Which substance causes mydriasis? Understanding dilated pupils

Over a dozen categories of medication and numerous recreational drugs can cause mydriasis, the medical term for dilated pupils. This occurs when substances interfere with the autonomic nervous system's control over the iris muscles, leading to significant pupil enlargement and potential light sensitivity.

Quick Summary

Mydriasis, or pupil dilation, is triggered by a wide range of substances, including prescribed and illicit drugs. These agents disrupt the autonomic nervous system's control of the eye, often by either stimulating the sympathetic nervous system or blocking the parasympathetic pathway.

Key Points

• Drug Classes: Various classes of drugs, including anticholinergics, sympathomimetics, and hallucinogens, are primary causes of mydriasis.

• Nervous System Action: Substances cause pupil dilation by either stimulating the sympathetic ('fight or flight') nervous system or blocking the parasympathetic ('rest and digest') system.

• Recreational vs. Prescription: Mydriasis can result from both illicit drugs like cocaine and LSD, as well as common prescription medications such as antidepressants and ADHD stimulants.

• Toxic Plants: Certain plants, like belladonna and Jimson weed, contain natural toxins that act as mydriatic agents.

• Beyond Drugs: Mydriasis is not exclusively caused by substances and can also be a sign of neurological issues, eye trauma, or emotional states.

• Diagnostic Tool: Ophthalmologists intentionally use mydriatic eye drops to dilate pupils during eye exams to get a clearer view of the retina and optic nerve.

• Context is Key: Judging the cause of dilated pupils requires considering a patient's full medical history and other symptoms, not just pupil size alone.

The Autonomic Nervous System and Mydriasis

To understand what causes mydriasis, it's crucial to know how the body controls pupil size. This process is managed by the autonomic nervous system, which is divided into the sympathetic ('fight or flight') and parasympathetic ('rest and digest') branches. The iris contains two sets of muscles that control the pupil: the sphincter muscle, which constricts the pupil, and the dilator muscle, which widens it.

• Sympathetic Stimulation: Certain substances activate the sympathetic nervous system, leading to the release of neurotransmitters like norepinephrine. This causes the iris dilator muscle to contract, pulling the pupil open.
• Parasympathetic Inhibition: Other substances block the effects of acetylcholine, the primary neurotransmitter of the parasympathetic nervous system. This inhibits the iris sphincter muscle from contracting, resulting in unopposed dilation.

Medications that Cause Mydriasis

Many prescription and over-the-counter drugs have mydriasis as a side effect. Some are used intentionally to dilate the pupils for medical examinations, while others cause it inadvertently.

Mydriatics (Ophthalmic Drops)

Used by eye doctors during exams, these drops contain powerful anticholinergic or sympathomimetic agents to dilate the pupils. Examples include:

• Tropicamide: A fast-acting anticholinergic.
• Cyclopentolate: Another anticholinergic used for diagnostic dilation.
• Phenylephrine: A sympathomimetic agent that works by stimulating adrenergic receptors.

Anticholinergics

These drugs block acetylcholine receptors and are used for various conditions, but they can cause systemic mydriasis.

• Atropine: Historically used as a mydriatic, it's also used to treat certain heart rhythm issues and in ophthalmology.
• Scopolamine: Prescribed for motion sickness, often as a transdermal patch.
• Antihistamines: Many older, first-generation antihistamines have anticholinergic effects.

Stimulant Medications

Prescribed for conditions like ADHD and narcolepsy, these increase norepinephrine and dopamine levels in the brain, activating the sympathetic nervous system.

• Amphetamines (Adderall)
• Methylphenidate (Ritalin)

Antidepressants

Some antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants, can cause pupil dilation due to their effects on neurotransmitter balance or anticholinergic properties.

Other Prescription Drugs

• Benzodiazepines: Prescribed for anxiety, they can cause mydriasis by affecting neurotransmitters.
• Botulinum Toxin (Botox): Accidental systemic exposure can cause mydriasis.
• Decongestants: Oral decongestants like pseudoephedrine and phenylephrine have stimulant-like properties.

Recreational Drugs Causing Mydriasis

Illicit substances often cause pronounced and prolonged pupil dilation, which can be a key indicator of use.

Stimulants

These drugs trigger the 'fight or flight' response more intensely than their therapeutic counterparts.

• Cocaine: Blocks norepinephrine reuptake, leading to significant dilation.
• Methamphetamine: Causes extreme dilation by increasing dopamine production.
• MDMA (Ecstasy/Molly): A stimulant with hallucinogenic properties that reliably causes mydriasis.

Hallucinogens

These substances primarily affect serotonin receptors in the brain, leading to alterations in perception and pupil size.

• LSD (lysergic acid diethylamide)
• Psilocybin (magic mushrooms)
• Mescaline

Other Illicit Substances

• Ketamine: A dissociative anesthetic that can produce moderate pupil dilation.
• Bath Salts: Synthetic cathinones with stimulant effects.
• Cannabis: May cause mild to moderate dilation in some users, though studies show conflicting results depending on factors like concentration.

Environmental and Toxin-Related Mydriasis

Exposure to certain natural substances can also induce mydriasis, often inadvertently.

• Toxic Plants: Several plants contain mydriatic alkaloids, such as atropine and scopolamine. Examples include:
  • Belladonna
  • Jimson weed
  • Angel's trumpet
• Accidental Exposure: Healthcare workers handling certain patches or chemicals or gardeners touching these plants and then their eyes can experience unilateral mydriasis.

Comparison of Mydriatic Substances

Substance Class	Examples	Mechanism of Mydriasis	Onset and Duration	Context	Other Effects
Ophthalmic Mydriatics	Tropicamide, Cyclopentolate, Phenylephrine	Topical application inhibits parasympathetic or stimulates sympathetic pathways directly in the eye.	Rapid onset (minutes), duration 4–24 hours depending on the drug.	Medical eye exams or procedures.	Blurred vision, light sensitivity.
Anticholinergics	Atropine, Scopolamine, Antihistamines	Systemic blockage of acetylcholine receptors, inhibiting parasympathetic tone.	Varies widely based on administration route and drug.	Accidental exposure, systemic medication.	Dry mouth, constipation, increased heart rate.
Stimulants (Illicit & Rx)	Cocaine, Methamphetamine, MDMA, Adderall	Increased release of norepinephrine and dopamine, activating the sympathetic nervous system.	Rapid onset, duration several hours (longer for certain drugs).	Recreational use, ADHD treatment.	Increased heart rate, blood pressure, euphoria.
Hallucinogens	LSD, Psilocybin, Mescaline	Interact with serotonin receptors in the brain, altering neurotransmitter balance.	Onset varies, duration can be up to 12 hours or more.	Recreational use.	Altered perception, heightened sensory experience.
Toxins (Plants)	Belladonna, Jimson Weed	Contain alkaloids like atropine that block muscarinic receptors.	Varies, can be prolonged.	Accidental or intentional poisoning.	Severe systemic toxicity.

The Role of Context in Evaluating Mydriasis

While observing dilated pupils can point to substance use, it is not a definitive diagnostic tool on its own. Many other factors can cause mydriasis, including neurological conditions, traumatic brain injury, eye trauma, and strong emotions. A dilated pupil that is uneven (anisocoria) or unresponsive to light can indicate a more serious underlying medical issue requiring immediate attention. Healthcare providers use the context of other symptoms, patient history, and toxicology screenings to arrive at an accurate diagnosis. It is essential to consult a medical professional for any unusual or persistent pupil dilation to rule out serious conditions.

Conclusion: The Bigger Picture of Dilated Pupils

In summary, mydriasis is a symptom with a wide range of potential pharmacological causes, stemming from a substance's interaction with the autonomic nervous system. From prescription mydriatic drops used for eye exams to illicit substances like stimulants and hallucinogens, different agents can either promote sympathetic activity or inhibit parasympathetic function to enlarge the pupil. The specific mechanism, duration, and associated side effects vary depending on the substance involved, highlighting the complexity of pharmacological effects on the human body. As with any unusual physical sign, persistent or asymmetric pupil dilation should prompt a visit to a healthcare professional for proper evaluation.

Frequently Asked Questions

Can over-the-counter medications cause mydriasis?

Yes, some over-the-counter medications can cause mydriasis. Oral decongestants like phenylephrine and first-generation antihistamines, such as diphenhydramine (Benadryl), have anticholinergic effects that can lead to dilated pupils.

How long does drug-induced pupil dilation typically last?

The duration of drug-induced pupil dilation varies depending on the specific substance and the individual's metabolism. While stimulant effects might last several hours, dilation from certain hallucinogens or long-acting anticholinergics can persist much longer.

Do all stimulants cause mydriasis?

Most stimulants, including cocaine, amphetamines, and methamphetamine, cause pupil dilation by activating the sympathetic nervous system. However, the degree of dilation can vary, and individual responses differ.

Can antidepressants cause dilated pupils?

Yes, certain types of antidepressants can cause pupil dilation as a side effect. This is particularly true for selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants, which affect neurotransmitter levels.

Are pinpoint pupils and dilated pupils the same thing?

No, they are opposites. Mydriasis refers to dilated pupils, while miosis describes pinpoint pupils. Miosis is most commonly associated with opioid use, including heroin and morphine.

Is it dangerous if my pupils are dilated after an eye exam?

Pharmacologic dilation during a standard eye exam is safe and intentional. While it can cause temporary light sensitivity and blurred vision, it is not considered dangerous unless you have pre-existing narrow angles that could predispose you to angle-closure glaucoma.

What should I do if I notice sudden or unusual pupil dilation?

Sudden or uneven pupil dilation, especially if accompanied by other symptoms like a headache or altered consciousness, could indicate a serious medical condition such as a traumatic brain injury. It is important to seek immediate medical attention to determine the underlying cause.