Strabismus Disorders



Strabismus Disorders


Scott E. Olitsky

Leonard B. Nelson



STRABISMUS, OR ABNORMAL ocular alignment, is one of the most common eye problems encountered in children. The misalignment may be manifest in any field of gaze, may be constant or intermittent, and may occur at distant or near fixation or both. Strabismus affects between 2% and 5% of the preschool population and is an important cause of visual and psychological disability (1,2). The word “strabismus” derives from the Greek word strabismos (meaning a squinting) and probably predates the geographer Strabo, whose “peculiarly horrible and unbecoming squint was famous in Alexandria during the Roman Empire.”

Strabismus involves a number of different clinical entities. Knowledge of the terms used to describe a strabismic deviation and the more common patterns of strabismus help to predict the cause of the strabismus and determine proper treatment.

Orthophoria is the condition of exact ocular balance. It implies that the oculomotor apparatus is in perfect equilibrium so that both visual axes always intersect at the object of visual regard.

Heterophoria is a latent tendency for the eyes to deviate. This latent deviation is normally controlled by fusional mechanisms which provide binocular vision or avoid diplopia. The eye deviates only under certain conditions, such as fatigue, illness, stress, or tests that interfere with the maintenance of these normal fusional abilities (such as covering one eye). If the amount of heterophoria is large, it may give rise to bothersome symptoms, such as transient diplopia or asthenopia.

Heterotropia is a misalignment of the eyes that is manifest. The condition may be alternating or unilateral, depending on the vision. In alternating strabismus, either eye may be used for seeing while the fellow eye deviates. Because each eye is used in turn, each develops similar vision. In unilateral strabismus, only one eye is preferred for fixation while the fellow eye deviates consistently. The constantly deviating eye is prone to defective central vision during the visually immature period of life.

A convergent deviation, crossing or turning in of the eyes, is designated by the prefix “eso-” (esotropia, esophoria). Divergent deviation, or turning outward of the eyes, is designated by the prefix “exo-” (exotropia, exophoria). Vertical deviations are designated by the prefixes “hyper-” and “hypo-” (hypertropia, hypotropia). In cases of unilateral strabismus, the deviating eye is often part of the description of the misalignment (left esotropia). Most vertical deviations are described in terms of the hypertropic eye. An exception to this general rule occurs when the lower, or hypotropic, eye is restricted in its movement. The deviation is then named according to the hypotropic eye.


OCULAR ALIGNMENT IN INFANCY

Ocular deviations during the first months of life do not necessarily indicate an abnormality. Because of oculomotor instability during this time, adequate assessment of alignment usually is not made until the patient is approximately 3 months of age and any angle of strabismus that is present is stable. Infants are often born with their eyes misaligned. During the first month of life, alignment may vary intermittently from esotropia to orthotropia to exotropia. Nixon and coworkers observed 1,219 alert infants in a newborn nursery and found that 40% seemed to have straight eyes, 33% had exotropia, and 3% had esotropia (3). Many had variable alignment and 7% were not sufficiently alert to permit classification. Other large population studies have confirmed that strabismus is common in early infancy (4).


ESODEVIATIONS


Pseudoesotropia

Pseudoesotropia is one of the most common reasons an ophthalmologist is asked to evaluate an infant. This condition is characterized by the false appearance of esotropia when the visual axes are aligned accurately. The appearance may be caused by a flat, broad nasal bridge; prominent epicanthal folds; or a narrow interpupillary distance (Fig. 8.1). The observer sees less sclera nasally than would be expected, which creates the impression that the eye is turned in toward the nose, especially when the child gazes to either side. This might be especially noticeable in
photographs. Pseudoesotropia can be differentiated from a true manifest deviation by use of the corneal light reflex and the cover-uncover test, when possible. Once pseudoesotropia has been confirmed, parents can be reassured that the child will “outgrow” the appearance of esotropia. As the child grows, the bridge of the nose becomes more prominent and displaces the epicanthal folds, so that the sclera medially becomes proportional to the amount visible on the lateral aspect. It should be stressed that it is the appearance of crossing that the child will outgrow. Some parents of children with pseudoesotropia may incorrectly believe that there is an actual esotropia that will resolve on its own. Because true esotropia can develop later in children with pseudoesotropia, parents and pediatricians should be instructed that reassessment is needed if the apparent deviation does not improve.






FIGURE 8.1. Pseudoesotropia caused by a wide nasal bridge and epicanthal folds. Note that the light reflex is centered in each pupil. (Reprinted with permission from Olitsky SE, Nelson LB. Pediatric clinical ophthalmologyA color handbook. London, UK: Manson Publishing, 2012, ISBN 9781840761511.)


Congenital Esotropia



Epidemiology

Congenital esotropia is a common form of strabismus. The sex distribution of congenital esotropia is equal. Transmission in many families seems to be as an irregular autosomal dominant trait; however, in others it may be recessive. The reported incidence of affected family members has varied widely (5). It is common to find a history of strabismus in the parents or siblings of affected patients. Reduced binocular function has been reported in parents of patients with congenital esotropia (6). The incidence of congenital esotropia is higher in patients with a history of prematurity, cerebral palsy, hydrocephalus, and other neurologic disorders.


Pathogenesis

Much clinical literature in this century has focused on the implications of two conflicting theories of pathogenesis for congenital esotropia. Worth’s “sensory” concept was that congenital esotropia resulted from a deficit in a supposed fusion center in the brain (7). According to his theory, the goal of restoring binocularity was considered hopeless, since there was no way to provide this congenitally absent neural function. Until the 1960s, results of surgical treatment almost universally supported this pessimistic view. Data on these patients were obtained at a time when surgery was rarely performed before 2 years of age.

Chavasse disagreed with Worth’s theory. He suggested that normal binocular vision may be achieved through facilitation of conditioned reflexes that depend on early ocular alignment (8). To Chavasse, the primary problem was mechanical. In this view, most children with congenital esotropia were potentially curable if the deviation could be eliminated in infancy. Only theoretical support was available for this “motor” theory until Costenbader, Taylor, Ing and Costenbader, and Parks began to report favorable binocular results in some infants operated on between 6 months and 2 years of age (9,10,11). These encouraging results became the basis for the theory of early surgery for patients with congenital esotropia.

Even advocates of early surgery have generally found imperfect binocularity in their postoperative patients. Parks defined the monofixation syndrome, in which “peripheral” fusion and vergence amplitudes capable of maintaining alignment within approximately 10 prism diopters (PD) may exist, despite deficient stereopsis and a central suppression scotoma in one eye during binocular viewing (12). Many strabismus surgeons accept this sensory state as the goal of treatment.



Clinical Manifestations

Visual Acuity. The association of amblyopia and congenital esotropia is well known. It is difficult to ascertain the exact incidence, however, especially in preverbal children. The incidence of amblyopia may be as high as 40% to 72%. Many infants spontaneously alternate their fixation and do not develop amblyopia. Others may “cross fixate,” using alternate eyes in the opposite fields of gaze, and appear to be protected as well.

Size of Deviation. The characteristic angle of congenital esodeviations is considerably larger than those acquired later in life (Fig. 8.2). Most patients have a deviation in the 40 to 60 PD range (13,14). Measurements tend to be similar at distance and near, although accurate distant fixation is difficult to achieve in the examination of infants. There is little short-term variability in the deviation size; it is generally unaffected by accommodation. Long-term changes may occur: increases seem more frequent than decreases.






FIGURE 8.2. A: Congenital esotropia. B: Three days following surgery for congenital esotropia. (Reprinted with permission from Olitsky SE, Nelson LB. Pediatric clinical ophthalmologyA color handbook. London, UK: Manson Publishing, 2012, ISBN 9781840761511.)

Refractive Errors. Children with congenital esotropia tend to have cycloplegic refractions similar to those of normal children of the same age. These observations contrast markedly with the characteristic hyperopia associated with accommodative esotropia, especially of the refractive types.

Ocular Rotations. Children with congenital esotropia will often appear to exhibit an apparent abduction deficit. This pseudoparesis is usually secondary to the presence of cross fixation. If the child has equal vision, there is no need to abduct either eye. He/she will use the adducted, or crossed, eye to look to the opposite field of gaze. In this case, he/she will show a bilateral pseudoparesis of abduction. If amblyopia is present, only the better-seeing eye will cross fixate, making the amblyopic eye appear to have an abduction weakness. A true unilateral or bilateral abducens nerve palsy is uncommon in infancy. To differentiate between a true abducens paralysis and a pseudoparalysis, two techniques may be used. The examiner can evaluate ocular rotations by rotating the infant’s head, either with the infant sitting upright in a moveable chair or using a doll’s head maneuver. Abduction testing can also be examined after covering infant’s one eye with a patch for a period of time.


Associated Findings

Dissociated Vertical Deviation. This consists of a slow upward deviation of one or alternate eyes. Occasionally, excyclotorsion can be demonstrated on upward drifting of the eye and incyclotorsion on downward motion. Dissociated vertical deviation (DVD) may be latent, detected only when the involved eye is covered, or manifest, occurring intermittently or constantly (Fig. 8.3). It can be differentiated from a true vertical deviation, because no corresponding hypotropia occurs in the other eye on cover testing. Bielschowsky’s phenomenon is another feature of DVD, characterized by downward movement of the occluded eye when filters of increasing density are placed before the fixing eye. The incidence of DVD in patients with congenital esotropia is high, ranging from 46% to 92%, which frequently develops during the second year of life (13,15). DVD appears to be a time-related phenomenon and is not related to successful initial surgery or the development of binocular vision (15,16).

Inferior Oblique Overaction. The incidence of overaction of one or both inferior oblique muscles in patients with congenital esotropia has been reported to be as high as 78%. Studies have shown the onset of inferior oblique overaction (IOOA) to be most frequent during the second and third year of life. There appears to be no correlation between the development of IOOA and age at surgery, time from onset of strabismus to surgery, or decompensation of ocular alignment. The presence of fundus torsion at the time of surgery may help to predict which patients will develop IOOA (17). IOOA and DVD are both conditions that can cause excessive elevation of one or both eyes
in adduction in patients with congenital esotropia. The differentiating features of these two conditions are listed in Table 8.1. IOOA results in elevation of the involved eye as it moves nasally (Fig. 8.4). DVD may also result in elevation as the eye moves nasally because the nose acts as a cover, dissociating the eyes. However, the vertical misalignment in DVD usually occurs equally in abduction, adduction, and primary position. When the adducting
eye in IOOA fixates, there is a corresponding hypotropia in the contralateral abducting eye. In DVD, contralateral hypotropia does not occur. IOOA and DVD frequently occur together in these patients. IOOA can be classified as Grades I to IV. Grade I represents 1 mm of higher elevation of the adducting eye in gaze up and to the side. Grade IV indicates 4 mm of higher elevation. These differences in elevation between the two eyes are measured from the 6 o’clock position on each limbus. A measurement of the degree of adduction that is required to elicit the overaction is also helpful when considering treatment. A moderatesize overaction that occurs with limited adduction may be more noticeable than a larger overaction that is seen only in extreme side gaze.






FIGURE 8.3. Bilateral dissociated vertical deviation (DVD). A: Right DVD when left eye is fixating. B: Left DVD when right eye is fixating. (Reprinted with permission from Tasman W, Jaeger E. The Wills Eye Hospital atlas of clinical ophthalmology, 2nd Ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2001.)








Table 8.1 DISTINGUISHING FEATURES OF DISSOCIATED VERTICAL DEVIATION AND INFERIOR OBLIQUE OVERACTION



































Dissociated Vertical Deviation


1. Causes elevation in adduction and abduction


2. Usually comitant, i.e., same in adduction, primary, and abduction


3. Variability of hyperdeviation


4. Usually not associated with a pattern


5. Same amount of hyperdeviation in upgaze and downgaze


6. Hyperdeviation may be associated with torsional movement and abduction


7. No corresponding hypotropia in abducted eye


Inferior Oblique Overaction


1. Causes elevation in adduction, not abduction


2. Incomitant, more in field of action of inferior oblique


3. Not variable


4. Commonly associated with V pattern


5. More hyperdeviation in upgaze than downgaze


6. Hyperdeviation not associated with torsional movement


7. Corresponding hypotropia in abducted eye







FIGURE 8.4. A-B: Overacting inferior oblique muscles. (Reprinted with permission from Tasman W, Jaeger E. The Wills Eye Hospital atlas of clinical ophthalmology, 2nd Ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2001.)

Nystagmus. Rotary nystagmus may occur in children with congenital esotropia and has been reported to be found in 30% of patients with congenital esotropia. In the authors’ experience, the incidence of this type of nystagmus is much less.

Latent nystagmus is a predominantly horizontal jerk nystagmus elicited by occluding either eye. The slow phase is toward the side of the occluded eye. Measurement of visual acuity in the uncovered eye while using opaque occlusion of the nonviewing eye produces the maximal nystagmus and the poorest vision. Alternative methods for evaluating visual acuity create a relative binocular state by partially occluding or blurring the image in the non-viewing eye.

Latent nystagmus is more common than rotary nystagmus in congenital esotropia. If a significant latent nystagmus is present, amblyopia treatment using occlusion therapy may be less effective as the nystagmus will decrease the central vision stimulation. Other forms of amblyopia treatment may be more efficacious if this occurs.


Differential Diagnosis

During the first year of life, a number of conditions can simulate congenital esotropia and cause diagnostic difficulty (Table 8.2). Because the management of these conditions may differ from the treatment of congenital esotropia, their clinical recognition is important. In general, a relatively small-angle deviation should raise doubt in assigning the diagnosis of congenital esotropia. Many of these other disorders can be ruled out following a thorough ophthalmologic evaluation. For this reason, all infants presenting with esotropia require a full evaluation, including a dilated funduscopic examination.



Postoperative Management


Over- and Undercorrection

Early successful alignment does not ensure long-term stability. The need for repeating observations throughout the first decade of life cannot be overemphasized.

Reduction of spectacle correction in hypermetropes and overcorrections in myopes has been used to treat small overcorrections. A large overcorrection associated with an adduction weakness in the immediate postoperative period should alert the surgeon to the possibility of a slipped muscle. Exploration of the suspected muscle should be undertaken. Consecutive exotropia > 15 PD approximately 6 weeks after surgery usually requires a secondary procedure.

Undercorrections < 10 PD may respond to correction of hypermetropia > +1.50 D, and a trial of spectacles is indicated. Patients with residual esotropia measuring > 15 PD, unless the deviation is responsive to antiaccommodative therapy, should be evaluated for secondary surgery after 6 weeks’ observation.


Accommodative Esotropia Following Congenital Esotropia

Accommodative esotropia may develop in children who were surgically corrected for congenital esotropia. These patients may be prone to the development of an accommodative esotropia secondary to their underlying poor binocular function and may respond to correction of small levels of hyperopia. Consideration for spectacle correction should be given to patients who redevelop esotropia even if the magnitude of their hyperopia is normal.


Dissociated Vertical Deviation

Patients with DVD usually do not complain of diplopia and are often asymptomatic. DVD is less frequently noted in adults with strabismus than in children, suggesting that DVD tends to improve with time. If the disorder is entirely latent, detected by the examiner only on cover testing, surgery is not indicated. If it is intermittent, surgery is dictated by the size and frequency of the deviation as well as the patient’s concern regarding its appearance.


Inferior Oblique Muscle Overaction

This rarely, if ever, causes symptoms; it is usually a problem only due to its appearance. Patients usually avoid the extreme lateral gaze necessary to elicit IOOA. Instead, they almost instantaneously turn the face to look laterally, minimizing the cosmetic appearance of IOOA. The indications for surgery for IOOA are different, depending on whether weakening the inferior obliques is the only surgery being contemplated, or whether weakening the inferior obliques in conjunction with horizontal strabismus surgery is being considered. If the inferior obliques alone are weakened, there should be a significant overaction present to justify surgery. When there is an obvious elevation of the adducting eye at about 30 degrees or less of lateral gaze, a reasonable cosmetic defect is present and the option of surgery could be offered. If however, the elevation on adduction is evident only on extreme lateral gaze, this minor cosmetic defect might be best left alone. If horizontal strabismus surgery is performed for congenital esotropia, smaller grades of IOOA may be corrected at the same time.

Surgery for Dissociated Vertical Deviation and Inferior Oblique Muscle Overaction. Three surgical approaches are generally advocated to correct DVD: recession of the superior rectus, recession of the superior rectus combined with a posterior fixation suture, and anterior transposition of the inferior oblique (ATIO).

An overacting inferior oblique muscle may be effectively weakened by recession, disinsertion, myotomy, myectomy, and denervation and extirpation. Inferior oblique recession, as popularized by Parks, is associated with low rates of complications and recurrences (29). The surgeon has the ability to grade the amount of recession according to the extent of overaction.

Scott, using a computer model, described anterior transposition of the inferior oblique muscle (30). Early reports of the procedure showed that it was more effective in treating IOOA, which was noteworthy given the fact that most of these studies utilized anterior transposition for larger degrees of overaction.

One study showed a decreased incidence of DVD development in a group of patients undergoing anterior transposition compared to a group that had undergone standard recession of the inferior oblique. The authors speculated that anterior transposition may decrease the risk for the development of DVD (31). However, the study groups were not comparable and there may have been a degree of bias introduced into the study. ATIO is effective in treating either DVD or IOOA or both when concurrently present (32).


Many surgeons now perform ATIO when either IOOA or DVD occurs alone in patients with a history of congenital esotropia. Many surgeons feel it is the procedure of choice when both motility disorders are present at the same time. It eliminates the need to operate on both the superior rectus and the inferior oblique.

When performing ATIO, placement of the new insertion of the inferior oblique is important. If the new insertion is brought too far anterior or spread too far laterally, the surgeon may induce an anti-elevation syndrome. This syndrome describes the inability to elevate the eye in the abducted position. It is caused by a restriction of the lateral fibers of the inferior oblique muscle when the eye is placed into this position. Because the contralateral adducting eye will demonstrate better elevation in this gaze position, it will appear as if there is an overaction of the inferior oblique muscle. Kushner first described this disorder and demonstrated that the treatment involves correcting the new insertion site so that it is neither too far anterior nor spread too far laterally. Surgery does not need to be directed at the apparent overacting inferior oblique (33).


Amblyopia

Amblyopia occurring postoperatively is common and must always be considered. Fixation preference testing should be performed at each postoperative visit until the visual acuity can be measured. Unfortunately, it is more difficult to recognize a fixation preference, the closer the eyes are to orthotropia. The 10 D fixation test for preverbal children with small angle or no deviation is useful. A 10 D vertical prism is placed in front of one eye to produce a vertical deviation, which facilitates the recognition of a fixation preference (34).

Once recognized, amblyopia should be treated promptly. Some patients may require maintenance therapy until visual maturity is reached, and susceptibility to amblyopia is eliminated as the sensitive period of visual development ends at approximately 9 years of age.


NYSTAGMUS BLOCKAGE SYNDROME

The nystagmus blockage syndrome is characterized by nystagmus that begins in early infancy and is associated with esotropia. The nystagmus is reduced or absent with the fixing eye in adduction. As the fixing eye follows a target moving laterally toward the primary position and then into abduction, the nystagmus increases and the esotropia decreases. A head turn develops in the direction of the uncovered eye when the fellow eye is occluded. This abnormal head posture allows the uncovered eye to persist in an adducted position.


ACCOMMODATIVE ESOTROPIA

Accommodative esotropia is defined as a convergent deviation of the eyes associated with activation of the accommodative reflex. Esotropia that is related to accommodative effort may be divided into three major categories: refractive, nonrefractive, and partial or decompensated.


Refractive Accommodative Esotropia

Refractive accommodative esotropia usually occurs in a child between 2 and 3 years of age with a history of acquired intermittent or constant esotropia. Occasionally, children who are 1 year of age or younger present with all the clinical features of accommodative esotropia (35,36). The refraction of patients with refractive accommodative esotropia averages +4.75 D (37). The angle of esodeviation is the same when measured at distance and near fixation, and is usually moderate in magnitude, ranging between 20 and 40 PD. Amblyopia is common, especially when the esodeviation has become more nearly constant.


Pathogenesis

The mechanism of refractive accommodative esotropia involves three factors: uncorrected hyperopia, accommodative convergence, and insufficient fusional divergence. When an individual exerts a given amount of accommodation, a specific amount of convergence (accommodative convergence) is associated with it. An uncorrected hyperope must exert excessive accommodation to clear a blurred retinal image. This in turn will stimulate excessive convergence. If the amplitude of fusional divergence is sufficient to correct the excess convergence, no esotropia will result. However, if the fusional divergence amplitudes are inadequate or motor fusion is altered by some sensory obstacle, an esotropia will result. Patients with lower levels of hyperopia but with significant anisometropia are also at an increased risk to develop an accommodative esotropia.



Nonrefractive Accommodative Esotropia

Children with nonrefractive accommodative esotropia usually present between 2 and 3 years of age with an esodeviation that is greater at near than at distance fixation. The refractive error in this condition may be hyperopic or myopic, although the average refraction is +2.25 D.


Pathogenesis

In nonrefractive accommodative esotropia, there is a high accommodative convergence to accommodation (AC:A) ratio: the effort to accommodate elicits an abnormally high accommodative convergence response. There are a number of ways of measuring the AC:A ratio: the heterophoria method, the fixation disparity method, the gradient method, and the clinical evaluation of distance and near deviation. Most clinicians prefer to assess the ratio using the distance-near comparison. This method allows the ratio to be evaluated more easily and quickly, since it employs conventional examination techniques and requires no calculations. The AC:A relationship is derived by simply comparing the distance and near deviation. If the near measurement in an esotropia patient is > 10 D, the AC:A ratio is considered to be abnormally high.

Jun 20, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Strabismus Disorders
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