The Test of Horizontal Gaze Nystagmus (HGN) for subjects is identical to the HGN test for alcohol-impaired subjects.

Session Overview – Introduction to Drugged Driving

DWI Detection and Standardized Field Sobriety Testing

The Test of Horizontal Gaze Nystagmus

(HGN) for subjects is identical to the HGN

test for alcohol-impaired subjects.

• First Clue: lack of smooth pursuit

• Second clue: distinct and sustained

nystagmus at maximum deviation

• Third clue: onset of nystagmus prior to

45 degrees

Horizontal Gaze Nystagmus (HGN)

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Horizontal Gaze Nystagmus

The test of Horizontal Gaze Nystagmus (HGN) for subjects is identical to the HGN test

for alcohol-impaired subjects.

• First Clue: Lack of smooth pursuit

• Second clue: Distinct and sustained nystagmus at maximum deviation

• Third clue: Onset of nystagmus prior to 45 degrees

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

• Inhalants

Session Overview – Introduction to Drugged Driving

DWI Detection and Standardized Field Sobriety Testing

The angle of onset becomes of special

interest when a subject is under the

influence of a Dissociative Anesthetic such

as PCP

PCP May Cause Immediate Onset

of Nystagmus

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PCP May Cause Immediate Onset of Nystagmus

PCP impaired subjects may exhibit immediate onset, i.e., the jerking begins virtually as

soon as the eyes start to move toward the side.

Sometimes, PCP- impaired subjects will exhibit resting nystagmus, i.e., the eyes jerk

while they are looking straight ahead.

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Session Overview – Introduction to Drugged Driving

DWI Detection and Standardized Field Sobriety Testing

Vertical Nystagmus

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Vertical Nystagmus

The Vertical Nystagmus test is very simple to administer.

• Position the stimulus horizontally. Approximately 12-15 inches (30-38 cm) in front of

the subject’s nose.

• Instruct the subject to hold their head still, and follow the stimulus with the eyes only.

• Raise the stimulus until the subject’s eyes are elevated as far as possible, hold for a

minimum of four seconds.

• Watch closely for evidence of jerking (up and down).

Vertical Nystagmus may be present in subjects under the influence of CNS depressants

or inhalants.

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Session Overview – Introduction to Drugged Driving

DWI Detection and Standardized Field Sobriety Testing

• Central Nervous System Depressants

• Central Nervous System Stimulants

• Hallucinogens

• Dissociative Anesthetics

• Narcotic Analgesics

• Inhalants

• Cannabis

Drug Categories and Their

Observable Effects

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C. Drug Categories and Their Observable Effects

Seven Categories of “Drugs”

Definition of “Drug”: Any substance that, when taken into the human body, can impair

the ability of the person to operate a vehicle safely.

Within this simple, enforcement-oriented definition, there are seven categories of drugs:

• Central Nervous System Depressants

• Central Nervous System Stimulants

• Hallucinogens

• Dissociative Anesthetics

• Narcotic Analgesics

• Inhalants

• Cannabis

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Source: DWI Detection and

Standardized Field

Sobriety Testing

March 2013 Edition

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