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Ophthalmology / Paediatric ophthalmology

Evidence-Based Medicine in Congenital Esotropia

Kammi B. Gunton, MD Bradley A. Nelson Reprinted from Gunton KB, Nelson BA. Evidence-Based Medicine in Congenital Esotropia. J Pediatr Ophthalmol Strabismus. 2003;40(2)70-73.
Article


Introduction

Congenital esotropia is a large-angle esodeviation that is present by age 6 months. It represents the most common form of strabismus, with an incidence of 1% to 2%.1 The accepted treatment is eye muscle surgery to realign the visual axes with the goals of attaining not only immediate postoperative alignment but also stable alignment with establishment of binocularity. With these goals in mind, several authors have tried to elucidate factors that may influence the outcome of treatment, including patient age at surgical alignment, the duration of misalignment prior to surgery, and the development of associated features of congenital esotropia.

Monofixation syndrome with alignment within 8 PD of orthotropia with some degree of binocularity has become the accepted optimum result following surgery for congenital esotropia.2,3 Yet, the stability of the alignment with monofixation syndrome is variable. Kushner and Fisher4 reported the type of microtropia achieved 6 months postoperatively can better predict long-term stability. After 5 years of follow-up, the percentage of patients maintaining alignment within 8 PD of orthotropia was greatest for patients whose alignment postoperatively was orthotropia (96%) compared with esotropia and exotropia < 8 PD (80% and 20%, respectively). Regarding establishing binocularity, patients whose postoperative alignment was orthotropia were more likely to exhibit stereopsis (65%) than if the postoperative alignment was either esotropia or exotropia < 8 PD (28% and 0%, respectively).

In a similar analysis of 60 patients with postoperative alignment within 8 PD of orthotropia and subnormal stereoacuity, Prieto-Diaz and Prieto-Diaz5 found 35% had a decompensation in their alignment.5 Approximately half of the patients with decompensation had developed consecutive exotropia, which appeared at a mean of 7.6 years after the initial procedure. Forty-three percent developed recurrent esotropia, which developed at a mean of 11 years after the initial surgery. This study stresses the importance of long-term follow-up to ensure postoperative alignment remains stable.

Etiology

Efforts to improve treatment outcomes have evolved from theories to understanding the pathogenesis of the disease. Understanding the etiology of congenital esotropia dates back to the early 1900s when the inherent defect that resulted in esotropia was believed to be a cortical fusional abnormality.6 It was theorized that in the absence of this fusional capacity, surgical treatment would not remain stable or allow for the development of binocular vision. This theory was later challenged by the idea of a muscular abnormality with reflex crossing from an overzealous convergence mechanism.7 It was hypothesized that recession of the medial rectus muscle therefore would allow proper alignment and the possibility of establishing stereoacuity, and surgical treatment indeed resulted in improved alignment of the visual axis and subnormal binocularity.

In 1993, Helveston8 proposed a theory combining the fusional and motor concepts. Helveston hypothesized normal development of fusion requires sensory input from the retina to a fusional center, which then signals motor alignment of the visual axes. During development, the sensory and motor components mature independently, but prior to the end of a critical period, the fusional control center develops and coordinates the two components, thereby ensuring binocularity. In congenital esotropia, the inherent defect is proposed to occur during the development of the sensory and motor components such that the fusional control center cannot develop. Thus, inaccurate signals are sent to the extraocular muscles, and esotropia results secondary to the strength of convergence mechanisms. Appropriate treatment of congenital esotropia must occur when the sensory input and motor response are immature and plastic if fusion is to develop.

Evidence supports the development of fusion following maturation of sensory and motor inputs. Studies of the normal development of binocularity demonstrate infants show an abrupt onset of stereopsis at age 3 to 5 months.9,10 At this age, there is sufficient maturation of the cortex, macula and vergence control mechanism to allow stereopsis to develop. Even infants with congenital esotropia when tested at age 3 to 5 months with appropriate prism correction can demonstrate stereopsis.11 Other studies also support a sensitive period for the development of binocularity beginning at several months after birth and peaking at ages 1 to 3 years.12

Several studies have supported the view that surgery for congenital esotropia in children prior to the age of 2 years establishes binocularity. Ing13 reported a significant decline in the percentage of patients who achieved binocularity if alignment occurred after the age of 2 years. Seventy-five percent of the patients in Ing's study with alignment by the age of 2 years had fusion on Worth 4 light and stereopsis by Polaroid Titmus vectograph. Stereopsis and fusion decreased to 12% in children who achieved alignment after the age of 2 years. In a similar study by Zak and Morin,14 the percentage of patients achieving stereopsis declined if realignment occurred after the age of 2 years. Fusion occurred most frequently in patients who achieved alignment by the age of 9 months (92%) and less thereafter.

Other studies have reported an increased frequency of binocularity in infants who achieved alignment before the age of 1 year.15,16 Birch et al16 reported 57% of 80 infants who had alignment before the age of 6 months had stereopsis compared to 27% of infants who had alignment by the age of 7 to 15 months. Birch et al17 also found a slight improvement in the percentage of children with stereoacuity if treated early (45% with alignment at ages 7 to 12 months compared with 32% with alignment at ages 13 to 24 months).

Although these studies demonstrate the increased frequency of binocularity from surgical alignment prior to the age of 2 years, the degree of binocularity expected following congenital esotropia surgery continues to be debated. Prior to 1994, there are only a few reports of patients treated for congenital esotropia who attained 40" of stereoacuity.18,19 However, these cases are believed to be exceptional.

Factors Affecting Binocularity

Patient Age at Surgical Alignment
Citing the physiologic evidence for the critical period of binocularity development, Wright et al20 proposed early surgery between the ages of 2.5 and 3 months for the correction of congenital esotropia. The Early Surgery for Congenital Esotropia trial and the Retina Foundation of the Southwest study group found a constant esotropia > 40 PD in patients aged 2 to 4 months did not spontaneously resolve.16 Similarly, the Pediatric Eye Disease Investigator Group reported infants with esotropia who were older than the age of 12 weeks were more likely to have a constant, large deviation > 40 PD that failed to resolve spontaneously.21,22 Such patients can be considered for treatment of congenital esotropia.

Wright et al20 reported 3 of 7 children surgically aligned at ages 2.5 to 3 months attained stereoacuity > 100" of arc on Titmus testing.20 However, in a multicenter independent study of 16 patients who underwent alignment before the age of 6 months, Ing23,24 found no improvement in the quality of binocularity compared with patients who underwent alignment at the age of 6 months. Although one patient who underwent surgical alignment at the age of 3 months had 40" of arc, the remaining patients did not exhibit a greater degree of stereoacuity compared with patients who underwent surgery at the age of 6 months.

Duration of Misalignment
Factors other than age at surgical alignment that may influence the degree of binocularity have been studied. Birch et al25 reported the age at initial surgery may not be as important as the duration of misalignment. In 129 surgically treated patients, after correcting for age at surgery, statistical analysis demonstrated the duration of alignment was the significant factor in determining the degree of stereoacuity. In patients with improved stereoacuity, there was also stability of the alignment. Fourteen percent of patients with stereoacuity required additional surgical procedures to maintain alignment versus 32% of those patients without stereopsis.

Ing and Okino26 also reviewed data on stereopsis with regard to duration of misalignment. They found patients who achieved alignment by the age of 1 year and patients whose duration of alignment was < 12 months had increased percentage of stereopsis. Moreover, the shorter duration of misalignment affected the quality of stereopsis more favorably than the age at which alignment occurred.

Associated Motility Disorders
Helveston et al27 reported the critical factor in achieving high-grade stereoacuity was not early age at surgical alignment, but the development of associated motility disorders in congenital esotropia, such as dissociated vertical deviation, latent or manifest latent nystagmus, accommodative esotropia, and other types of latent strabismus. Birch et al25 also found patients with stereopsis were less likely to have dissociated vertical deviation (25%) compared with those without stereopsis (63%).

The conflicting data may result from interpretation of the sensory results. There are several methods of measuring stereopsis, such as the Titmus fly and Random dot testing. These tests delineate the seconds of arc of binocularity and require mature, cooperative patients. Therefore, testing for stereopsis in young children is often difficult. Consistent interpretation of the results must be documented for reports to be accurate. Moreover, the result of a quantitative sensory test has been reported to either improve as the patient ages or decline if one of the clinical associations of congenital esotropia such as dissociated vertical deviation occurs.25,28 Nevertheless, comparing binocular results in treated congenital esotropia requires the use of these quantitative tests and accepting the inaccuracies of the tests.

Conclusion

The attainment of higher grades of binocular vision in the treatment of congenital esotropia appears possible but is uncommon. Several factors influence the treatment outcome, including patient age at surgical alignment, duration of misalignment, associated motility disturbances in congenital esotropia, and type of microtropia achieved postoperatively. Additional studies are needed to establish the critical factors to achieve stable, binocular vision in congenital esotropia.

References

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Posted May 2004