Horizontal Gaze Nystagmus is an involuntary jerking of the eye that becomes more pronounced with alcohol impairment. The NHTSA-validated test scores six clues, three per eye: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees. Four or more clues indicates likely BAC at or above 0.08. Illinois courts admit HGN as evidence of impairment but not as a numeric BAC substitute. Reliability requires proper administration, correct stimulus distance and speed, no background flashing lights, and screening for causes of nystagmus other than alcohol including inner-ear disorders, medications, and neurological conditions. A Chicago DUI lawyer attacks stimulus technique and medical alternatives.
