A Demographic Comparison of the Effects of Morphine and Butorphanol on Pupillometry and Reaction Times

Pain-related issues represent the most common reason for which people seek health care. 1 Opioids, such as morphine, are the most commonly prescribed class of drugs for treating moderate to severe pain. Morphine is a μopioid, δ-opioid and κ-opioid receptor agonist. Whereas, butorphanol is a synthetic opioid analgesic exhibiting partial agonist and antagonist activity at the μ-opioid receptor as well as agonist activity at the κ-opioid receptor. Also known as Stadol, butorphanol has been found to be very effective at reducing pain, particularly in women, and is often given during labor. 2 Both drugs act by stimulating opioid receptors in the central nervous system, thus providing pain relief. These drugs are prescribed intravenously according to the weight of the patient. However, the effects of the drug vary widely between individuals. In addition to reducing pain, opioids produce other biological effects such as miosis and altered reaction time (RT). Miosis, or constriction of the pupil, is caused by normal light reflex and occurs in response to μ-opioid and κ-opioid receptor agonists. 3 That opioids induce miosis in humans is well documented, but the factors contributing to variations in pupil size and reactivity are not as well studied. 4, 5 It has been shown that there is no light reflex change but there is a decrease in pupil size caused by opioids 6 . The pupillary effects of opioids can be observed just four minutes after administration. 7 The Pupillometer uses a brief retinal light to induce the pupillary light reflex which causes miosis. For this to occur, sympathetic activity from the ciliary ganglion is reduced and parasympathetic activity is increased through disinhibition. The leading theory is pupilloconstrictor neurons in the visceral nuclei of the oculomotor nuclear complex increase frequency of discharge in response to μ-opioid agonists. 6, 8 In general, opioid-induced miosis in the central nervous system is mediated by the parasympathetic nervous system. 9