Strabismus

What is strabismus?

Strabismus is a condition in which the eyes are not aligned properly and point in different directions. One eye may look straight ahead, while the other eye may turn inward, outward, upward, or downward. The eye turn may be constant, or it may come and go.

Strabismus is a common condition, about 2 percent of people have strabismus. It commonly presents in early childhood but can also occur later in life. It may run in families; however, many people with strabismus have no relatives with the problem.

What causes strabismus?

Movement of the eye is controlled by six eye muscles. For eyes to look straight all muscles in each eye must be balanced and work together. Importantly in strabismus eye muscles are usually normal, what is impaired is brain ability to coordinate these muscles.

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

  • Cerebral palsy;

  • Down syndrome;

  • Hydrocephalus;

  • Brain tumors;

  • Prematurity.

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.

Normally when both eyes aim at the same spot, the brain receives two images from two eyes. 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 images 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 and may develop amblyopia. 

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 misaligned eye. Children usually do not see double.

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.

What are the common types of Strabismus?

Esotropia is an in-turning eye.

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 and surgery is usually recommended for this type of srabismus.

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. This form of strabismus is often treated with glasses.

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

How is strabismus diagnosed?

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 pediatrician, 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 (Eye M.D.). 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.

How is strabismus treated?

Goals of strabismus treatment is to identify and treat amblyopia, straighten the eyes and restore binocular (two-eyed) vision. Depending on underlying cause of strabismus these goals may be achieved to varying degree. In children patching or blurring the strong eye to improve amblyopia is often necessary.

In some cases of strabismus (especially accommodative esotropia), eyeglasses can be prescribed for your child to straighten the eyes. Sometimes bifocal glasses are used to improve alignment.

Small angle of misalignment is often corrected with prism glasses.  Eye exercises are beneficial in patients with convergence insufficiency type of strabismus. 

Many forms of strabismus are treated with surgery to correct the unbalanced eye muscles or to remove a cataract.  Ophthalmologist will guide you as to timing of strabismus surgery.  In some cases it is advisable to do surgery soon upon diagnosis to improve the chance of restoring or promoting normal binocular vision; in other situations waiting until child reaches a certain age is desirable. 

How is strabismus surgery done?

The ophthalmologist makes a small incision in the tissue covering the eye to reach the eye muscles and reposition them to make them weaker or stronger.  Often surgery is done on both eyes to achieve good balance.  Patients are usually able to resume their normal activities within a few days after surgery.

After surgery, glasses or prisms may still be required. In some cases, more than one surgery may be needed to straighten the eyes. In some instances surgery may not be advisable at all.

In adults eye muscle surgery can restore normal appearance, improve binocular vision and minimize double vision. Eye contact is hugely important in human communication. Eye muscle surgery is considered restorative and is usually covered by medical insurances. 

Preoperative tests for strabismus surgery  

Before surgery, a specialized examination called a sensori-motor examination will be performed by ophthalmologist to assess the alignment of the eyes to determine which muscles are contributing to misalignment 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. 

Adjustable sutures  

In strabismus surgery, the muscle is weakened, strengthened or moved and a permanent knot is placed. In adults, there is the added advantage that an adjustable suture can be used. Instead of a permanent knot, a temporary knot is placed. After the surgery, with the patient awake, alignment can be reassessed, and if necessary, adjustments can be made before a permanent knot is placed to minimize the chance of an over-correction or under-correction.

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;

  • Anesthesia-related complications;

  • Need for more surgery.

How successful is strabismus surgery?

Strabismus surgery is a common procedure and most patients will see a large improvement in the alignment of their eyes after surgery.  However in month and years following surgery eyes may drift again and repeat surgery may be needed in some patients.