Opioid miosis: effects of lighting intensity and monocular and binocular exposure

Opioid miosis — that is, pupillary constriction caused by opioids — is one of the most sensitive and frequently assessed objective indices of opioid effects. Pupillary size is also affected by lighting intensity and monocular or binocular exposures. This study is the first systematic examination and quantitative characterization of the effects of lighting intensity and exposure on opioid miosis. Seven patients received their usual daily dose of methadone (50–60 mg p.o.). Reflected light intensities were manipulated among 4, 16, 40, 80, 160, 240 foot-lamberts (fl). Pupil photographs of the right eye were obtained with the left eye closed and both eyes open in random counterbalanced order at each fl. Pupil photographs were obtained 15 min before methadone and 5, 15, 30, 45, 60, 90, 120 and 180 min after methadone. Peak methadone miosis was best detected under moderately dim interior lighting (4, 16 fl) 90 min after methadone. Pupil diameters were systematically larger with one eye closed than with both eyes open and the average difference was 0.35 mm. Pupil diameter decreased 1.0 mm with each log unit increase in lighting intensity.