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What causes pupils to dilate?

I know some drugs make people have dilated pupils and the eye doctor does it, but what exactly causes dilated eyes?

5224 day(s) ago

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Excessive pupil dilation is scientifically known as mydriasis. Mydriasis can be caused by trauma, disease, drug use, or induced by a doctor for eye testing. Eyes dilate naturally in dim lighting to allow more light to reach the eye. The size of the iris, the colored part of the eye, is controlled by two muscles, one radial and one circular. These muscles are controlled by the sympathetic nervous system, meaning we donít have to think about it to contract the muscle. It is controlled automatically by the stimulus of light. When the pupil dilates unnaturally, it is due to a disturbance in the sympathetic nervous system or the parasympathetic nerves in the eye itself.

Trauma to the head or the orbits of the eyes can cause the eyes to dilate at inappropriate times. Testing for eye dilation is part of a common field examination by emergency responders or doctors for head injury or brain damage. If damaged severely enough, the eye may lose all of its reaction to light.

Certain drugs cause eyes to dilate by blocking the receptors in the brain that responsible for controlling the muscles of the iris. Many hallucinogens and dissociatives, along with cocaine, and opiates block the nerve receptors in the brain causing eye dilation.

Certain conditions or diseases cause the eyes to dilate by making the body lose control of the muscles. Typical medical conditions that can cause dilation are epilepsy and stroke.

Doctors use specialized drugs called mydriatics to cause the pupils to dilate so they can see more clearly into the pupil to view the retina. Tropicamide and phenylephrine are two such drugs that are used by doctors. Those who have been subjected to these drugs will not be able to tolerate bright light for several hours.

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