Unilateral Amblyopia, its Associated Risk Factors (A Current Study)

Background: Strabismus can cause unilateral amblyopia causing permanent deterioration of vision. The two most common types are esotropia and exotropia. Risk factors associated with it are low birth weight, prematurity and its complication in prenatal period or during delivery and maternal smoking in pregnancy.

Objective: To determine the frequency of strabismus and its types in pediatric age group attending outpatients department of tertiary care hospital. It was a cross sectional study at the Department of Ophthalmology, Liaquat National Hospital, Karachi, for six months from 9th May 2017 to 8th November 2017.

Material and Methods: Total 142 patients with deviation of eyes and with surface problems were included. Informed consent was taken. Age, sex, onset of strabismus, unilateral or bilateral involvement were recorded. Presence of strabismus was evaluated on the basis of hirschburg reflex and cover test after which findings were noted. Descriptive statistics were calculated. Stratification was done and post-stratification chi square test was applied. P-value ≤0.05 was considered as significant.

Results: The overall mean age was 6.73±3.15 years. Mean onset age was 38.72±28.30 months. Total 7.6% patients were found with strabismus family history. Strabismus was found in 47.2% children, among them 38.2% were found with exotropia and 61.8% with esotropia. There was significant association of strabismus with age.

Conclusion: The results showed 47.2% strabismus, esotropia with 61.8% was commonest followed by exotropia in female gender aging less than 6 years, and age onset more than 36 months were found as a risk factors.

Keywords: Strabismus, Pediatric, Exotropia, Esotropia

Received: Oct’2020   Accepted: Nov.2020



Strabismus is a common presenting problem in children coming to the outpatient patient department, around 3.55% in Asian population.1 But in Pakistan it was estimated to be around 5.4% in children below 15 years consist of 45% of total population. 2.5% children with this problem are below 5 years and 2.9% above 5 years of age.2 Additionally one study shows that in children, strabismus is third common ocular problem in Pakistan.3 It should be screened and diagnosed earlier as it can cause unilateral amblyopia,4 which affects 5% of the total population,5 causing permanent decrease in vision. Children with this problem have difficulty in their studies as well as have psychological and social stress due to cosmetic appearance.

So it is very important to detect earlier and correct it to enhance social interactions and promote vision.6


The results showed 47.2% strabismus, Esotropia

with 61.8% was commonest followed by Exotropia

in female gender aging less than 6 years,

and age onset more than 36 months were found

as a risk factors in Pakistan


Every patient with strabismus must be investigated for any associated systemic or ocular disease. Risk factors associated with it are low birth weight, prematurity and its complication in prenatal period or during delivery7 and maternal smoking in pregnancy.8 Family history is also of value as same types of squint are seen in members of same family.9 The 2nd most common types of strabismus are esotropia and exotropia. One study shows that esotropia was most common type accounts for prevalence of 2.2% and exotropia 0.4% in school going children10, aged from 6 months to 12 years.

The purpose of this study is to determine the current burden of disease in children attending outpatient department of tertiary care hospital because it is one of the common problems occurred in children. Strabismus cause lifelong impairment in socialization skills, earning power and quality of life. We can prevent this problem by screening, diagnosing and managing it at correct time. There are no such studies regarding statistics of Pakistan in any journal index. Patients presented with deviation of eyes (15 or > 15 degree considered as positive and < 15 degree considered as negative mentioned below in Hirschburg test) from its normal central position


Esotropia: Patients presented with inward deviation of eyes.

Exotropia: Patients presented with outward deviation of eye.


Strabismus and its types are assessed clinically by Hirschburg reflex and cover test. The distance of corneal light reflection from center of pupil to see angle of deviation (using torch), figure on the last page.




The study was conducted in Department of Ophthalmology, Liaquat National Hospital, Karachi for six months from 9th May 2017 to 8th November 2017. The sample size was calculated using WHO software using test percentage of strabismus (P) = 38.0%,(6) d=8% and 95% confidence level. The calculated sample size is 142 patients, non-probability consecutive sampling was used for the study. It was a descriptive cross-sectional study.


Inclusion Criteria: Children aged from 6 months to 12 years, male or female presented with deviation of eyes (assessed by Hirschburg and cover test), patient presented with surface problems like itching, watering, burning, redness for more than 1 week (confirmed on the basis of history).

Exclusion Criteria: History of ocular trauma.History of previous strabismus surgery (confirmed on the basis of past ocular surgical history and medical records by researcher) Paralytic strabismus: It occurs due to damage of extraocular muscles or its nerve.


Patient with other ocular pathology like retinoblastoma, retinitis pigmentosa (confirmed by past medical history and slit lamp examination). Patient with hypertropia and hypotropia - assessed by Hirschburg reflex and cover test as discussed in operational definition.

This study was conducted after approval of hospital ethical review committee and CPSP. The children visited to department of ophthalmology Liaquat National Hospital, Karachi and fulfilled the inclusion criteria were counseled and included in the study. Informed consent was taken from their parents. Age, sex, onset of strabismus, unilateral or bilateral involvement were recorded by taking history. After taking a detailed history, presence of strabismus is evaluated on the basis of Hirschburg reflex and cover test after which findings were noted in the given proforma.

Patient’s data were compiled and analyzed through statistical package for social sciences (SPSS) version 21. Frequency and percentage were computed for qualitative variables like gender, unilateral or bilateral involvement, family history of strabismus and outcome (strabismus and its types). Mean±SD were calculated for quantitative variable i.e. age and age at onset of strabismus. The stratification was done on gender, age, involvement of eye, family history of strabismus and age at onset of strabismus to see the effect of these modifiers on outcome using Chi-square test. P≤0.05 was considered as significant.




Total 144 Children aged from 6 months to 12 years of either gender meeting inclusion criteria of study were evaluated to determine the frequency of strabismus and its types in pediatric age group attending outpatient’s department of tertiary care hospital. Descriptive statistics were calculated using SPSS version 25. Stratification was done and post stratification Chi square test was applied to observe the effect of modifiers on outcome. P value ≤0.05 was considered as significant. There was 69 male and 75 female as presented in Table-1. The overall mean age of children was 6.73±3.15 years.

The overall mean onset age of children was 38.72±28.30 months. Among 144 children, 7.6% were found with strabismus family history. Detailed frequency distribution of strabismus family history is presented in Table-4. In our study, strabismus was found in 47.2% children as presented in Table-5. Among strabismus children, 38.2% were found with exotropia and 61.8% with esotropia as presented in Table-6. The detailed descriptive statistics of age and onset age according to strabismus are presented in Table-7 and Table-8. Stratification with respect to gender, age, onset age, eye involvement and family history was done to observe effect of these modifiers on strabismus. P-value ≤0.05 was considered as significant.

The results showed that there was significant association of strabismus with age (p=0.000) while no significant association was found with gender (p=0.062), onset age (p=0.863), eye involvement (p=0.915) and family history (p=0.078). The detailed results of associations are presented from Table-1-5.









The study was conducted to determine the frequency of strabismus and its types in pediatric age group attending, outpatient’s department of a tertiary care hospital. The prevalence of strabismus in a study by Kalikivayi V et al conducted in South India is 0.7%.11 Another study in North India by Gupta M et al gave a prevalence of 2.5%.12 In other countries, prevalence of strabismus found to be 4.3% in England by Adelstein AM et al while Graham PA gave a prevalence of 7.1%.13,14 In Africa, study by Ayanru JO showed a prevalence of 1.9%.15 In Japanese elementary school children, it was 1.28%.16 Rantanen A et al in Finland gave a prevalence of 4.6%.One of the factors responsible for this variation are the differences in the age group of children included in the study.17 Prevalence of strabismus in one study was found to be 0.3% in boys and 0.299% in girls as compared to Graham PA et al study which shows 7.3% are boys and 6.9% are girls.12 While other study done by AM et al shows male preponderance and CBO Yu et al, and Cass EE reported a female preponderance.12,18 One study shows that strabismus was more common in 3-10 years age group (prevalence of 0.38%) when compared to children in 11-16 (0.22%) year’s age group prevalence.

When they compared esotropia and exotropia separately, they found that Esotropia was more common in 3-10 years age group while exotropia was more common in 11-16 years age group.19 Mohney BG et al also proposed that esotropia is the most common form in the first six years of life; beyond this age exotropia predominates until the teenage years when the three forms have a similar but decreased incidence.20 In study by Abrahamsson M et al children were screened at 1 year and also the data of children under 8 years who attended ophthalmic clinics for various complaints were screened. Mohney BG et al included children under 19 years of age.21,22 According to writing committee for the multi-ethnic pediatric eye disease study and the baltimore pediatric eye disease study groups, esotropia was associated independently with prematurity, maternal smoking during pregnancy, older preschool age (48-72 months), anisometropia, and hyperopia.18 Exotropia was associated with prematurity, maternal smoking during pregnancy, family history of strabismus, female sex, astigmatism and anisoastigmatism.19

According to Remaly NA et al risk of strabismus increased with low birth weight.23 Maternal cigarette smoking during pregnancy also increased the risk of each type of strabismus. Maternal age was also a significant risk factor for esotropia. The risk of esotropia increased with increasing age until age 34 years. Cass EE stated that a family history of squint was present in only 28% of the cases.24 In study by Graham PA, 8.1% of children in control group had a family history of squint, while 19% of children with squint had a positive family history.14 In one study, 3.3% of children had a positive family history. History of prematurity was present in 3.3% of children (all of them were esotropes).19 The classical teaching about the distribution of esotropia and exotropia is that esotropia is more common than exotropia.25,26 This idea is based on studies conducted in western population. But, studies conducted on Asian population revealed that exotropia is more common than esotropia.19 Studies by Rachael H et al have shown that intensity and duration of exposure to sunlight may play a role in pattern distribution of strabismus along with racial factors.27 Thus, higher the intensity of light, higher the frequency of exotropia.19

Graham PA, suggests that accommodative esotropia was found to be more common than non-accommodative esotropia and also with Mohney BG.12,22 Similar results were also seen in study by Chia A et al.28 In their study, 53% had accommodative esotropia and 23% had congenital esotropia. In yet another study by Greenberg AE et al, on incidence and types of childhood esotropia, fully accommodative 36.4%; acquired non accommodative 16.6%; esotropia associated with an abnormal central nervous system 11.4%; partially accommodative 10.1%; congenital 8.1%; sensory, 6.5%; paralytic 6.5%; undetermined 3.4%; and other 1.0% which correlates with the current study.29 One study shows that Sensory strabismus was seen in 32% of the children in the current study. Among them majority (78% of children with sensory strabismus) had exotropia. Age of these children varied from 5 - 13 years in those with esotropia and 7 - 16 years in those with exotropia. Etiology in esotropia was found to be predominantly congenital causes while that in exotropia was predominantly due to acquired causes.19 Havertape SA et al, states that of patients with congenital vision loss, 67% developed sensory esotropia and 33% developed sensory exotropia.30

One study shows that the age distribution of children with sensory esotropia ranged from 5-13 years, while that in those with sensory exotropia ranged from 7-16 years. Thus there is a considerable overlap in the age distribution in both the groups. But because the age of onset cannot be accurately known, this discrepancy is not significant. Refractive errors were seen in 52% of children examined. Of these myopia was most common refractive error overall hypermetropia was most common error in children with esotropia, but also seen in 1 child with exotropia.19 In the study by Holmes JM et al on paediatric third, fourth and sixth nerve palsies, the most commonly affected nerve was the fourth (36%), followed by the sixth (33%), the third (22%), and multiple nerve palsies (9%).31 One study shows that of those with strabismus, only 21.4% knew that they had strabismus.

Neither the prevalence nor the self-known proportion of strabismus changed substantially with school grade, suggesting that the majority of strabismus afflicted children remain unrecognized during their elementary school years.19


Study Limitations: The main limitation of our study was the small sample size. Other limitations of the present study include a single-center experience and non-randomized study design. It was conducted with urban environment therefore, the results might not be generalizable to larger populations.




The study results showed 47.2% strabismus. The esotropia with 61.8% was commonest followed by exotropia. Further, female gender, age less than 6 years, and age onset more than 36 months were found as a risk factors. Timely diagnosis and treatment can improve prognosis and thus quality of life too. More research is needed in Extensive health education campaign among children as well as their parents are needed.



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