The eye is a marvel of engineering. Each eye is a compact but exquisitely sensitive biological camera that focuses images of the external world onto the retina, a ten-layer cellular membrane directly connected to the brain through the optic nerve. Before arriving at the retina (located at the back of the eyeball), light must pass through
- the cornea, a clear, highly sensitive structure that forms a domelike covering over the iris, whose pigment gives the eye its color;
- the pupil, the small black circular aperture (opening) in the center of the iris. The pupil dilates or constricts in response to the amount of light striking the eye;
- the lens, placed directly behind the iris, which focuses light on the retina;
- the vitreous humor, clear gelatinous material that fills the eyeball.
Vision disturbances can arise from abnormalities in the shape, function, or clarity of these structures.
Other important structures of the eyes are the sclera, the white surface of the eyeball; the conjunctiva, a membrane that covers the sclera and the inner surfaces of the eyelids; the lacrimal gland, which produces tears; and the nasolacrimal duct, through which tears travel from the inner corner of the eye into the nose.
Even though a baby’s eyes are not fully developed at birth, the intricate process of collecting visual information begins as soon as a newborn can open her eyes. The retina is not fully functional during the first few months, and the brain needs time to assemble visual images into meaningful patterns. Nevertheless, during the first few weeks of life, a baby will gaze intently at objects 8 to 15 inches from her face. She prefers to study plain, high-contrast, black-and-white images such as stripes, checks, or spirals, or a simple drawing of a face. But a newborn’s favorite subject to scrutinize will be a person’s face about a foot away frosts hers. She will not respond directly to a smile for the first few weeks.
The iris in a light-skinned newborn contains little pigment and is usually blue gray or blue brown. The final color of a baby’s eyes won’t be known for at least six months or longer, during which time pigment gradually forms. The iris is brown from birth in most dark-skinned infants. The lacrimal gland does not become fully functional for about four weeks, so the crying new-born makes few tears. The sclera is relatively thin at birth and may have a slightly bluish tinge because of the dark color of the tissue lying just beneath it. A rise in bilirubin level shortly after birth may cause the sclera to appear yellow.
By two or three months of age a baby will be able to coordinate her eye movements to stay locked on an interesting visual target that passes through a semicircle in front of her. She will also be interested in more complex shapes and patterns and will be able to hold her head steady enough to fixate on simple, high-contrast objects hung over her crib. By three months of age her depth of vision will have increased so that she will recognize you halfway across a room. Responses to color also develop over the first several weeks. At first, a newborn will pay attention to objects with bright, strongly contrasting colors. It will take a few months before the color vision has matured enough to distinguish a full palette of colors and shades.
All babies cross their eyes briefly as they develop their tracking skills. However, an infant who does not follow a face by the age of three months or whose eyes frequently wander or cross after five or six months may have a visual problem that should be addressed by the pediatrician or family doctor. The primary-care doctor will examine the eyes and, if appropriate, refer the patient to an ophthalmologist for evaluation. (Ideally this would be a pediatric ophthalmologist – a physician who specializes in children’s eye problems – although many ophthalmologists deal with patients of all ages.)By six months of age an infant is normally able to focus on people or objects several feet away and follow movement in all directions. Visual acuity, or ability to see objects, will improve from approximately 20/400 at birth to about 20/80. (The numbers used to express visual acuity arc based on the ability to identify letters at certain distance. A visual acuity of 20/80 means that the smallest letters a person can identify 20 feet away could be read by someone with normal (20/20) vision at a distance of 80 feet. Obviously infants cannot identify letters, but their visual acuity measured by other methods is still expressed in the 20/ format.) A six-month-old will also show interest in more complex patterns and subtle shades of color.
By seven or eight months, a baby’s visual capabilities will mature to the point that she can focus en people and objects across the room, though not quite with the clarity with which she sees whatever is directly in front of her. The average one-year-old can see well enough to see small objects across the room or planes flying over land, and by the age of two a child’s visual acuity can approach 20/30 or 20/20, although this can be difficult to establish objectively. A child age two or older that squints a lot or brings objects close to her face should have a formal eye exam, as should a stint, who is old enough to complain specifically of blurt vision or difficulty seeing the blackboard at school. Routine infant and childhood exams include a full check of the eyes: the clarity of each cornea, appearance of the eyelids, and responses of the pupils. Children young as three can be screened for visual acuity with an appropriate eye chart or other testing methods the child can comprehend.
Common Eye Problems during Infancy
Chronic mucous drainage (nasolacrimal duct obstruction) is caused by a blockage of a baby’s now lacrimal duct, which normally drains tears into the nose after they have entered small openings in the corner of each eye. Six to 10 percent of infuses are born with an obstruction of this duct. Within days or weeks after birth, tears will accumulate and overflow from the affected eye. The small lacrimal located just above the obstructed duct may become infected, causing a constant backflow of discolored mucus into the eye. This usually bothers parents more than the baby, but more severe signs of infection such as inflammation and swelling may develop in the awe-corner of the eye.
If the baby’s primary-care physician diagnosis has a problem, he or she might recommend warm soup, a massage of the tear duct up to six times per day (done by pressing the tip of a finger against the nose just above the lacrimal sac) (in the area where the upper and lesser eyelids come together) and pinning downward toward the nasolacrimal duct. Antibiotic drops or ointment may be prescribed if an infection is present.
More than 70 percent of obstructed nasolacrimal ducts will open before the child is three months old, 90 percent will open by nine months. If the problem does not resolve spontaneously, a probing, which involves placing a thin wire probe through the obstacle, can be done in the office (or hospital) between three and nine months.
Strabismus is a misalignment of the eye. An eye may squint inward (esotropia), outward (exotropia), up (pertropia), or down (hypotropia). Strabismus occurs in 4 to 5 percent of infants. Since eye movements are notably coordinated until the child is three to six month old, temporary deviations in any direction before the age of three months usually do not require specific treatment. However, persistent deviation after the age of six months should be checked by the baby’s physician, and usually the physician will refer the patient to an ophthalmologist. Appropriate treatment of strabismus is important because misalignment of the eyes causes ongoing troubled vision, causing the infant’s brain to suppress in – formation arriving from one eye or the other. This results in an impairment of vision called amblyopia, which develops in 2 to 4 percent of individuals. Depending on the extent of the strabismus, correction may require the use of special glasses or even muscle surgery on one or both eyes. The operation, usually done on an outpatient basis, modifies the attachment positioning one or more of the muscles that control eye movements.
Common Eye Problems during Childhood
So far the most common vision disorders in children are refractive errors, in which the retina does not receive a perfectly focused image. These affect 20 percent of children by the age of 16. Since genetic factors play a role in these disturbances, other family members may have similar problems. There are several types of refractive errors:
- Myopia (nearsightedness) – distant objects are out of focus. This condition usually develops as the eyeball grows and becomes longer than is ideal for focusing images on the retina. Normally myopia is not seen until a child is five to ten years of age, and it may become more pronounced during adolescence and early adulthood. A near-sighted child may squint because this helps bring distant objects into focus.
- Hyperopia (farsightedness) – distant objects are more easily brought into focus than those that are close. This condition is present in 80 percent of newborns, a consequence of the smaller size of a baby’s eye. Normal alterations in the shape of the newborn’s lens and cornea compensate for hyperopia early in infancy. Farsightedness may actually become more pronounced during the first few years of life, but a child’s eye has a greater capacity than an adult’s to adjust the shape of the lens (a process called accommodation), which effectively brings a close object into focus. However, hours of this effort can cause eyestrain, headaches, and crossed eyes.
- Astigmatism is an abnormality in which the cornea is more egg-shaped than round, producing blurry or distorted vision. Squinting may improve a child’s focus when astigmatism is present, but a severe distortion can cause eyestrain and headaches.
Refractive errors can be corrected with glasses or contact lenses. Mild hyperopia is usually not treated. Children who are highly active should use shatterproof lenses. Youngsters involved in sports can wear an elastic strap that attaches to the glasses and extends around the back of the head to keep them in place and prevent loss or damage.
Eye trauma, ranging from a scratch on the cornea to a severe injury that causes permanent visual impairment or loss, is not uncommon in childhood. According to the U.S. Public Health Service, more than 100,000 eye in-juries occur every year in the United States, of which 90percent are preventable. (Injuries due to darts, BB guns, and firecrackers are all too common.
A corneal abrasion is a scratch in the cornea’s outer cell layers, often caused by a seemingly minimal incident such as being brushed by a leaf or twig. Since the cornea is very sensitive, the pain of an abrasion is usually intense. After the eye is examined by a doctor or ophthalmologist, an antibiotic is usually placed on the eye and a patch applied to pre-vent the eyelid from opening and closing over the in-jury. The antibiotic helps prevent secondary infection that could lead to more serious damage of the cornea. The surface of the cornea normally heals completely within 24 to 48 hours.
Sometimes trauma will cause internal bleeding of the eye, resulting in extreme pain and clouded vision. When blood appears in the clear space behind the cornea (a condition called a hyphenate child should be taken immediately to an ophthalmologist or emergency room. To prevent further bleeding, the child must be kept at strict bed rest for up to five days. Enforcement of this low level of activity is critical. If the blood is not absorbed, an operation might be needed to remove it. A hyphema can cause scarring of the eye, which can in turn produce glaucoma (increased pressure within the eye). Further eye exams may be conducted to ensure that no additional damage has occurred.
If the eyeball has been penetrated or cut open, the child should be taken directly to the emergency room for immediate evaluation. If fluid is oozing front the eye, the child should be transferred while lying flat on her back so that additional fluid will not escape from the eye. A simple shield such as a small paper cup should be held over the eye (but without exerting pressure) to protect it. A chemical injury to the eye, especially from an alkaline substance such as drain cleaner, can be very damaging and difficult to treat. It is important to flush any such material out of the eye with prolonged irrigation. Immediately place the face under a water faucet or hose and rinse the eye. Then promptly see an ophthalmologist or go to a hospital.
A foreign object (such as a piece of dirt) in the eye will cause irritation, pain, and tearing. Sometimes a parent can locate the foreign body under the upper or lower eyelid and gently remove it. If the object’s size or location or the child’s discomfort prevents removal at home, the child’s doctor or an ophthalmologist should be contacted. Usually an anesthetic drop will be placed in the eye so the foreign body can be removed without pain. Eye infections can be caused by viruses, bacteria, or fungal agents. Common symptoms of an eye infection are redness, tearing, watery or mucous drainage, and mild sensitivity to light. Significant pain in one or both eyes, with or without sensitivity to light, should not be ignored. In such cases, the child should be examined by her physician or an ophthalmologist, not only to relieve the discomfort but to treat conditions that might damage the eye if left alone.
Any virus or a bacterium that infects the nose or throat can find its way into the linings of one or both eyes (or conjunctivae). Infections here, known as conjunctivitis, produce a reddish discoloration and thick, discolored drainage. When this material dries overnight, the eyelids may stick together with crusty debris, which you will need to remove gently with a warm, wet washcloth. Your child’s doctor will normally prescribe antibiotic drops or ointment for a few days. If the infection is caused by a virus (such as the common adenovirus, which is usually accompanied by an upper-respiratory-tract infection, fever, and sore throat), symptoms may not resolve for several weeks. Careful hand washing is a must for anyone handling a baby with conjunctivitis because the organisms involved can spread to others via contaminated fingers (see conjunctivitis).Various forms of the herpes virus can on rare occasion cause problems on the surface of (or deeper within)the eye. Chicken pox (varicella) occasionally causes conjunctivitis if the virus forms one or more small blisters (similar to those on the skin surface) on the conjunctiva of the eye. These normally resolve without damage. A reappearance of the chicken pox virus later on in life, known as herpes roster, or shingles, can also involve the cornea. In addition, the herpes simplex virus which causes common cold sores of the lips, may affect the eye. Treatment of any herpes infection involving eye will require an ophthalmologist to relieve discomfort and prevent rare complications that can cause scarring long-term visual impairment.
Allergies can cause itchy and runny eyes, a condition known as allergic conjunctivitis, which often respond to antihistamines. In some cases eye drops containing antihistamines or other anti-inflammatory substances can help reduce symptoms, although they do not provide cure. Thick mucus, crusting, and pain are not a part of this problem, and causes other than allergy should be considered. In more severe allergic reactions, swelling of the conjunctival membrane occurs, producing a bubble on the surface of the eye or inner lids. This can look frightening, but it resolves without harming the eye.
Problems with the Eyelids and the Iris
The eyelids can be the site of both inflammation and infection. Blepharitis is a chronic condition that causes swelling and redness of the eyelids and a scaling dandruff-like material from the eyelashes. Often blepharitis is caused by bacteria called staphylococci, but may also be related to an inflammatory condition called seborrhea. Treatment involves removing the crust with a moist cotton applicator (which may be dipped in baby shampoo diluted with an equal amount of water). Antibiotic ointment may also be useful.
A hordeolum, better known as a sty, is a staphylococcal infection of a gland in the eyelid. This usually begins as a tiny red spot that grows into a pustule, which will eventually drain. Warm com-presses may be applied three or four times a day, and antibiotic drops can be helpful. Inflammation of a different type of gland in the eyelid leads to a small lump called a chalazion. Warm soaks are also helpful, although this condition may take up to three months to resolve. Rarely a chalazion will not disappear, and surgical removal under anesthesia becomes necessary. The persistence of blepharitis or a chalazion is often discouraging to parents but most children will outgrow these problems. As mentioned earlier, persistent pain in the eye, especially when discomfort is aggravated by light, should be evaluated as soon as possible. One condition that may cause this combination of symptoms is iritis, or inflammation of the iris. Frequently this is accompanied by redness of the sclera, which may give the misleading impression that a child has the more common conjunctive pinkeye. But iritis must be treated quite differently to relieve pain and prevent scarring of the iris and other eye structures. In children, iritis is often associated medical conditions that affect other parts of the most commonly juvenile rheumatoid arthritis. Further evaluation to search for one of these disorders will probably be recommended.