What is Strabismus?
Strabismus is a visual defect in which the eyes are misaligned and point in different directions. Strabismus is a fairly common condition among children – approximately 3% of the population has an eye misalignment of some sort. Strabismus can also occur later in life, for a variety of reasons.
There are three general types of eye misalignment – the eyes can turn towards the nose (eso-deviation), towards the ears (exo-deviation), or one eye can be higher than the other (hyper-deviation). Notice that the plural “eyes” is used here–even though only one eye may appear to be deviating, in fact, it is almost always the case that the misalignment involves both eyes. Even when one eye appears to look straight ahead and the other eye appears to turn in another direction, it is more accurate – and useful – to consider the eyes as misaligned relative to each other.
For example, if both eyes actually turn in a little towards the nose (eso-deviation) but the right eye is in the straight-ahead position (it is the “preferred” eye), this will cause the left eye to shift towards the right (towards the nose) even more. Understandably most people would say, “the left eye is turned and the right is straight” but, in fact, the eyes are turned towards each other.
Strabismus can be present all of the time (a tropia), or only when the person is fatigued, sick, or under testing conditions (a phoria), or somewhere in between (an intermittent tropia). Therefore, someone can have an eso-phoria (eyes turn towards the nose only when tired, etc.), an intermittent eso-tropia (eyes sometimes turn towards the nose on their own), or an eso-tropia (eyes always turn towards nose).
Why Do We Need Properly Aligned Eyes?
With proper alignment of the eyes, both eyes look at the same object at the same time and send two separate (and slightly different) pictures to the brain. There are specialized brain cells (binocular cells) that take these two slightly separate pictures and put them together into a single three-dimensional (3-D) perception; only when both eyes point in the same direction at the same time is 3-D vision (depth perception) possible.
With strabismus, the eyes are looking in very different directions, so the brain is given two very different images to interpret. The binocular cells cannot combine these very different pictures into a single perception, so depth perception (3-D vision) is not possible; the brain perceives two separate pictures. When a young child develops strabismus, their brain avoids double vision by ignoring one of the pictures – normal binocular cells cannot develop, or are lost, in children who ignore one image. Such children lose their depth perception, sometimes irreversibly. Adults who develop strabismus cannot just ignore the second image, so they develop double vision (diplopia). Adults who develop strabismus typically have normal binocular cells but these cells are not being used when the eyes are misaligned.
By convention, when one eye is positioned higher than the other, the higher eye is the one that is labeled as being out of alignment – this is called a hypertropia and the higher eye is “hypertropic” relative to the other eye. This is the case even though the problem may actually be that the other eye is lower (hypotropia). Again, the important point here is that the problem is the alignment of the eyes relative to each other.
One of the most common causes of hypertropia is a problem with the eye muscle that rotates the eye down and in towards the nose (the superior oblique muscle). When the eye cannot be fully rotated down and in towards the nose, the muscle that balances the superior oblique (called the inferior oblique) is unopposed and does its job exceedingly well. The function of the inferior oblique is to rotate the eye up and out towards the ear. Therefore, when the superior oblique is weak (paretic) or not working at all (palsied), the inferior oblique is unopposed and it twists the eye up and out – to avoid double vision, people with this problem will tilt their head to make up for the eye being rotated.
Superior oblique palsy often presents in early childhood with a head tilt. When diagnosed in an infant, it is best to correct a superior oblique palsy before too much time has passed because a head turn before the age of 2 years is associated with permanent changes in the development of the face. Also, the neck muscles and bones can be affected by the head tilt. In older children and adults, most cases of superior oblique palsy have actually been present since childhood but have become more pronounced with time – these patients often complain of eyestrain or “headache” and may not realize that they are tilting their head. In older adults, superior oblique palsy can be associated with atherosclerosis, diabetes, or high blood pressure – this type of strabismus often gets better on its own. Regardless of the age, or the cause, superior oblique palsy can be managed very effectively when properly diagnosed.
Another common cause of hypertropia in adults is thyroid disease, which can affect the eye muscles (Graves’ ophthalmopathy). Graves’ ophthalmopathy can occur in people whose thyroid gland is overactive (hyperthyroid), underactive (hypothyroid), or even normal (euthyroid). Surgery is often needed to eliminate the double vision and abnormal head position.
At other times, a hypertropia is due to a misalignment of the horizontal muscles of the eye; getting the eyes horizontally aligned will then correct the hypertropia. Other causes of hypertropia include stroke, head trauma, tumor, and certain inflammatory disorders.
There are two major forms of childhood esotropia (and many less common causes).
A. Accommodative Esotropia
The most common form of childhood esotropia occurs because of high hyperopia (if this applies to your child, then Dr. Lichtenstein will give you information about hyperopia). With hyperopia, everything is blurry until the brain sends a signal to the lens inside of the eye to change shape – this puts images into focus and sends a clear picture to the brain. The process of the natural lens changing shape is accommodation. At the same time that the natural lens changes shape (accommodates), the eyes turn inwards (eso-deviate) – this is a hard-wired reflex and applies to everyone. With high hyperopia, the necessary lens change (accommodation) is large, so the corresponding eye turn (eso-deviation) is large. This type of esotropia typically presents around the age of 2 or 3 years and is called accommodative esotropia. Often, accommodative esotropia starts out as a “once in a while” problem and becomes more and more frequent; sometimes, babies as young as a few months can be affected.
Accommodative esotropia is managed by putting hyperopic glasses in front of the eyes to relax accommodation. If the lens inside the eye is not doing the work of accommodating, then the eyes will not turn in – thus, wearing glasses controls the esotropia. Sometimes, even when the appropriate hyperopic glasses are used full time, the eyes still turn in; this is called a residual esotropia and surgery may be indicated for correcting the residual amount of esotropia (remember, glasses are still needed after surgery to keep the eyes straight).
B. Infantile Esotropia
The other major category of esotropia is called infantile esotropia. As the name implies, the eyes turn in because of a problem that manifests early in life; by definition, these patients display their esotropia within the first 6 months of life. Many pediatric ophthalmologists believe that people with infantile esotropia have a basic predisposition in the brain for the problem and something in their environment causes the problem to manifest. The ideal treatment for this condition would therefore involve “tweaking” that part of the brain that is misbehaving. Unfortunately, the exact location of this defect remains unknown and the ability to “tweak” brain problems has not been developed yet.
Instead, this problem is treated by surgically aligning the eyes to a straight position. If this is done at an early enough age, the brain can learn where “straight-ahead” is for the eyes. In fact, there is evidence that some form of depth perception (binocularity) can be established by early surgery (before the age of 2 years, but earlier is better). It is important to remember that the basic problem remains; the brain’s control of eye alignment is essentially faulty. People with congenital esotropia are more likely than other people with strabismus to develop other types of strabismus and to require further surgeries. While on average, two surgeries are required, about 80% of people with congenital esotropia require just one surgery to achieve straight eyes.
Exotropia typically develops in a progressive course, as detailed below.
First, the eyes rarely turn outward, usually only when the patient is tired or sick. As you know, when strabismus occurs with fatigue, or only under testing conditions, this is called an exophoria.
Next, the eyes start to spontaneously deviate outwards, and then straighten out moments later. This is an intermittent exotropia. When an intermittent tropia is present, it means that the brain is unable to keep the eyes aligned all of the time, but has the capability to keep them aligned some of the time. At this point, depth perception is still present but is at risk of being diminished or lost.
Finally, as the intermittent tropia becomes less controlled by the brain, the eyes will start to turn outwards more and more frequently – once the eyes are constantly turned outwards, this is an exotropia.
It is preferable to operate on an intermittent exotropia because we know that the binocular cells (in the brain) are still functioning and have the capacity to keep the eyes aligned at least some of the time. Once the eyes are constantly exotropic (always turned out), the binocular cells may be damaged to the point that binocularity has been lost – in this case, there is no stimulus for the eyes to remain straight.
So, when is the best time to operate? As an intermittent exotropia becomes more frequent. The decision to operate is based on the frequency of the exo-deviation, not the size of the turn. For example, a small constant exotropia can be operated on at any time, but a large intermittent exotropia that occurs infrequently is probably not ready for surgery.
A few more things about exotropia: First, people who are myopic (near-sighted) and have an exotropia should wear their glasses all of the time, because this can help to limit the frequency of the exotropia. Second, there is one form of exotropia where eye exercises can help – if the eyes turn out more when looking at something up close than when looking at something far away. In this situation, which is called convergence insufficiency, specific eye muscle exercises can be effective if performed properly. Convergence insufficiency is the only type of strabismus that has been shown to respond to eye movement exercises (orthoptics).
Duane Syndrome is a type of strabismus where the eye rotations are abnormal due to abnormal innervation from the brain. In other words, the wiring to the eye muscles is misrouted and this causes the eyes to move in abnormal ways. This is a congenital (present at birth) problem and most of the time (70%) there is nothing else wrong. However, there can be associated abnormalities (Duane Plus) in approximately 30% of these patients. More commonly associated findings include abnormalities of the cervical spine and other parts of the skeleton (such as the forearm) as well as hearing deficits.
In terms of eye movement, Duane Syndrome is characterized by A) abnormal rotation of the eyes which leads to B) misalignment of the eyes (strabismus) and C) an abnormal head position (torticollis) to compensate for the strabismus. Because the eyes do not move properly the patient ends up seeing double and will turn their head in order to avoid this (torticollis). Typically, torticollis becomes apparent early in life, as soon as a baby can control their neck. Most babies have good neck control by 3 to 4 months. However, some parents do not notice the strabismus or the torticollis until the baby starts to walk.
By convention there are three general categories of Duane Syndrome. The most common is Duane Type 1 (70%). In these cases, the eye does properly rotate outwards (towards the ear) and the affected eye turns in. These patients turn their face in the direction that allows them to get rid of the esotropia. By looking towards the nose on the affected side and turning their face the esotropia goes away. For example, someone with Type 1 Duane of the left eye has an esotropia; their left eye wants to look to the right. By turning both eyes to the right, the esotropia goes away; the simplest way to do this is to turn the face to the left. If you straighten their head and have them look straight you will see the esotropia. Another finding is a change in the space between the lids depending on where the patient looks. When the eye is looking towards the ear, the space between the lids becomes wider; when looking towards the nose, the space becomes narrower. This is because of the innervational abnormalities.
Duane Type 2 shows an abnormality when looking towards the nose. As a consequence, these patients have the affected eye turn outwards (exotropia). For example, if the left eye has a Type 2 Duane it is exotropic (looking towards the left ear), so the patient avoids double vision by turning both eyes towards the left. To keep both eyes looking left, you would turn the face to the right. The abnormal eyelid findings occur here, too.
Duane Type 3 occcurs when both Type 1 and Type 2 occur on the same side. The eyes are typically straight when looking straight ahead, but to avoid double vision these people must to turn their face because one of the eyes does not move. Type 3 Duane is rare.
Duane Syndrome can affect one or both eyes. There can also be “up-shoots” and/or “down- shoots” when the affected eye(s) rotate. Finally, over time there is often contracture (“stiffening”) of the muscles around the eye(s) which can change the character of the strabismus.
The treatment of Duane Syndrome is surgical. There is no benefit in waiting if torticollis is present; in fact, waiting can lead to contractures and new surgical problems. Sometimes more than one surgery is necessary but this goes for all strabismus. Because each patient has their own “dose” of rotational abnormality, torticollis, and (possibly) contracture, surgical planning varies with the individual. However, there is one common goal: to provide a straight head with straight eyes at the same time. In general, repair is accomplished by releasing any contractures and differentially weakening muscles. Surgery can almost always make the problem better, but these patients cannot be cured. There will always be an abnormality in their eye movements.
Under normal circumstances, the eyes move together (converge) towards the nose when one reads or looks at something up close; one could say that the eyes “turn in” to this extent. Some people have difficulty getting their eyes to sufficiently come together (converge) when looking at something up close; they have a convergence insufficiency.
Symptoms (what the patient feels) occur when reading or looking at something up close – these symptoms include an achy feeling, headaches, blurred vision, words and letters “melting” or “blurring” into each other, closing one eye, avoidance of reading and visual other tasks, and double vision. Signs (what the doctor can observe) include an insufficient ability to converge the eyes (hence, the name of the problem) and, often, an exo-deviation (outwards turn of the eye) that is larger when looking at something up close than when looking at something far away.
Convergence insufficiency is the only type of strabismus that can be helped with eye exercises (orthoptics). There are two basic types of orthoptic exercises – one depends on the patient appreciating double vision and the other does not. We are experienced in managing this problem and will determine which type of orthoptic exercise(s) is best for a given patient.
The most important thing that you need to know about convergence insufficiency and orthoptic exercises is this: nothing will get better without doing the exercises. While orthoptic exercises can be effective, the most common reason for their failure is that the patient did not do them. Of course, there are cases when diligent patients do not get better even though they performed the orthoptic exercises the proper way; in these situations, surgery may be indicated.
Eye Muscle Surgery Benefits
Eye muscle surgery is performed to correct a misalignment of the eyes (strabismus). Strabismus is a broad term that includes any misalignment of the eyes, it can include eye drifting (exotropia), eye crossing (esotropia) or if one eye is higher or lower than the other (hypertropia or hypotropia). When thinking about strabismus it is important to remember that we have two eyes – strabismus describes an abnormal relationship between the eyes. When the eyes are pointing in different directions the brain has to deal with two very different images. In young children, the brain can (and does) “turn off” one of the misaligned eyes at any given moment; this damages depth perception (3-D vision) and can cause vision loss (amblyopia). In older children and adults the brain can not turn off one of the eyes so the misalignment causes double vision, which sometimes lead to abnormal head positions in an attempt to get rid of the double vision.
Depending on the type of strabismus, eye muscle surgery can be performed before the first birthday.. Most evidence indicates it is better to perform surgery early in life because the brain cells for binocular (stereo) vision are still developing. For adults with strabismus, surgery is effective at eliminating double vision and can often restore depth perception.
In addition to restoring binocular vision, strabismus surgery may be indicated to correct a head tilt or face turn. Reconstructing normal eye alignment also has important social and psychological benefits for many people. Children with misaligned eyes may start having a negative self-perception at around 4 or 5 years, when their peers might begin to point out and ask questions about physical differences. Most teenagers and adults with strabismus are self-conscious about it and this often affects their self-esteem. Regardless of how handsome or beautiful they actually are, many people with strabismus feel unattractive. In this regard, the benefits of eye muscle surgery are not cosmetic. Cosmetic surgery changes a normal body part. On the other hand, reconstructive surgery restores (or creates) a normal anatomy where it did not exist before. Looking like other people –whether it is having straight eyes, or a straight head, or both – is reconstructive, not cosmetic. This is not vanity and it is a big deal for these people.
Eye muscle surgery is performed as a same day procedure; there is almost never a need to stay overnight in a hospital.
For all children and some adults, eye muscle surgery is performed with the patient fully asleep (general anesthesia). Some adults are good candidates for sedation with local anesthesia if they prefer. Once the anesthesia has taken effect, the eye muscles (often of both eyes) are carefully repositioned; the eye is NEVER taken out of the socket! The eye muscles sit on the sclera (white of the eye) and are not actually “inside” the eyeball. Covering the muscles and the sclera is a thin skin, the conjunctiva.
The technique is as follows: The eye is carefully positioned, then a small opening is made in the conjunctiva. Surgery is performed through this small opening and the muscles are repositioned (recessed means moved back, resected means pulled forward). The amount of repositioning is based on published charts and the surgeon’s experience. At the end of surgery the conjunctiva is closed with stitches that dissolve on their own. Lasers are not used in eye muscle surgery.
Prisms are special type of lens that may help get rid of double vision in some cases. Prisms can never make the eyes straight, but they can often be used to control double vision. Eyeglasses are useful for certain types of strabismus and should be tried when indicated. Finally, there is ALWAYS the alternative of not doing surgery. When we recommend surgery, it is because we consider it the best option for achieving better alignment of the eyes and/or head as well as the preservation of depth perception.
Risks and Possible Complications
The eyes will be red for a few weeks after surgery. This is not a risk of surgery because it is a given. This is not a complication because the redness is expected and does not represent infection, a risk to vision, or a permanent problem. Over the course of several weeks (sometimes, a few months) the eyes return to their normal white color.
Scarring is a part of healing. Eye muscle incisions usually heal very well – it is rare that the scar can be seen without a microscope. To further hide the scar, the conjunctival incision is usually made so that the eyelid covers it. However, everyone heals differently and a scar may be visible. If this happens, and it is bothersome in any way, scars can be surgically revised.
Undercorrections and overcorrections can occur. While most people respond predictably to a given amount of surgery, some do not. Surgeons take individual factors into account which is why surgery is a science and an art.
Temporary double vision can occur because the brain must adjust to the new eye position. This does not happen often, but when it does, it usually last for several days to several weeks. Most children tolerate this well and are not bothered by it. Some adults are troubled by double vision for longer and may be prescribed prism in their glasses (or even a patch) to eliminate the double vision until the problem resolves on its own. If double vision does not resolve on its own, then additional surgery may be needed.
Re-operation is required in 10 to 20% of patients (and more over a long period of time). Re-operation is more likely if one of the eyes has very poor vision to start with. In this case, the likelihood of re- operation is about 50% over a lifetime. Of course, the surgeon’s goal is to achieve straight eyes that stay that way forever and every effort is made to achieve this goal. Re-operation is performed from 3 months to many years after the first surgery since it takes time to ensure that the results of the first surgery have stabilized. It is almost always possible to perform a re-operation if one is indicated.
Serious complications such as vision loss, infection, or bad anesthetic reactions are very rare. Eye muscle surgery is one of the safest of all eye operations – in terms of the risk of very bad complications, it is safer than cataract surgery (the most commonly performed operation in the U.S.) and safer than laser vision correction (PRK and LASIK).
Where do we perform surgery?
We perform our surgeries at these locations:
The New York Eye & Ear Infirmary (NYEEI) at 14th Street and 2nd Avenue.
Manhattan Eye, Ear, Throat Hospital (MEETH) at 64th Street between 2nd and 3rd Avenue.
Additional information from AAPOS: Strabismus.