The iris is the colored part of your eye. It is the muscular curtain that sits near the front between the cornea and the lens. The iris determines the size of the pupil (the small, black opening) to regulate how much light gets to the retina (the portion of the eye that initially processes visual information and delivers it to the brain).
The iris constricts when it’s bright out and opens up in lower light conditions. This is called the “pupillary light reflex.”
Disorders that affect the iris can occur due to genetic abnormalities or other conditions, such as glaucoma, Horner syndrome, Holmes-Adie syndrome, and more.
Anatomy
The iris is the part of the eye that makes up your eye color. A circular muscle with a hole in the middle, the iris contracts and expands to control the amount of light that gets into the pupil.
The iris sits in the uveal tract, which is the eye's middle layer. It is made up of the following parts:
- Iris pigment epithelium contains melanin granules and chromatophores that make up the eye color.
- Dilator and sphincter muscles that expand and contract to control the amount of light that gets in,
- Stroma is made up of connective tissue and blood vessels.
The iris sits in front of the lens within the coronal plane towards the front of the eye. Unbound in its middle to allow the pupil to change size, this structure is connected to the ciliary body—the part of the eye that produces the eye’s fluid (aqueous humor) and regulates contraction and constriction of the iris.
The iris splits the space between the cornea and lens into anterior and posterior chambers. The anterior chamber is bound by the cornea, while the posterior chamber connects with the ciliary bodies, zonules (a small anatomic band that holds the lens in place), and lens. Both chambers are filled with aqueous humor.
Anatomical Variations
The most common variation in iris anatomy is a condition called aniridia, in which the iris is incomplete or absent. Usually affecting both eyes, this congenital (occurring at birth) defect can be the result of either injury or mutations to the PAX6 gene.
Aniridia leads to a range of symptoms, including:
- Low visual acuity (sharpness of vision)
- Photophobia (sensitivity to light)
- Degeneration of the macular and optic nerves (associated with processing visual information)
- Glaucoma
- Cataracts (cloudy areas in the lens that affect vision)
- Changes in the shape of the cornea
- Involuntary eye movements (nystagmus)
This condition is associated with WAGR syndrome and Gillespie syndrome—two conditions characterized by disrupted organ function and intellectual disability.
Eye Color
Your eye color is determined by a combination of different pigments and saturation levels. Three main pigments are found in the iris:
- Melanin: A yellow-brown pigment that also determines skin tone.
- Pheomelanin: A red-orange pigment that causes red hair and is common in green or hazel eyes.
- Eumelanin: A black-brown pigment that determines how intense or dark the iris is.
Brown eyes have a higher amount of melanin, while blue eyes have very little pigment. The amount of pigment is often related to a person's genes, skin type, and hair color.
Function
Via dilation (opening) and constriction (closing), the iris plays a key role in regulating the amount of light that accesses the retina in the back of the eye.
- In low light, the iris dilates to maximize the available visual information by contracting the radial muscles
- In bright light, the iris uses the circular muscle of the eye to constrict and avoid overwhelming the visual sensory apparatus
Dilation and constriction of the pupil can also be influenced by physiological states, such as arousal and excitement.
In addition, this structure performs the “accommodation reflex,” which is the eye’s involuntary ability to switch focus from objects that are nearby versus far away. This activity is regulated by the parasympathetic nervous system and involves:
- Changing the aperture (opening) of the pupil
- Changing the shape of the lens
- Convergence: the ability of the eyes to work together when looking at nearby objects.
Along with the sphincter pupillae (structures at the borders of the iris that regulate its shape and motion), the iris can narrow the pupil to prevent blurring caused by light rays moving in different directions, such as car headlights.
Associated Conditions
Several health conditions can affect the iris, and, by extension, the visual system as a whole. The most common of these include:
- Anisocoria: Generally harmless, this occurs when pupils are different sizes, with one being either abnormally dilated or small. It can occur due to the onset of certain conditions, such as Horner’s syndrome (see below), or as a result of injury or certain surgeries.
- Albinism: A genetic condition with two main types: ocular albinism, which affects only the eyes, and oculocutaneous albinism, which reduces the amount of pigment in the eyes, hair, and skin.
- Glaucoma: Certain cases of this optic nerve-damaging condition called "angle-closure glaucoma" occur when disruptions to the movement of the aqueous humor push the iris out of position. In turn, due to increased pressure within the eye, the iris can fan forward and lead to eye pain, nausea, headaches, blurred vision, and other symptoms.
- Heterochromia: A congenital condition, often associated with other conditions, in which one eye is differently colored than the other. Beyond that difference, this condition is asymptomatic.
- Horner syndrome: A condition in which the sympathetic nerves of the face are damaged, leading to permanent constriction of the pupils. Constriction can arise from a range of conditions, including tumors, stroke, and injury; in rare cases, Horner’s syndrome is present at birth.
- Essential iris atrophy: A rare, progressive disorder, essential iris atrophy is characterized by the iris being out of place, under-developed, or perforated. This is usually a unilateral condition, meaning it affects only one eye.
- Holmes-Adie syndrome (Adie’s pupil): The hallmark of Holmes-Adie syndrome (also known as Adie’s pupil), is that one eye will have a pupil that is larger and less able to adjust to changes in light. This condition is believed to be an inflammatory response to a viral infection of the ciliary ganglion, the part of the brain that regulates eye movement.
- Iridoplegia: This condition occurs due to paralysis of the sphincter pupillae of the iris, which usually arises due to physical impact on the orbit but may also occur due to inflammation. There are three types: accommodative (an inability to constrict during accommodation); complete (the iris is unable to constrict at all); and reflex (the iris will not constrict due to light level, but can help with focusing).
- Iris coloboma:A congenital condition, colobomas are absences in pieces of the iris, which appear as gaps in the iris or an abnormally shaped pupil. These may appear in one or both eyes and, depending on where it’s located, may affect vision. In many cases, this condition leads to a “keyhole” appearance to the pupil.
- Traumatic mydriasis: The result of blunt trauma to the eye, traumatic mydriasis are tears in the fabric of the iris that can also lead to unusually shaped pupils.
- Uveitis: Inflammation inside the eye that causes pain and redness. If left untreated it can cause vision loss.
Testing
Checking the health of the iris as well as proper pupillary reflexes is an important part of vision care. Testing is needed to diagnose conditions and allows your healthcare provider to know if this part of the eye is functioning normally.
Eye specialists (ophthalmologists) and optometrists have several tests they can use, including:
- Pupillary observation: Your provider observes the iris and pupil as a whole, noting any differences in size or reactions to light. This is done by shining a light into the eye in a room that has low, ambient light.
- Light reflex test: To test how well irises respond to lighting conditions, your provider will ask you to focus on an object further away while shining a light into each eye individually. In doing so, the response by the iris is measured, with equal responses from each being considered healthy.
- Swinging flashlight test: This test assesses whether both irises are able to constrict properly and work together, with differences in response being documented as a condition called relative afferent pupillary defect or RAPD. This test is done by dimming ambient light and shining light into each eye individually and noting how well each eye constricts. This test can also assess if there is vision loss due to damage to the retina or cataracts.
- Near reflex test: This test checks the iris’ ability for accommodation: the ability to shift focus from objects far away to those close by. In a normally lit room, your provider will ask you to focus on an object far away, while bringing another object to a nearer point. This allows them to check the iris’s reaction to the shift in focus. A healthy eye will be able to shift seamlessly from focusing on further and nearer objects.
If your provider needs to examine a specific condition, they may order some imaging tests, such as:
- Magnetic resonance imaging (MRI)
- Computed tomography scan (CT scan)
- Ultrasound
- Optical coherence tomography (OCT)
Summary
The iris is the colored, muscular curtain of the eye. Located between the cornea and lens, the iris regulates how much light gets into the eye by dilating and constricting. Disorders that affect the iris can occur due to genetic abnormalities or other conditions. There are several tests your healthcare provider can use to assess eye health and diagnose eye conditions.
16 Sources
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By Mark Gurarie
Gurarie is a freelance writer and editor. He is a writing composition adjunct lecturer at George Washington University.
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