Chapter 1: 

The muscles that move the eye are called the extra-ocular muscles. There are six of them on each eye. They work together in pairs—complementary (or yoke) muscles pulling the eyes in the same direction(s), and opposites (or antagonists) pulling the eyes in opposite directions. Below are some basic movements for each of the pairs.

The medial rectus, or nose-side, muscles move the eyes inwardly; when working simultaneously, they converge, or cross, the eyes. The lateral rectus, or temple-side, muscles move the eyes outwardly; when they work simultaneously, they diverge, or splay apart, the eyes. When moving the eyes from side to side—their principal function, they work in teams, so that the normal movements are smooth and coordinated. For example, to gaze to the right, the nose side (medial rectus) muscle of the left eye, and the outside (lateral rectus) muscle of the right contract; their opposites (or antagonists)—the left lateral and right medial recti—relax.  

The vertical rectus muscles—superior moving the eyes upwardly, and inferior, moving them downwardly—are teamed in a similar fashion to the above-described horizontal recti. Because of the angle of the muscles on the eye, these muscles also have an effect on horizontal and torsion (or rotation) movements of the eyes. These secondary and tertiary actions are generally only of importance in complicated strabismus.

The most complicated muscles—both in anatomy and action—are the oblique muscles, superior and inferior. These muscles, like the rectus muscles, also work in antagonist pairs; the inferiors contract while the superiors relax, and vice versa. The inferior oblique muscles turn the eyes upwardly when the eye is looking inwardly, and rotates outwardly (extorts) the eye when looking outwardly. The superior oblique muscle turns the eye downwardly when the eye is turned inwardly, and rotates the eye inwardly when the eye is directed outwardly.

The muscles move in response to nerve impulses carried from the base of the brain (brain stem) by three cranial nerves, named the oculomotor (or III nerve), the trochlear (or IV nerve) nerve, and the abducens (or VI nerve). The trochlear nerve enervates the superior oblique muscle, the abducens nerve enervates the lateral rectus muscle, and the oculomotor nerve enervates the remaining four, plus the levator (or lifting) muscle of the upper eye lid.

Chapter 2:

Strabismus refers to eyes that are out of alignment. The eyes maybe converged (crossed), diverged (outwardly deviated), vertically (one eye higher than the other) or torsionally misaligned (one or both eyes rotated inwardly or outwardly). These planes of alignment (and misalignment) are like the types of movement of boats and airplanes—sometimes called yaw, pitch and roll. 

• Crossing (or deviation of one or both eyes toward the nose) is called esotropia.
• Outward (toward the ear) deviation of one or both eyes is called exotropia.
• Vertical deviation or divergence of the eyes is called hypertropia (higher) or hypotropia (lower).
• Torsional (rotational or tilting) misalignment is called cyclotropia.

Misalignment of the eyes may be constantly manifest—called a tropia—such as above, esotropia, esotropia, etc. Or the deviation may be intermittent, called intermittent esotropia, etc. Surgery may be appropriate for either constant or intermittent deviations, depending on a number of factors, including magnitude of the deviation, constancy or frequency of the misalignment, and the presence of other signs and symptoms.

One relatively simple way to think about strabismus is the concept of the position of rest of the eyes. Therefore, the problem of strabismus is not necessarily that of an abnormal eye. The problem is the angle of deviation between the two. Consequently, the eye that “drifts” is not necessarily abnormal; quite simply, the dominant or preferred eye is “straight”—that is, directed (pointed) toward the object being viewed—while the other assumes the position of rest. The importance of this concept is to explain that to correct the problem requires eliminating this angle of deviation between the eyes. For treatment, muscles on one or both eyes may be repositioned to eliminate or diminish this angle and eliminate the strabismus.

Chapter 3:

There are many medical conditions that are associated with strabismus, and a few of these associations are mentioned in this communication. In general, however, it can be said that the “causes” fall into the following categories: hereditary influences, neurological problems, selected medical conditions, and by far the largest category, unknown. The prevalence of strabismus in the general population is about 4 %. Please remember, not all strabismus requires treatment and a minority of strabismus requires surgery. 

Certain genetic syndromes—Down’s Syndrome being the most common—have a high incidence of strabismus. In some of these conditions, it may be so characteristic as to be expected in most cases. When there is a family history of strabismus (without a known genetic syndrome), with or without amblyopia, the prevalence in existing and subsequent family members can be as high as 25%. Please note two aspects about this number: the overall chances are actually against a child having strabismus if parents have it; and importantly, statistics only have meaning for populations and not individuals. And the hereditary patterns of strabismus are quite variable. In some families and for certain conditions, only a few are affected; sometimes called sporadic or recessive with low genetic expression (penetrance). In others, many family members are affected; and the pattern is more dominant, and/or with higher penetrance of genetic expression.  

Neurological problems encompass a diverse group of conditions that includes developmental problems and delays such as prematurity, cerebral palsy, and head trauma. Depending on the condition, the incidence of strabismus may be as high as 50% of cases. Please be sure to share with your doctor any concerns you may have about your child’s or your condition that may relate to neurological and developmental issues. Please make particular note of any instances of head trauma or fractures of head and face bones, even if in the distant past. There are many medical conditions that may affect the alignment of the eyes. Examples include:

• thyroid disease (also called Grave’s disease),
• myasthenia gravis,
• circulatory problems (including stroke)
• and diabetes. 

Again, please share all you know about your medical condition and its treatment, including any allergies you have. Please include allergies to foods, medicines and other substances, such as latex. Please make special note of any problems with anesthesia experienced by you ore related family members.

Finally, strabismus is, in most cases, unrelated to any of the above. It just happens to some and not others. Therefore, most of the time, there is no known cause or association with other medical conditions. While the prevalence is low—less than 4 per cent—the population is large, making this by far the largest category.

Chapter 4:

Signs of strabismus are those aspects that may be observed by the affected individual, parents or others. These include the misalignment itself (crossing, drifting, etc.), squinting of one eye closed, sometimes rubbing of one or both eyes, and a compensatory head posture.

Symptoms are the feelings or subjective observations of the affected individual. 
There may be no symptoms whatsoever, especially in young children or in persons whose strabismus is long standing. If there are symptoms, these may include double vision (diplopia, or two images seen for one object) or “split” vision (like seeing 1+1/2 images), unstable images, eyestrain or fatigue, headache and an awareness that an eye is moving about; it may feel as if one is “crossing” the eyes, yet the preferred eye feels fine. Importantly, there can be other sensations that are unpleasant relating to the affected persons sensitivity to their condition. These include the awareness that they are different and that others treat them differently as a consequence. This may affect one’s self-image and confidence. Some experience difficulties in a variety of areas including activities of daily living, such as reading and driving; work-related activities, including effectiveness, hiring and advancement; social interactions, including ability to communicate; and personal relationships and interactions, including the ability to maintain eye contact that may lead to embarrassment. 

In certain forms of strabismus, it is possible to control deviation of the eyes by positioning the head, called a compensatory head posture. The head may be turned from side to side, chin up or down, or tilted to right or left. Such head postures may be also effected to control nystagmus (shaking or dancing eyes), or to compensate for large refractive errors (that is, the need for eye glasses). Long standing abnormal head positions may lead to arthritic and other changes in the bones and muscles of the neck and spine.

Only the persons affected and/or their families, in consultation with their doctor(s), can determine the degree to which such signs and symptoms are sufficient to consider strabismus surgery. Doctors can help with information, the perspectives of existing knowledge, experience and then provide recommendations. The remainder of this booklet will provide additional information about the experience of strabismus surgery, so that children and adults with strabismus—and their families—will have additional information to make decisions about care.

Chapter 5:

General Comments About Strabismus Treatment
Many forms of strabismus can be managed with non-surgical treatment, including eyeglasses, prisms in eyeglasses, patching, and in certain circumstances, exercises. In general, each of these approaches has limitations. Eyeglasses may completely control strabismus, as in accommodative esotropia. In other cases, eye glasses may have no effect at all. Prisms are useful for relatively small, stable angles of deviation (strabismus); yet they may need to be made progressively stronger, are expensive, and there are practical limits to the power that may be applied (strong prisms turn light into rainbows!). In children, patching of a dominant eye to improve vision will sometimes, in conjunction with other treatment such as eyeglasses, facilitate improved eye alignment. In certain forms of early childhood strabismus, patching will improve alignment, although sometimes only for a period of time. Exercises may completely control some forms of strabismus and have no effect on others. Your doctor will try all appropriate non-surgical methods of treatment prior to recommending surgery. And many mild forms of strabismus—those with no or very mild symptoms—may require no treatment at all. Overall, only about 25% of strabismus warrants surgery. 

Some forms of strabismus require surgery, and non-surgical methods may only temporize and delay a recommendation for surgery. In other cases, non-surgical means may work for a period of time (even for several years), and then cease to be effective. Generally, deviations of a significant degree (usually 5 degrees of deviation or more, with or without symptomes) and smaller deviations (where symptoms cannot be controlled otherwise) are appropriate to consider surgery. Of course, all factors that are appropriate to a given individual’s needs should be considered in the context of the patient’s desires and best interests.   A decision to proceed with surgery should make sense to all parties to the decision!

 
Indications for Surgery 

Surgery is recommended when strabismus and its effects are “clinically significant”, meaning the angle of deviation is large enough, the condition is amenable, and the adaptations or consequences significant enough to promise improvements in alignment and function. These potential benefits are correlated to the particulars of a given person’s circumstances, be they a child or adult.

A person’s visual system develops and functions in a complex interplay between and among the vision in each eye, alignment of the eyes, the field of vision, fusion of the images from each eye, plus the affected person’s experience and perception of strabismus and its consequences. Early visual development occurs rapidly, and when strabismus occurs, adaptations—including decreased vision (amblyopia) and loss of binocular function (stereopsis or depth perception)—occur equally swiftly. The keys to successful treatment are: prevention if possible, early detection and prompt treatment.

In some individuals, strabismus can be overcome with a compensatory head posture, such as tilting or turning the head. In some of these cases, there may be long-term adverse consequences in the bones and muscles of the head and neck (please see following sections and appendix for examples of when surgery may be appropriate to align the head position). 

In adults, the conditions either are “held over” from childhood strabismus, or acquired later in life, generally spoken of as onset “after visual maturation” (generally after nine years of age). In either children or adults, the benefits of strabismus treatment may be multiple, depending on the type, severity and individual effects of the condition. Treatment is therefore promptly directed to:

• Improve Vision—The earliest adaptation to strabismus in a child is often the development of amblyopia, or decreased vision secondary to suppression of the image from the misaligned eye. Glasses and patching are often required to treat amblyopia. Elimination of the misalignment (strabismus) often will make the task of visual rehabilitation easier. Amblyopia does not occur in adults.
• Align the Eyes—Good alignment facilitates both good vision and binocular vision. As above, eyes that are out of alignment present a significant risk to the vision of young children and their developing sight. Moreover, misalignment precludes the development or maintenance of good binocular vision, including stereovision; persistent misalignment, for as short as three continuous months, may also cause adults to lose (irretrievably) their stereopsis (three dimensional vision). Establishing or reestablishing good alignment can improve binocular visual outcomes; in certain circumstances (not always predictable) binocular vision is completely normalized.
• Align the Head—Abnormal or compensatory head positions may occur with certain strabismus syndromes (for example, Duane’s syndrome, IV cranial nerve palsy, thyroid eye disease) or nystagmus. The long-term consequences may be orthopedic (arthritis, etc.), appearance (asymmetry of the face), social challenges (e.g., teasing), or practical (e.g., wearing glasses effectively). Such problems may be avoided by straightening the head—by moving the eye muscles—and in some instances of nystagmus, vision may be slightly improved.
• Improve Binocular Vision—Binocular vision refers to simultaneous teaming of the eyes and includes the functions of stereovision and depth perception. These functions occur in the brain; having good alignment is necessary (although not necessarily sufficient) for improving the quality of binocular vision. In general, the younger the child, both the risk and the opportunity for binocularity are increased.
• Eliminate Diplopia When Present—Diplopia, or double vision, occurs when eyes have previously been aligned and good binocular function obtained. When the eyes become subsequently misaligned, double vision occurs. This is relatively uncommon in very young children, but may be present in older children. It is common in acquired strabismus in adults.
• Improve the Field of Vision—Crossed eyes, technically called esotropia, will diminish the total field of vision by decreasing side or temporal vision in amounts that are directly related to the degree or amount of crossing of the eyes. Elimination of the misalignment will therefore increase the field of vision for these persons (and will decrease in exotropia).
• Improve Self Image—Persons with misaligned eyes of almost any age above three to four years will develop a sense of difference related to the significance of their strabismus. This awareness will often affect a child’s self-concept (image) and confidence, and may be reflected in shyness or withdrawal.
• Improve Social Interaction—Strabismus may affect the ways in which a family, friends and others will interact with a child, and thereby affect all of their relationships. Older children and adults may experience difficulty in communication related to difficulties in maintaining eye contact. Others may be distracted and behave differently towards persons with strabismus, and this may be hurtful to them.
• Improve Employment Opportunities—Children with strabismus may experience, perhaps in part related to the above, some perceived and actual limits to career options and advancement. Strabismus is a disqualifier for certain occupations.   Other persons will occasionally and quite inappropriately question the intellectual capacity of those with strabismus.

Strabismus at any age has multiple and complex effects. The sooner strabismus and its associated problems are identified, the longer and more profoundly the benefits may be enjoyed!