Indian Journal of Ophthalmology

: 2005  |  Volume : 53  |  Issue : 4  |  Page : 227--234

Profile of amblyopia in a hospital referral practice

Vimla Menon, Zia Chaudhuri, Rohit Saxena, Kulwant Gill, MM Sachdev 
 Strabismus and Amblyopia Services, Dr R P Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, Ansari Nagar, New Delhi, India

Correspondence Address:
Vimla Menon
Room No. 485, Dr R P Centre for Ophthalmic Sciences, All India


Objective: Evaluation of the clinical profile and distribution of different sub-types of amblyopia in a referral eye hospital in India. Methods: This was a prospective hospital-based observational study, evaluating the clinical profile of patients with amblyopia presenting to a referral strabismology practice. The examination included assessment of the visual acuity, the refractive status, the fixation pattern, the grade of binocularity, and evaluation of the associated strabismus, if any before treatment was started. Results: The average age of the patients at presentation was 7.976.18 years with 81 out of 683 patients (11.8%) presenting above the age of 20 years. The best-corrected visual acuity (BCVA) was less than 6/60 in the amblyopic eye in 121 out of 733 eyes (16.5%). Strabismic amblyopia was the most common sub-type of amblyopia seen (274/733 eyes, 37.38%). Though patients with anisometropic amblyopia presented at a later age (average of 10.036.92 years), they had better visual acuity, binocular functions, and centricity of fixation at all ages, relative to other sub-types of amblyopia. The BCVA did not show any co-relation with the age of presentation [co-relation co-efficient (CF) of 0.074], refractive status of the amblyopic eye (CF of 0.078), the type of amblyopia (CF of 0.196), or the type of strabismus present (CF of 0.079). However, a very significant co-relation was seen between the BCVA and the fixation pattern of the amblyopic eye (CF of 0.817). Conclusions: Lack of knowledge and awareness about amblyopia and its appropriate timely management has been the main cause for the late presentations and significant visual impairment associated with the condition.

How to cite this article:
Menon V, Chaudhuri Z, Saxena R, Gill K, Sachdev M M. Profile of amblyopia in a hospital referral practice.Indian J Ophthalmol 2005;53:227-234

How to cite this URL:
Menon V, Chaudhuri Z, Saxena R, Gill K, Sachdev M M. Profile of amblyopia in a hospital referral practice. Indian J Ophthalmol [serial online] 2005 [cited 2022 Jun 29 ];53:227-234
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Full Text

Amblyopia is one of the common causes of childhood[1],[2],[3],[4],[5],[6],[7],[8]and adult visual impairment, [9],[10],[11],[12],[13],[14] the prevalence of which is usually underestimated, often because of lack of awareness. The causes of amblyopia include strabismus, anisometropia, high-refractive errors, opacities of the ocular media, high astigmatism or a combination of two or more aetiologies in the same patient. In spite of different causes, the basic mechanisms of either abnormal binocular interactions between both the eyes or form deprivation in one or both eyes, both of which can lead to amblyopia remain the same in all cases. [15],[16],[17] The upper limit of the critical time when children are most vulnerable to these insults is around 8 years in humans. [15],[16],[17] Visual loss due to amblyopia can be permanent if corrective measures are not taken in time.[3],[9] The burden of disability due to this problem can become massive when one takes into account the duration of life with visual disability.[3]

The main focus of childhood blindness in developing countries like India has been on conditions like vitamin A deficiency, trauma, cataract and other causes related to malnutrition, infection, and poor health. [1],[2],[3],[4],[12],[18] Inadequate attention has been given to the prevalence of paediatric low vision due to amblyopia in our country. In a study done on 4029 school children in a south Indian state to determine the causes of visual impairment in children, 1.1% of the population was found to have amblyopia.[4] Similarly, in another study involving 1318 children in nine states in India, cataract, uncorrected aphakia and amblyopia comprised 12.3% of severe visual impairment.[3] A study of refractive errors prevalent in an urban population of India reported the prevalence of amblyopia to be about 4.4%,[12] while a similar study done on a rural population base reported it to be about 12%.[1] There has, however been no Indian study elucidating the clinical profile of patients with amblyopia, similar to what has been reported in western literature.[5],[8],[11],[19],[20] This leaves a certain lacunae in knowledge, as different sub-types of amblyopia are managed differently and the response to treatment is also dissimilar. Taking into consideration the Indian sub-continent, there has been a recent hospital-based Pakistani study that has elucidated the clinical profile of patients with amblyopia in a referral ophthalmology hospital set-up, similar to the study conducted by us.[21]

In this study, we prospectively analysed the clinical profile of patients with amblyopia, presenting to the Squint and Amblyopia clinic of our institution, so as to enable us to have objective measures of the demographic profile of these patients, the average age of presentation to an institution, the extent of visual impairment at presentation, the prevalence of eccentric fixation and other characteristics of amblyopic vision. We believe that this would enable us to plan future surveys and strategies to screen and implement appropriate therapeutic measures for treating amblyopia on a mass scale subsequently.

 Material and Methods

This was a prospective hospital-based observational study, evaluating the clinical profile of patients with amblyopia presenting to a referral strabismology practice. Amblyopia was defined as:

1.A difference in the best-corrected visual acuity (BCVA) between the two eyes of two or more Snellen lines/equivalent measure on the Teller's acuity chart or the Cardiff's chart in children less than 4 years in the absence of any organic lesion that could result in a decrease in vision.

2.A BCVA of less than 6/12 bilaterally on the Snellen's chart/equivalent measure on the Teller's acuity chart or the Cardiff's chart in children less than 4 years in the absence of any organic lesion that could result in a decrease in vision.

The assessment included a detailed history related to the age of onset as noticed by the patient or his guardians, age of presentation to the hospital, the subsequent clinical course, and any previous modality of treatment taken. A general physical and systemic examination was performed in every case to rule out any other associated problems.

Ocular examination included assessment of the unaided (UCVA) and the best-corrected spectacle visual acuity (BCSVA) with the help of Snellen's visual acuity chart in children more than 4 years of age and Teller's acuity chart/Cardiff's vanishing optotype charts in children less than 4 years of age. Refraction under appropriate cycloplegics depending upon the age of the patient, assessment of the ocular alignment, ocular motility and associated deviation if any, and slit lamp examination for the assessment of any anterior segment pathology was performed. A detailed fundus examination was done to rule out any posterior segment pathology and to determine the fixation pattern. Assessment of the binocular status of the eye was performed whenever possible with the help of the Worth's four-dot test, Bagolini's striated glasses, the synoptophore, the TNO test and the random-dot test.

Standard definitions of different subtypes of amblyopia were used for diagnosis.[22],[23] The criteria used for the diagnosis are listed below:

1.Strabismic amblyopia : This was defined as amblyopia in the presence of a heterotropia at distance or near fixation in the absence of any anisometropia meeting the criteria for a combined mechanism amblyopia. Patients with strabismus along with refractive errors of more than 1 D spherical equivalent in one or both eyes or eyes with regular astigmatism more than or equal to 1.5 D of astigmatism in any meridian were also included in this category.

2.Anisometropic amblyopia : This included patients who had amblyopia in the presence of anisometropia that was 1 D or greater in spherical equivalent, or a 1.5 D or greater difference in astigmatism between both the eyes that persisted for at least 4 weeks after spectacle correction, in the absence of any measurable heterotropia at distance or near.

3.Combined amblyopia : This included patients with either a heterotropia at distance or near along with anisometropia of 1 D or more in spherical equivalent or a 1.5 D or more difference in astigmatism in any meridian between both the eyes that persisted after at least 4 weeks of spectacle correction.

4.Sensory deprivation amblyopia : This group included patients with a known documented cause of sensory deprivation with no primary heterotropias or refractive errors that could be causally related to the amblyopia.

5.Ametropic amblyopia : Patients with refractive errors more than 1 D spherical equivalent in both eyes resulting in subnormal vision in one or both eyes and no associated strabismus or any other ocular pathology were classified under this category. Patients with significant anisometropia (as defined above) along with high refractive errors in both eyes were excluded from this category and were grouped under the anisometropic amblyopia group. Patients with heterotropias for distance or near with bilateral refractive errors more than 1 D spherical equivalent were included under strabismic amblyopia.

6.Meridional amblyopia : Patients with regular astigmatism 1.5 D of astigmatism in any meridian or those with irregular astigmatism in both eyes, resulting in a decrease in vision in one or both eyes and no associated strabismus were classified as having meridional amblyopia. Patients with significant anisometropia (as defined above) along with a difference of 1.5 D or greater astigmatism between the two eyes were excluded from this category and grouped under the anisometropic amblyopia group. Patients with heterotropias for distance and near with regular astigmatism more than 1.5 D in any meridian or irregular astigmatism were included under strabismic amblyopia.

The results were analysed using the relevant statistical methods (paired ' t ' test, student's ' t ' test and the Pearson's chi-square test) as indicated (Epi Info, Version 6.04D).


We evaluated the demographic and clinical profile of 683 patients who presented to the Squint and Amblyopia clinic at the Dr. Rajendra Prasad centre for Ophthalmic Sciences, AIIMS between September 2001 to December 2002.

Demographic profile

The average age of the patients at presentation was 7.976.18 years. There were 373 male (54.61%) and 310 female (45.38%) patients. Twenty-two patients (3.22%) had taken previous treatment for amblyopia in the form of conventional occlusion elsewhere, prior to their presentation to us but had either been very irregular or had discontinued their therapy. There was no difference in the mean visual acuity of amblyopic patients who had received prior amblyopia treatment and in those who had not.

The departments of paediatrics and paediatric neurology referred 45 patients. The remaining patients presented directly to the out-patients department of the Dr Rajendra Prasad center for ophthalmic sciences and were referred to the Squint and Amblyopia clinic. Twenty-four patients were diagnosed cases of cerebral palsy, 12 patients were mentally challenged, and 3 patients were diagnosed as having Goldenhar's syndrome. Two other patients were diagnosed as having Duane's syndrome. General physical and systemic evaluation in these patients confirmed the clinical findings suggestive of these conditions. Those patients who had such systemic problems and had not been seen by a paediatrician prior to their presentation to us were referred to the department of paediatrics for complete evaluation. In five patients with strabismic amblyopia, one of the parents had either strabismus or anisometropia.

Two hundred and sixty two patients out of 683 were residents of New Delhi, India. Two hundred and one patients were residents of different cities and towns in Northern India from the states of Punjab, Haryana, Himachal Pradesh, Uttaranchal, Uttar Pradesh, Bihar, Madhya Pradesh, and Rajasthan, all within a distance less than 500 km from New Delhi. Out of the remaining 128 patients, 12 patients were from the states of West Bengal, 2 from the state of Tamil Nadu, 19 from the state of Kerala, 23 from the state of Andhra Pradesh, 27 from the states of the North East including Assam, Manipur, Meghalaya, Nagaland, and Arunachal Pradesh, 18 from the state of Orrisa, 11 from the state of Maharashtra, and 16 from the state of Gujarat. Seventy-eight patients were from a completely rural background from different states of North India. Fourteen patients were residents of Nepal.

Fifty patients had bilateral amblyopia (44 patients with ametropic amblyopia and 6 patients with meridional amblyopia). Thus the total number of amblyopic eyes examined was 733.

Out of a total of 733 eyes, 274 eyes (37.38%) had strabismic amblyopia, 151 eyes (22.1%) had anisometropic amblyopia, 126 eyes (18.44%) had combined amblyopia, 88 eyes (12.88%) had ametropic amblyopia, 38 eyes (5.56%) had meridional amblyopia, while the remaining 56 patients (7.63%) had sensory deprivation amblyopia according to the criteria defined in the Material and Methods section. In most patients with strabismic and combined mechanism amblyopia, the poor vision was discovered when the patients presented for evaluation of the strabismus. In patients with anisometropic amblyopia, school health vision screening examinations were instrumental in detection of the poor vision in most cases. Most of the children were already in middle school by the time they presented to us. This probably accounted for the relatively late hospital presentation of patients with anisometropic amblyopia.

Age distribution

One hundred and forty-two patients (20.79%) presented before the age of 4 years [Table 1]. Only three (2.1%) out of these were cases of pure anisometropic amblyopia. The majority of patients (43.92%) presented between the ages of 4 and 10 years. All age groups showed an equal distribution of patients with strabismic, combined, and meridional amblyopia. Patients with anisometropic amblyopia were seen more commonly after the ages of 4 years, while those with ametropic and sensory deprivation amblyopia, presented more commonly in the age group between 4 and 10 years of age. Eighty-one patients (11.85%) were above the age of 20 years at presentation indicating that a large chunk of patients with amblyopia remained undiagnosed till late. A large majority of patients in this age group comprised of patients with strabismic, anisometropic, or combined amblyopia. The difference in the distribution of patients of different ages with respect to the specific subtype of amblyopia was however not statistically significant ( P = 0.227).

The average age of presentation of patients with anisometropic amblyopia was 10.036.92 years, which was significantly more than the average age of patients with strabismic amblyopia (7.675.55 years), combined amblyopia (8.626.23 years), ametropic amblyopia (8.225.93 years), and sensory deprivation amblyopia (8.33 6.63 years). The average age of presentation of patients with meridional amblyopia was 9.427.39 years, which was comparable to the average age of patients presenting with anisometropic amblyopia.

Visual acuity in the amblyopic eye

One hundred and forty-two patients (160 eyes; 124 with unilateral amblyopia;18 with bilateral amblyopia) were less than 4 years of age in whom the visual acuity was measured by the Teller's acuity chart or the Cardiff's charts [Table 2]. In the remaining 541 patients, assessment was done on the basis of the Snellen's visual acuity chart. To maintain the equivalence of visual acuity in the two groups assessed by two different methods, we calculated the Snellen fractions and equivalent measures as assessed by other methods and subdivided the groups on the basis of the log of these fractions.

The BCVA in the amblyopic eye showed a significant association with the diagnosed subtype of amblyopia with anisometropic amblyopia having the best visual acuities at presentation ( P Refractive status of the amblyopic eye

The refractive status of the amblyopic eyes (except in those with meridional amblyopia) was categorised according to the standard definitions of refractive errors as mentioned in the Material and Methods Section [Table 3]. Moderate hypermetropia of about 3 DS was the most common refractive error seen. Three hundred and fifty nine amblyopic eyes (51.65%) had a hypermetropic refractive error as compared to 236 eyes with myopia (33.95%), and 100 eyes (14.38%) with no significant refractive errors ( P Binocularity status

Binocularity could not be assessed in 72 patients (10.54%) who were uncooperative or unable to comprehend the tests. [Table 4] depicts the binocularity status in the remaining 611 patients. Thus though 475 patients (77.37%) had some grade of binocularity, only 205 patients of these had all grades of binocularity. Seventy-three out of these (35.6%) had anisometropic amblyopia ( P Fixation pattern

Based on the fixation pattern seen by the Linkz Star configuration of the standard Heine's direct ophthalmoscope, the fixation pattern of the amblyopic eyes were grossly divided into central fixation (foveal, unsteady foveal, perifoveal), where the reflex was definitely present within the central 3, eccentric fixation that took into account any fixed fixation point that was beyond the central 3 and last, wandering or no fixation [Table 5]. 114 eyes out of 151 eyes with anisometropic amblyopia (75.49%) had central fixation as compared to 87 out of 126 eyes (69.6%) with combined amblyopia, 184 out of 274 eyes (67.15%) with strabismic amblyopia, and 91 (50%) out of 182 eyes with form deprivation (sensory deprivation, ametropic and meridional) amblyopia.

This difference was significant when each of the other sub-types were compared with the eyes having anisometropic amblyopia. The eccentricity of fixation had a very high co-relation with the visual acuity in the amblyopic eye (depth of amblyopia). The more eccentric the fixation, lesser was the visual acuity.

Profile of strabismus in patients with amblyopia

Out of 683 patients, 425 patients (62.22%) had strabismus [Table 6]. This included 274 patients (40.11%) with strabismic amblyopia, 126 patients (18.44%) with combined amblyopia, and 25 patients (3.66%) patients with sensory deprivation amblyopia. Esodeviation was the most common deviation seen (56.47%) followed by exodeviations (36.23%) and primarily vertical deviations (7.29%). These patients with vertical deviations comprised of six patients with congenital third nerve palsy, nine patients with congenital fourth nerve palsy, three patients with Brown's syndrome, 11 patients with double elevator palsy (monocular elevation deficit), and two patients with dissociated vertical deviation.

Adult amblyopia

Eighty-one patients (49 males, 32 females) were more than 20 years of age (range was between 20 and 62 years) at the time of initial presentation to us. Surprisingly, none of them primarily came for the treatment of amblyopia. Amblyopia was either discovered by chance while they were being examined for some other eye complaint or because they were considered to be unfit for certain services. The oldest patient aged 62 years, had a refractive error of +8 DS in the amblyopic eye, which was only discovered when he started having a decrease in vision in the fellow eye due to progressive cataractogenesis. Another interesting finding was that 34 (41.97%) of these patients had pure strabismic amblyopia, where the strabismus had either been completely ignored in childhood and was considered to be untreatable or the patient had been misinformed that treatment was best if undertaken in adulthood.


Amblyopia is one of the most common causes of visual impairment in both children and adults with a prevalence varying between 0.2% and 12% depending on the subsets of the population studied. [1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21] Lack of adequate understanding or knowledge about this preventable and easily treatable condition, provided compliant treatment is started early, is often the reason why very few patients are referred to eye hospitals or specialist practices for the amelioration of the same especially in a developing country like India.[3]

Amblyopia and associated strabismus can have devastating psychosocial and economic fall-outs. Failure to develop binocular vision and unilateral or bilateral visual impairment may prevent the individual from pursuing certain occupations.[3],[9],[10] Severe amblyopia is also considered a significant risk factor for blindness in case an individual loses sight in the fellow eye.[24],[25] Amblyopia increases the chances of blindness in the healthy eye, mostly from trauma.[25]

Timely diagnosis and treatment is likely to reduce the prevalence of amblyopia as has been seen in many countries that have taken up mass education and visual screening as important community aids to increase knowledge and awareness about the condition.[6]-[9],[13],[14],[19],[20],[26],[27],[28],[29],[30],[31],[32]

There have been population-based regional studies in India related to childhood blindness and prevalence of refractive errors, where amblyopia has been noted to be one of the causes of visual impairment seen.[1]-[4],[12],[33],[34],[35]A recent Indian study, elucidating a population-based perspective on childhood blindness stresses on amblyopia after cataract surgery to be a major cause of preventable childhood low vision.[33] Similarly, another study by the same authors on planning low vision services in India, marks out amblyopia to be one of the main causes of paediatric low vision accounting for about 26% of the children with low vision examined.[34] There have, however, been no studies where amblyopia has been the primary focus of attention with the establishment of the patient profile, age of presentation, and other demographic details.

Though the present study suffers from a selection bias, as the data is hospital based, this analysis may form the basis of future population-based studies. However, one advantage of a study in a hospital referral practice with a very wide base of patients coming from all parts of the country, is the immense variety and numbers that can be seen, which can be utilised to elucidate the clinical profile of that condition under as ideal a circumstance as possible.

Another important factor that this study reveals is the relatively older ages of presentation to a speciality clinic irrespective of the sub-type of amblyopia present. Seven to 8 years is the critical time after which therapeutic measures for the treatment of amblyopia become less effective[6],[15],[16],[17] and that unfortunately was the average age of presentation of most patients to the hospital in our study. This reiterates that efforts to screen patients for amblyopia and educate personnel at every level to suspect, diagnose, treat or refer the patient as soon as possible. This effort could be directed towards starting vision screening programmes in schools and mother and child care clinics and training village health workers to assess for visual acuity, ocular movements, and the fixation behavior of the non-occluded eye after covering one eye in children presenting to them to a level adequate for them to recognise a problem and refer the patient as soon as possible. Paediatricians could be trained to look for delays in the development of visual milestones in children. They could also be trained to assess visual acuity in very small children with Cardiff or Teller's visual acuity charts. This would however call for easy availability of these expensive equipments and a short training course for paediatricians. The aim of training in all these quarters would be to enhance early referrals at a younger age group, which is of utmost priority while treating a patient with amblyopia. Institution of measures like the health-related quality of life (QOL) questionnaires which have been performed for the evaluation of many paediatric conditions including amblyopia, in the Western countries, could be of immense value to trial developers, who can use all these parameters to design appropriate randomised studies, which could demarcate areas where further action needs to be taken in the community.[36]

Early detection of amblyopia and institution of appropriate therapy is of immense value towards preventing the prevalence of life long visual morbidity. As mentioned earlier, we observed that the vast majority of patients with amblyopia in our study presented between 4 and 10 years of age. Associated strabismus was one of the reasons for early referral. Though patients with anisometropic amblyopia presented relatively later (when the child became verbal or when there was some other associated problems, the anisometropia being discovered as an incidental finding), these patients had better BCVA, higher prevalence of central fixation and higher grades of binocularity vis--vis patients diagnosed as having other subtypes of amblyopia especially those with form deprivation amblyopia (excluding meridional amblyopia). These findings are in agreement with other studies. [6],[7],[8],[31],[37]

Some important differences in our data relative to those published in the amblyopia treatment study 1 (ATS1) are possibly because the inclusion criteria were different in both the studies.[8] The present study was essentially hospital based and included patients with form deprivation amblyopia in addition to those with amblyopia secondary to abnormal binocular interaction. Other differences were that patients with bilateral amblyopia, those with dense amblyopia, patients of all age groups and patients having taken prior treatment were included. The ATS1 study took into consideration a specific age group of patients with moderate, unilateral amblyopia, who had not taken any prior treatment for amblyopia. Our results are however similar to two population-based studies conducted in the UK, where apart from the fact that they were community based, most of the other inclusion criteria for enrollment were the same.[6],[7] The study design of a recent hospital-based study in a referral eye institute in Pakistan, which evaluated the clinical profile of 316 verbal children presenting with unilateral amblyopia over a period of 2 years is very similar to our study. However, this study does not take into consideration older patients (> 14 years of age) and patients with bilateral amblyopia or form deprivation amblyopia.[21]

There are no other Indian studies which have prospectively evaluated the clinical profile of different sub-types of amblyopia, be it community based or hospital based. Knowledge about the sub-types of amblyopia is important because the clinical presentations, management and outcomes of these different types are different. Both in the context of Vision 2020, with the added stress on rehabilitation of paediatric low vision, of which amblyopia is a major preventable and treatable cause,[34],[38],[39] and the fact that untreated amblyopia is a major cause of monocular or binocular low vision in adulthood, [9],[10],[11],[13],[14] with the associated deterioration in the QOL indices, measures for the early detection and dedicated rehabilitation of amblyopia should be taken up on a priority basis and form another tenet of the evidence-based planning that has been the hallmark of the blindness control programme in India.[38],[39] The data in the present study could be used to enhance screening efforts in an organised manner in those health-care groups which come in regular contact with infants and young children. This includes village health workers, personnel at mother and child care clinics, paediatricians both in practice and in referral hospital services and general practitioners associated with school health programmes. Providing appropriate training to these personnel and incorporating their services could aid early detection and rehabilitation of patients. An ophthalmic referral to ophthalmologists in preschool children by paediatricians for assessment of visual acuity and the fundus, could be a key for early detection of amblyopia.

It is important to remember that amblyopia is often a diagnosis of exclusion. Other causes of low vision need to be evaluated before preemptorily diagnosing a patient as a case of amblyopia. In the present study, we found a high co-relation between the fixation pattern of the eye and the BCVA with increased centricity of the fixation pattern indicating better visual acuity. We recommend that evaluation of the fixation pattern, which is possible with the standard direct ophthalmoscope present with every ophthalmologist, should be an essential part of an ophthalmic examination in a child. This would be of great importance in preverbal children, in uncooperative children and mentally challenged children in the absence of the more expensive modalities of assessing paediatric visual acuity.

The present study was a cross sectional study defining the pattern and profile of patients with amblyopia in an apex tertiary referral eye hospital in India. Follow-up measures and response to treatment instituted in these patients after starting therapy is outside the preview of this report. However, it is important to stress that regular follow-up (after 2-3 weeks in children less than 3 years and after 4-6 weeks in older children and adults) is of utmost importance to monitor both the compliance to therapy and the response to treatment. Population-based studies at a future date would be useful to further validate the mass education measures that can be taken up to prevent and treat this condition.


1Dandona R, Dandona L, Srinivas M, Sahare P, Narsaiah S, Munoz SR, et al . Refractive errors in children in a rural population. Invest Oph Vis Sci 2002;43:615-22.
2Dandona L, Dandona R, Srinivas M, Giridhar P, Vilas K, Prasad MN, et al . Blindness in the Indian state of Andhra Pradesh. Invest Oph Vis Sci 2001;42:908-16.
3Rahi JS, Sripathi S, Gilbert CE, Foster A. Childhood blindness in India: causes in 1318 blind school children in 9 states. Eye 1995;9:545-50.
4Kalikiyavi V, Naduvilath TJ, Bansal AK, Dandona L. Visual impairment in school children in South India. Indian J Ophthalmol 1997;45:129-34.
5Thompson JR, Woodruff G, Hiscox FA, Strong N, Minshull C. The incidence and prevalence of amblyopia detected in childhood. Public Health 1991;105:455-62.
6Woodruff G, Hiscox FA, Thompson JR, Smith LK. Factors affecting the outcome of children treated for amblyopia. Eye 1994;8:627-31.
7Shaw DE, Fielder AR, Minshull C, Rosenthal AR. Amblyopia: Factors influencing the age of presentation. Lancet 1988;2:207-9.
8The Pediatric Eye Disease Investigator Group. The Clinical Profile of Moderate Amblyopia in Children younger than 7 years. Arch Ophthal 2002;120:281-7.
9Preslan MV, Novak A. Baltimore Vision screening project. Phase 2. Ophthalmology 1998;105:150-3.
10Attebo K, Mitchell P, Cumming R., Smith W, Jolly N, Sparkes R. Prevalence and causes of amblyopia in an adult population. Ophthalmology 1998;105:154-9.
11Ponte F, Giuffre G, Giammanco R. Prevalence and causes of blindness and low vision in the Casteldaccia Eye Study. Graefe's Arch Clin Exp Ophthalmol 1994;232:469-72.
12Murthy GV, Gupta SK, Ellwein LB, Munoz SR, Pokharel GP, Sanga L, et al . Refractive error in an urban population in New Delhi. Invest Oph Vis Sci 2002;43:623-31.
13Wang JJ, Foran S, Mitchell P. Age specific prevalence and causes of bilateral and unilateral visual impairment in older Australians: The Blue Mountains Eye Study. Clin Exp Ophthal 2000;28:268-73.
14Quah BL, Tay MT, Chew ST, Lee LK. A study of amblyopia in 18-19 year old males. Singapore Med J 1991;32:126-9.
15Keech RV, Kutschke PJ. Upper age limit for the development of amblyopia. J Pediatr Ophthal Strab 1995;32:89-93.
16Von Noorden GK. Factors involved in the production of amblyopia. Br J Oph 1974;58:158-64.
17Harwerth RS, Smith EL, Duncan GC, Crawford ML, Von Noorden GK. Multiple sensitive periods in the development of the primate visual system. Science 1986;232:235-8.
18Wedner SH, Ross DA, Balera R, Kaji L, Foster A. Prevalence of eye diseases in primary school children in a rural area of Tanzania. Br J Ophthalmol 2000;84:1291-7.
19Williams C, Harrad RA, Harvey I, Sparrow JM. The ALSPAC Study Team. Screening for amblyopia in preschool children: results of a population-based, randomized controlled trial. ALSPAC Study Team. Avon Longitudinal Study of Pregnancy and Childhood. Ophthal Epidemiol 2001;8:279-95.
20Brown SA, Weih LM, Fu CL, Dimitrov P, Taylor HR, McCarty CA. Prevalence of amblyopia and associated refractive errors in an adult population in Victoria, Australia. Ophthal Epidemiol 2000;7:249-58.
21Shah M, Khan MT, Khan MD, Rehman HU. Clinical Profile of amblyopia in Pakistani children age 3 to 14 years. J Coll Physic Surg Pak 2005;15:353-7.
22 In : Mein J, Trimble R. editors. Diagnosis and Management of ocular motility disorder, 2nd edn. Blackwell Scientific Publication: London, UK; 1991. p. 199.
23Von Noorden GK. Classification of amblyopia. Am J Ophthalmol 1967;63:238-44.
24Vereeken EP, Brabant P. Prognosis for vision in amblyopia after loss of the good eye. Arch Ophthalmol 1984;102:220-4.
25Tommila V, Tarkkanen A. Incidence of loss of vision in the healthy eye in amblyopia. Br J Ophthalmol 1981;65:575-7.
26Ohlsson J, Villarreal G, Sjostrom A, Abrahamsson M, Sjostrand J. Visual acuity, Residual amblyopia and Ocular pathology in a screened population of 12-13 year old children in Sweden. Acta Ophthal Scand 2001;79:589-95.
27Pareja RA, Martinez PA, Abreu Reyes JA, Serrano GM. A study of the visual acuity and amblyopia in infants aged 3 to 5 from El Hierro island. Arch Soc Esp Oftalmol 2000;75:397-402.
28Lim HC, Quah BL, Balakrishnan V, Lim HC, Tay V, Emmanuel SC. Vision screening of 4-year-old children in Singapore. Singapore Med J 2000;41:271-8.
29Schalij-Delfos NE, de Graaf ME, Treffers WF, Engel J, Cats BP. Long term follow up of premature infants: detection of strabismus, amblyopia, and refractive errors. Br J Ophthalmol 2000;84:963-7.
30Negrel AD, Maul E, Pokharel GP, Zhao J, Ellwein LB. Refractive Error Study in Children: sampling and measurement methods for a multi-country survey. Am J Ophthalmol 2000;129:421-6.
31Weakley DR. The association between anisometropia, amblyopia and binocularity in the absence of strabismus. Trans Am Oph Soc 1999;97:987-1021.
32Newman DK, East MM. Prevalence of amblyopia among defaulters of preschool vision screening. Ophthal Epidemiol 2000;7:67-71.
33Dandona R, Dandona L. Childhood Blindness in India: A Population based perspective. Br J Ophthalmol 2003;87:263-5.
34Dandona R, Dandona L, Srinivas M, Giridhar P, Nutheti R, Rao GN. Planning Low Vision Services in India: A Population based perspective. Ophthalmology 2002;109:1871-8.
35Datta H, Choudhuri BR, Datta S. Visual evoked response in different types of amblyopia before and after occlusion therapy. J Indian Med Assoc 1998;96:109-10.
36Clarke SA, Eiser C. The measurement of health-related quality of life (QOL) in pediatric clinical trials: A Systematic Review. Health Qual Life Outcomes 2004;2:66.
37Gottlob I. The detection, prevention and rehabilitation of amblyopia. Curr Opin Ophthal 1999;10:300-4.
38Murthy GV, Gupta SK, Bachani D, Jose R, John N. Current estimates of blindness in India. Br J Ophthalmol 2005;89:257-60.
39Khan SA, Shamanna B, Nuthethi R. Perceived barriers to the provision of low vision services among ophthalmologists in India. Indian J Ophthalmol 2005;53:69-75.