Strabismus (Crossed Eyes)
Strabismus is the medical term for what is commonly referred to as crossed eyes. According to the American Optometric Association, Crossed eyes, or strabismus, are conditions in which both eyes do not look at the same place at the same time. It occurs when one or both eyes turn in, out, up or down some or all of the time.
Strabismus is a severe problem with eye teaming, one of the three visual skill areas that make up functional vision. Because the eyes are pointing at different places, the brain has difficulty combining the images it sees into a single, 3D image.
Strabismus is commonly referred to as crossed eyes, but as the definition above indicates, it involves any type of eye turn, not just crossing.
Strabismus results when a person has difficulty developing the visual skill area of eye teaming. Infants learn this skill in the first few months after birth. Learning begins to happen very quickly in the first few weeks of life and then more gradually over several months. It continues over years as the visual skill of eye teaming is integrated with the rest of motor and sensory development.
Difficulty in developing the visual skill of eye teaming can lead to double vision. Strabismus is a solution to double vision in that if the child is having difficulty aligning the eyes in order to put the two images together they can instead learn to move them farther apart. This makes it much easier to ignore one of the images.
The majority of the time the underlying cause of the difficulty of learning to align the eyes is unknown.
It may be associated with family history of similar types of eye teaming problems, developmental delays and other visual conditions.
Children - Most strabismus occurs by age 6 and often by age 3, but can occur at any age. It often develops in the preschool years as the child is developing the functional visual skill areas of eye movement, eye focusing and eye teaming and integrating those with both large motor and fine motor skills. Interferences in this development can lead to strabismus.
Older Children and Adults - Functional strabismus can occur at any age. Sudden onset should be evaluated immediately (see below). Functional onset in teenagers and adults is often related to increased near visual stress. This could include increased demands for near work related to school or employment.
Types of Strabismus Many factors must be considered in determining the type of strabismus. The following are the most common:
1. Direction of the Eye Turn
The direction of the eye turn is designated by the following medical terms:
Esotropia - Inward
Exotropia - Outward
Hypertropia - Up
Hypotropia - Down
Cyclotropia - Rotational (not pictured below)
Types of Strabismus
Exotropia and esotropia are the most common types of strabismus. A patient may have any of the above in isolation, but could also have a combination such as one eye pointing inward and down (esotropia and hypotropia).
2. Frequency of the Eye Turn
The frequency of the eye turn is designated by the following medical terms:
Constant strabismus - Eyes are always misaligned
Intermittent strabismus - Eyes are misaligned only some of the time
Intermittent strabismus is usually easier to treat. The examination will include evaluating whether the eye(s) turn more when the patient looks far away or close up, and whether there are any other positions that are more difficult such as looking to the left, right, up or down. Additionally, we will want to determine whether there are any factors, which trigger the eye turn (e.g. reading, driving, fatigue, etc.).
Constant strabismus is more difficult to treat, as it is harder to retrain a visual system if the eyes never work together. The case history is especially important in these cases to determine how long the patient may have had eye teaming before it was lost. Recent onset of constant strabismus in adults (and some children) is often associated with systemic or ocular pathology.
3. Which Eye is affected?
Does one eye turn or both? Many people are surprised to learn that most of the time it is better if both eyes are turning as this usually means that the two eyes are functioning fairly equally.
4. Amount of Eye Turn
Is the turn severe or barely noticeable? How far the eye turns is an important consideration in the treatment program. However, it is important to note that a larger degree of turning is not necessarily more difficult to treat with optometric vision therapy than a smaller turn.
5. Sensory Issues
When the two eyes are not aligned (not looking at the same place at the same time) the person’s brain has to figure out what to do with the images of each eye to try to reduce confusion. This can result in one or more of the following:
Double vision - Could be constant or intermittent
Suppression - The person learns how to turn off/ignore the vision in one eye
Anomalous correspondence - The person could learn to match up in their brain the wrong part of one eye with the other eye
All of these factors must be assessed in a strabismus evaluation. When present, a term from each would be used to describe specifically the type of strabismus.
Example: 30-diopter Right Constant Esotropia with Suppression
Generally speaking, strabismic patients tend to be less symptomatic than patients who have non-strabismic functional vision problems. The reason is that they often turn off (suppress) the information from the eye that is turning, thus avoiding double vision and the symptoms that go along with it.
Patients with constant strabismus are typically the least symptomatic because they suppress an eye constantly whereas patients with intermittent strabismus may be more symptomatic as they suppress an eye intermittently and therefore may experience double vision when not suppressing.
Common symptoms of strabismus include: eye strain and/or pain, headaches, blurry and/or double vision, eye and/or general fatigue.
Strabismus Treatment for Adults and Children
The first step in treating strabismus is to ensure that the patient has the best glasses prescription. Our Developmental Optometrist use specialized testing to confirm that the patient’s prescription minimizes visual stress and maximizes visual function. In cases of accommodative esotropia, the right glasses prescription can actually reduce or eliminate the eye turn when they are worn.
The next step is to determine whether there are changes that can be made in the patient’s environment and behavior to improve their visual function. This is referred to as visual stress reduction or visual hygiene.
Depending on the patient’s goals, vision therapy may be recommended to treat the strabismus. There are two types of cures for strabismus: functional and complete.
A functional cure for strabismus means that the patient’s symptoms will be reduced or eliminated, but the strabismus itself may remain. The prognosis for a functional cure is generally very good in most cases of strabismus.
A complete cure is eliminating the strabismus all or part of the time and establishing binocular vision. The prognosis for a complete cure varies based on a number of factors, most importantly the type of strabismus. Generally speaking, a complete cure is more difficult to achieve and therefore takes longer.
Our Developmental optometrist takes time to discuss all of the treatment options available to the patient at the conclusion of the Functional Vision Exam.