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 Table of Contents  
ORIGINAL ARTICLE
Year : 2015  |  Volume : 28  |  Issue : 1  |  Page : 207-213

Incidence of amblyopia in the Menoufia University outpatient clinic


1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, National Eye Center Hospital, Cairo, Egypt

Date of Submission08-Apr-2014
Date of Acceptance02-Jun-2014
Date of Web Publication29-Apr-2015

Correspondence Address:
Sherif Mohammed Ibrahim Sakr
44 Horia Street, El-Salam City, Cairo Governorate
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1110-2098.155996

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  Abstract 

Objectives
The aim of the study was to estimate the incidence of amblyopia in children aged between 5 and 15 years and ascertain the most common causes of amblyopia in this age group in the Menoufia University outpatient clinic.
Background
Amblyopia is one of the leading causes of acquired preventable monocular visual impairment.
Methods
This is a cross-sectional clinical study that was carried out between March 2012 and September 2012 in children between the ages of 5 and 15 years (mean age 8.013 ΁ 3.381 years) attending the Menoufia University outpatient clinic. After obtaining patient consent all patients underwent refraction, vision, and subjective refraction tests. Amblyopia was defined as best-corrected visual acuity with a two-line interocular optotype acuity difference with no pathology.
Results
The study was conducted on 510 patients; 38 of them (6.3% of male patients and 8.6% of female patients) had amblyopia, with an estimated incidence of 7.45%. Amblyopia decreased significantly with age (P = 0.002). Of the 38 amblyopic patients, 26 (68.42%) had anisometropic amblyopia and 12 (31.58%) had strabismic amblyopia. The most common type of amblyopia was anisometropia, followed by strabismic amblyopia. Hypermetropia was the most common refractive error in anisometropic amblyopia.
Conclusion
The incidence of amblyopia in this study was 7.45%, which is high compared with other studies; however, the sample in this study was taken from diseased persons attending the outpatient clinic and not from a healthy population. Proper ophthalmological screening of preschool children should be carried out, including extraocular examination, fundus examination, and visual acuity testing, with special emphasis on detection of errors of refraction.

Keywords: Amblyopia, anisometropia, strabismus, visual acuity


How to cite this article:
El-Sobky HM, Said Ahmed KE, El-Sawy MF, Sakr SM. Incidence of amblyopia in the Menoufia University outpatient clinic. Menoufia Med J 2015;28:207-13

How to cite this URL:
El-Sobky HM, Said Ahmed KE, El-Sawy MF, Sakr SM. Incidence of amblyopia in the Menoufia University outpatient clinic. Menoufia Med J [serial online] 2015 [cited 2019 Jun 27];28:207-13. Available from: http://www.mmj.eg.net/text.asp?2015/28/1/207/155996


  Introduction Top


Amblyopia is a disorder of the visual system that is characterized by reduction in the best-corrected visual acuity (BCVA) in an eye with no organic pathology [1].

Anisometropia is a well-known cause of amblyopia [2].

Unequal refractive error between the two eyes produces abnormal binocular interaction and/or visual deprivation. Patients with anisometropic amblyopia usually have no identifiable ocular defects and the visual acuity of the sound eye is normal, which makes it difficult to diagnose the symptoms, and thus early treatment is often delayed. Because improvement in visual acuity with amblyopia treatment depends on the age at which treatment begins, earlier detection of children with anisometropic amblyopia is desired [3].

There is an increased risk for amblyopia with myopic anisometropia greater than 2 D, hyperopic anisometropia greater than 1 D, and astigmatic anisometropia greater than 1.5 D [4].

Hussein et al. [5] in a study of amblyopic children older than 6 years found that the risk factors for anisometropic amblyopia treatment failure were age above 6 years at the onset of treatment, presence of astigmatism of greater than 1.5 D in the amblyopic eye, poor compliance with treatment, and initial visual acuity in the amblyopic eye of 20/200 or less.

Amblyopia affects ~2-5% of the American population and is the most frequent cause of unilateral visual impairment in children and young adults in the USA and western Europe [6].

Vision screening is recommended between the ages of 3 and 5 years and is usually performed in schools or by primary care physicians. Amblyopia is most often detected during this routine vision screening. Despite these facts, adequate screening is believed to occur in only 21% of preschool children in the USA [7].

Treatment of amblyopia is less likely to be successful in children older than 6 years of age [8].

Anisometropic amblyopia is often detected later than other forms of amblyopia because vision is generally good in the fellow eye, the eyes are typically orthotropic, and the child functions well with the sound eye. The level of anisometropia required to cause amblyopia has been well studied. In general, anisomyopia of more than 2 D, anisohyperopia of more than 1 D, and anisoastigmatism of more than 1.5 D may result in amblyopia. A direct relationship between the degree of anisometropia and the severity of amblyopia has been reported. Studies on anisometropic amblyopia indicate a prevalence of amblyopia of 100% in hyperopes with 4.0 D of uncorrected anisometropia and in myopes with 6.0 D of uncorrected anisometropia. Anisometropia of more than 4 D is also believed to have a worse prognosis for successful visual outcome with traditional amblyopia therapy [9].

Although the acuity criterion varies among authors, the most common clinical definition of amblyopia assumes that the best possible far-point corrected acuity is 6/12 or worse, often with an additional qualifier of a difference of one or two lines between the two eyes [10].

The initial development of amblyopia from any cause rarely occurs in children older than 5.5 years, but once it has developed and been reversed by therapy it may reappear until about 9 or 10 years of age. Anisometropic amblyopia rarely occurs unless the anisometropia has been present for more than 2 years [11].

Children at birth frequently have modest amounts of astigmatism equal in each eye, which disappears without permanent effect by the age of 6 months. The critical period for the development of anisometropic amblyopia in humans is not known more precisely [12].

Amblyopia is primarily a defect of central vision; the peripheral visual field nearly always remains normal. Experimental studies on animals and clinical studies of infants and young children support the concept of critical periods of sensitivity for the development of amblyopia. These critical periods correspond to when the child's developing visual system is sensitive to abnormal input caused by stimulus deprivation, strabismus, or significant refractive errors. In general, the critical period for stimulus deprivation amblyopia occurs earlier than that for ocular misalignment or anisometropia. Several findings from both animals and humans suggest that the receptive fields of neurons in the amblyopic visual system are abnormally large. This disturbance may account for the crowding phenomenon (also known as contour interaction), whereby Snellen letters or equivalent symbols of a given size become more difficult to recognize if they are closely surrounded by similar forms, such as a full line or chart of letters. The crowding phenomenon sometimes causes the measured linear acuity of an amblyopic eye to drop several lines below that measured with isolated letters [13].


  Methods Top


This study was carried out between March 2012 and September 2012 on children between the ages of 5 and 15 years (mean age 8.013 ± 3.381 years) attending the Menoufia University outpatient clinic.

Exclusion criteria

Patients with structural ocular abnormalities, previous ocular surgery, ocular trauma, or neurologic disorders that could influence visual acuity were excluded from the study.

Clinical assessment

All patients underwent the following:

(1) Answering of a structural questionnaire to obtain the following data:

age, sex, history of spectacle correction, occlusion or penalization therapy, history of previous ocular surgery, and ocular trauma.

(2) An ocular examination, which consisted of the following:
  1. eye examinations, including assessment of eye alignment using the cover-uncover test at distant and near positions.
  2. Monocular examinations, in which the right eye was tested first. Uncorrected visual acuity and BCVA were determined. Cycloplegic refraction test was performed after application of 1% cyclopentolate eye drops. A fundus examination of the external eye was carried out to check for abnormalities.


Uncorrected visual acuity and best-corrected visual acuity

Visual acuity was measured at a 6 m distance using an international standard vision chart (Landolt C vision chart), and amblyopia was defined as a BCVA difference between the sound and amblyopic eyes of more than two lines on the vision chart.

Cycloplegic refraction

Refractive error was measure 40-60 min after instillation of two drops of cyclopentolate (1%) in each eye.

Examination of the external eye

Structural ocular abnormalities, signs of previous ocular surgery, and signs of previous ocular trauma were evaluated.

Fundus examination was performed with an indirect ophthalmoscope to detect any abnormalities.

Statistical analysis

Statistical analysis was carried out using the Statistical Package for the Social Sciences (SPSS, version 15). Quantitative variables were expressed as mean ± SD, whereas qualitative variables were given as numbers and percentage. Descriptive statistics were analyzed using the c2 -test. Correlation analysis was performed by calculating the Pearson correlation coefficient (r). Regression analysis was carried out to assess the different factors that can increase the incidence of amblyopia.


  Results Top


The ages of the patients ranged between 5 and 15 years (mean age 8.013 ± 3.381 years). Of 510 patients in this study 472 showed normal visual acuity in both eyes and 38 were amblyopic, an estimated incidence of 7.45%. Of the 38 amblyopic patients, 16 (42.1%) were male and 22 (57.9%) were female and 26 were anisometropic and 12 were strabismic, which demonstrates that anisometropia is the most common cause for amblyopia. Of the 26 anisometropic cases, 12 (46.15%) had amblyopia in the left eye and 14 (53.75%) had amblyopia in the right eye. Of the 12 strabismic cases, four (33.3%) had amblyopia in the left eye and eight (66.7%) had amblyopia in the right eye. Of 12 strabismic cases, eight (66.6%) presented with exotropia and four (33.3%) presented with esotropia.


  Discussion Top


Amblyopia is one of the leading causes of acquired monocular visual impairment, with an estimated prevalence in the pediatric population ranging from 4.7 to 7.5% [14].

Children with anisometropic amblyopia are typically diagnosed later than children having other types of amblyopia, likely reflecting the lack of noticeable physical abnormalities in these children, in contrast to children with strabismus, who usually have an obvious condition that is easily recognized [15].

Of 510 patients in our study 472 showed normal visual acuity in both eyes and 38 were amblyopic, an estimated incidence of 7.45%.

This incidence is higher compared with that seen in a questionnaire-based study of Japanese children aged between 1.5 and 12 years, in which the reported prevalence of amblyopia ranged between 0 and 0.2% [16].

However, in the USA the reported prevalence of amblyopia was 1.5% [17], whereas in the UK the prevalence was 3.6% [18].

In Japan the prevalence of amblyopia was 0-0.18% [16], whereas in Singapore it was 0.34% [19].

In Abha (Saudi Arabia) the amblyopia prevalence was 1.85%, whereas in the Sultanate of Oman it was 0.9% [20].

Of the 38 amblyopic cases 26 were anisometropic and 12 were strabismic, which demonstrates that anisometropia is the most common cause for amblyopia in this study. This is in agreement with the results of a study conducted by Chang et al. [21] in Taiwan, which reported that anisometropic amblyopia is more common than strabismic amblyopia, and in agreement with the results of the study conducted by Lim et al. [22] in South Korea. In contrast, amblyopia in the USA [23], UK [24], and Australia [17] was more likely to be associated with strabismus alone (26-44%) or combined strabismus and refractive error (20%), rather than refractive error alone (40-50%).

Of the 12 strabismic patients, eight (66.6%) presented with exotropia and four (33.3%) presented with esotropia. This is in agreement with the study by Yekta et al. [25] who reported that exotropia is the most common type of strabismic amblyopia.

In this study it was found that the amount of refraction, magnitude of anisometropia, and line acuity difference between both eyes correlated with the final BCVA. This is in agreement with the results of Cobb et al. [26] who found that the amount of refractive error and degree of anisometropia at presentation correlated strongly with final visual acuity.

It was found that the prevalence of anisometropic amblyopia is 68.42% (26 cases out of 38), whereas in the study by Goh et al. [27] an association of anisometropia with amblyopia was found in 47.6% of students (10 cases), similar to another study (51.1%) [28].

In contrast, in an Australian study in which 1736 children aged 6 years were examined, amblyopia was reported in 0.7% of patients, with strabismus constituting 37.5%, anisometropia 34.4%, or both 18.8% [23].

Few studies have involved East Asian children, in which the prevalence of myopia is the highest [16].

It was found that amblyopia is more frequent in hypermetropic anisometropia [16 of 26 (61.54%) cases] than in myopic anisometropia [eight (30.77%) cases]. This is in agreement with the study by McMullen [29] who found that amblyopia is more severe and more frequent in hypermetropic anisometropia than in myopic anisometropia.

In a study by Weakley [9] there was a tendency for the development of amblyopia at lower levels of hyperopic anisometropia (spherical hypermetropic anisometropia >1-2 D) compared with myopic anisometropia (spherical myopic anisometropia >−2 to −3 D). Kim et al. [30] found that hyperopic anisometropic patients have a higher risk for developing amblyopia. In contrast, Lee et al. [31] reported that there were no differences in the frequency of amblyopia development among patients having myopic spherical anisometropia, hypermetropic spherical anisometropia, or astigmatic anisometropia [Table 1],[Table 2],[Table 3],[Table 4],[Table 5],[Table 6],[Table 7],[Table 8] and [Table 9].
Table 1: Relation between amblyopia in the right eye and its cause

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Table 2: Relation between amblyopia in the left eye and its cause

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Table 3: Visual acuity in the right eye

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Table 4: Visual acuity in the left eye

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Table 5: Relation between sex and cause of amblyopia

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Table 6: Relation between spherical error in right eye and amblyopia

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Table 7: Relation between cylindrical error in right eye and cause of amblyopia

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Table 8: Relation between spherical error in left eye and cause of amblyopia

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Table 9: Relation between cylindrical error in left eye and cause of amblyopia

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  Conclusion Top


Amblyopia is a common disorder of the visual system without any structural anomaly.

Anisometropic amblyopia is a difficult type of amblyopia to be diagnosed, as the sound eye overlaps the visual defect of the amblyopic eye. Proper ophthalmological screening of preschool children should be carried out, including extraocular examination, fundus examination, and visual acuity testing with special emphasis on detection of errors of refraction.

Our study was conducted over 6 months among patients aged 5-15 years attending the outpatient clinic of Menoufia University.

A total of 510 patients were included in our study: 472 were normal and 38 were amblyopic, 26 had anisometropic amblyopia and 12 had strabismic amblyopia.


  Acknowledgements Top


Concepts, design, and definition of intellectual content done by Hoda Mohamed Kamel El-Sobky (MD); manuscript preparation, manuscript editing, and manuscript review done by Khaled El-Ghonemy Said Ahmed (MD); literature search done by Moataz Fayez El-Sawy (MD); clinical studies, data acquisition, data analysis, and statistical analysis done by Sherif Mohammed Ibrahim Sakr (MBBCh).

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9]



 

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