What Causes Dilated Pupils?

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In addition to being a response to low light, dilated pupils—also referred to as mydriasis—can be the result of taking some recreational drugs and medications, as well as trauma, and some serious brain conditions. You should see a doctor if your pupil or pupils dilate and don't return to their normal size.

Blue eye with dilated pupil

Radu Bighian / Getty Images

Definition

Pupils are the round, black circles located in the center of the iris (the colored part of the eye) which allow light to enter the eye. They are constantly changing size. When a pupil becomes enlarged, it is referred to as being dilated. This typically happens in conditions where there is low light, in order to let more light enter the eye.

On the other hand, pupils constrict (or get smaller) in conditions where there are higher levels of light, as a way to minimize the amount of light that enters the eye.

Along with reacting to light, pupils may also dilate following the use of some medications and recreational drugs—though the pupils typically return to their normal size once the effect of the drug wears off. These are normal reactions.

However, there are also times when one or both pupils become dilated for reasons that are abnormal and may be the sign of a serious health condition affecting the brain including:

  • Stroke
  • Internal bleeding
  • Tumor
  • Head trauma

If one or both of your pupils remains dilated—in situations where their change in size doesn't have to do with light or drug use—you should seek immediate medical attention.

Causes 

In addition to being a normal response to low light, pupils may become dilated for a number of additional reasons, including medication or drug use, an eye exam, injuries, and certain medical conditions, among others. Mydriasis (dilated pupils) can happen in both eyes, or only in one eye (in which case it is known as anisocoria).

Medications or Drug Use

Doctors are able to determine when certain medications or use of other drugs cause a person's pupils to dilate because their pupils are not responding normally to light-related stimuli—most notably, not constricting when the eye is exposed to a large amount of light.

Dilated pupils caused by medications or drug use are frequently accompanied by the following other symptoms:

  • Altered mental status
  • Dry skin
  • Fever
  • Flushing
  • Myoclonus (sudden, involuntary muscle jerks, shakes, or spasms)
  • Seizures
  • Urinary retention

In situations involving anticholinergic poisoning, symptoms might also include:

How Long Does It Take for Dilated Pupils to Return to Normal After Medication Use?

Once doctors confirm that a patient's extended period of pupil dilation was caused by medication or drug use, the patient can expect their pupils to return to normal as the impact of the drug wears off. There's no set timeframe for this to happen. The effects of various medications and drugs differ significantly, and the length of pupil dilation is no exception.

We do know, however, that the amount of time it takes a person's pupils to return back to normal is tied to the half-life of a particular medication or drug. So if a doctor is able to pinpoint the cause of the dilation, they should be able to provide you with information on the drug's half-life to give you a better idea of how long the dilation will last.

Eye Exam

During a comprehensive eye exam, the optometrist or ophthalmologist will likely put drops in a patient's eyes that cause their pupils to dilate. It takes approximately 20 to 30 minutes for the drops to take effect. When they do, it gives the eye doctor the opportunity to test for certain pupillary reactions—some of which can reveal neurological problems—as well as gain insight into the health of the internal structures of the eye, including the retina, vitreous, optic nerve, blood vessels (choroid), and macula.

It typically takes a few hours for the dilating drops to wear off, so it may be a good idea to have someone drive you to your appointment, if possible.

Injuries

Trauma to the eye or the brain can also cause dilated pupils. Typically, it is blunt closed trauma that damages the iris sphincter muscle—which is responsible for constricting the pupil—or one of the pathways in the brain that controls it. It can also cause bleeding inside the skull, which may result in dilated pupils.

Other than trauma, eye injuries can also result from intraocular surgery like cataract removal and corneal transplant, or following retinal procedures.

Medical Conditions

In addition to drugs and injuries, dilated pupils (in one or both eyes) may also be the result of a handful of medical conditions, including:

  • Aneurysm in the brain
  • Brain tumor or abscess (like pontine lesions) 
  • Excess pressure in one eye caused by glaucoma
  • Brain swelling
  • Intracranial hemorrhage
  • Acute stroke
  • Intracranial tumor
  • Increased intracranial pressure
  • Infection of membranes around the brain (meningitis or encephalitis)
  • Migraine headache
  • Seizure
  • Tumor, mass, or lymph node in the upper chest or lymph node
  • Horner syndrome
  • Diabetic oculomotor nerve palsy

Other causes

Finally, there are a few additional causes of dilated pupils. For example, recent research has indicated that increased levels of oxytocin—the "love" or "bonding" hormone—may result in dilated pupils in situations involving attraction, mood, or an emotional response to someone or something.

There is also evidence that a person's pupils can dilate in situations where they are concentrating very hard on something, including making a decision.

When to Call a Doctor

If you have persistent or unexplained changes in pupil size, then it's time to discuss it with your doctor. If any of these changes were sudden and/or recent — or have occurred following an injury to the eye or the head—then it could be the sign of a very serious condition.

If pupil dilation (in one or both eyes) is accompanied by certain symptoms, it could be the sign of an emergency, requiring you to seek medical treatment immediately. These symptoms include:

  • Blurred vision
  • Double vision
  • Eye sensitivity to light
  • Fever
  • Headache
  • Loss of vision
  • Nausea or vomiting
  • Eye pain
  • Stiff neck

A Word From Verywell

Not only do our pupils have the important function of regulating the light that enters our eyes, but they can also indicate a variety of other health issues—ranging from those that resolve on their own, to those that require immediate medical care.

Next time you take a look in the mirror, take note of the size of your pupils. You may also want to dim and raise the lights in the room to see how your pupils react. This will give you a basic idea of what your pupils usually look like and how they typically react to light. It could be useful moving forward, like if you notice your pupils aren't their usual size or reacting the way they normally would to light.

In those situations, it's best to consult your doctor about this symptom—unless, of course, it's accompanied by any of the symptoms above that could indicate a medical emergency. If that happens, seek medical attention immediately.

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Article Sources
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