There are other flaws with the HGN test result. Chemicals, drugs, and numerous substances can cause nystagmus (jerking of the eyes) without the presence of alcohol.
For example the ingestion of barbiturates, antihistamines, phencyclidine, depressants, and anticonvulsants can cause nystagmus. Other influential substances include caffeine, nicotine or aspirin. Even atmospheric pressure influences nystagmus.
Physiological Problems – Health and biological ailments can cause nystagmus, such as brain hemorrhage, inner ear disorder, certain congenital eye problems, measles, vertigo, influenza, streptococcus infection, epilepsy, syphilis, muscular dystrophy, motion sickness, sunstroke, hypertension, hypotension, Korsakoff’s syndrome, arteriosclerosis, and psychogenetic disorders. This list is not exhaustive, so consult a physician.
Tired drivers can exhibit nystagmus due to prolonged use of his eyes, insufficient lighting, or strained positions. Moreover, test results can be distorted by failing to remove all corrective lenses—hard or soft contact lenses or glasses—prior to testing.
Pupil Reaction Test – Less common though similar to the HGN, is the pupil reaction test. This measures the rate at which the pupil contracts when exposed to light—a slower contraction indicates greater alcohol impairment. Often officers will shine a flashlight in the defendant’s eyes to observe the pupil contraction. The test is flawed because it lacks scientific objectivity. Most officers lack training ophthalmology, and have no knowledge of other possible causes of slow pupil reaction. For instance, over-the-counter drugs, such as allergy and sinus medications, can cause dilated pupils, even in the absence of alcohol consumption