Pediatrics

Strabismus

OVERVIEW: What every practitioner needs to know

Are you sure your patient has strabismus? What are the typical findings for this disease?

1. The most common symptom of strabismus in children is the appearance of any ocular misalignment. Most commonly, the eyes are turned in (80%), less commonly turned out (15%), and least common one eye higher or lower than the other (5%). Strabismus can be categorized by the age at presentation, by its presence constantly or intermittently, by the direction of misalignment, or by the consistency or inconsistency of the misalignment in different positions of gaze.

2. Amblyopia is present in 50% of children with strabismus.

3. Double vision (diplopia) is uncommon in children less than 6-8 years of age.

4. Children with exotropia often close one eye in bright sunlight for reasons that we do not understand.

Strabismus by age at presentation

Primary infantile strabismus is defined by most ophthalmologists as documented ocular misalignment by 6 months of age. Few children are actually born with strabismus, and very few babies are documented to have misaligned eyes in the newborn nursery. Most specialists have ceased using the term congenital strabismus. Acquired strabismus presents after six months of age, and generally around 18-36 months of age.

Strabismus by part-time or full-time presence of deviation

Ocular misalignment that is constantly present and not controlled by fusional mechanisms is termed a "tropia." Esotropia equals turned in (cross-eye), exotropia equals turned out (wall-eye), and hypertropia describes one eye higher while hypotropia describes one eye lower than the other.

Most children with strabismus develop a deviation manifest mostly in one eye, the non-dominant eye. Some children with strabismus can alternate fixation, able to switch fixation using one eye sometimes and the other eye at times, so that the strabismus will appear to shift from one eye to the other. An intermittent tropia is a deviation that may sometimes be latent and controlled, but may at other times be spontaneously manifest, often with fatigue or illness. A "phoria" is defined as a latent deviation that is controlled by inherent fusional mechanisms and only present when one eye is blocked or covered.

Strabismus by the direction of deviation

The majority of strabismic children, probably 80%, manifest a convergent deviation, or an esotropia that a lay person would call "cross-eyed." Most infants with infantile esotropia present between 6 and 12 months of age. Accommodative esotropia is defined as a convergent deviation of the eyes associated with accommodative reflex from excessive hyperopia (far-sightedness). The onset is generally around 2-5 years of age. The condition is usually intermittent at onset and becomes constant with time without treatment.

The second most common presentation of strabismus in children, perhaps 15%, is a divergent strabismus, or an exotropia that a lay person would call "wall-eye."

Strabismus by the consistency of the misalignment in different positions of gaze

A comitant strabismus is one in which the ocular misalignment is measured the same in primary, or straight-ahead gaze, and also in left, right, up, and down gaze.

An incomitant strabismus is defined when the amount of strabismus differs depending on the position of gaze. Incomitant strabismus can result commonly from primary over-action of the inferior oblique muscles or, less commonly, the superior oblique muscles, and is most often associated with primary infantile strabismus. Incomitant strabismus can also result when a muscle is paretic or restrictive. The most common paretic incomitant vertical strabismus is secondary to superior oblique (cranial nerve VI) palsy. The most common incomitant horizontal strabismus results from lateral rectus or abducens (cranial nerve IV) palsy. A restrictive incomitant strabismic can occur if a muscle is trapped into an orbital wall fracture or in some systemic diseases like Grave's eye disease. When the strabismus is incomitant, patients may adopt an abnormal head position, a head tilt or head turn, in order to eliminate double vision and align their eyes.

What other disease/condition shares some of these symptoms?

Pseudoesotropia is a common presentation of "false" esotropia, most commonly occurring in young infants when a flat nasal bridge and prominent epicanthal folds tend to obscure the nasal portion of the conjunctiva and sclera. This optical illusion causes the patient to have an appearance of crossed eyes, and is most apparent when the eyes are in side gaze or focusing on a close object. A smaller than average distance between the two pupils can also give the appearance of pseudoesotropia.

Pseudoexotropia is less common but exists when the alignment of the eyes is straight but they appear to be wall-eyed. Pseudoexotropia exists when there is a large positive angle kappa, or a large angle between the two lines formed by the visual axis and the pupillary axis. A larger angle kappa is commonly seen when a disease process causes dragging of the fovea (center of the retina) temporally. This is seen in former premature infants who have recovered from proliferative retinopathy of prematurity. A larger than average distance between the pupils of the two eyes can also give the appearance of pseudoexotropia.

What caused this disease to develop at this time?

The cause of primary infantile strabismus remains unknown. Strabismus beginning less than 6 months of age used to be called "congenital;" however, most strabismologists have discontinued use of the term since it is very unusual for infants to present with ocular misalignment in the newborn nursery. Strabismus is present in approximately 2% of children. It is much more common in children with a neurological risk - prematurely born, seizures, cerebral palsy, developmental delay, chromosomal disorders, etc. There is also a familial association, with strabismus being more common among first degree relatives.

Pediatric patients with accommodative esotropia most often develop a convergent deviation on the basis of excessive accommodation in response to greater than normal degrees of hyperopia, or far-sightedness, generally between 3-5 years of age.

What laboratory studies should you request to help confirm the diagnosis? How should you interpret the results?

Currently, there are no laboratory studies that aid in the diagnosis of strabismus.

Would imaging studies be helpful? If so, which ones?

In a patient with strabismus who also has an abnormal red reflex from leukocoria (white reflex), a diagnosis of retinoblastoma should be considered. If retinoblastoma is the leading diagnosis, a MRI of the head can aid in the evaluation of a tri-lateral retinoblastoma, which is bilateral retinoblastoma and pineal tumor. A CT scan can also be done more quickly to evaluate for intraocular calcium, as this is a typical finding in retinoblastoma.

The presence of an incomitant strabismus, such as a lateral rectus palsy with horizontal incomitance or a superior oblique palsy with vertical incomintance, should prompt an exam of the optic nerves to look for papilledema. An abrupt onset of incomitant strabismus is an indication for a brain MRI.

If you are able to confirm that the patient has strabismus, what treatment should be initiated?

Almost any infant or child with strabismus deserves an evaluation by an eye physician, preferably a pediatric ophthalmologist, in a timely manner.

For infants with primary infantile strabismus, normal ocular alignment is rarely achieved without eye muscle surgery. However, surgery should be undertaken after correction of significant refractive errors and treatment of any existing amblyopia. Even with medical and commonly surgical treatment of infantile strabismus, these patients rarely establish binocularity and are at risks for other forms of strabismus later in life.

Some ophthalmologists use botulism toxin in the treatment of non-paretic strabismus. Multiple injections may be required. However, the long-term sensory and motor outcomes have not been shown to be superior to those from incisional surgery.

Treatment of refractive accommodative esotropia consists of correction of the hyperopia with spectacles, and treatment of any coexistent amblyopia.

Treatment of partially accommodative esotropia, that is, residual esotropia despite treatment of hyperopia and amblyopia, requires surgery for the residual strabismus.

Children with intermittent exotropia may be treated with part-time patching and atropine penalization for existing amblyopia or surgery. Children with constant exotropia or intermittent exotropia with poor control generally require surgical realignment. Most pediatric ophthalmologists will treat co-existent amblyopia, if present, very aggressively in an effort to maximize vision before realigning the eyes surgically.

What are the adverse effects associated with each treatment option?

Strabismus associated with retinoblastoma that goes unrecognized can be life-threatening.

Fifty percent of infants and children with strabismus develop amblyopia, which is the leading cause of decreased vision in young adults.

Approximately 10%-20% of infants and children treated with strabismus surgery can develop a recurrent ocular misalignment, either over- or under-correction. These children will require multiple surgeries. Other risks of strabismus surgery, although rare, include infection leading to a decrease in vision, loss of vision, or loss of eye if a severe infection were to occur.

Nystagmus occurs in up to one-third of patients with a history of primary infantile esotropia.

What are the possible outcomes of strabismus?

One-half of pediatric patients with strabismus can develop amblyopia. Some children need surgery, spectacles, amblyopia treatment, or a combination of these treatments. The earlier that an ocular misalignment can be corrected in children, the better the probability of a stable binocular result.

What causes this disease and how frequent is it?

The cause of infantile strabismus remains unknown. However, strabismus is present in approximately 2% of children. It is much more common in children with a neurological risk - prematurely born, seizures, cerebral palsy, developmental delay, chromosomal disorders, etc.

The debate regarding the etiology of strabismus is focused around two conflicting theories. Worth's "sensory" theory concept was that strabismus resulted from a deficit in a presumed fusion center in the brain. According to this theory, the goal of restoring binocularity was considered hopeless. Chavasse believed the primary problem was mechanical and potentially curable if the deviation could be eliminated in infancy. Some eye surgeons have reported favorable sensory results (depth perception and stereopsis), and these encouraging results are the basis for the practice of early surgery for patients with infantile strabismus.

There is a familial association with strabismus, being more common among first degree relatives.

Children with refractive esotropia, on the basis of excessive accommodation associated with above normal hyperopia/far-sightedness, generally present between 18 months and 26 months of age.

Children with increased intracranial pressure can develop strabismus, both comitant and incomitant, and sometimes can produce paralytic strabismus, most commonly an abducens palsy. The abducens nerve is the most susceptible to injury from elevated intracranial pressure due to its exposure along the brainstem.

Children with head trauma, serious central nervous system (CNS) infections, brain tumors, etc. often present with strabismus. All of the cranial nerves that control the extraocular muscles can be affected: CN III (oculomotor nerve), CN IV (trochlear nerve), or CN VI (abducens nerve). Paralytic strabismus is the most difficult strabismus challenge for the strabismus surgeon, especially CN III palsies, because so many of the extraocular muscles are affected.

How do these pathogens/genes/exposures cause the disease?

Although there is definitely a familial predisposition, there has been no confirmation of specific genetic markers that put a child at higher risk for strabismus. However, research is being done in this area. There is a higher incidence of strabismus in any neurologically at-risk children, such as prematurely born children, children with chromosomal syndromes, cerebral palsy, and seizures.

Other clinical manifestations that might help with diagnosis and management

Nystagmus occurs in up to one-third of patients with a history of early onset strabismus.

Dissociated strabismus may involve vertical (most common), horizontal, and torsional components. The strabismus is described as "dissociated" because the intermittent upturn, for example, is not associated with a corresponding down-turn of the other eye that is seen in true vertical strabismus. Dissociated vertical deviation (DVD) is an innervational disorder found in up to 50% of patients with infantile strabismus. DVD usually presents after 2 years of age, whether or not the horizontal deviation has been corrected with surgery, or glasses, or both. Either eye slowly drifts upward and outward with simultaneous extrusion, when occluded or during periods of visual inattention. Some patients may compensate by tilting the head. The condition is usually bilateral but generally asymmetric. The treatment generally involves further eye muscle surgery.

Overaction of the oblique muscles, inferior oblique most commonly and superior oblique less commonly, is common in patients with infantile strabismus, and has been reported to develop between ages 1 and 6 years in up to two-thirds of patients with infantile strabismus. In all but the mildest cases, a weakening procedure of the affected oblique muscles is required.

A- and V-pattern strabismus describe horizontal deviations that change in magnitude in upgaze and downgaze. V pattern describes a horizontal deviation that is more convergent or less divergent in downgaze compare with upgaze. An A pattern, less common than V-pattern, is present when a horizontal deviation shows a more convergent or less divergent alignment in upgaze than in downgaze. An A- or V-pattern is found in up to one-quarter of patients with horizontal strabismus. Clinically significant patterns typically are treated surgically, in combination with correction of the associated horizontal strabismus.

What complications might you expect from the disease or treatment of the disease?

Amblyopia is present in up to half of children with strabismus, and needs to treated concurrently with spectacles or eye muscle surgery.

In children requiring surgery to straighten the eyes, the risk of re-operation is generally considered to be between 10% and 20%. Children with esotropia (convergent squint or cross-eyed) generally are equally likely to develop a recurrent esotropia or a consecutive exotropia. Children undergoing surgery for exotropia are more likely to turn out again.

Diplopia - double vision - occurs rarely after strabismus surgery in older children.

Perforation of the sclera may uncommonly happen when the needle perforates through the sclera inside the eye during reattachment of the muscle. In most cases, this results in only a small chorioretinal scar than does not affect vision. Most perforations are unrecognized unless specifically looked for. Perforation can rarely lead to endophthalmitis.

Postoperative infections are not common after strabismus surgery. Intraocular infection is rare. Some patients develop mild conjunctivitis, which may be caused by allergy to suture material or post-operative topical ophthalmic medications, as well as by infectious agents. Preseptal cellulitis and orbital cellulitis are rare complications. These conditions usually develop 2-3 days after surgery and generally respond well to systemic antibiotics.

A foreign-body granuloma occasionally develops after strabismus surgery, usually at the site of muscle reattachment or the site of conjunctival closure with absorbable suture. It may respond to topical ocular steroids, and surgical excision may be necessary if the lesion persists.

A conjunctival inclusion cyst can appear after surgery if conjunctival epithelium is inadvertently buried during eye muscle reattachment to the sclera or closure of the conjunctival incision. Although they occasionally resolve spontaneously, and at times can respond to thermal cautery, persistent cases often require surgical excision.

Anterior segment ischemia can produce severe inflammation and corneal edema, and results from compromised circulation to the anterior eye structures. Most of the blood supply to the anterior segment of the eye comes through the anterior ciliary arteries that travel in the 4 rectus muscles. Although uncommon in children, simultaneous surgery on three rectus muscles, or as few as two rectus muscles in patients with compromised blood flow, may lead to anterior segment ischemia. No universally agreed upon treatment exists for anterior segment ischemia.

Changes in the position of the eyelids can occur most commonly after surgery on the vertical rectus muscles. This occurs because these muscles are attached to the lid retractors.

A muscle can slip out of the sutures or instruments during strabismus surgery. A slipped muscle can also result from inadequate suturing, with the muscle receding and retracting posteriorly within the capsule during the postoperative period. Clinically, the post-operative patient manifests a weakness of that muscle, with limited ductions and versions, and possibly decreased saccades in its field of action. Correction of a slipped muscle requires reoperation.

Postoperative nausea and vomiting are less common after eye muscle surgery with the use of newer anesthetic agents and ancillary drugs such as odansetron (Zofran), propofol, metoclopramide (Reglan) and decadron.

The oculocardiac reflex is a slowing of the heart rate caused by traction on the extraocular muscles, particularly the medial rectus muscles. In the most severe form, the reflex can produce asystole. Parenteral atropine and other agents can protect against this reflex.

Malignant hyperthermia (MH) may be triggered by many inhalational anesthetics and by the muscle relaxant succinylcholine. MH can occur sporadically or be dominantly inherited with incomplete penetrance. Disorders associated with MH include strabismus, ptosis, and other musculoskeletal abnormalities.

Changes in refractive error are most common when strabismus surgery is performed on two rectus muscles in one eye. An induced astigmatism of low magnitude usually resolves within a few months.

Are additional laboratory studies available; even some that are not widely available?

Currently, there are no laboratory studies available for strabismus.

How can this disease be prevented?

This disease cannot be prevented, however early diagnosis and treatment can reduce the risk of vision loss.

Ongoing controversies regarding etiology, diagnosis, and treatment

Most ophthalmologists agree that surgery should be undertaken early. Many authors have documented better outcomes when infants undergo surgery between 6 months and 2 years of age. Additionally, improved results are associated with less time between development of strabismus and ocular realignment.

These encouraging results became the basis for the practice of even earlier surgery for patients with primary infantile esotropia, as young as 4 months of age. This philosophy of intervention is not universally accepted, since most surgeons would prefer a longer period of observation, documenting a stable angle of misalignment before proceeding to the operating room.

Botulinum toxin injections into the rectus muscles have been used by some ophthalmologists in the treatment of infantile strabismus. However, multiple injections may be required, and the long-term sensory and motor outcomes have not been shown to be better than those associated with standard eye muscle surgery.

What is the evidence?

Enzenauer, RW, Hoehn, ME, Del Monte, MA, Rudolph, CD, Rudolph, AM, Lister, GE. "Rudolph's pediatrics". McGraw Hill Medical. 2011. pp. 2293-8.

Hoyt, CS, Taylor, D. "Pediatric Ophthalmology and Strabismus". Saunders Elsevier. 2013.

Villegas, V, Hess, D, Wildner, A, Gold, A, Murray, T. "Retinoblastoma.". Current Opinion in Ophthalmology. vol. Volume 24. Nov 2013. pp. 581-588.

Mulvihill, A, MacCann, A, Flitcroft, I, O'Keefe, M. "Outcome in refractive accommodative esotropia". Br J Opthalmol. vol. 84. 2000. pp. 746-9.

Holmes, JM, Beck, RW, Kip, KE. "Botulinum toxin treatment versus conservative management in acute traumatic sixth nerve palsy or paresis". J AAPOS. vol. 4. 2000. pp. 145-9.

Abroms, AD, Mohney, BG, Rush, DP. "Timely surgery in intermittent and constant exotropia for superior sensory outcome". Am J Ophthalmol. vol. 131. 2001. pp. 111-6.

Raab, E, Parks, MM. "Recession of the lateral recti: Early and late postoperative alignments". Arch Ophthalmol. vol. 82. 1969. pp. 203-8.

Ing, M, Costenbader, FD, Parks, MM, Albert, DG. "Early surgery for congenital esotropia". Am J Ophthalmol. vol. 61. 1966. pp. 1419-27.

McKeown, CA, Lambert, HM, Shore, JW. "Preservation of the anterior ciliary vessels during extraocular muscle surgery". Ophthalmology. vol. 96. 1989. pp. 498-506.

Preslan, MW, Cioffi, G, Min, YI. "Refractive error changes following strabismus surgery". J Pediatr Ophthalmol Strabismus. vol. 29. 1992. pp. 300-4.

Wright, KW, Edelman, PM, McVey, JH. "High-grade stereo acuity after early surgery for congenital esotropia". Arch Ophthalmol. vol. 112. 1994. pp. 913-9.

**The original source for this chapter was Dr Robert Enzenauer. The chapter was revised for this program by Dr Jasleen Singh.

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