Pedigree Analysis of Familial Primary Concomitant Horizontal Strabismus in Northern India

Presence of Copy Number Variants Associated With Esotropia in Patients With Exotropia

Importance

Strabismus is a common ocular disorder of childhood. There is a clear genetic component to strabismus, but it is not known if esotropia and exotropia share genetic risk factors.

Objective

To determine whether genetic duplications associated with esotropia are also associated with exotropia.

Design, Setting, and Participants

This was a cross-sectional study conducted from November 2005 to December 2023. Individuals with constant or intermittent exotropia of any magnitude or a history of surgery for exotropia were recruited from pediatric ophthalmic practices. Data were analyzed from March to December 2023.

Exposure

Genetic duplication.

Main Outcomes and Measures

Presence of genetic duplications at 2p11.2, 4p15.2, and 10q11.22 assessed by digital droplet polymerase chain reaction. Orthoptic measurements and history of strabismus surgery were performed.

Results

A total of 234 individuals (mean [SD] age, 19.5 [19.0] years; 127 female [54.3%]) were included in this study. The chromosome 2 duplication was present in 1.7% of patients with exotropia (4 of 234; P = .40), a similar proportion to the 1.4% of patients with esotropia (23 of 1614) in whom it was previously reported and higher than the 0.1% of controls (4 of 3922) previously reported (difference, 1.6%; 95% CI, 0%-3.3%; P < .001). The chromosome 4 duplication was present in 3.0% of patients with exotropia (7 of 234; P = .10), a similar proportion to the 1.7% of patients with esotropia (27 of 1614) and higher than the 0.2% of controls (6 of 3922) in whom it was previously reported (difference, 2.8%; 95% CI, 0.6%-5.0%; P < .001). The chromosome 10 duplication was present in 6.0% of patients with exotropia (14 of 234; P = .08), a similar proportion to the 4% of patients with esotropia (64 of 1614) and higher than the 0.4% of controls (18 of 3922) in whom it was previously reported (difference, 5.6%; 95% CI, 2.5%-8.6%; P < .001). Individuals with a duplication had higher mean (SD) magnitude of deviation (31 [13] vs 22 [14] prism diopters [PD]; difference, 9 PD; 95% CI, 1-16 PD; P = .03), were more likely to have constant (vs intermittent) exotropia (70% vs 29%; difference, 41%; 95% CI, 20.8%-61.2%; P < .001), and had a higher rate of exotropia surgery than those without a duplication (58% vs 34%; difference, 24%; 95% CI, 3%-44%; P = .02).

Conclusions and Relevance

In this cross-sectional study, results suggest that the genetic duplications on chromosomes 2, 4, and 10 were risk factors for exotropia as well as esotropia. These findings support the possibility that esotropia and exotropia have shared genetic risk factors. Whether esotropia or exotropia develops in the presence of these duplications may be influenced by other shared or independent genetic variants or by environmental factors.

Genetics of strabismus

Strabismus, or misalignment of the eyes, is the most common ocular disorder in the pediatric population, affecting approximately 2%-4% of children. Strabismus leads to the disruption of binocular vision, amblyopia, social and occupational discrimination, and decreased quality of life. Although it has been recognized since ancient times that strabismus runs in families, its inheritance patterns are complex, and its precise genetic mechanisms have not yet been defined. Family, population, and twin studies all support a role of genetics in the development of strabismus. There are multiple forms of strabismus, and it is not known if they have shared genetic mechanisms or are distinct genetic disorders, which complicates studies of strabismus. Studies assuming that strabismus is a Mendelian disorder have found areas of linkage and candidate genes in particular families, but no definitive causal genes. Genome-wide association studies searching for common variation that contributes to strabismus risk have identified two risk loci and three copy number variants in white populations. Causative genes have been identified in congenital cranial dysinnervation disorders, syndromes in which eye movement is limited or paralyzed. The causative genes lead to either improper differentiation of cranial motor neurons or abnormal axon guidance. This article reviews the evidence for a genetic contribution to strabismus and the recent advances that have been made in the genetics of comitant strabismus, the most common form of strabismus.

The Role of Heredity and the Prevalence of Strabismus in Families with Accommodative, Partial Accommodative, and Infantile Esotropia

Objectives:

To investigate the prevalence of strabismus in families of a proband with accommodative, partial accommodative, or infantile esotropia (IET), and to evaluate the mode of inheritance and the role of consanguineous marriages in this prevalence.

Materials and Methods:

Families of probands with comitant strabismus were invited to participate in the study. The family members of 139 subjects with accommodative esotropia (AET), 55 with partial accommodative esotropia (PAET), and 21 with IET agreed to participate. Detailed family trees were constructed. The first- and second-degree relatives were invited for a complete ophthalmological examination, and 518 individuals from 168 families were evaluated. The role of consanguinity, the presence of tropia, phoria (≥8 PD), microtropia, and hypermetropia (≥3.00 D) among first- and second-degree relatives were analyzed.

Results:

A non-Mendelian pattern was found in 49 families (23%), an autosomal dominant pattern in 39 families (18%), and an autosomal recessive pattern in 6 families (3%). The prevalence of consanguineous marriages among parents of probands was 18.1%, 22.6%, and 14.3% in the AET, PAET, and IET groups, respectively (p=0.652). The prevalence of strabismus in first-degree relatives was 58.9%, 45.5%, and 38.1%, respectively (p=0.07). The prevalence of microtropia in probands’ siblings was significantly higher in the AET group (p=0.034).

Conclusion:

Sporadic cases and non-Mendelian inheritance were more frequent than autosomal recessive inheritance. Autosomal recessive inheritance was found not to be frequent in consanguineous marriages. The prevalence of strabismus and microtropia was significantly higher in families of esotropia cases than in the general population.