Mydriasis is the medical term for pupil dilation, an unusual or excessive widening of the pupil. While this physiological change is a natural response to low light, when it occurs irrespective of ambient lighting, it often signals interference with the body’s control systems. Various substances, from therapeutic prescription medications to illicit compounds, can induce mydriasis by chemically altering the function of the eye’s internal muscles.
How the Pupil Size is Regulated
The size of the pupil is governed by two opposing smooth muscles within the iris. The iris sphincter muscle is circular and constricts the pupil (miosis). The iris dilator muscle is radial and contracts to widen the pupil (mydriasis).
This muscular movement is regulated involuntarily by the autonomic nervous system (ANS). The parasympathetic nervous system controls constriction by releasing acetylcholine, causing the sphincter muscle to contract. The sympathetic nervous system controls dilation by releasing norepinephrine, preparing the eye for a “fight or flight” response.
Drugs interfere with this balance by targeting these pathways. A substance can stimulate the sympathetic nervous system, increasing norepinephrine and causing dilation. Alternatively, a substance can block the parasympathetic nervous system, preventing acetylcholine action and stopping the sphincter muscle from constricting.
Illicit and Recreational Substances Causing Dilation
Certain recreational substances cause mydriasis by powerfully activating the sympathetic nervous system. Stimulant drugs achieve this by dramatically increasing neurotransmitter concentration.
Stimulants
Cocaine and methamphetamine, for example, block the reuptake of dopamine and norepinephrine. This leads to a sympathetic response, causing the iris dilator muscle to contract forcefully and resulting in noticeable pupil enlargement.
Hallucinogens
Hallucinogens, such as lysergic acid diethylamide (LSD) and psilocybin, frequently cause pronounced pupil dilation through interaction with serotonin receptors. They stimulate certain serotonin receptors that enhance sympathetic activity, producing both altered perception and physical dilation.
Dissociative Anesthetics
Dissociative anesthetics, such as ketamine and phencyclidine (PCP), contribute to mydriasis through complex mechanisms. While their primary action involves blocking N-methyl-D-aspartate (NMDA) receptors, they also affect neurotransmitters like dopamine. This cumulative effect overstimulates the central nervous system, leading to dilation.
Medical and Prescription Causes of Dilation
Mydriasis is often a controlled and desired effect of several prescription medications.
Ophthalmic Agents
Ophthalmic agents, such as tropicamide and phenylephrine, are routinely used by eye doctors during comprehensive examinations. These drops are applied directly to the eye to temporarily relax the sphincter muscle or stimulate the dilator muscle, allowing for a better view of the retina and optic nerve.
Anticholinergics
Anticholinergics are prescribed for conditions like Parkinson’s disease or bladder control issues. These drugs block the action of acetylcholine, the neurotransmitter responsible for signaling the pupil to constrict. Inhibiting this signal temporarily disables parasympathetic control, allowing the sympathetic system’s dilatory action to dominate.
Psychiatric Medications
Some psychiatric medications cause mydriasis as a side effect due to their broad effects on neurotransmitter systems. Older tricyclic antidepressants have anticholinergic properties that block pupil constriction. Newer selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) can also cause moderate dilation by affecting the balance of serotonin and norepinephrine pathways.
When Dilated Pupils Require Immediate Medical Attention
While drug-induced mydriasis is often temporary, certain signs indicate a medical emergency.
A warning sign is anisocoria, where the two pupils are unequal in size, especially if it occurs suddenly. This asymmetry can signal severe neurological issues, such as pressure on the third cranial nerve, potentially resulting from a brain aneurysm or a stroke.
Pupils that are fixed and non-reactive to light are also a serious concern, particularly when accompanied by signs of toxicity or overdose. A healthy pupil constricts immediately when exposed to light; a lack of this reflex suggests profound nervous system impairment.
Other accompanying symptoms requiring immediate medical intervention include severe headache, loss of consciousness, high fever, seizures, or a drooping eyelid (ptosis). Mydriasis combined with these symptoms suggests a severe reaction, trauma, or dangerous toxicity.