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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
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
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.
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-
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].
Table 1: Age and sex distribution of study population.
Age Range
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)
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%)
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
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.
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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 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 To assess the strabismus prevalence and associated factors among children aged ≤15 years. Methods This hospital-based cross-sectional study was conducted from March 2017 to October 2017 in the Department of Ophthalmology & Optometry, Hawassa University Comprehensive Specialized Hospital. Interviewer-administered questionnaires were used to collect relevant data and clinical examinations were performed for patient diagnosis. Results Overall, 582 children participated in the study (response rate, 97%). The prevalence of childhood strabismus was 17.9% [95% confidence interval: 14.6–21.1]. Additionally, 9.6%, 16.7%, and 9.6% of the children had anisometropia, amblyopia, and dense cataract, respectively. Among the 16.7% of children with amblyopia, 56.7% had strabismus; among the 22.5% of children with clinically significant refractive error, 52.7% had strabismus. Moreover, among the 9.6% of children with anisometropia, 58.9% had strabismus. The presence of amblyopia (adjusted odds ratio [95% confidence interval]: 3.9, 1.7–8.6), age <5 years (13.5 [5.0–36.1]), age 5 to 10 years (6.1 [2.3–16.3]), and clinically significant refractive error (13.3 [5.8–30.6]) were significantly associated with childhood strabismus. Conclusions The prevalence of strabismus was relatively high among patients in this study. Early screening for childhood strabismus is essential. A well-controlled community-based study is needed to confirm strabismus prevalence and predictors.
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Background: Strabismus (ocular misalignment) is frequently seen in clinical practice. Its prevalence varies in different parts of the world with no sex predilection. Stabismus is a significant cause of ocular morbidity. Esotropia is often the commonest form of presentation among children. Aim: To determine the frequency and clinical features of patients presenting with strabismus in the ophthalmic clinic a past 6-year period. Methods: This was a hospital based study of all the patients diagnosed having strabismus in University of Port Harcourt Teaching hospital between January 2007 and December 2013. The medical records of a total of 74 patients who visited the ophthalmic clinic and diagnosed as having strabismus were retrospectively reviewed and subsequently analysed using Statistical Package for Social Sciences (SPSS) version 17 computer soft ware package. Ethical clearance was obtained from the Ethics committee of the University of Port Harcourt Teaching Hospital. Results: A total of 12,334 case files were reviewed in this study. Five thousand one hundred and eighty-one (42%) were males and 7,153 (58%) females. A total of 74 cases had strabismus (0.6%). Twenty- three were males and 51 females (M:F=1:2.2). Twenty-eight (37.8%) had alternating squint, twenty- three (31.1%) had esotropia, twenty- one (28.4%) had exotropia and two (2.7%) had hypertropia. Conclusion: This study confirmed the relatively high frequency of alternating squint and esotropia in patients with strabismus.
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Purpose: The purpose of this cross-sectional observational study was to estimate the distribution and patterns of refractive errors, strabismus and amblyopia in children seen at a pediatric eye care. Patients and Methods: The study was conducted in a private hospital in Dammam, Kingdom of Saudi Arabia, from March to July 2013. During this period, a total of 1350 children, aged 1-15 years were seen at this center’s pediatric ophthalmology unit. All the children underwent complete ophthalmic examination with cycloplegic refraction. Results: Refractive errors accounted for 44.4% of the cases, the predominant refractive error being hypermetropia which represented 83%. Strabismus and amblyopia were present in 38% and 9.1% of children respectively. Conclusions: In this clinic-based study, the focus was on the frequency of refractive errors, strabismus and amblyopia which was considerably high. Hypermetropia was the predominant refractive error in contrast to other studies in which myopia was more common. This is attributed to the criteria for sample selection since it was clinic-based rather than a population-based study. However, it is important to promote public education on the significance of early detection of refractive errors, and have periodic screening in schools.
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Abstract: Background: Strabismus is a common condition in children, but there is paucity of studies done exclusively on it in Nigeria Objective: To describe the pattern of presentation of strabismus among children presenting at the eye clinic, Olabisi Onabanjo University Teaching Hospital (OOUTH) Sagamu, Nigeria Study design:- Retrospective . Results: Strabismus constituted 0.01% of paediatric ophthalmic presentations to OOUTH over a 5year period of between January 2008 and December 2012.Age range is from 0.75-16years with a mean of 4.9years. The male female ratio was 1:2. Esotropia was the most common type of squint while the predominant refractive error was astigmatism. Only 5(31.3%) of those whose onset was before 6months presented on or before 1 year There was very low uptake of treatment in these children. Conclusion: The prevalence of strabismus was low, intensive health education of parents and teachers is necessary to encourage early presentation and good uptake of treatment as this has a direct effect on visual and cosmetic outcome.
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Enteroviruses (EVs) have emerged as one of the important etiological agents as a causative organism for encephalitis, especially in children and adults. After the first report of EV encephalitis cases in 1950s, there have been increasing reports of regular outbreaks of EV encephalitis worldwide. Enteroviruses are RNA viruses of the family Picornaviridae that consists of more than 100 serotypes, which are characterized by a single positive-strand genomic RNA. The clinical features are pleomorphic and can be accompanied by mucocutaneous manifestations or isolated encephalitis only. The incidence of encephalitis in EV infection is reported to be about 3% and is associated with high mortality and morbidity. A number of newer therapeutic agents have been used in EV encephalitis with variable results. This review will focus on clinical features, pathophysiology, and newer treatment modality in EV encephalitis.
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IMPORTANCE Understanding the development of common strabismus is important in locating "at-risk" populations and implementing optimal treatment. This systematic review will bring together reported genetic and environmental risk factors for common strabismus to reveal relationships between risk factors and guide future research. OBJECTIVE To identify known environmental and genetic risk factors for comitant strabismus reported in the literature. DATA SOURCES A systematic literature search was performed in Medline, Embase, BioSciences Information Service Previews, Web of Science, and the OMIM database during a 2-week period in July 2011 (including all available years) using the following key words: gene, genetic environmental factor, inheritance, risk factor, esotropia, exotropia, strabismus, squint, convergent strabismus, and divergent strabismus. STUDY SELECTION No language restrictions were placed on the search. Exclusion criteria consisted of associated syndromes, strabismus not the primary outcome, poor study design or quality, and logarithm of the odds score less than 3. DATA EXTRACTION AND SYNTHESIS A study quality and extraction tool was used. Analysis was performed descriptively because of the variant characteristics of the study designs. MAIN OUTCOMES AND MEASURES Risk factor, twin, pedigree, and genetic studies. RESULTS Forty-one articles fulfilled the inclusion criteria set out by the study, which highlighted 4 subcategories: risk factor, twin, pedigree, and genetic studies. Significant risk factors for strabismus reported by the studies included low birth weight, cicatricial retinopathy of prematurity, prematurity, smoking throughout pregnancy, anisometropia, hyperopia, and inheritance. Inheritance was further supported by twin and pedigree studies, which revealed the complexity of the inheritance pattern. At present the STBMS1 locus is the only gene location that has been supported; however, others have been reported. CONCLUSIONS AND RELEVANCE Certain subgroups within the population are at higher risk of developing comitant strabismus and should be identified and monitored to allow for earlier detection. It is evident that a strong hereditary link is present particularly in intermittent and accommodative forms; however, further research is required to identify possible links between subtypes of strabismus. Further genetic research could also help to locate additional causative genes to aid the understanding of strabismus development.
Purpose: To determine the prevalence of strabismus and associated risk factors in southeastern Brazil. Methods: A cross-sectional, population-based study using a systematic sample from nine municipalities in a southeastern region of Brazil composed of 1852 individuals aged ≥1 and ≤12 years old was done. Visual acuity (VA), ocular alignment, and refractive error (RE) were evaluated. Strabismic individuals (strabismus group) were compared to orthotropic individuals (orthotropic group) to analyze risk factors linked to heterotropias. Results: Prevalence of strabismus was 0.81% in this population. In the strabismus group, we found 40% with hyperopia, 6.67% with astigmatism, 3.33% with myopia, 6.67% with amblyopia, and 8.33% with moderate anisometropia. In the orthotropic group, 6.85% had hyperopia, 18.12% astigmatism, 14.82% myopia, 0.19% amblyopia, and 4.37% moderate anisometropia. Conclusion: The prevalence of strabismus in southeastern Brazil was 0.81%. Strabismic individuals had more hyperopia. Amblyopia and moderate anisometropia were associated with strabismus.
Objectives: This study was conducted to determine the pattern of refractive errors among a group of strabismic children. Methods: A cross sectional survey was carried out among primary school children in Ilorin South Local Government area of Kwara State, Nigeria to isolate those with manifest strabismus. These children with manifest strabismus then underwent a full ocular examination followed by an objective refraction using a Topcon 8000R autorefractometer without cycloplegia and then a subjective refraction. Results: Of the 7288 children screened, there were 32 cases of strabismus (22 esotropia and 10 exotropia) consisting of 19 males and 13 females (M: F = 1.5:1). Hypermetropia was the commonest refractive error among the children (50%) followed by astigmatism (30%) and myopia (20%). There was more astigmatism among esotropic children (66.7%) compared to exotropic children (33.3%). Majority of the hypermetropia (85%) was associated with esotropia while most of the myopia (62.5%) was associated with exotropia. The prevalence of significant refractive error was found to be 34.4% (95% CI, 34.2 – 34.6). A great proportion of the refractive errors are in the mild and moderate category. It was only among hypermetropic children that refractive errors greater than ±5.0DS/Cyl in any meridian was found. Conclusion: There is a high prevalence of significant refractive error in the study population. Hypermetropia is the dominant refractive error seen in this study and most of it is found among the esotropic children. There was more myopia among exotropic children compared to esotropic ones but the difference is not statistically significant. Keywords: Refractive Error, Strabismus, Children, Ilorin
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.
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.