Iris of the Eye

8 sources cited
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Iris Anatomy & Functions

The iris is the colored part of the human eye and a component of the uvea. Also known as the uveal layer or uvea coat, the uvea is a pigmented layer found between the retina and the sclera (white of the eye).

In addition to the iris, the uvea also consists of the choroid and ciliary body

The choroid is a vascular layer found between the retina and the sclera. It supplies blood and oxygen to other parts of the retina.1

Fovea eye anatomy
Source: commons.wikimedia.org

The ciliary body is located behind the iris. It consists of muscles that shape the lens when the eye focuses. 

It also makes the aqueous humor, a clear fluid in the space between your cornea and iris. 

Within the iris is an opening called the pupil. This is a passage for light as it enters the eye. 

The iris controls pupil size and the amount of light getting into the eye.

Eye Color

Everyone, regardless of their eye color, has the same type of melanin pigment. Melanin is a natural skin color produced by cells known as melanocytes. Melanocytes are found in hair, skin, and the iris of your eyes.

Everyone has the same number of melanocytes.2 However, they produce different amounts of melanin. This is what causes different eye colors.

The color of your iris depends on the amount of melanin pigment in your iris. But how does this work?

Sufficient amounts of melanin will make your iris brown. According to scientists, everyone used to have brown eyes.3 However, an unusual mutation occurred about 10,000 years ago that turned off the pigmentation on the outer part of the iris. 

The reduced amount of melanin in the iris allows light to pass through and gets scattered by collagen fibers deeper in the iris. When the light is reflected back, it gives the eyes a bluer color. 

Depending on the extent of pigmentation, the color can range from light blue to dark brown. Common eye colors include blue, green, hazel, or brown. Nowadays, iris color is an inherited trait.

Pupil Size

The pupils are generally equal in size. A normal adult pupil ranges from 2 to 4 millimeters in diameter in bright light. In the darkness, the diameter may range from 4 to 8 millimeters. 

The pupil size may change due to other reasons such as:

  • Your emotions (e.g., happiness or sadness)
  • Underlying conditions (e.g., headache or double vision)
  • Drug abuse (e.g., cocaine, heroin, and LSD)

The change in pupil diameter is made possible by the iris. The iris has two types of smooth muscles: sphincter pupillae and dilator pupillae.

The sphincter pupillae make the pupil contract, whereas the dilator pupillae enlarges the pupil.4

This process is also made possible by the sympathetic and parasympathetic nervous systems. These are nerves that send signals to the muscles to either contract or expand.5 

Accommodation Reflex

The iris regulates the light entering the eye by changing the size of the pupil based on the situation.

For example, when it’s dark, the iris muscles expand the pupils to allow more light into the eye. Similarly, it contracts to reduce the amount of light entering the eye when in bright light.

The iris muscles enable the eye to shift focus based on near or far objects. The ciliary muscle changes the shape of the eye lens to help you see clearly.

Common Iris and Pupil Disorders

Below are common eye conditions that affect the iris and pupil:

Narrow-Angle Glaucoma

This condition causes the iris to bulge forward, blocking or narrowing the cornea-iris angle. It affects the movement of eye fluid, resulting in increased eye pressure.

Narrow-angle glaucoma is considered a medical emergency if the angle becomes completely blocked. This is called an acute angle-closure attack.

Iris Melanoma

This is cancer of the iris. The condition usually has no symptoms, but the tumor may be noticeable during a routine eye exam.

See your doctor if you notice pigmented spots in your iris or an abnormally enlarged or misshapen pupil.

Iritis

Iritis is the inflammation of the iris. The infection usually develops in one eye, although both eyes can be affected.

Symptoms of iritis include red, painful eyes, headache, blurred vision, and sensitivity to light. Cataracts, glaucoma, and vision loss are common complications.

Anisocoria

This is a harmless condition where the pupils are different sizes.

Anisocoria can be genetic. Different family members may have differently sized pupils.6

However, it may also result from underlying conditions such as Horner’s syndrome and other diseases of the brain, nerves, and blood vessels.

Heterochromia

Heterochromia is when a person has two different colored eyes. It can be either complete or partial heterochromia. 

Complete heterochromia is when one iris is different from the other.

Partial heterochromia is when a part of one iris is a different color than the other.

Horner Syndrome

Horner syndrome occurs due to damaged sympathetic nerves of the face and eyes.7 These are the nerves that enable the pupil to enlarge. When they’re damaged, permanent pupil contraction occurs.

Horner Syndrome can arise from injury, tumors, and other diseases. Symptoms may include decreased pupil size, lack of sweating on the affected side of the face, and a drooping eyelid.

Essential Iris Atrophy

Essential iris atrophy is damage to the iris due to displacement, degeneration, or formation of large holes. 

The underlying cause of this condition is not fully understood. Researchers believe that chronic inflammation and infection (such as from herpes virus) may contribute to the development of this disease. 

The condition usually affects one eye and progresses over time.

Iris Coloboma

Iris coloboma is a hole in the iris that may appear as an irregularly shaped iris. The condition can result from eye trauma or after eye surgery. It can also be inherited. 

Treatment for Iris and Pupil Problems

Routine eye exams are essential. They help detect eye issues before they become worse. 

Your doctor can also provide medical advice on eye care. This helps prevent avoidable eye problems.

Treatment options for iris disorders include:

  • Artificial Iris. When you have a dysfunctional iris, you can get an implant. For example, CustomFlex Artificial Iris was approved in 2018 for treating congenital aniridia.8
  • Steroid eye drops. These are often used to reduce inflammation.
  • Dilating eye drops. These are prescribed to dilate your pupil and reduce eye pain, such as in the case of iritis.
  • Antibiotic or antiviral eye drops. These are prescribed to prevent infection, especially after eye trauma.
  • Contact lenses. Your doctor may prescribe contact lenses if your iris problem causes poor vision. There are also prosthetic colored contact lenses to hide iris defects and improve visual symptoms like glare.
  • Laser surgery. Sometimes, your eye doctor may suggest surgery to prevent damage caused by some conditions. Laser Peripheral Iridotomy (LPI) is used to treat narrow-angle glaucoma.

Other Parts of the Eye

  • The cornea. The clear bulging surface of the eye.
  • Retina. This is the light sensitive lining at the back of the eye.
  • Sclera. The white part of the eye.
  • Eyelids. The protective skin that closes over the eye.
  • The lens of the eye. Clear part of the eye that helps you focus.
  • Pupil. An opening where light rays pass into the eye.
  • Vitreous Humor. The fluid filled the space between the lens and the retina.
  • Macula. Part of the retina focused on central vision.
8 Cited Research Articles
  1. Choroid,” National Cancer Institute
  2. What Controls Variation in Human Skin Color?,” National Center for Biotechnology Information (NCBI), 13 Oct. 2003
  3. Is eye color determined by genetics?,” National Library of Medicine, 12 May 2021
  4. Bloom J. Motlagh M. et al.,“Anatomy, Head and Neck, Eye Iris Sphincter Muscle,”  National Center for Biotechnology Information (NCBI),25 Jul. 2021
  5. McDougal D. and Gamlin P. “Autonomic control of the eye,” National Center for Biotechnology Information (NCBI), Jan. 2015
  6. Anisocoria,” National Library of Medicine, 30 Nov. 2021
  7. Horner syndrome,”  Mayo Foundation for Medical Education and Research (MFMER), 02 Apr. 2020
  8. CustomFlex™ Artificial Iris: Professional Use Information,” The United States Food and Drug Administration (FDA)
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