Similar to a Digital Camera
The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly.
A. Light is focused primarily by the cornea — the clear front surface of the eye, which acts like a camera lens.
B. The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture).
C. The eye’s crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.
D. Light focused by the cornea and crystalline lens (and limited by the iris and pupil) then reaches the retina — the light-sensitive inner lining of the back of the eye. The retina acts like an electronic image sensor of a digital camera, converting optical images into electronic signals. The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight.
- Keep the front surface of the eye moist and lubricated.
- Keep the inner surface of the eyelids moist and lubricated so they open and close easily without friction or causing eye irritation.
- Protect the eye from dust, debris and infection-causing microorganisms.
The conjunctiva has many small blood vessels that provide nutrients to the eye and lids. It also contains special cells that secrete a component of the tear film to help prevent dry eye syndrome.
Conjunctivitis (pink eye): inflammation of the conjunctiva.
Conjunctival pallor: unhealthy pale appearance to the palpebral conjunctiva that can be a sign of anemia.
Injected conjunctiva: red eye caused by dilation of blood vessels in the conjunctiva.
Conjunctival cyst: thin-walled clear sac in the conjunctiva that contains clear fluid. It resembles a small, clear blister on your skin. A conjunctival cyst or sac can occur as a result of an eye infection, inflammation or other causes.
Conjunctival hemorrhage: bleeding from a small blood vessel on the front surface of the eye, over the sclera. Because the leaking blood spreads out under the conjunctiva, it causes the white of the eye to appear bright red. More accurately called a subconjunctival hemorrhage, this type of red eye is harmless and typically resolves on its own within a couple weeks.
Conjunctival lymphoma: tumor of the front surface of the eye that usually appears as a salmon-pink, “fleshy” patch. Conjunctival lymphomas typically are hidden behind the eyelids and painless; therefore they may be present for quite some time before they are discovered — especially in people who don’t have routine comprehensive eye exams. If you have a growth on your eye that resembles this description of a conjunctival lymphoma, immediately see an ophthalmologist who can evaluate it and perhaps perform a biopsy to determine the proper treatment.
Conjunctival hemangioma: benign (noncancerous) tumor of tiny blood vessels that creates a red, blood-filled sac in the conjunctiva. Large conjunctival hemangiomas can be surgically removed if they cause irritation.
Conjunctival nevus: common, benign growth in the bulbar conjunctiva. In fact, conjunctival nevi (plural of nevus) are the most common growth that occurs on the surface of the eye. A conjunctival nevus can range in color from yellow to dark brown and can darken or lighten with time. In most cases, no treatment is needed for a conjunctival nevus, but if a nevus is growing in size, it can be surgically removed.
Conjunctival melanoma: elevated, dark or relatively clear cancerous growth in the bulbar conjunctiva. Conjunctival melanomas are uncommon but potentially lethal. The cancer cells from a conjunctival melanoma can infiltrate the eyeball and spread via the lymphatic system or bloodstream to the lungs, liver, brain and bones.
In some cases, a conjunctival melanoma can arise from a benign conjunctival nevus. To be safe, if you notice any type of growth or dark spot on your eye, or an unusual appearance to your conjunctiva, see your eye doctor immediately for an evaluation.
The sclera, along with the intraocular pressure (IOP) of the eye, maintains the shape of the eyeball.
The tough, fibrous nature of the sclera also protects the eye from serious damage — such as laceration or rupture — from external trauma.
The sclera also provides a sturdy attachment for the extraocular muscles that control the movement of the eyes.
Scleral icterus (yellow eyes): also called icteric sclera — is a yellowing of the white of the eye. It is associated with hepatitis and other liver disease.
- There is some controversy about the accuracy of the name of this condition. Some researchers have stated that the yellowing of the eyes (jaundice) actually takes place in the conjunctiva, not the sclera itself, and that the condition should therefore be called conjunctival icterus instead. Despite this, many doctors continue to call yellow eyes or yellowing of the eyes “scleral icterus” because it’s the color of the underlying white sclera that is altered by the condition.
- Increased blood serum levels of bilirubin (an orange-yellow pigment formed in the liver) is commonly associated with scleral icterus. If you develop yellow eyes, you should have blood tests to see if you have this condition and associated liver problems.
Blue sclera: when a normally white sclera has a somewhat blue color. Blue sclera is caused by a congenitally thinner-than-normal sclera or a thinning of the sclera from disease, which allows the color of the underlying choroidal tissue to show through it.
- Congenital and hereditary diseases associated with blue sclera include osteogenesis imperfecta (brittle bone disease) and Marfan’s syndrome (a connective tissue disorder). Acquired diseases such as iron deficiency anemia also can be associated with blue sclera.
Episcleritis: inflammation of the episclera that lies atop the sclera and under the conjunctiva. Episcleritis is relatively common and tends to be benign and self-limiting. It has two forms: nodular episcleritis where the redness and inflamed tissue occurs on a discrete, elevated area overlying the sclera, and simple episcleritis, where dilated episcleral blood vessels occur without the presence of a nodule.
- The cause of most cases of episcleritis is unknown, but a significant minority (up to 36 percent) of people who get the eye condition have an associated systemic disorder — such as rheumatoid arthritis, ulcerative colitis, lupus, rosacea, gout and others. Certain eye infections also may be associated with episcleritis.
- Most episodes of episcleritis will resolve on their own within two to three weeks. Oral pain medication and refrigerated artificial tears may be recommended if discomfort is a problem.
Scleritis: inflammation of both the episclera and the underlying sclera itself. Scleritis is a more serious and typically more painful red eye than episcleritis. Up to 50 percent of cases of scleritis involve an underlying systemic disease, such as rheumatoid arthritis.
- Generally, the onset of scleritis is gradual, and most patients develop severe, piercing eye pain over several days. This pain tends to worsen with eye movements. In most cases, the inflammation begins in one area and spreads until the entire sclera is involved.
- Scleritis can cause permanent damage to the eye and vision loss. Frequent complications include inflammation of the cornea (keratitis), uveitis, cataract and glaucoma.
- Scleritis typically is treated with oral non-steroidal anti-inflammatory drugs (NSAIDs) and corticosteroids. In some cases, immunomodulatory therapy may also be prescribed. Scleritis may remain active for several months or even years before going into long-term remission.
As already mentioned, the clear cornea allows light to enter the eye for vision. But it has another very important function as well — the cornea provides approximately 65 to 75 percent of the focusing power of the eye.
The remainder of the focusing power of the eye is provided by the crystalline lens, located directly behind the pupil.
Most refractive errors— nearsightedness, farsightedness and astigmatism — are due to a less-than-optimal curvature or symmetry of the cornea. Presbyopia, on the other hand, is due to an aging change in the crystalline lens.
In addition to allowing light to enter the eye and providing most of the focusing power of the eye, individual parts of the cornea have specialized functions:
Corneal epithelium. The corneal epithelium provides an optimal surface for the tear film to spread across the surface of the eye to keep it moist and healthy and to maintain clear, stable vision.
Bowman’s layer. The dense nature of Bowman’s layer helps prevent corneal scratches from penetrating into the corneal stroma. Corneal abrasions that are limited to the outer epithelial layer generally heal without scarring; but scratches that penetrate Bowman’s layer and the corneal stroma typically leave permanent scars that can affect vision.
Corneal endothelium. The single layer of cells that forms the endothelium maintains the fluid content within the cornea. Damage to the corneal endothelium can cause swelling (edema) that can affect vision and corneal health.
Arcus senilis. As people get older, a white ring often develops in the periphery of the cornea. This is called arcus senilis (also called corneal arcus), and it’s the most common aging change in the cornea. Arcus senilis typically is separated from the limbus by an area of clear cornea. The white ring — which is composed of cholesterol and related compounds — can be barely noticeable or very prominent.
- In older individuals, corneal arcus typically isn’t related to blood cholesterol levels; but if it occurs in a person under age 40, blood tests should be performed to check for hyperlipidemia (abnormally high concentration of fats or lipids in the blood).
Corneal abrasion. A scratched cornea can be very painful and can lead to an eye infection.
Chalazion is a fibrous growth that starts on the outer sclera of the eye but can encroach upon the cornea, causing irritation, vision problems and disfigurement of the front of the eye.
Dry eyes. Though the cause of dry eyes typically begins in the tears gland and eyelids, it can lead to damage of the corneal epithelium, which causes eye discomfort and vision disturbances.
Corneal ulcer. A corneal ulcer is a serious abscess-like infection of the cornea that can lead to significant pain, scarring and vision loss.
Corneal dystrophy. A dystrophy is a weakening or degeneration of a tissue. The most common corneal dystrophy — called Fuch’s dystrophy — affects the corneal endothelium, causing corneal swelling, foggy vision, light sensitivity and other problems.
Acanthamoeba keratitis. This is a very serious and painful corneal infection that can cause significant pain and vision loss.
Fungal keratitis. This is another dangerous corneal infection that (like Acanthamoeba keratitis) tends to affect contact lens wearers more often than people who wear glasses.
Keratoconus. This is a thinning and deformation of the cornea that causes vision problems that can’t be corrected with regular eyeglasses or contact lenses. In some cases, vision problems from keratoconus can be corrected with scleral contact lenses or hybrid contacts. But in severe cases, a cornea transplant may be required.
Corneal ectasia. This is thinning and deformation of the cornea that resembles keratoconus but occurs as a rare complication of LASIK or other corneal refractive surgery.
Each component of the uvea has a specific function:
Iris: In addition to giving the eye its color, the iris acts like the diaphragm of a camera and controls the size of the pupil. One muscle within the iris constricts the pupil in bright light (full sunlight, for example), and another iris muscle dilates (enlarges) the pupil in dim lighting and in the dark.
Ciliary body: The ciliary body holds the lens of the eye in place. It is connected to the lens with a network of many tiny ligaments (called ciliary zonules or zonules of Zinn) that suspend the lens in place behind the pupil. The ciliary body also secretes the clear aqueous fluid that fills the space in the anterior segment of the eye between the cornea and the iris and lens, and it contains the muscle that controls accommodations of the eye.
Choroid: The posterior portion of the uvea — the choroid — contains many tiny blood vessels and has the vital role of nourishing the retina.
A number of things can go wrong with the uvea. Some uvea problems are genetic, while others are age-related conditions or are associated with other health problems.
Here’s a short list of conditions and diseases of the uvea:
Uveitis: inflammation of the uvea. When confined to the iris, it’s called iritis. If the inflammation affects the iris and the ciliary body, it’s called anterior uveitis or iridocyclitis. There are many potential causes of uveitis, but often the cause cannot be determined. Symptoms include a painful red eye, sensitivity to light and decreased visual acuity.
Synechia: Pronounced “si-NECK-ee-ah,” this is when parts of the iris adhere to the back surface of the cornea or the front of the lens. Synechiae (plural; pronounced “si-NECK-ee-ee”) can occur because of trauma to the eye, iritis or other causes. Synechiae are dangerous because they can lead to certain types of glaucoma
Iris coloboma: This is a congenital disorder where a portion of normal iris tissue is missing, causing a misshapen “keyhole” or “cat-eye” appearance to the pupil. In some cases, iris colobomas can cause blurred vision, decreased visual acuity, double vision and ghost images. Often, people with iris colobomas choose to wear contact lenses to improve the appearance of the eye and decrease any visual symptoms.
Uveal melanoma: This is a cancerous growth within the iris, ciliary body or choroid. If the growth occurs in posterior uvea, it’s called a choroidal melanoma; if in the iris, it’s called an iris melanoma. The tumors are called melanomas because they develop in pigment cells (melanocytes) located within the uvea. Uveal melanomas are the most common type of cancer inside the eye, and the tumors can metastasize to other parts of the body. There are treatment options, but in some cases, removal of the affected eye (enucleation) may be necessary.
Choroidal nevus: A choroidal nevus is a flat, benign pigmented freckle in the choroid. If a choroidal nevus is detected during a comprehensive eye exam, your eye doctor typically will take a photograph or other type of image of the interior of your eye to document the shape and size of the freckle and monitor it over time to insure no changes occur.
Choroideremia: This is a hereditary, progressive degeneration of the choroid that primarily affects men. Choroideremia (“ko-roy-duh-REE-me-ah”) is characterized by night blindness, decreased visual field test and (eventually) blindness.
Iris nevus: This is a freckle (localized concentration of pigment) in the iris of the eye. Like freckles on your skin, iris nevi (plural of nevus) almost always are stable and harmless. However, if you have an eye freckle that appears to be getting larger, see your eye doctor immediately. In rare cases, an iris nevus can transform into a malignant growth.
During an eye exam, your doctor will inspect your pupils and perform testing of pupil function.
- Pupil testing is performed in a dimly lit room. While you are looking at a distant object, the doctor will briefly direct the beam of a small flashlight at one of your eyes a few times. While doing this, the response of the pupil of both eyes is observed.
- The doctor will then alternately direct the light at each eye and again observe the pupil responses of both eyes. This is called Marcus Gunn pupil testing, which is sometimes called the “swinging flashlight test.”
- Pupils normally react both directly and indirectly to light stimulation. The reaction of the pupil of the eye receiving direct illumination is called the direct response; the reaction of the other pupil is called the consensual response.
- The doctor may then turn up the room lights a bit and have you focus on a hand-held object while moving that object closer to your nose. This is a test of the accommodative response of your pupils.
If your pupils appear normal and respond normally, the optometrist may record this popular acronym in your medical chart: PERRLA, which is an abbreviation for “pupils are equal, round and reactive to light and accommodation.”
A pupil is abnormal if it fails to dilate in dim lighting or fails to constrict in response to light or accommodation.
The size of the pupil varies from person to person. Some people have large pupils, and some people have small pupils. Also, pupil size changes with age — children and young adults tend to have large pupils, and seniors usually have small pupils.
Generally, normal pupil size in adults ranges from 2 to 4 millimeters (mm) in diameter in bright light to 4 to 8 mm in the dark.
In addition to being affected by light, both pupils normally constrict when you focus on a near object. This is called the accommodative pupillary response.
Together, the iris and pupil control how much light enters the eye. Using the analogy of a camera, the pupil is the aperture of the eye and the iris is the diaphragm that controls the size of the aperture.
The size of the pupil is controlled by muscles within the iris — one muscle constricts the pupil opening (makes it smaller), and another iris muscle dilates the pupil (makes it larger). This dynamic process of muscle action within the iris controls how much light enters the eye through the pupil.
In low-light conditions, the pupil dilates so more light can reach the retina to improve night vision. In bright conditions, the pupil constricts to limit how much light enters the eye (too much light can cause glare and discomfort, and it may even damage the lens and retina).
Adie’s tonic pupil. This is a pupil that has nearly no reaction to light (direct or consensual) and there is a delayed reaction to accommodation. Adie’s tonic pupil (also called Adie’s pupil, tonic pupil, or Adie’s syndrome) usually affects only one eye, with the affected pupil being larger than the pupil of the unaffected eye. The cause of Adie’s pupil usually is unknown; but it can be caused by trauma, surgery, lack of blood flow (ischemia) or infection.
Argyll Robertson pupil. This is a pupil that is not reactive to light (direct or consensual), but reaction to accommodation is normal. Argyll Robertson pupil usually affects both eyes, causing smaller-than-normal pupils that do not react to light. The condition is rare and the cause usually is unknown, but it has been associated with syphilis and with diabetic neuropathy.
Marcus Gunn pupil. Also called relative afferent pupillary defect (RAPD) or afferent pupillary defect, this is an abnormal result of the swinging-flashlight test where the patient’s pupils constrict less (therefore appearing to dilate) when the light is swung from the unaffected eye to the affected eye. The most common cause of Marcus Gunn pupil is damage in the posterior region of the optic nerve or severe retinal disease.
Trauma. Penetrating eye trauma that affects the iris is a common cause of abnormally shaped pupils. Similar trauma can occur in complications of cataract surgery. Pupillary responses to light and accommodation often remain normal or nearly normal.
Sexual arousal. Recent research has confirmed that sexual arousal elicits a pupil dilation response, and that this response may be useful in sexuality research to evaluate sexual orientation.
Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.
In the visual cortex of the brain (which, ironically, is located in the back of the brain), these signals are converted into images and visual perceptions.
There is a wide variety of retina problems, conditions and diseases. Here is a short list of the more common retina problems:
Macular degeneration. Age-related macular degeneration (AMD) is the most common serious, age-related eye disease.
Diabetic retinopathy. One of the devastating consequences of diabetes is damage to the blood vessels that supply and nourish the retina, leading to significant vision loss.
Macular edema. This is an accumulation of fluid and swelling of the macula, causing distortion and blurred central vision. Macular edema has several causes, including diabetes. In some cases, swelling of the macula can occur after cataract surgery.
Central serous retinopathy. This is when fluid builds up under the central retina, causing distorted vision. Though the cause of central serous retinopathy (CSR) often is unknown, it tends to affect men in their 30s to 50s more frequently than women, and stress appears to be a major risk factor.
Hypertensive retinopathy. Chronic high blood pressure can damage the tiny blood vessels that nourish the retina, leading to significant vision problems. Risk factors for hypertensive retinopathy are the same as those for high blood pressure, including obesity, lack of physical activity, eating too much salt, a family history of hypertension and a stressful lifestyle.
Solar retinopathy. This is damage to the macula from staring at the sun, which can cause a permanent blind spot (scotoma) in your visual acuity. The risk of solar retinopathy (also called solar maculopathy) is greatest when viewing a solar eclipse without adequate eye protection.
Detached retina. A retinal detachment — a pulling away of the retina from the underlying choroid layer of the eye that provides its nourishment — is a medical emergency. If the retina is not surgically reattached as soon as possible, permanent and worsening vision loss can occur.