Strabismus is the general medical term for eye misalignment. Strabismus is a major concern in younger patients because it can lead to vision loss from amblyopia, which is discussed in another section of this website. However, it may also be an issue of concern in teenagers and adults as well since it can cause double vision, produce difficulty with reading or near work, reduce or eliminate depth perception, or become the source of social issues.
There is often confusion discussing strabismus with patients because of terminology. Non-medical people may sometimes refer to strabismus as “lazy eye.” That term is also frequently used by the general public to describe amblyopia, and thus can lead to miscommunication. Although often related, strabismus and amblyopia are two separate issues. Additionally, outside the United States, strabismus is often referred to as “squint.” Here that term generally refers to the squeezing of the eyelids to improve vision or to reduce glare in bright light.
It is impossible to cover all aspects of strabismus in this page. However, to provide a basic overview, it most helpful to organize it into five basic categories:
- Horizontal – The eyes turn either inward (esotropia) or outward (exotropia).
- Vertical – The eyes turn either upward (hypertropia) or downward (hypotropia).
- Paralytic – One or more of the eye muscles does not work properly because the nerve controlling it is damaged.
- Restrictive – One or more of the eye muscles is prevented from working properly.
- Special Syndromes – Types of eye misalignment that are unique enough to be listed separately.
- Horizontal Strabismus:
The most commonly encountered type of strabismus is a horizontal deviation. These patients can be divided into two large groups depending on whether the eye turns in (esotropia) or out (exotropia). Each of those groups has several subcategories as well.
Esotropia: The two most common types of esotropia are essential (also known as congenital or infantile) and accommodative. Essential esotropia occurs within the first few months of life. These children usually have little or no refractive error (need for glasses), a large inward eye turn, and generally require surgical correction.
Accommodative esotropia is an eye turn that is caused by high hyperopia, or farsightedness (a type of refractive error discussed in another section of this website). The hyperopia forces the individual to actively focus the eye’s lens to obtain clear vision even at a distance. This focusing is called accommodation. Normal people only accommodate to see near objects, much like a camera lens is adjusted to take a close up picture. There is a strong, built-in brain reflex, called accommodative convergence, which crosses the eyes inward when focusing the eye lenses. This keeps both eyes pointed at the near object. Because hyperopic people need to accommodate even to see at a distance, the reflex to turn the eyes inward is activated even when it should not be. Patients with this type of esotropia are therefore treated with glasses. Surgery is not indicated unless an eye turn continues to be present even when glasses are worn. Additionally a small group of children with this issue require bifocal glasses because the eye turn is much worse when they perform near work.
Exotropia: Exotropia is an outward wandering of the eyes. Although the tendency may be present from birth, it is not typically noticed until the age of 3-5 years and begins as an occasional, intermittent turn. (In fact, although a large constant exotropia can occasionally occur in infancy, it is uncommon enough to require a workup for other possible neurologic problems.) Typically exotropia is noted mostly when the patient is tired such as late in the day, is inattentive, or is upset. Often, people with exotropia will close one eye in conditions of glare, such as a sunny day at the beach, to avoid double vision which occurs when the eye begins to drift. Patients with exotropia frequently are also myopic (nearsighted). Treatment includes glasses if needed underlying refractive errors, patching for amblyopia if present, or surgery depending on the frequency of the turn. Exercises can be used in mild cases to delay other interventions or to reduce near vision issues temporarily, but these do not eliminate the eye turn.
- Vertical strabismus:
In these cases, one eye is higher relative to the other eye. In most cases, attention is drawn to the higher eye. There can be many causes to a vertical misalignment. Two of the most common are discussed in this section. Two others, thyroid ophthalmopathy and Brown Syndrome, are discussed in later sections.
Superior Oblique Palsy: A very common type of hypertropia is called a superior oblique palsy (also called fourth nerve palsy or trochlear nerve palsy). The superior oblique muscle is one of the four muscles that control vertical movement of the eye. It is supplied by a nerve called Cranial Nerve IV, also called the Trochlear Nerve (hence the alternate names). If the nerve is not working properly, the muscle will not work properly, and the eye will deviate upwards when it is turned in towards the nose. This is seen commonly both as a congenital problem but also after head trauma. Patients with this problem will often tilt their head to in an effort to keep the eyes properly aligned. In fact, in many cases the initial symptom of this problem is a head tilt and an eye turn is not suspected. Because of this, it can often be confused with torticollis, a problem with the neck muscles which also results in a head tilt. When required, treatment is surgical, but the need for treatment depends on the severity of the problem.
Dissociated Vertical Deviation: This problem, often abbreviated DVD, is a condition where one or both eyes will occasionally wander upward while the opposite eye remains completely straight. It has this name because unlike most other types of strabismus, the wandering eye is moving with complete lack of any movement in the other eye. The cause is unknown, but this is often seen in patients with other forms of strabismus, such as esotropia. Treatment is surgical, but it is often not needed as long as the eye remains controlled most of the time.
- Paralytic Strabismus:
As the name implies, these types of eye misalignments occur with some type of nerve damage. These differ from other types of eye misalignments in the pattern noted on examination. Common causes for paralysis of an eye muscle or nerve include trauma, strokes, and damage to the controlling nerve from damage to the small blood vessels seen in diabetes and hypertension (high blood pressure). Names often associated with this are 3rd Nerve Palsy, 4th Nerve Palsy and 6th Nerve Palsy. Treatment in many of these patients is delayed for months because there can be improvement after the initial event as the nerve repairs itself.
- Restrictive Strabismus:
Anything which prevents the eye muscles from moving properly will cause an ocular misalignment by restricting or limiting eye rotation. Two common causes of this are orbital (eye socket) fractures and thyroid related ophthalmopathy. In the former, bone displacement physically restricts muscle function. In the latter, an auto-immune process often seen in association with certain types of thyroid disease, causes progressive damage to and swelling of the eye muscles which become fibrotic (stiffer) and fail to move properly.
- Special Syndromes:
Two types of strabismus are listed here because of their unique nature, even though they have characteristics of some of the general types of strabismus listed above. These are Brown Syndrome and Duane Syndrome.
Brown Syndrome: This is a restrictive type of strabismus where the superior oblique tendon sheath is scarred. As mentioned above, the superior oblique muscle is one of the muscles responsible for vertical eye movement. This muscle is unique because of its anatomy. It actually begins behind the eye and then passes through the trochlea, a curved band of tissue near the rim of the orbit (eye socket) before heading back into the orbit to attach to the back of the eye. In essence, it is like a rope going through a pulley at about a 45-degree angle. In these patients, the muscle tendon is scarred or abnormal, and it cannot move properly through the trochlea to allow the muscle to fully relax, like a rope with a knot in it getting stuck in a pulley. This results in an inability to raise the eye when it is looking in at the nose. (The opposite of a superior oblique palsy described above.) This problem is most often congenital (present at birth), but sometimes can be acquired. Treatment is surgical, but the decision to treat depends on the severity of the problem. Patients with no head tilting or vision problems when looking straight ahead are usually only observed.
Duane Syndrome: This entity is actually a microscopic malformation in the brainstem of the two nerves which control horizontal eye movements, the 3rd and 6th cranial nerves. Basically, these nerves are short-circuited. This results in a variety of possible eye movement problems which can include inability to move an eye outward, inward, or horizontally at all. At times, the eye can literally be pulled into the eye socket slightly, causing the lids to seem to close. There are also sometimes significant vertical movements with attempted side gaze, called up-shoots or down-shoots. Surgery cannot eliminate this issue, since the nerves cannot be repaired or “rewired”. However, some of these patients may need to keep their head in a turned position to align the eyes properly. In such cases, surgery can be done to reduce or eliminate the head turn.