The Anatomy of the Iris

A pigmented curtain regulating the amount of light entering the eye

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The part of the eye that determines its color, the iris is the muscular curtain that sits near the front between the cornea on the outside and the lens. Primarily, by determining the size of the eye’s “window,” or pupil, this structure serves to regulate the amount of light that gets to the retina (the portion of the eye that initially processes visual information and delivers it to the brain). As such, it displays what is called a “pupillary light reflex,” wherein it constricts when it’s bright out while opening up in lower light conditions.

A number disorders can affect the iris; these can occur due to genetic abnormalities or other diseases. Notable among these are anisocoria (in which pupils are different sizes), impaired pupillary light reflex (where eyes are unable to adjust to light), as well as a range of other conditions, such as glaucoma, Horner syndrome, Holmes-Adie syndrome, as well as a number of others.

Anatomy

The iris is a circular, colored structure that 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. It splits the space between the cornea and lens into anterior and posterior chambers. The former of these is bound by the cornea, while the latter 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 seen in iris anatomy is a condition called aniridia, in which the iris is incomplete or absent. Usually affecting both eyes at once, this congenital defect can be the result of either injury or mutations to the PAX6 gene. This then leads to a range of symptoms, including low visual acuity, degeneration of the macular and optic nerves (associated with processing visual information), cataracts (cloudy areas in the lens that affect vision), and changes in the shape of the cornea. This condition is associated with two disorders characterized by disrupted organ function and intellectual disability: WAGR syndrome and Gillespie syndrome.

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. When there’s low light, it will dilate to maximize the available visual information, and when it’s very bright out, it constricts to prevent overwhelming the visual sensory apparatus. The former is performed by contraction of the radial muscles, whereas the latter activity involves the circular muscle. This activity is regulated by the cortex, and 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, which entails changing the aperture (opening) of the pupil, the shape of the lens, and convergence (the ability of the eyes to work together when looking at nearby objects), is regulated by the parasympathetic nervous system. Alongside the sphincter pupillae—structures at the borders of the iris that regulate its shape and motion—this part of the eye can narrow the pupil to prevent blurring due to divergent light rays accessing the eye.

Associated Conditions

A number of disorders, diseases, and other medical conditions can affect the iris, and, by extension, the visual system as a whole. The most common of these include:   

  • Anisocoria: Generally harmless, this is when pupils are different sizes, with one being either abnormally dilated or small. It can occur due to the onset of certain diseases, such as Horner’s syndrome (see below), or as a result of injury or certain surgeries.
  • 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: This disease, in which the sympathetic nerves of the face are damaged, leads to permanent constriction of the pupils. It can arise from a range of conditions, including tumors, stroke, injury, or other diseases; 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 inflammation response to 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 can also happen due to inflammation. There are three types: accommodative, meaning an inability to constrict during accommodation; complete, where the iris is unable to constrict at all; and reflex, where it will not constrict due to light level, but can help with focusing. 
  • Iris coloboma: Congenital and arising at birth, 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, can sometimes 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 rips in the fabric of the iris that can also lead to unusually-shaped pupils.

Tests

Checking for the health of the iris as well as proper pupillary reflexes is an important part of care; not only are these needed to diagnose conditions, they also allow doctors to know if this part of the eye is functioning normally. Luckily, eye specialists (ophthalmologists) and optometrists have a number of tests they can use, including:

  • Pupillary observation: The doctor will need to observe 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, doctors will ask patients 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 flagged as problematic (a condition called relative afferent pupillary defect or RAPD). This is done by dimming ambient light and shining light into each eye individually and noting how well each is able to constrict. 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, the doctor will ask the patient to focus on an object far away, while bringing another object to a nearer point. This allows the doctor to check the iris’s reaction to the shift in focus. Healthy patients will be able to shift seamlessly from focusing on further and nearer objects.
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