Understanding Strabismus
Strabismus is a condition manifested by the misalignment of the two eyes. Commonly used terms describing the different appearances of this problem include "squint", "walleyed", "cross-eyed", and "lazy-eyed." In each case, it becomes obvious to the observer that a strabismus patient seems to be looking in one direction with one eye and some other direction with the fellow eye. The human brain can only attend to one visual image at a time, and so it acts as if these patients are using only one eye. The turned eye, although receiving light rays and converting them into neural transmissions heading for the brain, does not really see because the brain is ignoring the incoming messages! Strabismus can develop at any age from the newborn period to old age. The implications of strabismus vary dramatically with the history of when and how the turn began.
Strabismus in Childhood
Sometimes children under two months of age have misalignment of the eyes. Usually this is seen as a slight or variable outward turning of the eyes. Crossed eyes in an infant is much less common, and is abnormal if it has not resolved spontaneously by two months of age. In fact, any misalignment of the eyes that has not resolved by three months of age is unlikely to resolve spontaneously. In all cases, children with misaligned eyes should be brought to the attention of a Pediatric ophthalmologist, an ophthalmologist with extra training in the diagnosis and treatment of childhood eye disease. The pediatric ophthalmologist will assess the vision and refractive error (need for spectacles) and determine whether one eye is weaker than the other, a condition called amblyopia. He will also measure the degree to which an eye is turned and whether the eyes are capable of working together. Should the doctor find that one eye is weak the doctor may prescribe patching or medically blurring the stronger eye so that the weaker eye can "catch up". If the misalignment of the eyes persist despite efforts to improve the vision in the eye, surgery may be required to align the eyes.
Amblyopia
Amblyopia, also called "lazy eye", is a decrease in vision in an apparently healthy eye. It is caused by the abnormal development of the brain's connections for the eye that is not getting a clear image, either because the light cannot get through the eye (such as when it is blocked by a droopy eyelid or cloudy lens), or the light cannot be focused by the eye (such as when the eye is very nearsighted, far-sighted or astigmatic), or because the eye is turned and not seeing the same things as the fellow eye, forcing the brain to ignore the image from the turned eye. Amblyopia can be detected in the pediatrician's or family practitioner's office when the vision is carefully assessed. These vision screenings should be performed before the child is three years old, since school screenings may not detect a problem, and the closer to age 9 years the problem is detected, the lower the possibility that good vision can be returned to the eye. This is because the visual pathways complete development by this time. Recently, a new device called the MTI Photoscreener has become available for examining very young children. It uses instant photography to identify children at risk for amblyopia by as early as 6 months of age, and may therefore permit earlier intervention. Treatment for amblyopia involves improving the vision in the weaker eye by whatever means are necessary, and temporarily decreasing vision in the stronger eye so that the brain is forced to attend to the weaker eye. With care, good vision to the weaker eye can be restored in days to weeks.
Strabismus During the Adult Years
When the eyes become misaligned in the adult years, they can do so either suddenly or gradually. When gradual, the turn is usually an outward movement of an eye, and the turned eye usually has poor vision from a cataract or other intra-ocular disease. When the eye becomes misaligned suddenly, it turns in almost any direction, and there is usually a history of significant trauma to the head or serious systemic illness such as diabetes, cancer, or inflammation, so that the nerves controlling one or more of the eye muscles becomes damaged. Sudden misalignment of the eyes should be considered an emergency. Although in many cases a patient can be offered simple reassurance and expect the eyes to re-align within weeks to months, other times a grave situation exists where every hour counts. When an adult develops a misalignment from poor vision, the therapy is to improve the vision if possible, so that the normal visual system can re-align the eyes. If this does not occur, the patient can be offered surgery to re-align the eyes or can try special glasses that re-align whatever the patient might be looking at! In cases where a poorly seeing and turned eye cannot be made to see well, there is also surgery available to turn the eye so that the misalignment is less cosmetically displeasing.
Visual Training Exercises to Correct Strabismus
When a child's amblyopic eye is turned, it may be difficult to determine which came first, the misalignment or the poor vision. In either case however, the pediatric ophthalmologist will try correct the vision in the weak eye as much as possible with spectacles, patching of the good eye, etc., so that the brain will attend to the amblyopic eye. Eventually, the brain may become equally satisfied with the vision in both eyes so that it naturally aligns the eyes permitting them to work together.
When an adult's eye turns out in association with fatigue or headaches when performing close visual tasks, they may have a problem called convergence insufficiency. In this condition, the patient often has a tendency to be walleyed, and the brain is already doing all it can to keep the eyes straight for distance, finally giving out when it has to make the eyes come together (i.e., converge) for reading. In this situation, exercises can be used to strengthen the muscles controlling the eyes and the patient may resume normal studies. These exercises are easy to do and can be done at home, after an eye doctor has measured the deficit and assured the patient that nothing more ominous is occurring.
Spectacle Correction of Strabismus
When a child develops crossed-eyes around the age of 2 to 3 years old, it often is because of a condition called accommodative esotropia. In this condition, the patient is usually rather far-sighted and the brain must use a great deal of energy to thicken the natural lenses to help the eyes adjust to see something close (i.e., accommodate). The process of accommodation also involves the eyes turning in toward each other, but in accommodative esotropia the eyes often turn too much because they are responding to the overly-powerful accommodation signals sent to the lens. This condition is often alleviated with spectacles that relieve the lenses of the work of accommodating, i.e., reading glasses. This reduction of accommodative effort will often reduce the signal for the eyes to turn in too much and the esotropia will completely resolved. In cases like this, a child may often be weaned off the spectacles as they grows. In other cases, the spectacles will not completely resolve the esotropia, putting the patient at risk for amblyopia. This residual strabismus is usually correctable with surgery and therapy should be sought as quickly as possible.
Surgical Correction of Strabismus
Strabismus surgery is essentially using a mechanically-oriented therapy to alleviate a neurophysiologic disorder. When the eye muscles are not balanced, the eyes will be misaligned. In cases where the imbalance cannot be restored by medication, spectacles or patching, surgery is required. Here, the muscles that move the eye are detached, occasionally shortened, and secondarily reattached after they are repositioned in such a way that their normal (or abnormal) tone helps the eyes realign. It is crucial for the ophthalmologist to make careful measurements of the misalignment of the eyes before surgery so that the eye muscles can be manipulated correctly. A small error can lead to very bad results, while good measurements can give a patient the chance to use both eyes together and enjoy the blessings of good depth perception.
