From the Guidelines
Miosis in a sick individual is most likely caused by opioid overdose, as evidenced by the strong correlation between miosis and response to naloxone administration. According to the 2024 American Heart Association and American Red Cross guidelines for first aid 1, miosis is strongly correlated with response to naloxone administration, suggesting that opioid overdose is a common cause of pupil constriction in sick individuals.
Some key points to consider when evaluating miosis in a sick individual include:
- Certain medications, such as opioids, cholinergic drugs, and some antipsychotics, can cause miosis by directly affecting the pupillary muscles
- Neurological conditions, including Horner's syndrome, pontine strokes, or brain hemorrhages, can result in miosis due to damage to the sympathetic nerve pathway
- Infections affecting the central nervous system, such as meningitis or encephalitis, may lead to pupil constriction
- Metabolic disorders, including liver failure with hepatic encephalopathy or uremia from kidney failure, can cause miosis as toxins accumulate and affect brain function
However, the most recent and highest quality study suggests that opioid overdose is a primary concern in cases of miosis 1. This is supported by the fact that naloxone, a safe and effective treatment for opioid overdose, has been shown to reverse miosis in individuals with opioid-induced respiratory depression. Therefore, in cases of miosis, it is essential to consider opioid overdose as a potential cause and administer naloxone if suspected.
From the FDA Drug Label
Morphine causes miosis, even in total darkness. Pinpoint pupils are a sign of opioid overdose but are not pathognomonic (e. g., pontine lesions of hemorrhagic or ischemic origins may produce similar findings).
Miosis, or small pupils, in a sick individual can be caused by opioid use, such as morphine, as it directly affects the central nervous system and causes pinpoint pupils 2. However, it's essential to note that pontine lesions of hemorrhagic or ischemic origins may also produce similar findings, and marked mydriasis rather than miosis may be seen due to hypoxia in overdose situations 2 2.
From the Research
Causes of Miosis in Sick Individuals
Miosis, or small pupils, can be caused by various factors when an individual is sick. Some possible causes include:
- Opioid use or overdose, which can lead to respiratory depression and miosis 3, 4, 5, 6, 7
- Stimulation of opioid receptors in the central nervous system (CNS), which can cause the release of endogenous opioids and lead to miosis 3
- Activation of μ-opioid receptors at specific sites in the CNS, including the pre-Bötzinger complex, a respiratory rhythm generating area in the pons 4, 7
- Use of certain medications, such as clonidine, which can cause miosis as a side effect 3
Mechanisms of Opioid-Induced Miosis
The mechanisms underlying opioid-induced miosis are complex and involve the activation of opioid receptors in the CNS. Some key findings include:
- Opioids can cause respiratory depression by activating μ-opioid receptors in the pre-Bötzinger complex and other brainstem respiratory centers 4, 7
- The effects of opioids on breathing are determined by the transfer of the opioid to the receptor site, as well as the receptor kinetics, including dissociation kinetics 7
- Naloxone, an opioid receptor antagonist, can reverse opioid-induced respiratory depression and miosis, but may also reverse analgesia 3, 4, 5, 6, 7
Relevance to Sick Individuals
In sick individuals, miosis can be a symptom of various underlying conditions, including opioid use or overdose. It is essential to consider the potential causes of miosis and to provide appropriate treatment, such as naloxone, if necessary. Additionally, research is ongoing to develop new therapeutic agents that can reverse opioid-induced respiratory depression and miosis without affecting analgesia 6, 7.