The iris is the colored part of the eye surrounding the pupil. It automatically regulates the amount of light entering the eyes by regulating the size of the pupil.
Light is the primary stimulus our eyes process to make sense of the world. Too little or too much light can affect our ability to see.
The iris and pupil play a crucial role in vision. They control the amount of light entering the eye, which helps us see no matter the amount of light in the environment.
This article discusses the iris, its anatomy, and its functions. We also explore various iris conditions and when to speak to a doctor.
The iris is a thin, round, highly pigmented circular membrane between the cornea and the lens. It divides the eye’s anterior chamber into an anterior and posterior chamber.
It has a center opening, called the pupil. The muscles in the iris narrow (constrict) and widen (dilate) the pupil to regulate the amount of light reaching the retina.
Learn more about the different sizes of pupils here.
Genetics determine a person’s eye color. The color of the eye depends on the pattern and amount of melanin, a dark brown pigment the iris contains.
Dark brown eyes contain the most melanin. Green, hazel, and light brown eyes contain slightly less. Lighter eye colors, such as blue and gray, contain only a little.
The front of the eye contains several structures:
- Sclera: The white portion of the eye.
- Cornea: A clear dome that transmits and focuses light into the eye.
- Anterior chamber: A fluid-filled space that contains aqueous humor. This clear fluid keeps the eyes inflated and nourishes them.
- Iris: This has two layers that both contain pigment. The sphincter muscles constrict or dilate the pupil, regulating the light coming in.
- Lens: A clear part of the eye found behind the iris. The ciliary muscle changes its shape to focus light on the retina.
The retina absorbs and converts the light it receives into electrical impulses. Optic nerves transmit these signals to the brain, where the visual cortex interprets this information to give us sight.
The primary function of the iris is to control the pupil’s diameter as a response to the intensity of light coming into the eye.
The dilator muscles of the iris make the pupil larger to let in more light in low-light settings. The sphincter muscles found at the margin of the pupil shrink the pupil’s size to let in less light.
This process is called pupillary light reflex, an involuntary action controlled by the brain.
The iris also shrinks the pupil whenever the eye focuses on a nearby object as part of the accommodation reflex.
Several conditions affect the iris of the eye.
The uvea is the eye’s middle layer between the sclera and the retina. It consists of the iris, ciliary body, and choroid.
Iritis, also called anterior uveitis, refers to the inflammation of the iris and the ciliary body. It is the most common type of uveitis.
Symptoms of iritis include:
- blurry vision
- eye pain
- eye pain upon exposure to bright light (photophobia)
- red eye
Blunt trauma causes about
It can also be related to infectious causes such as herpes simplex, toxoplasmosis, syphilis, varicella-zoster virus, and tuberculosis.
Doctors typically prescribe steroids as drops, pills, or injections to reduce eye inflammation. Prescription eye drops that dilate the pupil can help relax the iris to improve comfort.
Heterochromia refers to the presence of different eye colors in a person. The difference can be within one eye (partial) or between the eyes (complete).
It can occur at birth (congenital) or later in life (acquired). It is most often genetic. Factors that may cause acquired heterochromia include:
- trauma or injury to the eye
- taking certain medications for glaucoma
- certain eye drops
- Fuchs heterochromic iridocyclitis
Rarely, it can also be related to conditions such as Waardenburg syndrome, Horner’s syndrome, Hirschsprung disease, tuberous sclerosis, and Sturge-Weber syndrome.
There is no need to treat the condition unless another condition causes it. In this case, the focus of treatment will be the underlying cause or condition.
Waardenburg syndrome is a group of rare, inherited genetic conditions that cause hearing loss and changes in the pigmentation of the hair, eyes, and skin.
It has six subtypes. However, types I and II are commonly reported worldwide.
Common features include:
- hearing loss
- pale blue eyes or eye colors that do not match (heterochromia)
- pale skin
- white streak or patch of hair
There is no single treatment for the condition. Treatment addresses specific presentations in each individual.
In children with Waardenburg syndrome, the goal is to improve hearing difficulties to promote mental development.
Individuals also need sun protection since they are susceptible to sun damage. Genetic counseling is also critical since the condition tends to run in families.
The following symptoms should prompt a person to seek medical attention immediately:
- eye pain
- headaches with blurred vision
- trauma or injury to the eye
- red-eye in the absence of trauma
- light sensitivity
- vision loss
Receiving an early diagnosis and prompt treatment may prevent severe complications such as total vision loss.
The iris is more than just a person’s eye color. It plays a vital role in vision, allowing us to see in various lighting environments and focusing on objects better.
Conditions affecting the iris may be due to a range of factors, including heredity, viral infections, and injuries and trauma. Sometimes, they develop with no apparent cause.
A person who suspects having any condition stated above or is bothered with something concerning their irises may want to consider talking with an eye specialist.