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Risk Factors of Strabismus in Children in a Southern Nigerian Tertiary Hospital

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Risk Factors of Strabismus in Children in a Southern
Nigerian Tertiary Hospital
H Nwachukwu1, AA Onua2* and AO Adio2
1Department of Ophthalmology, University of Port Harcourt Teaching Hospital, Nigeria
2Department of Ophthalmology, University of Port Harcourt, Nigeria
Introduction
Normal binocular single vision is the ability of the visual
cortex to fuse and integrate the image from each eye into a single
perception which develops after birth from early infancy and
is completed with fusion and stereopsis by age 8-10years [1].
Strabismus is a misalignment of the eyes such that the visual axes
of both eyes are not simultaneously directed at the object of regard
[2]. Ocular misalignment is common in newborns because humans
are often born with a slight exodeviation thought to represent
the anatomic position of the divergent orbit [3]. A study done in
university of Liverpool found the prevalence of strabismus to be
about 73.1% in one-month old babies, reducing to 49% in two
months old and virtually disappearing in normal four-month-old
     
with one eye and squint with the other, amblyopia is bound to set in
especially in young children due to the continuous abnormal visual
stimulation from the weaker eye during early visual development
with subsequent disruption of neurodevelopment of the visual
centers in the brain [5,6].
Strabismus is a relatively common condition worldwide
especially among newborns with a prevalence of 1.3% -5.7% in
all children with an increased prevalence associated with assisted
delivery, low birth weight, prematurity and associated neuro-
developmental disorders [7,8].
In America, a population-based study of strabismus among
Native American children showed a prevalence of 3.8% [9]. Asian
population reveal a lower prevalence of 0.7-1.9% [10,11]. About
3-4% of Caucasian children have also been reported to be affected
with strabismus [12]. The prevalence in Africa is generally low
compared to Caucasians and Asians as seen in various studies
[13,14]. In Nigeria the prevalence of strabismus is between 0.01-
2.4% in different populations [15-22]. In a retrospective study
conducted in the university of Port Harcourt Teaching Hospital
*Corresponding author: AA Onua, Department of ophthalmology, University of Port
Harcourt, Nigeria.
Received Date: August 13, 2019
Published Date: August 22, 2019
ISSN: 2641-6360 DOI: 10.33552/WJOVR.2019.02.000541
World Journal of
Ophthalmology & Vision Research
Research Article Copyright © All rights are reserved by AA Onua
This work is licensed under Creative Commons Attribution 4.0 License WJOVR
.MS.ID.000541.
Abstract
Background: Strabismus is a misalignment of the eyes affecting one or both eyes. The deviation of the eyes could be esodeviation, exodeviation,
hyperdeviation or hypodeviation. Its prevalence is low globally and varies in different regions of the world. In our local environment esotropia is
the commonest form of presentation. The possible risk factors that predispose a child to developing strabismus is necessary for early mitigation.
Objective: The aim of this study is to determine the possible risk factors of strabismus among children attending the Paediatric Ophthalmology
Clinic in the university of Port Harcourt Teaching Hospital ( a tertiary hospital) in the Niger Delta Region, Nigeria.
Method:
University of Port Harcourt Teaching Hospital from October 2016 to March 20 18 were recruited for the study.
Results: The prevalence of manifest strabismus was 6.6%. Refractive error (hypermetropia and astigmatism) was the most prevalent ocular
risk factor seen among these children. Other risk factors were amblyopia, prematurity, low birth weight, family history of strabismus and female sex.
Conclusion: Adequate cycloplegic refraction should form a baseline clinical procedure for children presenting with strabismus.
Keywords: Children; Risk factor; Strabismus
World Journal of Ophthalmology & Vision Research Volume 2-Issue 4
Citation: H Nwachukwu, AA Onua, AO Adio. Risk Factors of Strabismus in Children in a Southern Nigerian Tertiary Hospital. W J Opthalmol &
Vision Res. 2(4): 2019. WJOVR.MS.ID.000541. DOI: 10.33552/WJOVR.2019.02.000541. Page 2 of 4
the prevalence of strabismus was 0.6% and majority was found
between the age group of 0-10 years (75.7%) [16].
Risk factors of strabismus in children include refractive error,
positive family history, history of maternal cigarette smoking
during pregnancy, low birth weight, prematurity, developmental
ocular abnormalities such as craniofacial abnormalities (e.g.
Crouzon Syndrome), cataract, retinoblastoma and retinopathy of
prematurity [23-31]. Hypermetropia was most commonly found
among children with esotropia and myopia among children with
exotropia [23,32]. Other risk factors of strabismus include systemic
disorders such as meningitis, encephalitis, neonatal jaundice,
cerebral palsy, Down’s syndrome and fetal alcohol syndrome [33-
41].
Strabismus affects a child’s psychology and social interaction,
and this cannot be neglected. Patients with strabismus are said to
have lower levels of psychological well-being, are less accepted by
their peers and the society at large [42,43]. This underscores the
importance of this study- to identify the risk factors associated with
strabismus in our locality and hence improve in the management of
our patients.
Materials and Method
The study design is a hospital based descriptive cross-
sectional study conducted over a period of twenty-four months.
The sampling technique used was a consecutive sampling method
in which all children with manifest squint attending the Pediatric
Ophthalmology Clinic, university of Port Harcourt Teaching Hospital
from October 2016 to March 2018 were recruited consecutively
into this study. The university of Port Harcourt Teaching hospital
serves as a catchment as well as a referral Centre for inhabitants of
Rivers State and neighboring Bayelsa, Imo and Abia states.
An interviewer-administered questionnaire was administered
in the clinic to the assenting patients whose parent/guardian has
given consent. Relevant information on socio-demography such
as birth history, age at onset of squint, history of previous trauma,
previous eye surgery and family history of squint were obtained
from either parent/guardian via interviews. Visual acuity (VA) was
tested in preverbal children (0-2yrs) using preferential looking,
       
bright light or bright colored toy. While in verbal children (>2yrs),
visual acuity was done with a picture optotype test e.g. Kay’s
pictures test; which uses pictures of objects like cars, trains houses,
bikes, animals. Snellen acuity chart or illiterate E-chart asking
    
testing was done for subjects with VA of 6/12 and worse.
    
   
cornea with reference to the pupil was noted. Bruckner test was
performed using the direct ophthalmoscope in a dark room to
          
Cover and uncover test was also done monocularly at both near
and distance. It was done to detect presence of manifest squint and
to differentiate between phoria and tropia. Alternate prism cover
test was done to measure the total deviation which is the degree
     
Krimsky’s test which is similar to Hirschberg’s test except that the
      
          
deviated eye.
Anterior segment examination was done with a pen torch
and slit lamp looking out for the eyelids, size of the globe and
extraocular muscles motility, conjunctiva and pupils. A dilated
fundoscopy was done for all children using a binocular indirect
ophthalmoscope to assess the macula, optic disc, and peripheral
retina for any pathology such as retinoblastoma, glaucoma, optic
disc coloboma, toxoplasmosis etc. All data generated was entered
into a proforma and were analyzed using commercially available
statistical data management software- Statistical Package for Social
Sciences (IBM-SPSS) version 23.
Consent and Ethical Clearance
Ethical clearance was obtained from the Ethical Committee of
University of Port Harcourt Teaching Hospital. Informed written
consent and assent were obtained from each patient’s parent before
enrolment into the study in accordance with Helsinki Declaration
involving human subjects [44].
Results
(Table1)
Table 1: Age and sex distribution of study population.
Age Range
(Yrs.)
Sex
Male (n %) Female (n%) Total (n%)
<1 year 6 (10.7) 11 (15.9) 17 (13.6)
1-3 years 15 (26.8) 22 (31.9) 37 (29.6)
4-6 years 14 (25.0) 13 (18.8) 27 (21.6)
7-9 years 9 (16.1) 12 (17.4) 21 (16.8)
10-12 years 4 (7.1) 7 (10.1) 11 (8.8)
13-15 years 5 (8.9) 2 (2.9) 7 (5.6)
16-18 years 3 (5.4) 2 (2.9) 5 (4.0)
Total 56 (44.8) 69 (55.2) 125 (100.0)
Fisher’s exact test = 4.235; p-value = 0.661
Sixty-nine were females (55.2%) while 56 were males (44.8%).
Male to female ratio was 1:1.2. Mean age was 5.53±4.42years. Age
range of 1-3years had the highest proportion 37 (29.6%) while
those 16-18 years had the least representation 5(4.0%). The
differences in the proportion of age categories between the males

(Table 2)
Esotropia has a prevalence of 4.3%, 38 (2.0%) subjects had
exotropia out of which 15 were male and 23 were female while that
for vertical deviation was 0.27% (hypertropia 0.16%, hypotropia
0.11%). The overall prevalence for heterotropia was 6.6%.
Citation: H Nwachukwu, AA Onua, AO Adio. Risk Factors of Strabismus in Children in a Southern Nigerian Tertiary Hospital. W J Opthalmol &
Vision Res. 2(4): 2019. WJOVR.MS.ID.000541. DOI: 10.33552/WJOVR.2019.02.000541.
World Journal of Ophthalmology & Vision Research Volume 2-Issue 4
Page 3 of 4
Table 2: Prevalence and gender distribution of types of strabismus.
Type of Strabismus Males Females M: F Total Prevalence (95% CI)
Horizontal
Esotropia 39 43 01:01.1 82 4.32% (3.48% - 5.31%)
Exotropia 15 23 01:01.5 38 2.00% (1.49% - 2.77%)
Vertical
Hypertropia 0 3 0:03 3 0.16% (0.04% - 0.43%)
Hypotropia 2 0 2:00 2 0.11% (0.02% - 0.35%)
Total 56 69 01:01.2 125 6.59% (5.54%-7.78%)
M: F-Male to Female ratio; CI-Condence Interval
Discussion
This was a cross sectional study conducted among 125 children
with strabismus attending the pediatric eye clinic of the University
of Port Harcourt Teaching Hospital. The aim of the study was to
unravel the risk factors associated with strabismus among children
in our locality.
The prevalence of strabismus in this study was 6.6% which
was similar to the hospital-based study carried out in Tanzania
(5.9%) [45] and Australia (7.3%) [46]. It was slightly higher than
the global prevalence of 3–5% [47] and those reported in different
parts of Nigeria [12-14,17-22]. Most of the studies done in Nigeria
were primarily to assess ocular diseases in children, therefore
standardized methods of examining strabismus may not have
been applied which could have led to cases of missed strabismus
[15,16,48]. However, the high prevalence obtained in this study is
          
[49]. In developed countries like China, there are better awareness
to ocular health, improved health care facility, maternal nutrition
and perinatal child health care; these factors probably account for
the low prevalence of strabismus. The relative high prevalence
of strabismus in our study, could be attributed to the fact that all
children presenting with strabismus were included in this study
(not just healthy children) and the prevalence was calculated in
relation to the total number of children presenting within the study
period. In addition, this study was hospital-based and more yield of
patients with strabismus was expected.
In the course of this study different ocular and systemic
morbidities were found to be implicating risk factors in manifest
strabismus. These risk factors include refractive error, prematurity,
family history and female sex. Refractive error was noticed in
64.6%. Hypermetropia was the commonest type seen amongst the

by Al- Tamini [32], Zhu [49], Baiyeroju [20], Bodunde [17] and
Azonobi, et al. [15]. Hypermetropia was found to be more common
in subjects with esotropia in this study, which is similar to studies
done in other parts of the world such as United States 47 China
50 and Australia 51. This was followed by astigmatism (22%).
The multiethnic pediatric eye disease study/Baltimore pediatric
eye disease study observed that astigmatism was implicated
in exotropia26 but in this study astigmatism was implicated in
esotropia. Also, in this study, 16% of the subjects were myopic and
was found to be more prevalent among subjects with esotropia.
             
who observed that myopia was predominant among subjects with
exotropia. In our study, maternal smoking in pregnancy was not
considered a risk factor because smoking among females is rare in
our locality.
Amblyopia is a cause of visual impairment in children especially
those with strabismus. The prevalence of amblyopia in this study
was 16.0%. It is the second most implicating risk factor seen in this
study. It is more prevalent among the esotropic subjects. Systemic
co-morbidities found among the subjects in this study included
cerebral palsy /birth asphyxia, neonatal jaundice, oculocutaneous
albinism, low birth weight, meningitis, and sickle cell disease.

almost equally found among subjects with esotropia and exotropia.
Prematurity/ low birth weight was also seen among subjects with
esotropia in this study.
Acknowledgement
None.


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World Journal of Ophthalmology & Vision Research Volume 2-Issue 4
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Vision Res. 2(4): 2019. WJOVR.MS.ID.000541. DOI: 10.33552/WJOVR.2019.02.000541. Page 4 of 4
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... 26 These variations might be attributable to disparities in genetic vulnerability, environmental factors, and Additionally, the present study showed that 16.7% of the children had amblyopia. This finding is comparable with the results of a study conducted in a tertiary hospital in southern Nigeria (16%) 27 and a study conducted in an ophthalmology clinic in a large hospital in Brazil (18.6%). 28 However, lower prevalences were observed in other studies: 0.82% in schoolchildren in rural southwest China, 29 2.3% in a populationbased cross-sectional study in Iran, 30 and 0.8% (in an urban area) and 0.2% (in a rural area) in schoolchildren in India. ...
... In the present study, children with a family history of strabismus were 8.1-fold more likely to have strabismus, compared with children in families without a history of strabismus. Similar findings were reported in studies from southwestern Nigeria, 7 southern Nigeria, 27 Saudi Arabia, 9 and Baltimore (USA), 11 as well as in a twin study conducted by Sanfilippo et al. 40 These findings imply that strabismus can be inherited from a child's parents. Notably, a cross-sectional study conducted by Hashemi et al. 41 indicated no association between age and strabismus. ...
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Objective: This study was conducted to evaluate the visual pattern of strabismic children. Methods: A cross-sectional survey was carried out among primary school children in Ilorin South Local Government Area. A cluster random sampling technique was used to select the subjects for this study. The pattern of vision of children found to have ocular misalignment was studied. Results: A total of 7288 school children were screened and this yielded 22 cases of esotropia and 10 cases of exotropia. Their ages ranged from 2 to 16 years; 78.1% of the study group had normal vision; 3.1% had low vision and another 3.1% was blind. Eight-two per cent (82%) of the children with esotropia had normal vision, while 4.5% were blind; 70% of the children with exotropia had normal vision; 10% of exotropics had low vision. Twenty-five per cent(25%) of the strabismic population was amblyopic, and the majority of the cases of amblyopia (62.5%) were associated with esotropia. The prevalence of strabismic amblyopia was found to be 0.11%. Conclusion: The majority of strabismic children had normal vision. The prevalence of low vision and blindness was low. The prevalence of strabismic amblyopia was also low in the study population and most of it was associated with esotropia.
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Purpose: Investigate the relationship between the extorsion of the rectus muscle pulleys and the V-pattern exotropia and "overelevation in adduction" observed in Crouzon syndrome. Methods: Twenty children with Crouzon syndrome had assessment of eye alignment. The horizontal and vertical positions of the four rectus muscle pulleys were estimated from coronal CT images. Eye alignment was simulated in Orbit 1.8 software by shifting the corresponding location of the rectus muscle pulley array. Results: Eleven of the 20 patients had a V-pattern exotropia with displacements of each rectus muscle pulley ranging from 2 to 7 mm. The remaining nine patients were orthotropic with <2 mm displacement of the rectus muscle pulleys. Simulated displacements (>2 mm) of either the horizontal or vertical rectus muscle pulleys produced a similar strabismus pattern. The amount of V-pattern exotropia observed clinically was highly correlated with the amount predicted by pulley displacements in Orbit 1.8 (r(2) = 0.63; P < 0.0001). The displacement of vertical and horizontal rectus muscle pairs was significantly higher for patients having overelevation in adduction. Conclusions: Rotation of the four rectus muscle pulleys relative to the corresponding rotation planes of the globe changes the direction and magnitude of their active and passive pulling forces in a gaze-dependent manner. Extorsion of the horizontal and vertical rectus muscle pulleys in Orbit 1.8 reproduces the pattern strabismus observed in Crouzon syndrome. The high correlation between the predicted magnitude of the V-pattern exotropia and observed exotropia indicates that extorsion of the rectus muscle pulleys primarily accounts for the pattern strabismus.