Strabismus occurs when the visual axes of the eyes are misaligned. It is one of the most common disorders encountered in pediatric ophthalmology, estimated to affect approximately 3% to 5% of children. The type of strabismus is defined by the direction of misalignment and whether the deviation is latent, intermittent, or constant. A phoria describes a latent strabismus that is present when one eye is covered. The eyes return to normal alignment when the eye is uncovered and the patient views under normal binocular viewing conditions. A tropia describes strabismus that is present when both eyes are viewing. Intermittent strabismus is present when the eyes vary between being misaligned and straight.
When the eyes are horizontally misaligned, they are esotropic if they are turned toward each other (“cross-eyed”) and exotropic if they are directed away from each other (“wall-eyed”). If the eyes are vertically misaligned, the type of strabismus is described by the deviating eye. If the deviating eye is lower than the straight eye, it is hypotropic. If it is higher than the straight eye, it is hypertropic. Vertical strabismus is much less common than horizontal strabismus in children.
If the eyes are aligned normally, they are orthophoric. A deviation is comitant if the amount of misalignment does not change in different positions of gaze. This is the most common situation in primary strabismus. A deviation is incomitant if the amount of misalignment changes as the eyes move in various directions. This occurs in patients with cranial nerve palsies or other forms of strabismus with limited extraocular movements. Horizontal eye movements are called adduction when the moves in an inward direction and abduction when it moves outward. Pseudostrabismus is present when the eyes appear misaligned, but they are optically straight.
Eye movements result from contraction of the extraocular muscles. Horizontal movements are produced by the medial and lateral rectus muscles. Vertical movements are produced by the inferior and superior rectus muscles. The superior oblique muscle causes downward and intorsional movements. The inferior oblique muscle causes upward and extorsional movements (Figure 34–1).
Movements of the extraocular muscles are controlled by inputs from the third, fourth, and sixth cranial nerves. The third cranial nerve controls 4 of the 6 extraocular muscles (medial rectus, inferior rectus, superior rectus, and inferior oblique muscles), in addition to the sphincter muscle of the iris and the levator muscle of the eyelid. It begins in the rostral midbrain at the level of the superior colliculus, where it is composed of separate subnuclei that subserve the different muscles controlled by the nerve. The fourth cranial nerve controls the superior oblique muscle. It begins in the caudal midbrain at the level of the inferior colliculus. It has the longest intracranial course of any cranial nerve. The sixth cranial nerve controls the lateral rectus muscle. It begins in the caudal pons adjacent to the fourth ventricle.
The third, fourth, and sixth cranial nerves eventually converge in the cavernous sinus and travel through the orbital fissure to the eye. The third cranial nerve further subdivides into the superior division, which innervates the superior rectus muscle and the levator muscle of the eyelid, and the inferior division, which innervates the sphincter muscle of the iris and the medial rectus, inferior rectus, and inferior oblique muscles.
The extraocular muscles form from mesodermal tissue within the developing orbit. Differentiating muscle cells are present beginning at approximately the fifth week of gestation.
Strabismus may be caused by a number of conditions. The eyes are normally maintained in alignment by the binocular vision centers of the brain. Some patients have a latent tendency for the eyes to drift, which manifests when one eye is covered (a phoria). When the eye is uncovered, the binocular vision areas of the brain recognize the misalignment and send signals to the eye muscles to realign the eyes. In the most common forms of childhood strabismus (infantile esotropia and intermittent exotropia), the extraocular muscles themselves are normal. The eyes become deviated due to abnormal signals from the binocular vision and motion processing centers of the brain. Sensory strabismus may develop when the vision is decreased (for any reason), and the binocular vision center has less impetus to keep the eyes aligned. In some disorders the eye muscles themselves are abnormal, creating a restrictive strabismus.
Cranial nerve palsies may arise from abnormalities occurring anywhere from the cranial nerve nuclei to the sites where the nerves connect to the extraocular muscles. They may be acquired or congenital. Intracranial mass lesions may directly compress a cranial nerve. Increased intracranial pressure alone may also cause dysfunction.
Proper eye alignment is necessary for normal binocular vision. Each eye sends visual information to the occipital cortex. Because the 2 eyes are slightly separate, these images are slightly different. The brain is able to compare the images and evaluate depth (stereopsis). If the eyes are misaligned, this binocular activity cannot function. In adults with acquired strabismus, the eye misalignment usually produces diplopia (double vision) because the brain is unable to reconcile the disparate images. Unlike adults, most children with strabismus suppress (ignore) one of the images, rather than experience diplopia.
Strabismus presents with misalignment of the eyes. This is most often first noticed by the parents. Because young children with strabismus usually do not experience diplopia, they usually have no complaints. Older children, particularly those with cranial nerve palsy and no previous history of strabismus, may report that they see 2 images of the same object. Eye deviations must reach a threshold angle to be visible to observers, usually approximately 10˚. Angles between 5˚ and 10° may not be clinically obvious, but have the potential to interrupt binocular vision. Pediatricians may detect small angles of strabismus during well-child visits by noting asymmetry of the corneal light reflex.
Strabismus may present acutely, with the onset of a large deviation occurring over a brief period (days). Alternatively, it may begin intermittently, in which case it often gradually progresses in duration and frequency. Intermittent strabismus is most noticeable when children are fatigued, ill, or daydreaming. In young children with esotropia, the eye crossing is often most noticeable at near, whereas intermittent exotropia is usually more noticeable when the child is viewing distant objects.
The presence of associated signs and symptoms may help discriminate between primary strabismus and strabismus secondary to cranial nerve palsies or other acquired disorders. The specific findings are discussed in the individual sections below.
During the first 2 months of life, normal infants may have brief episodes of strabismus. This is most commonly esotropia, but intermittent exotropia may also occur. This should resolve by 3 to 4 months of age. Constant crossing is not normal, even in neonates. Any infant with constant strabismus, or intermittent strabismus that persists beyond 2-3 months of age, should be referred to an ophthalmologist.
This term refers to a condition in which the eyes appear to be deviated, but the visual axes of the eyes are actually aligned. The most common form is pseudoesotropia, in which the eyes appear to be crossed. This is common in infancy, and results from the broad, flat nasal bridge that is a normal feature of most infants’ faces. Patients may have epicanthal folds, folds of tissue of the inner eyelid. Epicanthal folds block visualization of the normal white inner sclera, which creates an optical illusion in which the eyes appear to be crossed. The esotropia appears to be worse when the child looks to the side, because more of the white of the eye is blocked (Figure 34–2A and B). Pseudostrabismus is diagnosed by finding a symmetric corneal light reflex despite the appearance of eye crossing. Ophthalmological consultation may be indicated if the diagnosis is uncertain. The diagnosis is confirmed if no strabismus is found with cover testing and normal binocular vision responses are present.
A less common form of pseudostrabismus is pseudoexotropia. In one type of this condition the fovea of one eye is displaced temporally, most commonly due to retinopathy of prematurity (with dragging of the fovea toward the temporal retina). Because the fovea is displaced, the eye must rotate laterally to bring the fovea into alignment with the visual axis (Figure 34–3). The eye therefore appears exotropic, and the corneal light reflex is decentered nasally. This is known as positive angle kappa. The diagnosis is established by the absence of any shift of the eyes with an alternate cover test, which indicates that there is no ocular misalignment (Figure 34–4A–C). Visible changes of the retina may be noted with ophthalmoscopy (Figure 34–5), but some children with pseudoexotropia have normal retinas.
FIGURE 34–4
Pseudoexotropia (positive angle kappa). (A) The left eye appears slightly exotropic, and the left corneal light reflex is nasal to the center of the cornea. There is no change in the reflex and the eyes do not move when (B) the right eye is covered or (C) the left eye is covered. This confirms the diagnosis of pseudostrabismus.
Infantile esotropia is not present at birth, but usually is first noted within the first 2 months of life. It may start intermittently, but rapidly progresses to constant crossing. If only one of the eyes is constantly crossed, infants develop amblyopia. This occurs in approximately half of affected patients. If the crossing spontaneously alternates, this indicates that the vision in both eyes is equal or nearly equal (Figure 34–6A and B). Some infants with large deviations have cross-fixation. This means that the child uses the crossed left eye to view things on their right side and the crossed right eye to view things on their left side. Other eye movement abnormalities in patients with infantile esotropia include smooth pursuit asymmetry (the eye moves more evenly when tracking objects from lateral to central gaze than vice versa) and latent nystagmus (the patient develops nystagmus when one eye is covered, with the nystagmus appearing to beat toward the uncovered eye).
FIGURE 34–6
Infantile esotropia with alternate fixation. (A) The patient is using the right eye and the left eye is crossed. (B) During the examination, the patient spontaneously shifts fixation to the left eye and the right eye becomes crossed. This indicates that the vision is equal or nearly equal between the eyes.
There is a genetic component to infantile esotropia, but it is not inherited as a classic mendelian trait. A history of infantile esotropia in other family members helps in establishing a diagnosis. Infantile esotropia also occurs more commonly in conditions such as prematurity, Trisomy 21, and other disorders associated with developmental delay.
The goal of treatment of infantile esotropia (and, indeed, of all strabismus therapy) is to align the eyes so that binocular vision can function. The infant brain has plasticity. In general, the more promptly the eyes are realigned the better the outcome. However, even with good alignment, children with infantile esotropia almost never develop the same degree of binocular vision and depth perception that normal individuals have. Usually the best outcome is monofixation syndrome, in which the eyes are closely, but not perfectly, aligned and there is some degree of binocular vision.
The most common treatment for infantile esotropia is surgery, specifically weakening of the medial rectus muscles. For strabismus in general, and infantile esotropia in particular, the need for more than one surgery is common, with approximately one-third of patients requiring 2 or more procedures. Reasons a patient may need additional surgery include recurrent esotropia, consecutive exotropia (exotropia that develops after treatment of esotropia), or vertical misalignment (dissociated vertical deviation and inferior oblique muscle overaction—discussed below in the section on vertical strabismus).
Almost all children in the first few years of life are farsighted (hyperopic). This means that the lens of the eye must change its shape to focus (accommodation). This is easily accomplished in normal children with typical mild farsightedness. In addition to lens focusing, the accommodative response consists of inward movement of the eyes and constriction of the pupils. In normal patients the degree of inward movement corresponds appropriately to the amount of focus needed to view near objects. If the eyes are more farsighted than normal (which occurs in accommodative esotropia), the increased effort required to focus the image may be accompanied by a greater-than-normal convergence of the eyes, which produces esotropia. This can be explained to the parents as “the eyes cross because they have to focus too much.”
Accommodative esotropia may develop in infancy, but usually begins after 1 year of age. It typically gradually worsens over 1 to 2 months. It is initially intermittent and most noticeable when children are viewing near objects (due to the increased focusing requirement at near). The diagnosis is established by measuring the degree of farsightedness after using dilating drops that prevent accommodation (cycloplegic refraction) and noting resolution of the esotropia when the children wear glasses to correct the farsightedness (Figure 34–7A and B).
Normal farsightedness in young children usually decreases with age. The farsightedness in children with accommodative esotropia often does not resolve, and usually changes very little as children grow older. It is important that the parents of children with accommodative esotropia understand that their child will probably continue to require glasses as they grow older, and that the eyes will usually continue to cross when they are not wearing their glasses (such as while bathing or at bedtime).
Some children have features of both infantile and accommodative esotropia. In these patients the eye crossing improves while wearing spectacles, but does not fully resolve. The treatment is surgery to correct the portion of the esotropia that persists while the glasses are worn.
A less common form of esotropia develops relatively rapidly in older children (3–5 years or older). This may occur spontaneously, or after a period of occlusion of one eye (for example, when one eyelid is swollen shut after trauma or an insect bite). In most such patients the esotropia is thought to result from a previously unrecognized small-angle strabismus, which decompensates to a constant deviation. However, the possibility of an intracranial lesion or other disorder responsible for the strabismus must be considered in this situation. The absence of associated neurological symptoms and a positive family history of strabismus make this less likely. A comitant deviation (the angle of esotropia does not change in lateral gaze) also is reassuring, because this indicates that a sixth cranial nerve palsy is not present. However, acute comitant esotropia has been reported in older children with Arnold-Chiari malformation or central nervous system (CNS) tumors.1 The presence of gaze limitation due to sixth nerve palsy (with incomitant strabismus), nystagmus, pupil defects, or optic nerve changes suggests an underlying CNS abnormality.
Imaging of the CNS (usually magnetic resonance imaging [MRI]) is sometimes performed in older children with acute esotropia due to the possibility of associated abnormalities. If no other problems are found, prompt surgical alignment of the eyes is indicated. The prognosis for improvement in binocular vision after successful surgical restoration is better than in patients with infantile esotropia.2
This is discussed below in the section on cranial nerve palsies.
Cyclic esotropia is a very unusual form of strabismus in which the eye alignment varies between straight and large-angle esotropia. In most children the cycles alternate daily (i.e., the eyes are straight one day and crossed the next). When the eyes are straight, the depth perception is usually normal. The patients are usually otherwise healthy. The condition responds well to surgery, with the amount of correction based on the angle of esotropia that is present on the day the eyes cross.
Intermittent exotropia is present when the eye varies between straight and outwardly deviated (Figure 34–8). It usually begins in children after 1 year of age. As the name denotes, it starts as an intermittent strabismus, but with time it may progress to a constant deviation.
There are 3 important differences between infantile esotropia and intermittent exotropia in terms of their effects on vision:
- First, amblyopia is uncommon in intermittent exotropia. Most patients have normal vision in both eyes.
- Second, when the eyes are aligned, the depth perception is usually normal (unlike infantile esotropia, in which the binocular vision is subnormal even after successful surgical realignment).
- Third, unlike infantile esotropia, in which there is a higher incidence in patients with developmental delay, most children with intermittent exotropia are developmentally normal. An important exception to this, however, is intermittent exotropia that develops within the first year of life, which is associated with an increased incidence of developmental delay.3
The decision regarding timing of treatment for intermittent exotropia is based on the degree and frequency of the strabismus. The visual acuity in most patients is normal. If amblyopia or a significant refractive error is found, treatment of the amblyopia and the refractive error is initiated. If this does not improve the strabismus, surgery may be considered. In general, if the family reports progressive strabismus, and the examination shows an easily elicited strabismus that the patient has difficulty controlling, surgery is recommended.
This is a specific type of intermittent exotropia that usually does not develop until later in childhood, often during the teenage years. In convergence insufficiency the exotropia is greater when viewing near objects, particularly with reading. Patients may report diplopia, but more commonly describe vague symptoms of eye fatigue or strain (asthenopia) with prolonged reading. Examination reveals good alignment when viewing distant objects and an intermittent exotropia when fixating on a near object. This type of strabismus may improve with orthoptic exercises to improve convergence.4 Surgery may be indicated if symptoms persist despite exercises.
If a patient has decreased vision in one eye due to a structural abnormality of the eye, there is an increased risk of strabismus developing in that eye. Sensory strabismus can also develop if patients have an ocular disorder that affects both eyes (Figure 34–9). Any ocular condition that causes decreased vision may cause strabismus. In young children sensory strabismus is most commonly esotropia. In older children and adults, it is most commonly exotropia. In patients with sensory strabismus the strabismus is usually constant, the vision is decreased, and the underlying ocular defect is found on examination. Although most children with eye misalignment do not have sensory strabismus, every patient with strabismus requires a complete ophthalmic examination to rule out the presence of structural eye abnormalities.
These 2 forms of vertical strabismus are usually seen in patients with infantile esotropia. They are the most common type of vertical strabismus in children. They are usually not present in early infancy (when the babies first present with infantile esotropia), but develop later during the first 1 to 2 years of life. They occur more frequently in patients whose esotropia is not corrected, but may also develop in patients whose horizontal misalignment has been successfully surgically repaired.
Dissociated vertical deviation is an intermittent slow updrifting of the eye. Inferior oblique muscle overaction (also termed overelevation in adduction) is an elevation of the eye when it is turned toward the nose (Figure 34–10A and B). Both forms of strabismus benefit from surgical correction. A history of infantile esotropia helps distinguish inferior oblique muscle overaction from superior oblique palsy, which has some of the same characteristics (see below).
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