In 2003 the Fifty-sixth World Health Assembly (WHA56.26), recognized that 45 million people in the world were blind and that a further 135 million people were visually impaired and acknowledged that 90% of the world’s blind and visually impaired people lived in the poorest countries of the world. It also noted the significant economic impact of this situation on both communities and countries and that most of the causes of blindness are avoidable and that the treatments available are among the most successful and cost-effective of all health interventions. Avoidable blindness and vision loss refer to vision impairment due to conditions that are potentially preventable through the modification of known risk factors, or for which effective treatments exist to restore sight or prevent further vision loss. Amblyopia is an avoidable vision impairment that if detected in the early childhood years can in most cases be effectively treated. Developed countries have the opportunity to adopt early childhood vision screenings that target amblyopia and its risk factors and so that treatment can be instigated. Restoration of visual acuity in an amblyopic eye will mean that future occupational choices are not limited due to inability to meet vision standards, and also reduce the risk of visual impairment later in life.
Amblyopia is the most common cause of reduced vision in children and young people, affecting approximately 3% of the population [1, 2], with significant costs both to the individual and the community for screening and treatment. Clinically, amblyopia is usually defined by a difference between eyes of two or more lines in visual acuity (VA) (logMAR 0.2) . In patients with amblyopia, postnatal development of the visual system is altered because of abnormal visual experience in early childhood. There is no obvious eye or visual pathway pathology underlying the reduced vision, but rather there is some predisposing condition that interferes with the normal development of the visual system. The most common conditions leading to amblyopia are strabismus (which disrupts binocular fusion), refractive error (particularly anisometropia or high hyperopia), or more rarely an obstruction in the optical pathway that reduces retinal image quality (such as congenital cataract). The potential negative impact on visual development is greater when the predisposing condition is present very early in life, and the level of amblyopia is generally more profound for longer duration of abnormal visual experience. While data for the natural history of untreated amblyopia are still scarce, randomised controlled treatment trials , and reviews of patients who have not been compliant with treatment  indicate that amblyopia does not recover without intervention.
Recent randomised treatment trials provide direction for appropriate treatment of amblyopia. Initially, any substantial refractive error needs to be corrected and vision followed up in spectacles until no further improvement in VA is recorded, which can take up to 6 months, before further treatment is considered. Residual VA deficit may warrant treatment that penalises the non-amblyopic eye either by patching or atropine drops, forcing use of the amblyopic eye by the visual system. Prolonged or intense penalisation treatment programs may not be necessary with as little as one to two hours per day patching or twice weekly atropine drops achieving successful visual acuity outcomes .
Patients with amblyopia usually have poor visual acuity in one eye and poor stereo-acuity (depth judgement). Other visual functions can also be reduced including contrast sensitivity, vernier acuity (ability to judge relative position), steadiness of fixation, motion perception and temporal processing . Amblyopic eyes can have decreased accommodative amplitude, increased accommodative lag and inaccurate or unsteady eye movements. While the magnitude of functional disability created by degradation in these various aspects of visual function has not been fully explored, recent studies have examined the impact of amblyopia on fine motor skills, both in children and in adults with impaired binocular vision. Amblyopic children have been found to perform worse than age-matched control children on fine motor tasks, particularly those manual dexterity tasks that require speed and accuracy , demonstrating that amblyopia has a functional impact that goes beyond the monocular VA deficit and loss of binocular function that define the condition. Amblyopia is reported to affect accommodation and fixation stability and deficits in reading performance have also been reported . Future studies that measure accommodation characteristics and fixation stability and how they relate to recorded eye movements may provide insight into the reported deficits in reading speed.
Poor visual acuity in one eye can impact on ability to achieve the minimum visual standard required for some career choices and may prevent qualification for a commercial driver’s licence. Patients with amblyopia are excluded from a wide range of jobs, which increases with the severity of the amblyopia  (Table 1). The ability to appreciate the depth of one object relative to another is often required for efficient and safe job performance. While the monocular cues to depth (e.g. overlay, motion parallax, and atmospheric haze) are often all that are required, when precise judgment of relative depth is essential to job performance there may be justification for a binocular vision (i.e., stereopsis) requirement. Examples of occupations in which stereopsis may be required include crane and forklift operators. A demonstration of a moderate degree of stereopsis (i.e., 80 arc seconds) has been suggested as a binocular vision standard for occupational safety.
Tab. 1: Visual standards required to enter UK occupations (Adams and Karas 1999).
The potential for visual disability due to loss of visual function of the non-amblyopic eye can be calculated and is an argument for amblyopia treatment that aims to maximise visual potential. From a population perspective, studies have now described the association between amblyopia and lifetime risk of vision impairment [11, 12]. The lifetime risk of bilateral vision impairment is nearly double that for amblyopes than for controls; being as high as eighteen percent for those with amblyopia, compared with ten percent in non-amblyopes .
For many years, early childhood vision screening programs have aimed to provide a safety net by identifying children with risk factors for amblyopia while they are still with in the critical period of treatment efficacy (traditionally believed to be up to about 8 years of age). Refractive error and non-cosmetically obvious strabismus have been targeted conditions of vision screenings, with the premise that risks factors of media opacity or large angle strabismus would have been identified by a parent or primary health care provided during infant or early child health assessments.
The two main objectives of early detection and treatment of amblyopia or its predisposing conditions are, firstly, to improve acuity in the affected eye and improving potential for binocular vision and, secondly, to reduce the risk of visual impairment later in life if the non-amblyopic eye loses its function through disease or injury. Debate continues regarding the benefits of community funded early childhood vision screening programs, with the cost-effectiveness of screening found to be dependant on the long-term utility effects of loss of vision in one eye . The benefit of an improvement in visual ability in the amblyopic eye can also be quantified by improved quality of life scores and indicate positive cost-analysis of treatment [14, 15].
The optimal time for visual screening has also been the subject of debate. Findings that early screening for amblyopia risk factors lowers the later prevalence and severity of amblyopic deficits [16, 17] support the call for preschool vision screening, at age 3 to 4 years, rather than screening at school entry when children are generally 4 to 5 years of age. However, Clarke et al.’s (2003) finding that treatment efficacy is not reduced if treatment is deferred, suggests that screening and treatment is not critical at 4 years of age and could be delayed until during the first year of school .
Randomised controlled treatment trials have provided evidence for the development of effective treatment plans for optimal vision outcomes following detection of amblyopia. However, questions still exist regarding whether the condition or its treatment negatively impact on patient quality of life. Penalisation (eye patching) in amblyopia treatment can create negative changes in behaviour and self-esteem in children and has an impact on family life, more so in children with greater level of amblyopia. As the child’s behaviour influences compliance, it is essential to address this issue with the parent supervising treatment when instigating penalisation therapy. Whilst the balance between possible disabilities attributable to the condition versus the treatment has not been established, the potential disability of incapacitating vision loss later in life, due to loss of visual function of the non-amblyopic eye, can be calculated and is an argument for treatment to maximise visual potential in each eye [12, 18].
The studies of the Pediatric Eye Disease Investigator Group (PEDIG) group have provided clinicians with more distinct guidelines with respect to penalisation therapy in amblyopia (Table 2) . The importance of parental education and support to maximise compliance with treatment has now been established and should be part of the treatment plan. Evidence-based guidelines for treatment of amblyopia are available so that clinicians can provide optimal treatment with minimal negative side-effects for the patient .
Tab. 2: Current treatment recommendations (adapted from Holmes and Clarke 2006).
Developed countries could support the World Health Assembly goal of implementing strategies to avoid preventable vision impairment by establishing programs that target childhood amblyopia. In addition to effectively identifying children with amblyopia or its risk factors early in life, vision screening programs need to ensure appropriate referral pathways to treatment that are affordable for both the individual and community. Amblyopia can be, and should be, avoided in vigilant communities.