Comitant strabismus

Esodeviations are the most common type of ocular misalignment in children and represent over 50% of all ocular deviations.

Congenital esotropia

Few children who are diagnosed with congenital esotropia are actually born with crossed eyes. Most pediatric ophthalmologists consider infants with confirmed onset of esotropia earlier than 6 months of age as having congenital esotropia. Some pediatric ophthalmologists prefer the term infantile esotropia to differentiate those children whose eyes were not crossed at birth.

ETIOLOGY

The cause of congenital esotropia is unknown. Theories include both a primary defect in sensory development of the brain that leads to the abnormal alignment as well as a primary ‘motor’ theory in which the ocular misalignment is the primary abnormality which then leads to a secondary disruption of binocular vision. It is likely that both causes exist and may also be equally responsible for the development of the disorder in many children.

CLINICAL PRESENTATION

The characteristic angle of congenital esotropia is large and constant (43). Because of the large

deviation, cross-fixation is frequently encountered. This is a condition in which the child looks to the right with the left eye and to the left with the right eye. With cross-fixation, there is no need for the eye to turn away from the nose (abduction) as the adducting eye is used in side gaze. Cross-fixation may therefore simulate a 6th nerve palsy. Abduction can be demonstrated by the doll’s head maneuver or by patching one eye for a short time. Children with congenital esotropia tend to have refractive errors similar to those of normal children of the same age. This contrasts with the characteristic high level of farsightedness associated with accommodative esotropia. Amblyopia is common in children with congenital esotropia.

DIFFERENTIAL DIAGNOSIS

During the first year of life, a number of conditions can simulate congenital esotropia (Table 4). Because the management of these conditions may differ from the treatment of congenital esotropia, it is important to recognize these other possibilities. In general, a relatively small angle deviation should raise doubt when contemplating the diagnosis of congenital esotropia. The majority 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.

43

43 Congenital esotropia.

DIAGNOSIS

The diagnosis is generally made when an infant presents prior to 6 months of age with a large, constant esotropia, full abduction, a normal level of hyperopia, and no underlying ophthalmic disorder that could lead to vision loss and a secondary strabismus. It is not infrequent to encounter a child who meets some but not all of these criteria. Some children may present with all the findings of congenital esotropia but with a higher than average level of farsightedness. In order to rule out the possibility that the farsightedness is the cause of the strabismus (accommodative esotropia), a trial of glasses may be necessary. If the glasses do not significantly change the deviation, the diagnosis of congenital esotropia can then be made. In children, or even adults, who present later in life, the typical history of early crossing in association with the other expected findings can also be used to confirm this diagnosis.

MANAGEMENT/TREATMENT

The treatment for congenital esotropia consists of strabismus surgery (44). The primary goal of treatment in congenital esotropia is to eliminate or reduce the deviation as much as possible. Ideally, this results in normal sight in each eye, in straight-looking eyes, and in the development of binocular vision. Surgery is performed after any associated amblyopia, if present, is treated. It is important to treat amblyopia prior to surgery. It is much easier to follow the progress of amblyopia in a preverbal child while their eyes are crossed. In addition,

44

44 Three days following surgery for congenital esotropia.

parental compliance with amblyopia treatment tends to be much lower once the eyes are straightened and appear ‘normal’.

PROGNOSIS

It is important that parents realize that early successful surgical alignment is only the beginning of the treatment process for children with congenital esotropia. Many children with a history of congenital esotropia may redevelop strabismus or amblyopia and therefore they need to be monitored closely during the visually immature period of life. Early treatment is more likely to lead to the development of binocular vision, which helps to maintain long-term ocular alignment. Even with successful surgical alignment, it is common for vertical deviations to develop in children with a history of congenital esotropia. These generally develop months or years after the initial surgery has been performed. The two most common forms of vertical deviation to develop are inferior oblique muscle overaction (IOOA) and dissociated vertical deviation (DVD). In IOOA, the overactive inferior oblique muscle produces an upshoot of the eye closest to the nose when the patient looks to the side. In DVD, one eye drifts up slowly with no movement of the other eye. Surgery may be necessary to treat either or both of these conditions. In addition, a significant number of children may develop a recurrent esotropia where glasses will help to eliminate the crossing (accommodative esotropia) even if they did not prior to surgery.

Accommodative esotropia

Accommodative esotropia is defined as a ‘convergent deviation of the eyes associated with activation of the accommodative (focusing) reflex’.

ETIOLOGY

The mechanism of accommodative esotropia involves uncorrected hyperopia, accommodation, and accommodative convergence.The image entering a hyperopic (farsighted) eye is blurred. If the amount of hyperopia is not significant, the blurred image can be sharpened by accommodating (focusing of the lens of the eye). Accommodation is closely linked with convergence (eyes turning inward). If a child’s hyperopic refractive error is large, esotropia may develop.

CLINICAL PRESENTATION

Accommodative esotropia usually occurs in a child who is between 2 and 3 years of age and who has a history of acquired intermittent or constant crossing. Occasionally, children under 1 year of age may present with all the clinical features of accommodative esotropia. They may also present with a history suggestive of congenital esotropia but have a higher than expected level of farsightedness. Amblyopia occurs in a large percentage of patients.

MANAGEMENT/TREATMENT

To treat accommodative esotropia, the full hyperopic (farsighted) correction is initially prescribed. These glasses eliminate a child’s need to accommodate and therefore correct the

esotropia (45, 46). Although many parents are initially concerned that their child will not want to wear glasses, the benefits of binocular vision and the decrease in the focusing effort required to see clearly provide a strong stimulus to wear glasses, and they are generally accepted well. The full hyperopic correction sometimes straightens the eyes at distance fixation but leaves a residual deviation at near fixation; this may be observed or treated with bifocal lenses, antiaccommodative drops, or surgery.

PROGNOSIS

Most children maintain straight eyes once initially treated. Because hyperopia generally decreases with age, many patients outgrow the need to wear glasses to maintain alignment. In some patients, a residual esodeviation persists even when wearing their glasses. This condition commonly occurs when there is a delay between the onset of accommodative esotropia and treatment. In others, the esotropia may initially be eliminated with glasses but crossing redevelops. The crossing that is no longer correctable with glasses is the deteriorated or nonaccommodative portion. Surgery for this portion of the crossing may be indicated to restore binocular vision.

It is important to tell parents of children with accommodative esotropia that the esodeviation may appear to increase without glasses after the initial correction is worn. Parents frequently state that before wearing glasses, their child had a small esodeviation, whereas after removal of the glasses the esodeviation becomes quite large. Parents often blame the increased esodeviation on the glasses. This apparent increase is due to a

child using the appropriate amount of accommodative effort after the glasses have been worn.

Congenital exotropia

Congenital exotropia behaves in a very similar fashion to congenital esotropia. It typically occurs early in life and presents with a large, constant out-turning. Amblyopia is common. Treatment for congenital exotropia requires strabismus surgery. As with congenital esotropia, early surgery gives the best chance of developing binocular vision although vertical strabismus (IOOA and DVD) frequently occurs later in life. Congenital exotropia may be associated with neurologic disease or abnormalities of the bony orbit, as in Crouzon syndrome.

Intermittent exotropia

Intermittent exotropia is the most common exodeviation in childhood.

CLINICAL PRESENTATION

The age of onset of intermittent exotropia varies but is often between age 6 months and 4 years. It is characterized by outward drifting of one eye, which usually occurs when a child is fixating at distance (47). The deviation is generally more frequent with fatigue or illness. Exposure to bright light may cause reflex closure of the exotropic eye. Because the deviation generally begins with distance fixation and is only seen when the child is tired, it is often not seen when examined by a primary medical doctor at close distance or during a well child visit. A history reported by the parents of an out-turned eye which occurs mostly when tired or sick is so typical that this history alone is highly suggestive for the disorder, and consultation with an ophthalmologist may be advisable. With time, the deviation usually becomes manifest more frequently.

DIAGNOSIS

The diagnosis of intermittent exotropia is made based upon the history and findings at the time of the motility examination. While the exam findings may help to determine the level of control that is present to maintain proper alignment, the history from the family may be

crucial in deciding how often the deviation is present. This may be especially helpful when determining when treatment should be suggested.

MANAGEMENT/TREATMENT

While there are some forms of medical treatment that may be used by some ophthalmologists, most pediatric ophthalmologists agree that strabismus surgery is needed in the overwhelming majority of patients with intermittent exotropia and that medical treatments serve only to delay surgery in most patients.

The decision to perform eye muscle surgery is based on the magnitude and frequency of the deviation. If the deviation is small and infrequent, it is reasonable to observe the child. If the exotropia is large and increasing in frequency, surgery is indicated to maintain normal binocular vision.

PROGNOSIS

Because the eyes initially can be kept straight most of the time, visual acuity tends to be good in both eyes and binocular vision is initially normal during early stages of the disease. Left untreated, patients with intermittent exotropia show a typical progression: the deviation becomes more frequent until it becomes constant. Strabismus surgery has a high success rate although some patients may redevelop an exotropia later in life.

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47 Exotropia.