Comitant Strabismus

Comitant strabismus is the most common type of strabismus. The individual extraocular muscles usually have no defect. The amount of deviation is constant, or relatively constant, in the various directions of gaze.

Pseudostrabismus (pseudoesotropia) is one of the most common reasons a pediatric ophthalmologist is asked to evaluate an infant. This condition is characterized by the false appearance of strabismus when the visual axes are aligned accurately. This appearance may be caused by a flat, broad nasal bridge, prominent epicanthal folds, or a narrow interpupillary distance. The observer might see less white sclera nasally than would be expected, and the impression is that the eye is turned in toward the nose, especially when the child gazes to either side. Parents often comment that when their child looks to the side, the eye almost disappears from view. Pseudoesotropia can be differentiated from a true misalignment of the eyes when the corneal light reflex is centered in both eyes and when the cover-uncover test shows no refixation movement. 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, and the medial sclera becomes proportional to the amount visible on the lateral aspect. It is the appearance of crossing that the child will outgrow. Some parents of children with pseudoesotropia erroneously believe that their child has an actual esotropia that will resolve on its own. Because true esotropia can develop later in children with pseudoesotropia, parents and pediatricians should be cautioned that reassessment is required if the apparent deviation does not improve.

Esodeviations are the most common type of ocular misalignment in children and represent >50% of all ocular deviations. Congenital esotropia is a confusing term. Few children who have this disorder are actually born with an esotropia. Most reports in the literature have therefore considered infants with confirmed onset earlier than 6 mo as having the same condition, which some observers have designated infantile esotropia.

The characteristic angle of congenital esodeviations is large and constant (Fig. 615-1). Because of the large deviation, cross-fixation is often 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) because the adducting eye is used in side gaze; this condition simulates a 6th nerve palsy. Abduction can be demonstrated by the doll’s head maneuver or by patching 1 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.

Figure 615-1 Congenital esotropia. Note the large angle of crossing.

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. Early treatment is more likely to lead to the development of binocular vision, which helps to maintain long-term ocular alignment. Once any associated amblyopia is treated, surgery is performed to align the eyes. Even with successful surgical alignment, it is common for vertical deviations to develop in children with a history of congenital esotropia. The two most common forms of vertical deviations to develop are inferior oblique muscle overaction and dissociated vertical deviation. In inferior oblique muscle overaction, the overactive inferior oblique muscle produces an upshoot of the eye closest to the nose when the patient looks to the side (Fig. 615-2). In dissociated vertical deviation, 1 eye drifts up slowly, with no movement of the other eye. Surgery may be necessary to treat either or both of these conditions.

Figure 615-2 Inferior oblique muscle overaction.

It is important that parents realize that early successful surgical alignment is only the beginning of the treatment process. Because many children redevelop strabismus or amblyopia, they need to be monitored closely during the visually immature period of life.

Accommodative esotropia is defined as a “convergent deviation of the eyes associated with activation of the accommodative (focusing) reflex.” It usually occurs in a child who is between 2 and 3 yr of age and who has a history of acquired intermittent or constant crossing. Amblyopia occurs in the majority of cases.

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 or if the amount of convergence that occurs in response to each unit of accommodative effort is great, esotropia can develop.

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 (Fig. 615-3). 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 eye position at distance fixation but leaves a residual deviation at near fixation; this may be observed or treated with bifocal lenses, antiaccommodative drops, or surgery.

Figure 615-3 Accommodative esotropia; control of deviation with corrective lenses.

It is important to warn parents of children with accommodative esotropia that the esodeviation might appear to increase without glasses after the initial correction is worn. Parents often state that before wearing glasses, their child had a small esodeviation, whereas after removing the glasses, the esodeviation becomes quite large. Parents often blame the increased esodeviation on the glasses. This apparent increase is due to a child’s using the appropriate amount of accommodative effort after the glasses have been worn. When these children remove their glasses, they continue to use an accommodative effort to bring objects into proper focus and increase the esodeviation.

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 and is not correctable with glasses. 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.

Exodeviations are the second most common type of misalignment. The divergent deviation may be intermittent or constant. Intermittent exotropia is the most common exodeviation in childhood. It is characterized by outward drifting of one eye, which usually occurs when a child is fixating at distance. The deviation is generally more common with fatigue or illness. Exposure to bright light can cause reflex closure of the exotropic eye. 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.

The age at onset of intermittent exotropia varies but is often between ages 6 mo and 4 yr. The decision to perform eye muscle surgery is based on the amount and frequency of the deviation. If the deviation is small and infrequent, it is reasonable to observe the child. If the exotropia is large or increasing in frequency, surgery is indicated to maintain normal binocular vision.

Constant exotropia may rarely be congenital. Congenital exotropia may be associated with neurologic disease or abnormalities of the bony orbit, as in Crouzon syndrome. Exotropia that occurs later in life might represent a deterioration of an intermittent exotropia that was present in childhood. Surgery can restore binocular vision even in long-standing cases.

Noncomitant Strabismus

When an eye muscle is paretic, palsied, or restricted, a muscle imbalance occurs in which the deviation of the eye varies according to the direction of gaze. Recent onset of paresis can be suggested by the symptom of double vision that increases in one direction, the findings of an ocular deviation that increases in the field of action of the paretic muscle, and an increase in the deviation when the child fixates with the paretic eye. It is important to differentiate a noncomitant strabismus from a comitant deviation because noncomitant forms of strabismus are often associated with trauma, systemic disorders, or neurologic abnormalities.

Third Nerve Palsy

In the pediatric population, 3rd nerve palsies are usually congenital. The congenital form is often associated with a developmental anomaly or birth trauma. Acquired 3rd nerve palsies in children can be an ominous sign and can indicate a neurologic abnormality such as an intracranial neoplasm or an aneurysm. Other less-serious causes include an inflammatory or infectious lesion, head trauma, postviral syndromes, and migraines.

A 3rd nerve palsy, whether congenital or acquired, usually results in an exotropia and a hypotropia, or downward deviation of the affected eye, as well as complete or partial ptosis of the upper lid (Fig. 615-4). This characteristic strabismus results from the action of the normal, unopposed muscles, the lateral rectus muscle, and the superior oblique muscle. If the internal branch of the 3rd nerve is involved, pupillary dilation may be noted as well. Eye movements are usually limited nasally in elevation and in depression. In addition, clinical findings and treatment may be complicated in congenital and traumatic cases of 3rd nerve palsy owing to misdirection of regenerating nerve fibers, referred to as aberrant regeneration. This results in anomalous and paradoxical eyelid, eye, and pupil movement such as elevation of the eyelid, constriction of the pupil, or depression of the globe on attempted medial gaze.

Figure 615-4 Third cranial nerve palsy of the left eye.

Fourth Nerve Palsy

Fourth nerve palsies can be congenital or acquired. Because the 4th nerve has a long intracranial course, it is susceptible to damage resulting from head trauma. In children, however, 4th nerve palsies are more commonly congenital than traumatic. A palsied 4th nerve results in weakness in the superior oblique muscle, which causes an upward deviation of the eye, a hypertropia. Because the antagonist muscle, the inferior oblique, is relatively unopposed, the affected eye demonstrates an upshoot when looking toward the nose. Children typically present with a head tilt to the shoulder opposite the affected eye, their chin down, and their face turned away from the affected side. This head position places the eye away from the area of greatest action of the affected muscle and therefore minimizes the deviation and the associated double vision. Long-standing head tilts can lead to facial asymmetry. Because the abnormal head posture maintains the child’s ocular alignment, amblyopia is uncommon. Because no abnormality exists in the neck muscles, attempts to correct the head tilt by exercises and neck muscle surgery are ineffective. Recognition of a superior oblique paresis can be difficult because deviation of the head and the eye may be minimal. Eye muscle surgery can be performed to improve the ocular alignment and eliminate the abnormal head posture.

Sixth Nerve Palsy

Sixth nerve palsies produce markedly crossed eyes with limited ability to move the afflicted eye laterally. Children often present with their head turned toward the palsied muscle, a position that helps preserve binocular vision. The esotropia is largest when the eye is moved toward the affected muscle.

Congenital 6th nerve palsies are rare. Decreased lateral gaze in infants is often associated with other disorders, such as congenital esotropia or Duane retraction syndrome (Chapter 585.9). In neonates, a transient 6th nerve paresis can occur; it usually clears spontaneously by 6 wk. It is thought that increased intracranial pressure associated with labor and delivery is the contributing factor.

Acquired 6th nerve palsies in childhood are often an ominous sign because the 6th nerve is susceptible to increased intracranial pressure associated with hydrocephalous and intracranial tumors. Other causes of 6th nerve defects in children include trauma, vascular malformations, meningitis, and Gradenigo syndrome. A benign 6th nerve palsy, which is painless and acquired, can be noted in infants and older children. This is often preceded by a febrile illness or upper respiratory tract infection and may be recurrent. Complete resolution of the palsy is usual. Although not uncommon, other causes of an acute 6th nerve palsy should be eliminated before this diagnosis is made.

Monocular Elevation Deficiency

A monocular elevation deficiency is an inability to elevate the eye in both adduction and abduction. It can represent a paresis of both elevators, the superior rectus and inferior oblique muscles, or a possible restriction to elevation from a fibrotic inferior rectus muscle. When an affected child fixates with the nonparetic eye, the paretic eye is hypotropic and the ipsilateral upper eyelid can appear ptotic. Fixation with the paretic eye causes a hypertropia of the nonparetic eye and a disappearance of the ptosis (Fig. 615-5). Because the apparent ptosis is actually secondary to the strabismus, correction of the hypotropia treats the pseudoptosis.

Figure 615-5 Double elevator palsy of the right eye. Note the disappearance of the apparent ptosis when fixating with the involved eye.

Duane Syndrome

Duane syndrome (Chapter 585.9) is a congenital disorder of ocular motility characterized by retraction of the globe on adduction. This is attributed to the absence of the 6th nerve nucleus and anomalous innervation of the lateral rectus muscle, which results in co-contraction of the medial and lateral rectus muscles on attempted adduction of the affected eye. Within the spectrum of Duane syndrome, patients can exhibit impairment of abduction, impairment of adduction, or upshoot or downshoot of the involved eye on adduction. They can have esotropia, exotropia, or relatively straight eyes. Many exhibit a compensatory head posture to maintain single vision. Some develop amblyopia. Surgery to improve alignment or to reduce a noticeable face turn can be helpful in selected cases.

Duane syndrome usually occurs sporadically. It is sometimes inherited as an autosomal dominant trait. It usually occurs as an isolated condition but can occur in association with various other ocular and systemic anomalies.

Möbius Syndrome

The distinctive features of Möbius syndrome (Chapter 585.9) are congenital facial paresis and abduction weakness. The facial palsy is commonly bilateral, often asymmetric, and often incomplete, tending to spare the lower face and platysma. Ectropion, epiphora, and exposure keratopathy can develop. The abduction defect may be unilateral or bilateral. Esotropia is common. The cause is unknown. Whether the primary defect is maldevelopment of cranial nerve nuclei, hypoplasia of the muscles, or a combination of central and peripheral factors is unclear. Some familial cases have been reported. Associated developmental defects include ptosis, palatal and lingual palsy, hearing loss, pectoral and lingual muscle defects, micrognathia, syndactyly, supernumerary digits, and the absence of hands, feet, fingers, or toes. Surgical correction of the esotropia is indicated, and any attendant amblyopia should be treated.

Brown Syndrome

In Brown syndrome, elevation of the eye in the adducted position is restricted (Fig. 615-6). An associated downward deviation of the affected eye in adduction can also occur. A compensatory head posture may be evident.

Figure 615-6 Brown syndrome of the right eye.

Various causes have been described. Some cases have been attributed to structural abnormalities such as a tight superior oblique tendon, congenital shortening or thickening of the superior oblique tendon sheath, or connective tissue trabeculae between the superior oblique tendon and the trochlea. Acquired Brown syndrome can follow trauma to the orbit involving the region of the trochlea or sinus surgery. It can also occur with inflammatory processes, particularly sinusitis and juvenile rheumatoid arthritis.

Acquired inflammatory Brown syndrome might respond to treatment with either nonsteroidal medications or corticosteroids. Surgery may be helpful for children with true congenital Brown syndrome.

Parinaud Syndrome

Parinaud syndrome is a palsy of vertical gaze, isolated or associated with pupillary or nuclear oculomotor (3rd cranial nerve) paresis. It indicates a lesion affecting the mesencephalic tegmentum. The ophthalmic signs of midbrain disease include vertical gaze palsy, dissociation of the pupillary responses to light and to near focus, general pupillomotor paralysis, corectopia, dyscoria, accommodative disturbances, pathologic lid retraction, ptosis, extraocular muscle paresis, and convergence paralysis. Some patients have associated spasms of convergence, convergent retraction nystagmus, and vertical nystagmus, particularly on attempted vertical gaze. Combinations of these signs are referred to as the sylvian aqueduct syndrome.

A principal cause of vertical gaze palsy and associated mesencephalic signs in children is tumor of the pineal gland or 3rd ventricle. Differential diagnosis includes trauma and demyelinating disease. In children with hydrocephalus, impairment of vertical gaze and pathologic lid retraction are referred to as the setting-sun sign. A transient supranuclear disorder of gaze is sometimes seen in healthy neonates.