Squint (Strabismus)

Squint is the common name for ‘strabismus’ which is the medical term used to describe eyes that are not pointing in the same direction. Squint is relatively common in children, and 2-3% of the population suffers from squint. Strabismus is a failure of the two eyes to maintain proper alignment and work together as a team. If you have strabismus, one eye looks directly at the object you are viewing, while the other eye is misaligned inward (esotropia or ‘cross-eyed’), outward (exotropia or ‘wall-eyed’), upward (hypertropia) or downward (hypotropia).

Strabismus can be constant or intermittent. The misalignment also might always affect the same eye (unilateral strabismus), or the two eyes may take turns being misaligned (alternating strabismus).To prevent double vision from congenital and early childhood strabismus, the brain ignores the visual input from the misaligned eye, which typically leads to amblyopia or “lazy eye” in that eye. Strabismus is a common condition among children. It can also occur later in life. It may run in families; however, many people with strabismus have no relatives with the problem.

Infantile esotropia, where the eye turns inward, is the most common type of strabismus in infants. Young children with esotropia cannot use their eyes together. Accommodative esotropia is a common form of esotropia that occurs in children usually 2 years or older. In this type of strabismus, when the child focuses the eyes to see clearly, the eyes turn inward. This crossing may occur when focusing at a distance, up close or both.

Exotropia. Notice the outward-turning eye.

Exotropia, or an outward-turning eye, is another common type of strabismus. This occurs most often when a child is focusing on distant objects. The exotropia may occur only from time to time, particularly when a child is daydreaming, ill or tired. Parents often notice that the child squints one eye in bright sunlight.

Strabismus Causes

Each eye has six external muscles (called the extraocular muscles) that control eye position and movement. In each eye, one muscle moves in the eye to the right, and one muscle moves the eye to the left. The other four muscles move it up or down and at an angle.

For normal binocular vision, the position, neurological control and functioning of these muscles for both eyes must be coordinated perfectly. In order for the eyes to move together, the muscles in both eyes must be coordinated. The brain controls these muscles. With normal vision, both eyes aim at the same spot. The brain then combines the two pictures into a single, three-dimensional image. This three-dimensional image gives us depth perception.

When one eye is out of alignment, two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or better-seeing eye. The child then loses depth perception.

Strabismus occurs when there are neurological or anatomical problems that interfere with the control and function of the extraocular muscles. The problem may originate in the muscles themselves, or in the nerves or vision centers in the brain that control binocular vision.

Genetics also may play a role: If you or your spouse has strabismus, your children have a greater risk of developing strabismus as well.

Occasionally, when a farsighted child tries to focus to compensate for uncorrected farsightedness, he or she will develop a type of strabismus called accommodative esotropia, where the eyes cross due to excessive focusing effort. This condition usually appears before 2 years of age but also can occur later in childhood. Often, accommodative esotropia can be fully corrected with eyeglasses or contact lenses.

Adults who develop strabismus often have double vision because their brains have already learned to receive images from both eyes and cannot ignore the image from the turned eye. A child generally does not see double.

Strabismus is especially common among children with disorders that may affect the brain, such as:

• Cerebral palsy;
• Down syndrome;
• Hydrocephalus;
• Brain tumors;

A cataract or eye injury that affects vision can also cause strabismus. The vast majority of children with strabismus, however, have none of these problems. Many do have a family history of strabismus.

Strabismic amblyopia

Good vision develops during childhood when both eyes have normal alignment. Strabismus may cause reduced vision, or amblyopia, in the misaligned eye.

The brain will pay attention to the image of the straight eye and ignore the image of the crossed eye. If the same eye is consistently ignored during early childhood, this misaligned eye may fail to develop good vision, or may even lose vision. Strabismic amblyopia occurs in approximately half of the children who have strabismus.

Amblyopia can be treated by patching or blurring the stronger eye to strengthen and improve vision in the weaker eye. If amblyopia is detected in the first few years of life, treatment is usually successful. If treatment is delayed, amblyopia may become permanent. As a rule, the earlier amblyopia is treated, the better the result for vision.

 Strabismus Symptoms and Signs

The primary sign of strabismus is a visible misalignment of the eyes, with one eye turning in, out, up, down or at an oblique angle.

When the misalignment of the eyes is large and obvious, the strabismus is called “large-angle,” referring to the angle of deviation between the line of sight of the straight eye and that of the misaligned eye. Less obvious eye turns are called small-angle strabismus.

Typically, constant large-angle strabismus does not cause symptoms such as eye strain and headaches because there is virtually no attempt by the brain to straighten the eyes. Because of this, large-angle strabismus usually causes severe amblyopia in the turned eye if left untreated.

Esotropia (crossed eyes) needs to be treated early in life to prevent amblyopia.

Less noticeable cases of small-angle strabismus are more likely to cause disruptive visual symptoms, especially if the strabismus is intermittent or alternating. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading and unstable or “jittery” vision. If small-angle strabismus is constant and unilateral, it can lead to significant amblyopia in the misaligned eye.

Both large-angle and small-angle strabismus can be psychologically damaging and affect the self-esteem of children and adults with the condition, as it interferes with normal eye contact with others, often causing embarrassment and awkwardness.

New-borns often have intermittent crossed eyes due to incomplete vision development, but this frequently disappears as the infant grows and the visual system continues to mature. Most types of strabismus, however, do not disappear as a child grows.

Routine children’s eye exams are the best way to detect strabismus. Generally, the earlier strabismus is detected and treated following a child’s eye exam, the more successful the outcome. Without treatment, your child may develop double vision, amblyopia or visual symptoms that could interfere with reading and classroom learning.

The main sign of strabismus is an eye that is not straight. Sometimes children will squint one eye in bright sunlight or tilt their head to use their eyes together.

Unlike true strabismus (top of page), note here the symmetrical light reflection of pseudostrabismus.

Pseudostrabismus
The eyes of infants often appear to be crossed, though actually they are not. This condition is called pseudostrabismus. Young children often have a wide, flat nose and a fold of skin at the inner eyelid that can make eyes appear crossed. This appearance of pseudostrabismus may improve as the child grows. A child will not outgrow true strabismus.
An ophthalmologist can distinguish true strabismus and pseudostrabismus.

Strabismus Diagnosis

Strabismus can be diagnosed during an eye exam. It is recommended that all children between 3 and 3½ years of age have their vision checked by their paediatrician, family practitioner or an individual trained in vision assessment of preschool children. Any child who fails this vision screening should then have a complete eye exam by an ophthalmologist. If there is a family history of strabismus or amblyopia, or a family history of wearing thick eyeglasses, an ophthalmologist should check vision even earlier than age 3. After a complete eye examination, an ophthalmologist can recommend appropriate treatment.

Strabismus Treatment

Treatment for strabismus works to straighten the eyes and restore binocular (two-eyed) vision. In some cases of strabismus, eyeglasses can be prescribed for your child to straighten the eyes. Other treatments may involve surgery to correct the unbalanced eye muscles or to remove a cataract. Patching or blurring the strong eye to improve amblyopia is often necessary.

Non-Surgical Strabismus Treatment

In some cases of intermittent and small-angle strabismus, it may be possible to improve eye alignment non-surgically with vision therapy.

For example, convergence insufficiency (CI) is a specific type of intermittent exotropia in which the eyes usually align properly when viewing a distant object, but fail to achieve or maintain proper alignment when looking at close object, such as when reading, resulting in one eye drifting outward. Convergence insufficiency can interfere with comfortable reading, causing eye strain, blurred vision, double vision and headaches.

Sometimes, a strabismus surgeon may recommend a program of vision therapy for a period of time after strabismus surgery to treat amblyopia and minor binocular vision problems that might remain after surgery. In these cases, the term “orthoptics” (“ortho” = straight; “optics” = eyes) rather than “vision therapy” might be used to describe this treatment, which may be provided by an orthoptist working closely with the surgeon rather than by an optometrist.

Surgical Strabismus Treatment

In most cases, the only effective treatment for a constant eye turn is strabismus surgery. If your general eye doctor finds that your child has strabismus, he or she can refer you to an ophthalmologist who specializes in strabismus surgery.

Very young children with esotropia usually require surgery to realign the eyes.

For accommodative esotropia, glasses reduce the focusing effort and often straighten the eyes. Sometimes bifocals are needed for close work. If significant crossing of the eyes persists with the glasses, surgery may be required.

With exotropia, though glasses, exercises, patching or prisms may reduce or help control outward-turning of the eye in some children, surgery is often needed.

Questions to Ask

When consulting with your eye doctor or strabismus surgeon prior to treatment, here are a few important questions to ask:

• If surgery is recommended, inquire whether one surgery will suffice or if additional procedures are likely to be necessary.
• Ask the eye surgeon about the success rates for the type of strabismus and the surgery he or she is recommending.
• Ask what criteria are used to determine if the treatment is a success. In other words, is “success” defined as reducing the eye turn so the eyes are better aligned and look more natural in appearance, or is success defined as eyes that are perfectly aligned with normal visual acuity, eye teaming and depth perception.
• For optometrists or orthoptists, ask about the success rate, likely duration and costs of vision therapy (or orthoptics).
• Remember, children do not “outgrow” strabismus. For best visual outcomes and to prevent developmental delays and other problems, seek treatment for strabismus as soon as possible. 

Strabismus Surgery

Strabismus surgery also can effectively align the eyes of adults with long-standing strabismus. In most cases of adult strabismus, however, a significant degree of amblyopia is likely to remain even after the affected eye is properly aligned. This is why early treatment of strabismus is so important.

In some cases of strabismus in children and adults, strabismus treatment consists of glasses, prisms, patching or blurring of one eye, botulinum toxin injections, or a combination of these treatments. Other times, eye muscle surgery is necessary to straighten the eyes.

In children with some types of constant strabismus, early surgery may be recommended to improve the chance of restoring or promoting normal binocular vision. The earlier strabismus is treated surgically, the more likely it is that the affected eye will develop normal visual acuity and the two eyes will function together properly as a team.

In adults, eye alignment surgery is not strictly cosmetic. Cosmetic surgery is enhancement surgery, such as restoring youthful appearance in a normal aging person. Eye alignment surgery restores normal appearance and is considered reconstructive. There are many other benefits beyond restoring normal appearance: improved depth perception or binocular vision, improved visual fields, eliminating or minimizing double vision and improved social function — as eye contact is hugely important in human communication. It is important to discuss the goals and expectations of the surgery with your ophthalmologist.

During strabismus surgery, one or more of the eye muscles are strengthened, weakened or moved to a different position to improve alignment. Strabismus surgery is usually performed as an outpatient procedure and does not require an overnight hospital stay.

Preoperative tests for strabismus surgery  

Before surgery, a specialized examination called a sensorimotor examination will be performed to assess the alignment of the eyes to determine which muscles are contributing to the strabismus and which muscles need to be altered (weakened, strengthened, or moved) to improve the alignment of the eyes. Prisms are used to measure the degree of the strabismus. These preoperative tests help guide the surgeon in determining the surgical plan. Often both eyes require surgery, even if only one is misaligned. Sometimes the exact surgical plan is determined based on findings at the time of the surgery, especially in reoperations.

The strabismus surgery procedure

Strabismus surgery in children requires general anaesthesia. In adults, the procedure can be done with general or local anaesthesia. Either way, the patient must fast for about eight hours before the procedure.

The eyelids are gently held open with a lid speculum. A small opening is made through the conjunctiva (the mucous membrane surface of the eye) to reach the eye muscles. The eye muscles are detached from the wall of the eye and repositioned during the surgery, depending on which direction the eye is turning. It may be necessary to perform surgery on one or both eyes.

Most strabismus surgeries are less than one to two hours; however, the patient will be at the surgery centre for several hours including pre-operative and post-operative care. Recovery is usually rapid. Children are usually able to resume their normal activities within a few days.

After surgery, glasses may still be required. In some cases, more than one surgery may be needed to straighten the eyes.

After surgery

Any patient that has surgery, whether under general anaesthesia or local anaesthesia with sedation, needs to be monitored after surgery. Children can return to school after two days. Adults should not drive the day of surgery or the day after and may need up to a week before returning to work. You may have double vision that can last hours to days or a week or more, rarely longer. Exercise caution with activities like driving if you have double vision.

Pain is minimal and usually over-the-counter medicines, such as ibuprofen or acetaminophen and cool compresses are adequate. Adults and older children may need prescription pain medicine.

The main restriction after strabismus surgery is not swimming for two weeks.

The eye will be red for one to two weeks, rarely longer, especially if it is a reoperation.

Potential risks of strabismus surgery  

The chance of any serious complication from strabismus surgery that could affect the sight or well-being of the eye is exceedingly rare. However, there are risks with any surgery, including:

• Sore eyes;
• Redness;
• Residual misalignment;
• Double vision;
• Infection;
• Bleeding;
• Corneal abrasion;
• Decreased vision;
• Retinal detachment;
• Anaesthesia-related complications.

Success of a Strabismus surgery

Strabismus surgery is a common procedure and most patients will see a large improvement in the alignment of their eyes after surgery. The success of strabismus surgery depends on many factors, including the direction and magnitude of the eye turn. In some cases, you may need additional surgery or prism glasses to optimally align the eyes. Each case of strabismus is unique and should be discussed with your Eye M.D. to understand the goals and expectations of surgery.