office can check to see if both eyes are tracking equally. If they don’t (i.e., if one eye tracks the stimulus, but the other fails to move, or lags behind the stimulus) there is the possibility of a pathological disorder. If a person has sight in both eyes, but the eyes fail to track together, there is a possibility that the person is suffering from an injury or illness.
Horizontal Gaze Nystagmus
The NHTSA claims that Horizontal Gaze Nystagmus (HGN) is the most reliable field sobriety test. Especially when used in combination with the divided attention tests, it will help law enforcement officers correctly identify subjects who are impaired. The officer is looking for involuntary jerking of the eyes occurring as the eyes gaze to the side. It is normally the first test administered in the SFST battery.
Involuntary jerking of the eyes becomes readily noticeable when a person is impaired. As a person's blood alcohol concentration increases, the eyes will begin to jerk sooner as they move to the side.
In administering the HGN test, the officer has the subject follow the motion of a small stimulus with the eyes only. The stimulus may be the tip of a pen or penlight, or an eraser on a pencil, whichever contrasts with the background.
When the HGN test is administered always begin with subject's left eye. Each eye is examined for three specific clues.
- As the eye moves from side to side, does it move smoothly or does it jerk noticeably? (As people become impaired by alcohol, their eyes exhibit a lack of smooth pursuit as they move from side to side.)
- When the eye moves as far to the side as possible and is kept at that position for four seconds, does it jerk distinctly? (Distinct and sustained nystagmus at maximum deviation is another clue of impairment.)
- As the eye moves toward the side, does it start to jerk prior to a 45 degree angle? (Onset of nystagmus prior to 45 degrees is another clue of impairment.)
As a person's blood alcohol concentration increases it is more likely these clues will appear. The maximum total number of clues is six. The maximum number of clues that may appear in one eye is three.
If the eyes track equally, but “jerk” while they are moving, then the possible presence of three categories of drugs should be noted:
- Central Nervous System Depressants;
- Dissociative Anesthetics; and,
There are a number of reasons that nystagmus might occur other than alcohol or drug impairment. Fatigue and medications treating seizures, barbiturates, and depressants can lead to nystagmus. If you failed your HGN “test” it may be important to hire a neurologist to testify that alcohol consumption was not responsible for the nystagmus.
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