Epidemiology of Pediatric Strabismus

Core Messages

■Recognition and diagnosis of the individual forms of childhood strabismus are important for the best preservation of visual function.

■Esotropia is the most common form of pediatric ocular deviation in the West, whereas exotropia predominates in the East.

■Accommodative esotropia is the most prevalent form of strabismus in the West, comprising half of all esodeviations.

■Congenital, or infantile, esotropia accounts for less than 10% of all pediatric esotropia, a figure much smaller than once widely believed.

■Intermittent exotropia is the second most common form of childhood strabismus in the West

and the most commonly diagnosed form of exodeviation worldwide.

■Hyperdeviations are uncommon, with fourth cranial nerve palsy being the most prevalent etiology.

■Major independent risk factors associated with strabismus development include: prematurity, central nervous system (CNS) impairment, low birth weight, family history, and refractive error.

■Recent studies have reported a decline in the number of surgeries performed for strabismus; however, population-based data of congenital esotropia in the United States confirms a more stable rate.

1.1Introduction

Strabismus, or squint, is a disorder of ocular alignment. This overarching term may be further characterized by the direction of the misalignment: the prefix esodescribes an inward ocular deviation; exo-, an outward deviation; and hyper-, a vertical deviation. Descriptive su xes include -tropia, a manifest deviation in which fusional control is not present, and -phoria, a latent deviation that is controlled by fusion.

Strabismus detection, classification, and treatment are especially important in pediatric populations as strabismus is a leading factor in the development of amblyopia, or a loss in visual function resulting from inadequate or abnormal visual system stimulation. This strong connection with amblyopia di erentiates pediatric from adultonset strabismus, wherein vision and stereopsis are less likely to be irreversibly harmed. In children, strabismus should be corrected to decrease the occurrence of amblyopia, to maximize the potential for stereopsis, and to straighten the visual axes of the eyes.

This chapter will review recent data on the epidemiology of pediatric strabismus. The information will focus

solely on tropic deviations rather than phorias and will encompass worldwide incidence and prevalence as well as clinical characteristics of the various strabismus subtypes.

1.2Forms of Pediatric Strabismus

1.2.1Esodeviations

Esodeviations are characterized by an intermittent or constant inward deviation of the eye or eyes (Fig. 1.1). Esotropia comprises approximately 60% of all strabismus in the West [1] whereas only about 30% in the East [2]. In the United States, children are diagnosed with esotropia at a mean age of 3.1 years [3], and 90% of esodeviations occur by 5 years of age [4]. Esotropia is more commonly associated with amblyopia than either exoor hypertropia, occurring in one of three esotropic children vs. 1 of 12 exoor hypertropic children [5]. There is no significant gender predilection among any of the following subtypes of childhood esotropia.

21 Epidemiology of Pediatric Strabismus

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Fig. 1.1 A child with esotropia

1.2.1.3Acquired Nonaccommodative Esotropia

Acquired nonaccommodative esotropia defines children whose deviation develops after 6 months of age and is not associated with accommodative e ort. This subtype has typically been thought of as uncommon and as portending underlying neurological disease. However, a recent population-based study showed that it is the second most common form of childhood esotropia [3], with an incidence of 1 in 257 children and is rarely the result of neurologic disease [8].

1.2.1.1Congenital Esotropia

Congenital esotropia, also known as infantile or essential infantile esotropia, is generally defined as a neurologically intact child with a constant nonaccommodative esotropia that develops by 6 months of age. This term is often confusing as children do not typically present at birth with their deviation. Moreover, esotropia measuring up to 40 prism diopters (PD) between weeks 4 and 20 of life has been reported to resolve in 27% of children [6].

Congenital esotropia has, for decades, been considered the most common form of strabismus. However, more recent reports have demonstrated that congenital esotropia is much less common than once believed. In a recent incidence study among children born over a 30-year time period in the US, 1 in 403 live births developed congenital esotropia [7]. Other recent reports from the same population reported similar results, with infantile esotropia making up only 8.1% of all forms of esotropia [3].

1.2.1.2Accommodative Esotropia

Accommodative esotropia is characterized by an acquired constant or intermittent deviation that is corrected or reduced 10 PD or more after wearing hyperopic spectacles full time for at least 3 weeks. Patients can further be classified as having fully accommodative esotropia, in which the deviation is reduced to ≤8 PD, or partially accommodative esotropia, in which there is a residual deviation of 10 or more PD. Accommodative esotropia, including both the partially and fully accommodative forms, comprises approximately one half of all pediatric esotropia in the United States and is the most prevalent form of childhood strabismus in the West [3]. This form of esodeviation has been reported to occur in 1 in 92 children [3].

1.2.1.4Abnormal Central Nervous System Esotropia

Esotropic children with a developmental or neurologic disorder may be classified under central nervous system (CNS) defects regardless of the age at onset or form of esotropia. The most commonly associated conditions include cerebral palsy, developmental delay, Down syndrome, and seizure disorder. CNS-associated esotropia makes up approximately 10% of all diagnosed esodeviations [3].

1.2.1.5Sensory Esotropia

Sensory esotropia includes patients with a unilateral or bilateral ocular condition that prevents normal fusion. This form of esodeviation is commonly associated with anisometropic amblyopia as well as with disorders of deprivation such as cataract, corneal scarring, and retinal or optic nerve disorders [3].

Summary for the Clinician

■Accommodative esotropia comprises approximately half of all pediatric esotropia.

■Acquired nonaccommodative esotropia is the second most common form of esodeviation in the West and is rarely associated with neurologic disease.

■Congenital esotropia, once thought to be the most common esodeviation, makes up less than 10% of all esotropia diagnosed in childhood.

■Amblyopia occurs in one of three children with esotropia, a rate significantly higher than in children with either exotropia or hypertropia.