Strabismus

"Why do you have that patch on? Did your eye fall out?" Kindergartner Kimberly May answered the jeer with a shrug. In 1974, it was Kim's first day of school in Gaithersburg, Md., and she and the boy harassing her were waiting with other children for the building to open. Standing nearby, Kim's 7-year-old brother Erik replied with gusto: "You dumb thing. She's got amblyopia, and she needs the eye patch so she can see better. So there."

Although Erik couldn't explain how Kim's amblhopia (decreased vision) resulted from strabismus (eye misalignment), he defended his sister with the few terms and facts he'd overheard at home. And although Kim couldn't find comfort in knowing that many other youngsters wear eye patches for amblyopia, she took heart from her brother's support.

Strabismus affects approximately 4 percent of U.S. children under age 6. Amblyopia occurs in about 2 percent of the general population.

Anne May, a registered nurse, discussed her daughter's condition with the teacher, emphasizing that strabismus would not hamper Kim's ability to do class work. With agreement from Kim and her teacher, May also spoke to the class.

"I told them one eye sometimes is weak but can often be strengthened by patching," May says. "We took the patch off to show them her eye was OK under there. After that, there were only one or two remarks, from students absent that day."

In Kim, strabismus occurred as crossed eyes. In others, it may manifest as eyes that turn out, up or down. Its name can be traced to the Greek word strabismos, to look obliquely or with unstraight eyes; some use the terms "squint" and "lazy eye." Strabismus can disable sight in one eye, yet leave the other with 20/20 vision. Strabismus can be acquired from diverse causes at any age. There are more than a dozen variations. (See accompanying article.)

Sight: A Team Effort

Healthy eyes move together to send similar images along the optic nerve to the brain for fusion into a single 3-dimensional picture at the brain-vision junction, or visual cortex. Toward this end, six muscles (see illustration) attached to the outside of each eye contract and relax to move the eyes in perfect synchronization, permitting fusion, or binocular vision, across a large area of the visual field.

Strabismic eyes, on the other hand, do not move in unison. A muscle may pull too weakly or too strongly against its opposing muscle, creating an imbalance that causes one eye to drift from parallel alignment with its mate; more than one pair of muscles may be imbalanced.

Since each eye fixates on an object at a different point in space, the images received by the brain are dissimilar. The brain is unable to fuse the dissimilar images, resulting in double vision, which can be very disturbing. To avoid this disturbance, the brain may suppress vision in the deviating eye, allowing clear sight to develop solely in the straight eye. Decreased vision in the suppressed eye is called amblyopia. Prolonged amblyopia causes a loss in 3-dimensional viewing and depth perception.

The "squint" or turn usually is constant but may be intermittent and may occur in only one eye or alternate between the two eyes. Vision in people with alternating strabismus generally remains good i each eye individually.

While strabismus clearly stems from muscle imbalance, the causes of such imbalance are many and are not all completely understood.

"There's a strong genetic influence, but there are also many anatomic and neuromuscular reasons," says John F. O'Neill, M.D., an ophthalmologist (a physician who specializes in eye disease) and director of the Pediatric Ophthalmology and Strabismus Service at Georgetown University's Center for Sight in Washington, D.C. "One group of children may have eyes that turn, usually inward, from the day they are born. Another group may have perfectly healthy and straight eyes their first few years of life. However, as these children mature and start focusing more carefully on objects, the effort to see clearly causes their eyes to cross. Another group of children with neurologic conditions, such as cerebral palsy, not only may have poor movement of their arms and legs, but the eye muscle system is affected as well."

Strabismus can be associated with many other conditions that cause poor vision in one or both eyes--for example, cataract, Down syndrome, thyroid disese, eye tumor, damage to the fetal central nervous system from toxoplasmosis (a parasitic infection that can pass from the mother during pregnancy), damage to a nerve supplying the eye muscle (perhaps from birth trauma), or eye disuse due to a high refractive error (such as extreme farsightedness) or different refractive errors in each eye (such as nearsightedness in one eye and farsightedness in the other).

Is It Strabismus?

Maybe not. Some children have facial features that make the eyes look crossed when they aren't, and some normal newborns have a temporary outward drift.

Eye alignment is not fully mature at birth. A true developmental eye drift typically shows up from birth to 3 or 4 months of age, when normal eyes are straightening, but may occur through childhood or later. Misalignment that persists to age 5 or 6 months or occurs later should be assessed by an eye specialist. Early diagnosis is vital to detect and treat underlying causes and prevent severe vision disability.

"The first thing we do in examining children is to assess their vision, to determine the degree of visual attentiveness in each eye separately," O'Neill says. "We observe the child's ability to fixate on an object, and then we test how well each eye alone, and both eyes together, can follow that object in different directions and at different distances."

Many techniques are used to check the alignment and movement of the eyes to test for strabismus.

A quick screening method is the Hirschberg corneal reflex test, says Walter Sloane, M.D., an ophthalmologist with FDA's Center for Devices and Radiological Health, which regulates medical devices, including those used to test and treat the eye. The examiner directs an examining light onto the child's cornea (the transparent covering that admits light through the pupil) as the child looks at the light. In normal eyes, the light reflection appears in the center of both pupils. An eye that reflects light from a different place may be strabismic and should be further examined. Sloane says.

Another corneal reflex test is the Krimsky method. It compares the location of the reflected examining light in each pupil; again, light reflection that are dissimilar indicate strabismus. Prisms placed over one or both eyes align the reflections to estimate the amount of the turn.

The alternate cover test quickly spots misalignment. "The examiner rapidly covers and uncovers each eye, shifting back and forth from one eye to the other like a windshield wiper," Sloane says. "If the child has any deviation, the eye will immediately move as the cover is shifted to the uncovered eye."

The cover-uncover test differentiates serious types of strabismus called tropias from latent drifts called phorias, which seldom require treatment.

"The brain immediately overcomes a phoria drift," Sloane says. "So when the drift is a phoria, we see movement immediately after the eye is uncovered as it responds to control by the brain.

"But when the eye with a tropia is covered, it becomes unhooked, so to speak, from the brain's control so that it drifts--in any direction--and remains turned in that position when uncovered until we cover its fellow eye, which has been staring at the object. When the fellow eye is covered, the turned eye is reconnected to the brain. The turned eye then moves to fixate on the object, as if to say, 'Oops, I was facing the wrong direction.'"

One new method, a "preferential looking technique," uses Teller acuity cards. These devices are similar to educational flash cards, but instead of letters or numbers they have black and white stripes ranging in patterns from very broad to very narrow, simulating large to small pictures or letters.

"Vision is gauged," O'Neill says, "by how attentively a child, even a baby, looks at each pattern."

The use of drops to dilate the eyes allows inspection of the back of the eye to detect eye disease that may be contributing to the drift.

Depending on the findings, other tests may be required.

Best Chance to See

Prompt attention to correcting eye misalignment will provide the most satisfactory outcome of treatmentm. Indeed, if some cases of strabismus are left untreated until age 6, permanent visual impairment can result.

Treatment has three primary goals, O'Neill says: foremost, to obtain the best possible vision in each eye; second, to gain the best possible alignment of each eye alone and as a pair; and, finally, to provide the best opportunity for binocular vision. Corrective eyeglasses, patching, or both are the mainstay therapies, with about 30 percent of patients needing surgery, he says.

Corrective eyeglasses can help children as young as 6 months of age. They're most effective when there is significant farsightedness and the eyes turn in, and they're the only therapy needed in about a third of these patients whose eyes turn in. Prisms incorporated in eyeglasses may relieve double vision in some older patients.

To force use of a "lazy" eye while preserving vision in the preferred eye, patching is very effective.

"If a child doesn't develop vision equally in each eye early in life," O'Neill says, "it may never develop fully. For a 4-month-old child, patching might be used only an hour or two a day. A child that age probably takes 2 or 3 naps a day, so I'd patch for only one of those waking periods. But you have to be cautious. When you patch an eye at this early stage of development, you inactivate it. If you cover it for too long a period at] this time, the child can lose sight in that eye, and the loss could be irreversible."

In 1969, when Kim May was first patched at age 6 months, doctors didn't have as much information about early development of vision.

"We were told to patch her straight eye 24 hours a day for an entire week, removing the patch only for changing it and for bathing," her mother says. "And then we were to patch the other eye. But at the end of that week, when I took the patch off and started covering her crossed eye, the straight one was basically blind. She couldn't see her baby bottle. I had to put it into her hand. Her brain had totally switched over to the crossed eye."

Sight did return to Kim's temporarily vision-disabled left eye. She had surgery on it in September 1970 and on the right eye in 1975 and 1980 to realign the imbalanced muscles. With corrective eyeglasses, Kim today at age 22 has 20/30 vision in her left eye and 20/25 in her previously crossed right eye.

Surgery Can Help

Some 60,000 to 80,000 operations are performed each year to correct strabismus. When the eyes turn out, up or down, correction usually requires surgery. Sometimes a second operation is required. With current knowledge and techniques, it's uncommon for a patient (about 1 in 20) to need a third operation, O'Neill says. The need for further corrective surgery, he says, depends on the stability of the muscle system and the degree of muscle response (over- or under-response) to the surgical adjustment.

Complications related to strabismus surgery are infrequent. Besides general risk such as bleeding and infection that accompany any surgery, complications may include postoperative double vision, and--rarely--excessive tissue reaction with scarring.

By weakening or strengthening an eye muscle (or muscles), strabismus surgery alters the muscular pull on the afflicted eye in order to align its movements with the other eye. The ophthalmologist can weaken or strengthen a muscle function by repositioning the muscle on the outside of the eye (never cutting into the eyeball) and also can strengthen a muscle by cutting out (resecting) a small piece of its tendon. Techniques with adjustable sutures allow additional muscle repositioning within the first day following surgery.

When the patient is a child, general anesthesia is always used. Some adults may have local anesthesia.

After surgery, bandages frequently are unnecessary and there is just redness in the eye. The parents are given an antibiotic ointment to put in the child's eyes, and the doctor generally will see the patient again in two or three days.

New Treatment

Following a number of years of investigations, FDA, in December 1989, licensed a therapy for strabismus patients age 12 and older: Oculinum, an injectable form of sterile, purified Botulinum toxin, type A. Before FDA granted approval, the agency's Center for Biologics Evaluation and Research reviewed safety and effectiveness data on Oculinum.

Wiley Chambers, M.D., an ophthalmologist with FDA's Center for Drug Evaluation and Research who contributed to this review, points out, "Oculinum can be used effectively to treat certain adults with strabismus, but its effect in children hasn't been adequately evaluated.

"We limited it to patients over age 12," he says, "because children under that age have a chance of developing amblyopia, and more information is needed to reliably assure muscle balance and prevent the risk of amblyopia. When amblyopia isn't consideration, a lot of people think it's worth trying, to avoid an operation."

Unfortunately, effectiveness is unlikely in opposing-muscle weakness, severe misalignment, and certain other circumstances.

The toxin derives from Clostridium botulinum bacteria and is very potent. If accidentally eaten in contaminated foods, it can cause botulism poisoning that may result in paralysis, even death. In the treatment of strabismus, however, it is used in extremely dilute concentrations, and there have been no reports of botulism poisoning from Oculinum use in patients with strabismus or blepharospasm, an eye spasm disorder the product also is licensed to treat. It some 8,340 injections, nine accidental punctures and 16 instances of excess bleeding occurred. None resulted in vision loss. The most common side effects are eyelid droop and eye irritation.

Oculinum is injected into an eye-turning muscle, outside the eye, through an electromyographic needle that guides placement by recording the muscle's electrical activity. Anesthetic eye drops generally are used before the injection.

The toxin "turns off" the muscle by paralyzing it. Scientists theorize the paralysis affects muscle pairs by causing the injected muscle to lengthen, thus prompting the opposing muscle to shorten.

About half of patients require repeated treatments. In a recent study of 677 patients, 55 percent showed improvement six months later. Correction may be permanent, provided the injected muscle is paralyzed well enough and long enough and the opposing muscle is intact.

Another new therapy may benefit patients who acquire crossed eyes after age 6 months. It involves the use of eyeglasses overlaid with thin plastic prisms.

In September 1990, the University of Iowa Hospitals and Clinics in Iowa City announced the results of a six-year study led by William Scott, M.D., in which 14 medical centers tested the efficacy of treatment with prism eyeglasses before surgery in patients who had no previous eye surgery.

"By knowing the exact prism power that corrects the misalignment," Scott says, "we can more accurately determine the surgical adjustment needed on the eye muscles, thus reducing the possible need for additional surgery." The eyes of about 83 percent of patients who used the special eyeglasses were straightened by the surgery, compared with 72 percent of patients without them.

Appropriate management offers strabismus patients the best possible circumstances for improvement.

"The key most often is early detection and treatment," says Georgetown's O'Neill. "Without proper care, strabismus in an infant or in a child early in life will generally get worse, not better. Children do not outgrow strabismus when the eyes truly are 'crossed'."

Dixie Farley is a staff writer for FDA Consumer.

COPYRIGHT 1991 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group