Figure 1. Congenital esotropia at age four months - left eye convergent
Strabismus is primarily a condition of the childhood years. With early identification and proper treatment, most cases can be solved and limited to this period. Failure to obtain adequate treatment early in life, however, often results in a lifelong pattern of poor vision and/or abnormal ocular alignment.
Children with strabismus are generally referred to as being "crosseyed." Although this term seems to imply an observed crossing direction, experience quickly teaches that neither parents nor most physicians seem to distinguish easily between children whose eyes actually turn in (are convergent) and those whose eyes deviate outward (are divergent) or those with the less common vertical or torsional deviations. Parents' descriptions mav suggest that the child's eyes have a "vague staring appearance" or "don't track well together" or that there may be a "cast" in one eye. Other observations are that the child's eyes "just don't look quite right" or "don't move properiy."
Strabismus may be present for many reasons, and it often becomes the pediatrician's responsibility to determine whether there is an actual deviation of the eyes (Figure 1) or whether this may merely be another case of Pseudostrabismus - that very common condition in which a young child's eyes appear to be crossed but are actually quite straight (Figure 2). In the former cases consultation with an ophthalmologist to determine the nature and proper management of the deviation is essential at the earliest possible time; in the latter, simple observation by the pediatrician with reassurance of the parents may be all that is necessary.
Since strabismus is associated with such a high incidence of poor vision (amblyopia, or lazy eye), it becomes extremely important that those responsible for children's primary care be familiar with the elements of family history and the signs and symptoms that will permit early recognition of these problems.
Although most children obviously have good vision and straight eyes, it is the rare person, child or adult, who maintains perfect ocular alignment in all seeing tasks. These minimal tendencies for eyes to turn slightly in or out occur so commonly that we generally ignore the fleeting phenomenon of double vision. Diplopia or blurred or overlapping images occur most often during times of fatigue, stress, or illness. That strabismus occurs in only 1 to 3 per cent of the population attests to the inherent normality of the numerous mechanisms that modify and control our ocular alignment, including visual acuity.
Figure 2. Pseudostrabismus. Broad, flat nasal bridge with prominent epicanthal folds.
It is appropriate to state here a very basic and obvious principle of eye care: the human condition is best served when two healthy eyes possess good vision and are properly aligned early in development so that each person gains the opportunity to see with both eyes simultaneously, affording a single integrated binocular image with fusion and depth perception.
When the presence of any type of ocular deviation in children is discussed, it is helpful to explain this principle to the parents carefully, stating what I consider to be the primary goals of visual development for every child. Ideally, we are trying to ensure the proper development of three distinct but intimately interdependent functions.
First, and of primary importance, is the development of good vision in each eye separately. This means the attainment of the best possible visual acuity by whatever means necessary.
Second, each child should acquire the best possible ocular alignment, and the eyes should be essentially straight, particularly in the primary or straight-ahead position and the eyes-down reading position.
Third, with good vision and straight eyes the child will have the opportunity to develop fusion and depth perception. Our visual system is constantly making adjustive movements to maintain each eye in its proper orientation for single binocular vision and fusion, which is the cortical integration of the two separate images into a single visual perception.
It is also helpful for parents to understand that we can obtain a great deal of information from even the youngest of infants. Specifically, we can assess the growth and development of the ocular structures, the functional integrity of the visual mechanisms, the central nervous system functions that are intimately related to ocular movement and controls, the refractive or optical status of each eye, and even the fusion potential. The quality and quantity of information elicited and its reliability will obviously depend on the child's age and attention span. In children below the age of two and a half, we rely primarily on objective testing methods; beyond this age we are generally able to elicit subjective responses.
With this basic information an ophthalmologist accustomed to working with children can accurately determine the nature and cause of most types of strabismus and mobilize whatever specific treatment measures may be indicated. Such measures may include the use of corrective lenses, occlusion or patching to stimulate the poorly seeing eye, medications, orthoptics or visual exercises, and, in certain instances, more definitive surgical treatment. Most important for the parents to keep in mind are the final goals of good vision: straight eyes and the development of fusion and depth perception.
In order to understand the types of deviations most frequently encountered and to obtain some appreciation of both the underlying causes and the specific treatment measures found to be effective, we must proceed with an explanation of certain basic visual and ocular considerations.
BASIC VISUAL AND OCULAR CONSIDERATIONS
There are numerous factors that influence the alignment and movement of the eyes. Four deserve special consideration, since any significant variation from normal will constitute a major obstacle to the development or maintenance of stable ocular alignment.
1. The basic position of rest of each eye.
2. The development of normal vision in each eye.
3. The neurologic innervation.
4. The fusion mechanisms.
The basic position of rest of each eye is its natural anatomic orientation as would be reproduced in deep sleep or under general anesthesia. The direction of its anteroposterior axis is unrelated to the fellow eye and is determined by its gross anatomy and tissue structure exclusive of any tonic, innervational, visual, or stress influences. This position obviously depends on several basic physical considerations, including the size, shape, and direction of each orbital cavity in its craniofacial bony casing; the anatomic development and symmetry of each globe independently; and the structure, attachments, and elasticity of each of the six extraocular muscles. Should the orbits be misdirected or misshapen in their development or should the ocular muscles be abnormal in size, position, or insertion or be fibrous rather than elastic, these anomalies would be serious obstacles to the development of normal binocular vision. Ideally, the position of rest need be such that with normal neurologic innervation and muscle tonus the visual axes of the two eyes will be approximately parallel in the straight-ahead or primary position.
Figure 3. Accommodative esotropia controlled with glasses. A: Eyes are crossed as a result of effort of overcoming moderate farsightedness. B: Eyes are straight when farsightedness is neutralized by glasses.
The development of normal vision is probably the single most important factor in maintaining straight eves through an entire lifetime. The optical or refractive status of the eyes is determined primarilv by our genetic background; significant degrees of nearsightedness, farsightedness, or astigmatism, particularly if uncorrected, will obviouslv influence visual development and ultimately ocular alignment.
Farsightedness (hyperopia), present in a moderate degree, is the normal refractive status in childhood. Its presence in greater degrees is the most important factor in the causation of convergent strabismus or crossed eyes. Farsighted children experience blurred images that can be improved or sharpened only by expending additional focusing effort (accommodation). This increased accommodative effort, in turn, initiates excessive degrees of convergence, resulting in proportionate but excessive crossing of the eyes. Even minimal amounts of accommodative effort initiate this type of overresponse and crossing effect in certain susceptible children who have a poorly balanced accommodation-convergence relationship. Early identification and treatment with appropriate corrective lenses will restore good vision and are imperative to the proper management of this type of strabismus (Figure 3).
Myopia and astigmatism are less important as specific causes of strabismus. It is important to remember, however, that any significant variation from the normal range of vision predisposes to ocular deviations, because blurred or dissimilar images cannot be easily fused or "locked together" securely. Blurred or dissimilar images that are present through the early years of development must be identified and treated early if amblyopia and irrecoverable visual loss are to be avoided. The visual system requires two separate clear pictures as the first and most important goals of visual development; without good vision in each eye, there can be no "fusion anchor."
There are two associated corollaries: First, eyes that do not see well tend to deviate or drift (Figure 4). The poorly seeing eve will most often cross or turn inward in early childhood and progressively diverge in later years. The second and more important related fact is that a deviating eye in a child or adult may be the first indication of poor vision. The limited vision may be longstanding or gradually decreasing and often is undetected and unknown by the person himself or by parents, teachers, physicians, or other observers until the changing alignment is noted. This situation may be due simply to a variation in the refractive status between the two eyes, or it may have a much more serious implication - as with cataracts, retinal abnormalities or detachments, or even tumors that might be either intraocular or intracranial (Figure 5).
Figure 4. Congenital esotropia secondary to cataract in right eye with poor vision (rubella syndrome).
Neurologic innervation. There are four separate central nervous system functions that mediate vision and control the oculomotor mechanisms.
Figure 5. Acquired strabismus (esotropia). A: Secondary to poor vision, right eye. Congenital toxoplasmosis with retinal lesions B: Retinal photograph of toxoplasma chorioretinitis in same patient.
Figure 6. Cranial nerve paralysis - complete - acquired in early childhood. The cause could not be determined by extensive neurologic examinations, including angiography. A: Flaccid paralysis of left upper eyelid with complete ptosis. B: With eyelid supported, shows dilated fixed pupil with marked divergence (only lateral rectus and superior oblique function).
The first and most important relates to cranial nerve II, the optic nerve, which is not a peripheral nerve but an anterior projection of the cerebral pathways. This cerebral tract transmits all vision through photoelectric impulses from the retina through its synapses to the visual cortex of each occipital lobe.
Second, the exclusive function of cranial nerves III, IV, and VI is the motor innervation of the six extraocular muscles. Cranial nerve III innervates the medial, superior, and inferior rectus muscles and the inferior oblique (Figure 6), while IV controls the superior oblique muscle and VI, the lateral rectus muscle. An acute paresis or paralysis involving any of these nerves may cause an immediate and severe ocular deviation. Any sudden onset of double vision in a child with consistent symptoms and appropriate functional disturbance must be considered an extremely serious indicator of CNS abnormality. The differential diagnosis must include the possibility of an active meningitis, encephalitis, or space-occupying intracranial lesion.
The third area involves the integrity and function of the entire central nervous system. It has been well documented that children who are brain damaged and exhibit evidence of cerebral palsy, mental retardation, seizure disorders, or other cerebral dysfunction syndromes often have associated abnormalities of their oculomotor system functions. As in the case of poor vision in one eye, the deviations are usually convergent in the very early years, with progressive divergence later. The incidence of strabismus has been reported in a range of 30 to 80 per cent in different studies of children with cerebral palsy and other conditions of cerebral dysfunction.
It is important to remember that people see not merely with their eyes. The eyes are the primary receptor organs and require intact cerebral pathways and occipital cortex to receive, process, integrate, and store the afferent data into meaningful visual perceptions. A child can have entirely normal eyes and visual apparatus but poor, distorted, or even absent visual perception when the optic pathways or visual cortex is abnormal.
Figure 7. Congenital esotropia at age three months - convergence of both eyes with cross-fixation. Head is held in position by mother.
The fourth of the important neurologic innervation functions are the specific oculomotor nuclei and the midbrain centers that control convergence, divergence, and the involuntary fusional reflexes.
Fusion mechanisms. As I have stated, a very high percentage of human beings have some tendency for their eyes to "drift" - that is, to turn in or out to small degrees at certain times or under certain circumstances. Such tendencies for deviation are not necessarily abnormal if the basic position of the eyes is essentially straight and the subject possesses adequate control mechanisms to maintain single binocular vision without excessive effort or related symptoms.
The process by which we develop and maintain single binocular vision is fusion, the essential nature of which is the simultaneous cortical integration of the images of each eye into a single visual perception. This fusion mechanism is probably the only true "anchor" available to keep our eyes locked securely in position. Most infants appear to possess a fusion potential, this inherent desire for binocularity, although it deteriorates gradually when the opportunity to use both eyes simultaneously is not present or if it is not stimulated during the early months and years of development.
In addition to fusion, which is considered a sensorv function, a motor feedback component also exists to maintain binocularity, fusion, and its associated depth perception. This system consists of the corrective maneuvers automatically initiated through the ocular muscles to resist misalignment or regain proper ocular alignment when the eyes tend to deviate to small degrees from the straight position. When fusion and depth perception develop properly and securely during the first several years of life, this innate fusion mechanism should become sufficiently reinforced and strengthened so that only a major visual catastrophe could disrupt it and cause strabismus in later years.
CLINICAL TYPES OF STRABISMUS
The etiology of strabismus is complex. We have discussed numerous factors related to visual and ocular structure and physiology that influence the position and stability of the eyes. Similarly, there are many classifications of strabismus, and several ways to categorize these numerous factors related to ocular deviations will be mentioned.
The simplest classification merely records the most basic of information - "which way the eyes turn and how often," the direction and frequency of the deviation. Other classifications may be based on the time of onset (congenital or acquired), etiology (anatomic, innervational, or refractive), or the response to treatment (visual, medical, or surgical).
The terminology used to indicate the direction and frequency of a deviation is basic to any discussion of these problems. The term phoria indicates a transient deviation, one in which there is a tendency for the eyes to be misaligned but the straight position is maintained through the action of the fusion mechanism. Loss of alignment occurs with fatigue, illness, excitement, or other stress phenomena that cause the control mechanisms to be less effective.
The term tropia describes deviations that are constant, present under all visual conditions and physical circumstances.
In the second part of this classification, the direction of deviation is indicated by the prefix eso- for convergence, crossing, or turning in (Figure 7), while exo- signifies divergence, or turning outward (Figures 8 and 9). Vertical misalignment is noted by using the term hyperpreceded by right or left to indicate the higher eye (Figure 10).
Figure 8. Divergent strabismus (exotropia). A: Fixing with right eye. B: Fixing with left eye.
Figure 9. "A" pattern exotropia. Illustration of horizontal deviation (exotropia) present in lower field of gaze only.
Figure 10. Abnormality of rotation of the eyes. Structural anomaly of the right superior oblique muscle (Brown's syndrome) with left hypertropia on left gaze. A: Primary position. B: Right gaze. C: Left gaze. Basic problem is an inability to elevate the right eye on the left field of gaze.
Combinations of these terms are obviously necessary and are routinely used.
Esophoria or esotropia is intermittent or constant convergent strabismus.
Exophoria or exotropia is intermittent or constant divergent strabismus.
Hyperphoria or hypertropia (right or left) indicates which eye is elevated and whether on an intermittent or constant basis.
I should also comment here on the phenomenon of Pseudostrabismus. Many children have a relatively flat and broad nasal bridge with prominent epicanthal folds that obscure the inner canthal angle and contribute to the appearance of esotropia (Figure 2). This illusion is further increased on side gaze when the following eye becomes hidden under the nasal fold, exaggerating the impression that one eye crosses and that the eyes generally "don't track together properly." This illusion is the basis of that frequently heard parental comment that "Johnny's eyes crossed as a boy, but he gradually grew out of it." In fact, ocular deviations almost always get worse rather than better as children mature.
Any practical discussion of the management of strabismus requires a basic understanding and what I have earlier termed the "primary goals of visual development." The fundamental principles of these goals are the attainment of the best possible vision, the straightest possible alignment of the eyes, and the opportunity to develop fusion.
The treatment goals must often be modified, depending on the child's general health, neurologic stability, and several specific ocular factors. Any deviation of the eyes in a severely brain-damaged noneducable child, for example, would be of little importance either functionally or as a general developmental aid. Similarly, an essentially normal child, who may have no correctable vision in one eye, has no possibility of acquiring fusion (a binocular function); therefore any degree of strabismus present may need be considered only from a psychologic, social, or cosmetic standpoint.
One consideration that must be continuously stressed is the importance of early identification and treatment. Recent investigative work by Hubel and Weisel has demonstrated that visual deprivation in the early weeks and months of life will cause amblyopia or visual loss that is irreversible - that is, it will not respond to treatment or be recoverable at any time in later life. These effects were demonstrated in kittens, but experience strongly suggests that the same mechanisms are operative in human beings. A similar effect has been demonstrated in the binocular visual mechanisms, suggesting that the failure to acquire fusion in the early developmental period will preclude its possible development at any later stage of visual maturation.
Because of these facts, the general philosophy of treatment of strabismus that has evolved in recent years strongly proposes that the earlier that good vision and ocular alignment are obtained for any child, the greater will be that child's opportunity to develop secure binocular vision and to maintain straight eyes with fusion and depth perception through his lifetime.
Our treatment measures, then, may be aimed at the attainment of as many of these basic goals as is possible - good vision, straight eyes, and fusion. The primary treatment measures available consist of:
2. Occlusion therapy, when amblyopia is present.
4. Orthoptics - visual or oculomotor training.
5 Surgery of the extraocular muscles.
Glasses. Many children's eyes cross primarily because of refractive errors and associated poor vision. For these types of strabismus, corrective lenses in the form of spectacles or occasionally contact lenses are often the only definitive measures required. As has been stated previously, farsightedness is an important factor in the development of convergent strabismus; this type of deviation generally starts between the ages of two and three years, although it may occur later. Specific treatment requires careful refraction and the prescription of appropriate lenses to neutralize the farsightedness, which in turn eliminates the convergence (Figure 3). With proper glasses the child will see sharply, clearly, and equally without expending any accommodative effort. The glasses do the focusing, and since no convergence is initiated, the child will see well with his eyes straight. Occasionally these youngsters have a residual and excessive tendency for their eyes to cross on near-fixation. This is one of the very few cases in children's eye care in which a bifocal may be required to eliminate convergence in the reading area.
Glasses also play an important role in establishing the best possible vision in the correction of myopia, astigmatism, and anisometropia, which implies a difference in the refractive status between the two eyes. When less good vision is present in one eye, fusion is poor, the corrective mechanisms generally do not function well, and the eye with limited vision tends to deviate.
Federal laws now require that all lenses dispensed be impact resistant for maximum safety - that is, they are made of either hardened or tempered glass or plastic. Athletic straps or headbands are often helpful for very young children or those participating in vigorous sports activities. Spectacle frames should be of a durable material and construction. Plastic frames are generally more satisfactory than the finer metallic frames, which are easily bent and difficult to maintain in adjustment for active children.
We rarely prescribe corrective lenses for children unless there is an obvious and demonstrable need. We usually find that it is the rare child who will not tolerate and willingly wear glasses after gaining appreciation of the visual benefit and obtaining a sense of satisfaction from their use. As a rule, if a child refuses to wear glasses, it is because there is insufficient need for corrective lenses, the glasses may not be properly fabricated, or, in some children, the vanity and psychosocial factors may be major obstacles.
Occlusion therapy. The only therapeutic value of occlusion is in the treatment of amblyopia, and it is only indirectly beneficial in the treatment of strabismus. Poor vision in one eye in the absence of any organic lesion or structural anomaly and uncorrectable by lenses in medically termed amblyopia or, in common usage, a "lazy eye."
There are two common causes of amblyopia. The first is strabismus, in which one eye is not properly aligned with the other. The resulting double or eccentric image must be centrally suppressed, tuned out, and not registered as an innate defense mechanism for avoiding double vision. The second cause of amblyopia is anisometropia ("different measure of vision"), which means that one eye structurally and optically has a different focal length than the other, as when one eye has a normal refractive status and the other eye is markedly nearsighted or farsighted.
Occlusion or patching of the better-seeing eye forces the patient to stimulate and, it is hoped, to improve the vision in the poorly seeing one. It has been stated several times that secure alignment of our visual system depends on good vision in each eye. Occlusion, therefore, is an indirect or secondary method of treatment of strabismus, aimed specifically at either initially developing or later restoring the best possible vision to the amblyopic eye.
The use of an appropriate corrective lens or lenses is usually necessary to obtain the proper focus of each eye simultaneously, and along with it some type of eye patch or lens cover is necessary to "strengthen" the vision of the amblyopic eye. The numerous methods of amblyopia treatment are extensively discussed in another section of this magazine. Medications. Several drugs have been found effective in the treatment of accommodative esotropia. These medications are generally reserved for use in infants too young to tolerate spectacles, for short-term therapy, or for certain selected children who have a very labile or abnormal accommodative-convergence relationship (Figure 11). The drugs are used in the form of topical eye drops, given once or twice daily; occasionally, they may effectively control an accommodative esodeviation when used at intervals of three, four, or five days or, in rare instances, on a weekly basis.
Figure 11. Congenital esotropia. A: At age four months. B: Treated with phospholine iodide eye drops daily for two weeks. Eyes are much straighter but gradually deteriorated to constant marked esotropia requiring surgery Note: Pupils are miotic from medication. C: Two weeks after surgery following bilateral medial rectus recessions.
The two drugs most frequently used are phospholine iodide and isoflurophate (Floropryl®), both of which are potent anticholinergic drugs that should be used in the lowest effective concentration and for as brief a period as possible. These medications are not considered appropriate for long-term therapy.
Orthoptics - visual training. Visual training methods have well-defined but limited value in the treatment of ocular deviations. I have discussed fusion as the recognition and integration of two separate images into a single visual perception. This mechanism is our major visual ally in maintaining our eyes straight and resisting misalignment. Any available methods that will reinforce or strengthen the fusion mechanism may help to maintain single binocular vision when tendencies to drift do occur. It is in this area that visual training methods or orthoptics provide their greatest benefits.
Training that includes "diplopia recognition," coupled with techniques to expand the fusional vergences, can reinforce the motor fusion mechanisms when they are inadequate or inefficient. These techniques are particularly helpful for those who have a convergence insufficiency, in which there is difficulty maintaining the eyes properly turned in for reading and close work.
In children such training requires not only visual maturity but also sufficient understanding and cooperation in following directions and the necessary interest to maintain fixation. This usually requires a patient of at least school age or, preferably, seven to eight years.
This type of training does not strengthen the ocular muscles but, rather, reinforces the fusion mechanism. It is an education process to ensure that the desire to see together will offset the tendency to deviate. Orthoptics are not a substitute for other methods of treatment, but they may be a helpful adjunct in the treatment of certain types of strabismus.
A question often raised by parents and teachers concerns the role that vision plays in the learning process. More specifically, the question is whether poor vision or poor "visual perception" is an important factor in the acquisition of basic learning skills for those legions of children who are considered to have learning disabilities.
Obviously, some children have difficulty learning because they do not see well. A certain number of these children require glasses or other types of visual assistance. They may have difficulty reading the printed material in books or seeing the blackboard from the back of the classroom. It is also obvious that some learn to read with speed, proficiency, and good comprehension even though they have no vision whatever - by learning to read Braille with their fingers.
In the extensive experience of those working with children's visual and ocular disorders, however, it has been found that only rarely do children have difficulty acquiring or using these basic learning skills because of "what they see" or "the way that they see." They may read, copy, or transcribe letters or words in reversed patterns or even as mirror images, and they may have great difficulty with spatial orientation, left-right discrimination, chronology, sequencing, etc.; but these patterns are much more often manifestations of minimal cerebral dysfunction (MCD) syndromes than they are visual abnormalities. We find no greater proportion of abnormalities of visual acuity, alignment, or fusion in these youngsters than we find in normal children.
Whether these children are "lateralized" as to their eye and hand dominance also has no bearing on their visual efficiency or learning. Early concerns about so-called crossed-dominance patterns in children who were right-handed and lefteyed, or vice versa, are merely part of the insecure cerebral dominance patterns that many of these MCD children exhibit. The eye that is used as the dominant eye is almost always the better seeing or the one with the least refractive error. Visual training as an isolated treatment measure for children with "poor visual perception" has little value unless it is integrated into the child's total educational program to assist in the acquisition of basic learning skills.
Surgery. Surgery of the ocular muscles plays a major role in the realignment of deviated eyes. Since the causes of strabismus are numerous and any patient's response to the same procedure may vary, ocular muscle surgery is not an exact science. When strabismus is approached with careful attention to preoperative testing and with refined surgical techniques, an improvement can be assured in the vast majority of cases and the total desired effect obtained in many patients.
In the congenital or infantile type of esotropia, in which a large angle of deviation is present from birth or in the first few months of life, surgery affords the only effective means of obtaining parallelism and should generally be considered as soon as visual fixation has been equalized in each eye and the deviation is reasonably constant. In cases in which the deviation is present from early infancy, surgical correction is most advantageously attempted in the fourth or fifth month of life and certainly before age one if the child is otherwise healthy and neurologically normal (Figure 12).
Children born with their eyes severely crossed can be greatly helped with initial surgery, but they may have residual horizontal deviations that require further corrective procedures before optimum alignment can be obtained. This group has also been noted to develop various types of vertical deviations in later months and years, sometimes asymmetrically and seemingly in relation to their basically unstable alignment mechanisms and poor fusion potential.
Figure 12. Congenital esotropia treated by surgery. A: Marked esotropia of both eyes at age four months B: At age five and a half months, eyes are straight following surgery. Recession of both medial rectus muscles
Some cases of convergent strabismus of the later-onset type (accommodative esotropia) persist even after good vision has been established and maximal benefit gained from the use of corrective lenses. Such residual deviations can then be eliminated only by surgical intervention to "put the eyes in the most favorable position," so that the fusional mechanisms then have the opportunity to lock the eyes in place.
We have discussed what constitutes a normal visual and ocular system in children and the inherent mechanisms that help each person develop and maintain normal visual function. This ideal status should include good visual acuity in each eye, straight eyes, and the development of fusion. Strabismus represents an obstacle to obtaining these goals and may be caused by poor vision, refractive errors, or anatomic, structural, or innervational abnormalities. Each of these conditions is treatable and correctable when discovered early. With early identification, proper treatment can be instituted and the patient can have the opportunity to develop optimal vision with fusion and depth perception - a lifetime benefit.
Several other types of strabismus respond poorly to the use of glasses, prisms, exercises, or medications and can be effectively treated only by surgical methods. Most divergent and vertical deviations fall into this group, as do ocular deviations associated with paretic or structurally abnormal muscles.
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