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We’ve been talking about the horizontal gaze nystagmus on our blog in the past week. One of the things that can go wrong in an HGN is that the subject has a medical condition that causes bad results. 

After positioning the object, but before conducting the test, the officer should always check for signs of medical impairment. 

First, the officer checks for equal tracking by moving the object quickly across the subject’s whole field of vision to see whether their eyes follow the object simultaneously. The officer then checks for equal pupil size. If the subject lacks equal tracking or equal pupil size, it may indicate a glass eye, blindness in one eye, a medical disorder or an injury. If the subject exhibits lack of equal tracking or equal pupil size, the officer should discontinue the HGN test since the results would be unreliable.