The extraocular muscles attach via tendons to the sclera (the white, opaque, outer protective covering of the eyeball) at different places just behind an imaginary equator circling the top, bottom, left, and right of the eye. The other end of each of these muscles attaches to a part of the orbit (the eye socket in the skull). These muscles enable the eyes to move up, down, to one side or the other, or any angle in between.
Normally, both eyes move together, receiving the same image on corresponding locations on both retinas. The brain fuses these matching images into one three-dimensional image. The exception is in strabismus, which is a disorder where one or both eyes deviate out of alignment, most often outwardly (exotropia) or toward the nose (esotropia). In this case, the brain receives two different images, and either suppresses one or allows the person to see double (diplopia). By weakening or strengthening the appropriate muscles to center the eyes, a person can correct this deviation. For example, if an eye turns upward, the muscle at the bottom of the eye could be strengthened.
The main purpose of eye muscle surgery is thus to restore straight eye alignment. The surgery is performed to align both eyes so that they gaze in the same direction and move together as a team; to improve appearance; and to promote the development of binocular vision in a young child. To achieve binocular vision, the eyes must align so that the location of the image on the retina of one eye corresponds to the location of the image on the retina of the other eye.
In addition to being used to correct strabismus, eye muscle surgery is also performed to treat such other eye disorders as nystagmus or such special types of congenital strabismus as Duane syndrome. Nystagmus is a condition in which one or both eyes move rapidly or oscillate; this condition can be improved by moving the eyes to the position of least oscillation. Duane syndrome is a disorder in which there is limited horizontal eye movement; it can sometimes be relieved by surgery that weakens an eye muscle.
According to doctors at Wills Eye Hospital, Philadelphia, the most common divergent strabismus in childhood has a variable onset, often between six months and four years. The disorder occurs in 1.2% of children by seven years of age and occurs equally in males and females.
Duane syndrome commonly affects girls more often than boys, and the left eye more often than the right eye.
Congenital nystagmus is thought to be present at birth, but is usually not apparent until the child is a few months old. Acquired nystagmus occurs later than six months of age, and can be caused by stroke, such diseases as multiple sclerosis, or even a heavy blow to the head. It is not known how many people suffer from nystagmus, but it is thought to be one in 1,000 adults, and one in 640 children in the United States, according to the Nystagmus Network.
The procedure used by the surgeon depends on the condition that needs correcting. During surgery, eye muscles can be:
- Weakened. This usually involves recessing the eye muscle or moving it posteriorly on the eye to elongate the muscle and allow the muscle tissue to relax.
- Tightened. Muscles are tightened by resection, which involves removing a piece of the muscle near its point of insertion and then reinserting the muscle into its original location. By removing a piece of muscle, the muscle is shortened and therefore strengthened.
- Repositioned. For some forms of strabismus, the eye muscles are neither weakened nor strengthened, but repositioned: i.e., the muscle's point of insertion is moved to a different location.
There are two methods to alter extraocular muscles. Traditional surgery can be used to strengthen, weaken, or reposition an extraocular muscle. The surgeon first makes an incision in the conjunctiva (the clear membrane covering the sclera), then puts a suture into the muscle to hold it in place, and loosens the muscle from the eyeball with a surgical hook. During a resection, the muscle is detached from the sclera, a piece of muscle is removed so that the muscle is now shorter, and the muscle is reattached to the same place. This strengthens the muscle. In a recession, the muscle is made weaker by repositioning it. More than one extraocular eye muscle might be operated on at the same time.
Eye muscle surgery is performed with the eye in its normal position and usually takes an hour and a half. At no time during the operation is the eye removed from the socket. The surgeon determines where to reattach the muscles based on eye measurements taken before surgery. Most of the time, it can hardly be seen except with magnification.
Depth perception (stereopsis) in humans develops around the age of three months. For successful development of binocular vision and the ability to perceive three-dimensionally, eye muscle surgery should not be postponed past the age of four years. The earlier the surgery, the better the outcome, so an early diagnosis is important. Surgery may even be performed before the child is two years old.
Patients (or their caregivers) should make sure their doctors are aware of any medications that they are taking, even over-the-counter medications. Patients should not take aspirin, or any other blood-thinning medications for 10 days prior to surgery, and should not eat or drink after midnight the night before.
After surgery, the eyes feel scratchy, but not very painful. Postoperatively, the eyes are also a little red and watery. There may be some hemorrhage under the conjunctival membrane over the white of the eye that usually settles over a period of two to three weeks. It usually takes on a yellowish discoloration similar to a bruise as it clears. Sometimes there is some thickening of the membranes over the eye, which can take several more weeks to clear. Very fine dissolving sutures are used to reposition the conjunctival membrane at the end of surgery and, until these sutures dissolve, there may be some scratchiness in the eyes. This feeling usually disappears after two or three weeks.
There will also be some swelling and discharge after the surgery. The swelling is usually minor, and patients should be able to open their eyes within the next two days, as the swelling should gradually disappear.
Patients will need someone to drive them home after the operation. They should continue to avoid aspirin and other nonsteroidal anti-inflammatory agents for an additional three days, but they can take acetaminophen (e.g., Tylenol). Patients should discuss what medications they can or cannot take with the surgeon. Pain will subside after two or three days, and patients can resume most normal activities within a few days. Again, the period of recovery may vary with the patient and the patient can discuss with the surgeon when to return to normal activities. Patient's should not get their eyes wet for three to four days and should refrain from swimming for 10 days. Operated eyes will be red for about two weeks.
Adults and children over the age of six often experience double vision for a limited period of time after surgery. Children younger than six sometimes will have double vision for a short period of time. Double vision is rarely permanent.
Patients generally do not have to wear patches after surgery, although occasionally a temporary patch may be recommended. They are usually required to use eye drops for a week until the follow-up examination. If the eye is healing on schedule, then the eye drops are usually discontinued at that stage. A further postoperative appointment is usually made for six to eight weeks later, by which time the eye will have stabilized.
After surgery for strabismus, the patient usually needs corrective lenses and eye exercises (vision therapy) if binocular vision is to develop.
As with any surgery, there are risks involved. Eye muscle surgery is relatively safe, but very rarely a cut muscle cannot be retrieved. This, and other serious reactions, including those caused by anesthetics, can result in vision loss in the affected eye. Occasionally, retinal or nerve damage occurs. Permanent double vision is also a risk of eye muscle surgery. The success rate of this surgery varies from person to person and depends on each person's particular condition.
Some infrequent complications include, but are not limited to, allergy to the sutures, bleeding, and change in pupil size.
The major risk of eye muscle surgery is failure to achieve a satisfactory alignment of the eyes. This may be an undercorrection or an overcorrection, with the eyes turning the other way after the operation. Surgeons aim to achieve perfect alignment, but this is not always possible. If the alignment is still unsatisfactory at the final postoperative visit, then a second operation may be required.
Infection is an unusual postoperative complication and can be treated with antibiotic drops.
Because an incision is made through the conjunctiva and muscle, there is always some residual scarring. Usually, this is detectable only under a microscope, although it may be possible to see it on close examination.
As with any eye surgery, there is a potential risk of visual loss from strabismus operations, but this is a very rare complication.
Vision therapy
Vision therapy is an individualized, supervised, non-surgical treatment program designed to correct eye movements and visual-motor deficiencies. Vision therapy sessions include procedures designed to enhance the brain's ability to control:
- eye alignment
- eye teaming
- eye focusing abilities
- eye movements
- visual processing
Visual-motor skills and endurance may be developed through the use of specialized computer and optical devices, including therapeutic lenses, prisms, and filters. During the final stages of therapy, the patient's newly acquired visual skills are reinforced and made automatic through repetition and by integration with motor and cognitive skills.
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