Small pupils, medically termed miosis, refer to the constriction or narrowing of the eye’s central opening, the pupil. This phenomenon is the opposite of pupil dilation, or mydriasis. While various factors can cause miosis, this article focuses on specific drugs that lead to pupil constriction.
How Pupil Size is Controlled
The autonomic nervous system, which manages involuntary bodily functions, controls pupil size. This system has two main branches: the sympathetic, causing pupils to dilate for “fight or flight” responses, and the parasympathetic, promoting “rest and digest” activities and leading to pupil constriction. This constriction occurs when the parasympathetic system stimulates the iris sphincter muscle, causing it to contract and reduce the pupil’s diameter. Drugs can interfere with this delicate balance, either by enhancing parasympathetic activity or by inhibiting sympathetic activity.
Common Drugs Causing Small Pupils
Opioids are widely recognized for causing miosis, often leading to “pinpoint pupils.” These drugs, including substances like heroin, morphine, oxycodone, and fentanyl, bind to mu-opioid receptors in the brain. This binding stimulates the parasympathetic nervous system, which then causes the iris sphincter muscle to contract. The resulting miosis is a direct effect of this central nervous system stimulation and is a common indicator of opioid use or overdose. The degree of pupillary constriction can be dose-dependent, with higher doses often leading to more pronounced miosis.
Sedatives and hypnotics can also contribute to small pupils, though typically to a lesser extent than opioids. These medications, such as some barbiturates and benzodiazepines, generally depress the central nervous system. While benzodiazepines paradoxically may not always cause miosis, other sedatives can induce miosis proportional to their sedative effects, as a decrease in arousal is often accompanied by pupil constriction.
Other Substances and Medications
Beyond opioids and some sedatives, other substances and medications can also lead to constricted pupils. Cholinergic drugs, also known as parasympathomimetics, directly stimulate the parasympathetic nervous system. Examples include pilocarpine, a medication used to treat glaucoma, which directly causes the iris sphincter muscle to contract. Certain insecticides, such as organophosphates, are also cholinergic agents and can induce miosis.
Some antipsychotic medications, particularly second-generation or atypical antipsychotics like risperidone, haloperidol, and olanzapine, can cause miosis as a side effect. This effect is often related to their impact on various neurotransmitter systems in the brain. Furthermore, while alcohol typically causes pupil dilation at moderate to high doses, very high levels of alcohol consumption can sometimes lead to pupil constriction due to enhanced parasympathetic activity.