Which muscle dilates the eye

The dilator muscle is arranged in a radial pattern. Its contraction pulls the iris outward, bunching it up like a curtain drawn open. The iris sphincter is arranged in a circular pattern, similar to a purse string.

Its constriction pulls the iris inward and pulls it flat, like a curtain drawn closed. These iris muscles are under the control of the autonomic nervous system, which deals with involuntary reflex actions.

Sympathetic output, which is associated with arousal, stimulates the dilator muscle to constrict, opening our pupils during a "fight or flight" situation. Parasympathetic output, more closely associated with calming mechanisms, stimulates the iris sphincter to constrict, shrinking our pupils.

Dilating drops work by blocking parasympathetic receptors in the iris sphincter, allowing the iris dilator to act unopposed and enlarge the pupil. The pupil dilates mostly because it can't constrict.

This parasympathetic input also controls accommodation, or changing focus of the eye for near objects. Dilating drops block that input too, making near objects appear blurry when the drops are active.

These drops are anticholinergic agents, blocking the effects of acetylcholine, the neurotransmitter released by parasympathetic nerve cells. Modern dilating drops are just synthetic cousins of their ancestor atropine, an extract of Atropa belladonna also known as deadly nightshade. Atropine is a notorious poison, responsible for the famous quartet of signs that indicate someone has ingested the toxin: "red as a beet, dry as a bone, blind as a bat and mad as a hatter.

One would only have to rub an eye after preparing this extract to discover its pupil-dilating effects. Apparently this was exploited, particularly in Italy, by women who wanted to adopt a doe-eyed appearance by creating large pupils. The sight of one's beloved with pupils enlarged had the desired effect of communicating arousal. After all, the sympathetic nervous system is more than just fighting or fleeing. A large pupil created by dilating drops allows the doctor to examine the inside of the eye parts of the eye behind the iris.

This is necessary to diagnose many different eye problems. This is a normal pupillary response to light but does not allow for an examination of the inside of the eye. Also, relaxing the focusing muscles of the eye with the drops is the best way for the doctor to check whether your child needs glasses refractive error ; glasses for children.

In some cases, dilating eye drops are necessary to treat eye problems, such as amblyopia, inflammation, and progressive nearsightedness progressive high myopia. Dilating eye drops used for examination of the eyes usually last anywhere from 4 to 24 hours. This depends on the strength and type of the drop, and on the individual patient. Dilation can last longer in people with lighter colored eyes, and in some cases, may last longer than 24 hours.

Children need stronger drops than adults to accurately measure their need for glasses refractive errors in children. Weaker drops are used for premature babies or patients with a seizure history. Dilating eye drops used to treat certain eye diseases usually last longer than those used for the eye exam, often lasting up to 2 weeks.

Even though they last longer, your doctor may still ask you to use the drop daily or twice a week for effective treatment. Light sensitivity and blurry vision at near are common side effects.

Both these gradually go away within 4 to 6 hours. Sunglasses can help decrease sensitivity to light after a dilated eye exam. Children can go back to school, but teachers should expect them to have blurry vision while reading.

Allergic reactions are rare but can include eyelid swelling and red eyes. Systemic side effects from Atropine a stronger and longer acting drop include fever, dry mouth, flushing of the face, and a fast heartbeat. Rarely, Atropine can cause new eye crossing esotropia , or make an existing esotropia worse.

When treating progressive nearsightedness progressive high myopia with Atropine, a very weak concentration is used. This reduces potential side effects. Like most eye drops, there may be some stinging immediately after the eye drop is used. This usually lasts only a few moments. A numbing eye drop can be used before use of the dilating drop to decrease stinging but may cause stinging of its own.

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