The pupil is the round black center of the eye. It is the opening that lets light into the eyeball. The colored iris surrounds the pupil. According to Ted M. Montgomery, O.D., the eye's pupil widens when the dilator muscles of the iris contract. This normally occurs when the eye detects low light levels, during sexual arousal or when looking at something far away. Drugs, trauma or disease can also cause pupil dilation, also known as mydriasis.
Drugs
The Pediatric Glaucoma & Cataract Family Association says that the dilator muscles open the pupil and the sphincter muscles close down the pupil. Two types of drugs dilate the pupil: one that stimulates the dilating muscles, and one that paralyzes the sphincter muscles. Drugs commonly used by ophthalmologists to stimulate the dilator muscles include Tropicamide, Cyclopentolate, Homatropine and Atropine. Phenylephrine is a drug that paralyzes the sphincter muscle. Ophthalmologists frequently mix these drugs. According to the website WD, some non-ophthalmic drugs that dilate the pupil include antihistamines, amphetamines, muscle relaxants, tricyclic antidepressants, caffeine, cocaine and psychedelics.
Plants
Accidental contact or ingestion of certain plants can cause pupil dilation. The WD website notes that Angel's Trumpet is a garden plant that contains alkaloids that act as an anticholinergic substance. Anticholinergic substances interfere with nerve impulses, essentially paralyzing the muscles. The belladonna plant contains similar alkaloids that dilate the pupils.
Trauma
Several types of trauma cause pupil dilation. The website informaworld says that a sphincter tear, a foreign body in the eye and head trauma can cause pupil dilation. Since the sphincter muscles encircle the pupil, a tear in any muscle area disrupts the entire dilation process in that eye. A foreign body in the eye can cause neuropathy, or numbness in the muscle's nerves, that prevents the pupil from constricting. Any type of head trauma can affect the nerves that control the constricting or dilating of the pupil.
Diseases
Many diseases--some of which have no apparent direct connection to the eye--cause pupil dilation. According to WD, Adie's syndrome causes sudden loss of pupil constriction in one eye. Damage to the nerves of the dilator muscles causes this problem. Aortic arch syndrome also causes pupil dilation late in its development. Botulism causes pupil dilation because it is neurotoxic and paralyzes the muscles that control the pupil's movement. Carotid artery aneurysm causes pupil dilation because the aneurysm puts pressure on the nerves that control pupil dilation. Acute angle-closure glaucoma causes pupil dilation by blocking the drainage of fluids from the eye. Pressure builds up, disrupting the nerve impulses to the dilator muscles. Oculomotor nerve damage prevents impulses from reaching the sphincter muscles, which prevents the pupil from narrowing.
