Squint & Orthoptics
For detection and surgical correction of squint, eye muscle disorders and amblyopia
Strabismus or squint is a misalignment of the eyes where the two eyes are pointed in different directions. Though it is a common condition which affects children, it may appear later in life.
Before attempting to understand squint, its effects, and how it is treated, it is important to understand the function of eye muscles and their role in maintaining eye alignment and binocular vision.
Eye movements are controlled by eye muscles, much like reins control a horse’s head. There are six eye muscles attached to the outside of each eye, which control its movement. In each eye, two muscles move the eye to the right or left side; the other four muscles move the eye up or down, and control tilting movements. In order to line up and focus both eyes on a target, all eye muscles of each eye must be balanced and working together with the corresponding muscles of the opposite eye. When the eye muscles do not work together, then misalignment of the eyes or strabismus results.
The eyes are designed to focus images clearly on the retina and then to relay that image to the brain. If both eyes are lined up on the same target, the visual portion of the brain can fuse the two pictures into a single 3-dimensional image. This creates binocular vision and depth perception, which helps the eyes work together to transmit one “picture” to the brain.
When one eye turns as in strabismus, two different pictures are sent to the brain. In the young child, the brain learns to ignore the image of the misaligned eye and see only the image from the straight or better-seeing eye. The image of the worse eye is suppressed. This causes loss of depth perception. Adults who develop strabismus usually have double vision because their brain is already trained to receive images from both eyes and cannot ignore the image from the weaker or turned eye.
Normal alignment of both eyes during early childhood is necessary to allow good vision to develop in each eye. Abnormal alignment as in strabismus may cause reduced vision or amblyopia (“lazy eye”), which is not correctable by glasses or contact lenses. Amblyopia occurs in approximately one-half of children with strabismus. The brain will recognize the image of the better-seeing eye and ignore the image of the weaker or amblyopic eye. Amblyopia often can be reversed by patching the preferred or better-seeing eye in order to strengthen and improve the vision of the weaker one. If amblyopia is detected before the first few years of life, treatment is often successful. If adequate treatment is delayed until later, amblyopia or reduced vision generally becomes permanent. As a rule, the earlier amblyopia is treated, the better the visual result.
Amblyopia (lazy eye) and strabismus are not the same condition. Strabismus is one of the causes of amblyopia. Other causes are anisometropia (highly different spectacle prescriptions in each eye), and obstruction of vision in an eye due to injury, cataract, lid droop, etc.
Strabismus is caused by misaligned eye muscles. However, the exact reason for the misalignment of the eyes leading to strabismus is not fully understood. Many factors can be responsible for strabismus. They include:
• Inappropriate development of the “fusion center” of the brain, problems with the “eye movement” centers of the brain, and injury to or disease of the eye muscles or nerves. This explains why children with cerebral palsy, Down’s syndrome and hydrocephalus often have strabismus. Even a brain tumor may cause strabismus.
• Another factor is genetics, and it is known that strabismus may run in families. However, in many patients there are no relatives with the problem. The condition occurs equally in males and females.
• Associated eye conditions may also give rise to strabismus. In cases of cataract, injury or tumor within the eye, the eye may frequently turn in or out.
The primary symptom of strabismus is an eye that is not straight. The misalignment may be permanent and always noticeable (constant strabismus), or it may come and go, appearing normal at times and abnormal at others (intermittent strabismus). One eye may be directed straight ahead while the other eye is turned inward, outward, upward, or downward. In other cases, the turned eye may straighten at times, and the straight eye may turn (alternating strabismus).
Sometimes a youngster will close one eye in bright sunlight. Faulty depth perception may be present. Some children turn or tilt their heads in a specific direction in order to use their eyes together.
Up to the first 6 months of age, intermittent strabismus is a normal developmental milestone. After 6 months, it needs to be evaluated.
A child should be examined by the family doctor, pediatrician, or an ophthalmologist during infancy and preschool in order to detect any potential eye problem, particularly if a relative has had strabismus or amblyopia. Even the most observant parent may not discover strabismus without a doctor’s help. It is often difficult to determine the difference between eyes that appear to be crossed and true strabismus.
Young children usually have a wide, flat nose and a redundant fold of skin at the inner eyelid that tends to hide the eye during side gaze and cause concern about strabismus. An ophthalmologist can readily distinguish this from true strabismus.
It is never too early to have a child’s eyes examined. Fortunately, an ophthalmologist can test even a newborn infant’s eyes. In general, research suggests that the maximum “critical period” in humans for the development of binocular vision with resultant depth perception is from just after birth to 2 years of age. Any disruption of binocular vision in this period will therefore result in strabismus and/or amblyopia. If the eye examination is delayed until the child enters school, it may be too late to properly correct strabismus and amblyopia.
Occasionally, a misaligned eye may be caused by a cataract or tumor within the eye, as mentioned earlier. It is important to know about such conditions as early as possible, so that both the underlying condition and resulting strabismus can be corrected.
Parents often get the false impression that a child may “outgrow” the problem. Though fatigue or illness may worsen strabismus, children do not outgrow strabismus. Once a child has a suspected turning of an eye, an examination by an ophthalmologist is necessary to determine the cause and to begin treatment.
It is often asked at what age should treatment no longer be attempted. The answer is, everyone deserves a chance. Age should not be a deterrent, though treatment under 6 years of age (especially before 2) is ideal and allows better results than later treatment. After age 6, age is not important. However, every attempt should be made to improve strabismus and lazy eye, even though treatment might not be as effective after the age of six, and definitely requires more work.
The goals of treatment are to preserve vision, straighten the eyes, and restore binocular vision. Treatment of strabismus depends upon the exact cause of the misaligned eyes. It can be directed towards unbalanced muscles, cataract removal or other conditions that are causing the eyes to turn. After a complete eye examination, including a detailed study of the inner parts of the eye, an ophthalmologist can recommend appropriate optical, medical or surgical therapy. Covering or patching the good eye to force use of the amblyopic eye may be necessary to ensure equal vision.
Constant strabismus must be dealt with immediately if one wants to re-establish proper use of the eyes. Treatment for this condition needs to be early and aggressive. If the eye turn is constant and simple things like patching, glasses (bifocal, prismatic, etc) do not eliminate the eye turn, surgery needs to be considered.
With intermittent strabismus, the eye does not turn in all the time, so the brain is probably receiving appropriate stimulation for the development of binocular vision. Children with intermittent eye turns should be handled with judicious patching, special glasses, and/or orthoptics (special eye excercises designed to encourage binocular vision). Surgery, if considered at all, should be a last resort.
The two most common types of strabismus are esotropia, where an eye turns in and exotropia, where an eye turns out.
There are various types of ESOTROPIA (inward turning of eyes). Infants developing esotropia within the first three months of life (Congenital or Infantile Esotropia) usually have a large inward turn, which is easily noticed. The chances of developing normal binocular vision with normal depth perception are not good and may lose vision in the weaker eye. However, the best chance is with early aggressive surgery. Treatment after the age of 2 decreases the chances of improvement of vision, and the ability to use the two eyes together. Additionally, the cosmetic defect resulting from “crossed eyes” can have a negative effect on a child’s self-confidence. Both the parent and surgeon have to be committed to multiple procedures to obtain perfect alignment. The aim of eye surgery is to adjust the muscle tension on one or both eyes in order to pull the eyes straight. In surgery for esotropia, the tight inner muscles are placed further backward which weakens their pull and allows the eyes to move outward. Sometimes the outer muscles may be tightened by shortening the muscle length, which further pulls the eye outward.
Strabismus surgery is usually a safe and effective treatment, but is not a substitute for glasses or amblyopia therapy. During surgery, the eyeball is never removed from the socket. A small incision is made within the tissues covering the eye to allow access to the eye muscles. Selection of eye muscles to be operated upon depends upon the direction the eye is turning. Despite a thorough clinical evaluation and good surgical technique, the eyes may be closely aligned after surgery, but not perfect. In these cases, fine adjustment is dependent upon the coordination between the eye and the brain. Sometimes patients may require the use of prisms or glasses following eye muscle surgery. Over-corrections or under-corrections can occur and further surgery may be needed.
One or both eyes may be operated upon. General anesthesia is required in children. Some adults may prefer local anesthesia. Recovery time is rapid and the patient is usually able to return to normal activity within a few days. As with any surgery, eye muscle surgery has certain risks. There is a small risk of infection, bleeding, excessive scarring, and other rare complications, which can lead to loss of vision.
Another common form of esotropia that occurs in children usually after age two is caused by a need for glasses (accommodative esotropia). These children are farsighted (hypermetropia or plus power in spectacles). They have the ability to focus their eyes enough to adjust for the farsightedness, which allows them to see well for both distance and near. Some children excessively cross their eyes when they focus, which causes one eye to turn in. Wearing glasses equal in strength to their farsightedness reduces the need to focus and straightens their eyes. Sometimes the addition of bifocals is necessary to further reduce the need to focus when looking at objects up close. Occasionally, eye drops and special lenses, called prisms, can be used to help the eyes focus properly. Rarely, special eye exercises (orthoptics) are necessary to help older children control the eye misalignment.
EXOTROPIA or an outward turning of an eye is another common type of strabismus. Most commonly this occurs when a child is focusing at distant objects. Often the exotropia will occur intermittently, particularly when the child is daydreaming, ill, or tired. Parents often note that the child squints one eye in the bright sunlight. Although glasses and prism therapy may reduce the amount of outward turning in some patients, surgery is usually needed.
Treatment for strabismus is most effective when the child is young. It becomes more difficult to treat strabismus and establish binocularity as the child grows older, but cosmetic straightening of the eyes remains possible at any age. There is no known prevention for strabismus, but misaligned eyes can be straightened, and loss of sight from amblyopia is preventable if treatment is begun early.
• Children with strabismus do not outgrow the condition.
• Treatment for strabismus may be non-surgical and include eye drops or glasses.
• If surgical treatment is indicated, it is wise to align the eyes when the child is young in order to allow more normal use of the eyes together.
Besides the conditions mentioned above in children, there are certain disorders in adults which are associated with strabismus. These include:
• Thyroid disease
• After cataract surgery
• After retinal detachment surgery
• Myasthenia Gravis
• Paralysis of eye muscles due to Diabetes and Hypertension
• Orbital fracture