What Is Miosis?

With miosis, the eye's dark center (the pupil) is smaller than it should be. Usually, the pupil expands and contracts to allow less or more light into the eye as needed. But with miosis, the pupil remains at just 2 millimeters or less, even in conditions that usually would call for allowing more light into the eye to see optimally. This may affect one or both eyes.

In this article, you will learn what miosis (pupil constriction) is, what symptoms to watch for, possible causes, what to know about treatment, and more.

Miosis Symptoms

Detecting whether you have a constricted pupil may or may not be obvious. The type of symptoms you have can vary depending on what's causing the miosis. Some signs to watch for include the following:

• Two small pupils
• One small pupil and one normal-sized one (anisocoria)
• A drooping eyelid
• Less sweating on the side of the face with the smaller pupil
• Redness
• Pain
• Light sensitivity
  

Causes

While the pupils usually contract in very bright light, they may also be small due to conditions affecting your health. A variety of conditions can cause pinpoint pupils. At the root can be medications, substance overuse, disease, or physical trauma.

Medication Effects

New medication may be causing your miosis. Various drugs found over the counter or even some prescribed by a healthcare provider can cause your pupils to contract. Medications that can cause pinpoint pupils include the following:

• Glaucoma-related miotic eye drops like Alphagan P (brimonidine) and Vulty (pilocarpine)
• Medication for controlling high blood pressure, such as Catapres (clonidine)
• Antidepressant noradrenergic serotonergic medication such as an overdose of mirtazapine
• Antipsychotic medication such as Haldol (haloperidol) and Seroquel (quetiapine)
• Cholinergic medication such as Mestinon (pyridostigmine) used for muscle weakness in myasthenia gravis (an autoimmune condition affecting the muscles)
• Opioids such as morphine, tramadol, fentanyl

Drug Overuse

Overuse or misuse of certain substances can also cause pupils to constrict. Drugs that can cause this effect include:

• Sedatives such as benzodiazepines or barbiturates
• Gamma hydroxybutyrate (GHB), a nervous system depressant also known as the "date rape" drug
• Phencyclidine (PCP), also known as angel dust
• Tobacco

Toxins

Coming into contact with certain environmental toxins may also cause pupil constriction. For example, organophosphate pesticides can cause miosis. Contact with plants such as Jimsonweed or belladonna, as well as atropine, hyoscine, scopolamine, and hyoscyamine, can all lead to pinpoint pupils.

Health Conditions

Certain health conditions can also cause constricted pupils. Inflammation in the eye can lead to what's known as acute anterior uveitis, which is a sudden onset of symptoms at the front of the eye. Conditions that can cause this include:

• Infection with the herpes simplex virus that causes cold sores
• Having inflammatory arthritis such as ankylosing spondylitis
• Having bowel inflammation or colitis
• Having an autoimmune condition such as sarcoidosis that inflames tissues and causes lumps to form or vasculitis involving inflammation of the blood vessel wall.

Horner's Syndrome

An injury to the nerves responsible for sending the signal to widen or narrow the pupil can cause Horner's syndrome. These sympathetic nerves are located in the brain, neck, and upper chest.

These same nerves are associated with lifting the eyelid. The pupil on the same side that the injury occurred on will be affected. Also, sometimes the lid on that side will droop.

Trauma

Head or neck trauma or a bleed in the brain from a broken blood vessel, such as a brain aneurysm, can potentially lead to a pinpoint pupil. These are serious issues and should be immediately addressed.

Diagnosis

To determine if there is an issue with constricted pupils, your eye doctor, either an ophthalmologist or optometrist, will need to examine your eyes. The eye doctor will begin by taking a history to look for anything new such as medication, that may be causing the pinpoint pupil. Then they will examine your eyes in both light and dark conditions and see how each of the pupils responds.

If the situation calls for it, you may be asked to undergo neuroimaging tests such as either magnetic resonance imaging (MRI) or a computed tomography (CT) scan.

If a healthcare provider thinks the situation is not just eye related, you may be asked to undergo some general testing, such as blood work or urinalysis.

Treatment

Your healthcare provider will recommend treatment based on your diagnosis.

If you have an infection, your eye doctor will prescribe medication to alleviate this. If a medication itself is the source of the constricted pupils, then your healthcare provider may either adjust the dose or select another medication without the same side effects.

If a substance use disorder is at the root of the constricted pupils, you may be directed to a treatment program that will help you to overcome it.

If you have acute anterior uveitis, your ophthalmologist or optometrist may recommend treatment with steroids to alleviate inflammation. Also, they may prescribe eye drops to help relax your eye muscles and enlarge your pupils.

Any underlying infection will also be treated with either antibiotics or antifungal medication. In rare cases, immunosuppressants may be prescribed for those who don't respond to the usual treatment.

Prognosis

How you ultimately fare depends upon the cause of the pupil constriction. If medication is the cause, then switching it may correct the issue. But if pupil constriction is caused by trauma, there may be no way to cure it, although your healthcare provider may recommend a strategy to compensate for the pupil constriction, such as the use of dilating (widening) drops.

Summary

While the dark pupil at the center of the eye usually expands and contracts depending upon the amount of light, sometimes the pupil becomes constricted and stays that way even when the lighting changes.

The cause of this pupillary constriction can involve illness, injury, medication or substance use, or contact with toxins. Because causes can be so varied, diagnosis may affect just the eye or may involve more extensive testing. Any treatment you receive will address the underlying cause so the pupil can respond as needed.