Miosis means excessive constriction (shrinking) of your pupil. In miosis, the diameter of the pupil is less than 2 millimeters (mm), or just over 1/16th of an inch.

The pupil is the circular black spot at the center of your eye that allows light to enter. Your iris (the colored part of your eye) opens and closes to change the size of the pupil.

Miosis can occur in one or both eyes. When it affects only one eye, it’s also called anisocoria. Another name for miosis is pinpoint pupil. When your pupils are excessively dilated, it’s called mydriasis.

There are many causes of miosis. It can be a symptom of certain brain and nervous system conditions. It can also be induced by many types of drugs and chemical agents. Opioids (including fentanyl, morphine, heroin, and methadone) can produce miosis.

Constricted or dilated pupils can be an important clue to help your doctor diagnose your condition.

The size of your pupil is controlled by two counteracting muscles — the iris dilator and the iris sphincter. Usually miosis or pupil contraction is caused by a problem with your iris sphincter muscles or the nerves that control them.

The iris sphincter muscles are controlled by nerves that originate near the center of your brain. They’re part of the parasympathetic or involuntary nervous system. To reach your eye, these nerves pass along your third cranial nerve, also called the oculomotor nerve.

Any disease, drug, or chemical agent that affects these nerves, or the parts of the brain and head that they pass through, can cause miosis.

Diseases or conditions that can cause miosis

Diseases or conditions that can cause miosis include:

Drugs and chemicals that can cause miosis

Some of the commonly used drugs and chemicals that can cause miosis are opioids, including:

  • fentanyl
  • oxycodone (Oxycontin)
  • codeine
  • heroin
  • morphine
  • methadone

Other drugs and chemicals that can cause miosis include:

  • PCP (angel dust or phencyclidine)
  • tobacco products and other nicotine-containing substances
  • pilocarpine eye drops used to treat glaucoma
  • clonidine, which is used to treat high blood pressure, ADHD, drug withdrawal, and menopausal hot flashes
  • cholinergic drugs used to stimulate the parasympathetic nervous system, including acetylcholine, carbachol, and methacholine
  • second-generation or atypical antipsychotics, including risperidone, haloperidol, and olanzapine
  • phenothiazine-type antipsychotics used to treat schizophrenia, including prochlorperazine (Compazine, Compro), chlorpromazine (Promapar, Thorazine), and fluphenazine (Permitil, Prolixin)
  • organophosphates, found in many insecticides, herbicides, and nerve agents

Age-related miosis

Both newborns and older adults may have small pupils. It’s normal for a newborn to have small pupils for up to two weeks.

As you get older, your pupils tend to grow smaller. This is usually due to weakness of the iris dilator muscles, not to a problem with the iris constrictors.

Because miosis can be triggered by a variety of diseases and conditions, there are many possible accompanying symptoms. Here we’ll break down some of the common causes of miosis and their accompanying symptoms:

Cluster headaches. A cluster headache produces very severe pain around or above the eye, in your temple or forehead. It occurs only on one side of your head, and recurs at different intervals, depending on the type of cluster headache you have (chronic or episodic).

Miosis is one of the common accompanying symptoms. Other cluster headache symptoms can include:

  • drooping eyelid
  • eye redness
  • tearing
  • runny nose
  • sensitivity to light and sound
  • confusion
  • mood change
  • aggressiveness

Intracranial hemorrhage and brain stem stroke. Miosis in both pupils is a common symptom of an intracranial hemorrhage or a brain stem (Pontine) stroke. A hemorrhage or stroke happens when the blood supply to your upper brain stem (Pons) is cut off by a burst artery or a blockage.

A brain stem stroke does not produce the same symptoms as a typical stroke. The most common symptoms are dizziness, vertigo, and weakness on both sides of the body. It can occasionally produce jerking or shaking that looks like a seizure, slurred speech, or sudden loss of consciousness.

Horner’s syndrome. Horner’s syndrome is a collection of symptoms resulting from damage to the nerves connecting the brain to the face or eye. Decreased pupil size (miosis) and drooping eyelid on one side of the face are typical symptoms.

Horner’s is sometimes the result of a stroke, brain tumor, spinal cord injury, or shingles (herpes zoster) infection.

Iris inflammation (iridocyclitis). Decreased pupil size (miosis) can be a symptom of inflammation of your iris, the colored portion of your eye. Iris inflammation can have many causes. These include:

  • HIV
  • rheumatoid arthritis
  • psoriasis
  • tuberculosis
  • shingles (herpes zoster)

Iris inflammation can also be called iridocyclitis iritis or uveitis.

Neurosyphilis. When an untreated syphilis infection progresses to the brain, it’s called neurosyphilis. Syphilis can invade the nervous system at any stage of the infection.

The infection can affect the midbrain and cause a specific type of miosis called Argyll Robertson pupil. In Argyll Robertson, the pupils are small but don’t contract further when exposed to light. However, they contract when focusing on a near object.

Lyme disease. Lyme disease is caused by infection with a corkscrew-shaped bacterium similar to the syphilis spirochete. Except for the genital rash, untreated Lyme can produce many of the same symptoms in the nervous system as syphilis. When the infection affects the third cranial nerve, it can cause miosis and Argyll Robertson pupil.

Your doctor will examine your pupils, usually with the aid of a flashlight or other light source. They’ll look at your pupils in a dimly lit place, because it’s natural for pupils to be constricted in a brightly lit location, especially outdoors.

Miosis is defined as a pupil size of 2 mm (a little over 1/16th inch) or smaller.

Once the miosis is identified, your doctor will look for specific signs:

  • Does it affect one eye (ipsilateral) or both (bilateral)?
  • Does the pupil size change in response to light?
  • Does the pupil size change in response to a near object?
  • How long does it take for the pupil to respond?

The answer to each of these questions can help identify the possible cause of the miosis.

Miosis is a symptom of something else and not a disease in itself. It can provide an important clue to your doctor in finding the underlying cause.

If your miosis is the result of prescription drugs, such as for glaucoma or high blood pressure, your doctor may be able to find a substitute drug that will reduce or eliminate the symptom.

Miosis can be a result of use of opioid drugs, including fentanyl, oxycodone (Oxycontin), heroin, and methadone. Severe miosis could be a sign of an overdose. In that case, emergency treatment with naloxone drug could save your life.

If drug use is ruled out, miosis could be a sign of organophosphate poisoning. Organophosphates are the most widely used class of insecticides in the United States. These products are no longer for sale for home use, but they’re still used in commercial agriculture and insect control. Organophosphates are also contained in nerve agents such as Sarin.

Organophosphate poisoning produces serious symptoms including:

  • salivation
  • tearing
  • stomach disorder
  • violent muscle contractions
  • accelerated or reduced heart rate
  • shock

Miosis is a relatively minor symptom of organophosphate poisoning, but may help in diagnosis. Acute organophosphate poisoning is treated in a hospital or emergency setting. The drug pralidoxime (2-PAM) can be used to treat organophosphate poisoning.

As a symptom of disease

When miosis is a symptom of an underlying disease, the treatment addresses the underlying disease. Some of the common disease causes and their treatments include:

Cluster headaches. Acute cluster headaches are treated with oxygen inhalation, triptans, ergotamine, and topical lidocaine nose drops.

Preventive treatments include:

  • corticosteroids (prednisone)
  • lithium carbonate
  • the blood pressure medication verapamil
  • melatonin in doses of 9 milligrams per day

Injection of a mixture of methylprednisolone and lidocaine into the greater occipital nerve (back of your neck) can serve as a preventive.

Intracranial hemorrhage and brain stem stroke). Miosis can be a sign of a brain stem (Pontine) stroke. Because the symptoms are different from a classic stroke, it may be misdiagnosed. Doctors use an MRI to confirm it. Treatment involves either dissolving the blockage with drugs or insertion of a stent, or surgery to stop the bleeding and restore blood flow to the brain.

Horner’s syndrome. There’s no specific treatment for Horner’s syndrome. If your doctor can find the underlying condition, they’ll treat that. It could be due to stroke, brain tumor, spinal cord injury, or shingles — or there may be no discoverable cause.

Neurosyphilis and ocular syphilis. If the ocular symptoms occur in earlier stages (primary, secondary, or latent) of the infection, a single intramuscular injection of benzathine penicillin is recommended.

The tertiary stage of syphilis requires multiple doses of penicillin, and existing damage to the nervous system won’t be repaired.

Lyme disease. Early detection of Lyme disease is crucial for a good outcome. If caught in the first few weeks, antibiotic treatment for up to 30 days will usually cure the infection. In later stages of Lyme, long-term antibiotic therapy is needed. The causes and treatment of late stage or chronic Lyme is controversial.

Miosis or pinpoint pupil can be a symptom of many underlying disease conditions or a reaction to drugs.

The condition isn’t normally painful or dangerous in itself. But it can be a marker for some serious conditions including stroke, drug overdose, or organophosphate poisoning.

Be sure to consult a doctor if you notice the signs of miosis.