Specific subtypes of amblyopia can be organised into two groups: functional and organic. (Olson and Scott, 1997) defined organic amblyopia as visual loss as a result of a subtle abnormality of the optic nerve, macular or associated media, which is untreatable. By contrast, functional amblyopia is a condition which is not the result of pathology or abnormality of the visual pathway and cannot be immediately corrected by refractive correction (Powell and Hatt, 2009). The most common forms include refractive amblyopia which is a result of uncorrected refractive error. Anisometropic, is a difference in refractive error in the two eyes (1.00DS and 1.50DC of astigmatism) as specified by (Singh et al., 2008). Finally, strabismic, which is amblyopia caused by strabismus.
‘’Amblyopia is a frequent cause of lifelong unilateral visual impairment’’ (Attebo et al., 1998). The prevalence of amblyopia is estimated to lie between 1.6-3.6% of the population (Simons, 2005). It is thought that this variation is due to methods used by researchers when measuring VA. Amblyopic subjects perform better when letters are presented in isolation. Hence, the use of single letter optotypes can inflate the VA scores thus underestimating the true prevalence. The use of crowded tests, for example, LogMAR, provide more accurate results as amblyopia is sensitive to the crowding phenomenon (Hiscox et al., 1992). Successful treatment of amblyopic children means that 1.9% will remain amblyopic as adults (Thompson et al., 1991). This small change in prevalence between childhood and adulthood suggests that either few amblyopes actually get treated or that the current treatment available is ineffective. (Maconachie and Gottlob, 2015) claimed that the treatment of amblyopia is not fully understood due to a lack of scientific evidence.
For the purpose of this report, treatment of only functional Amblyopia will be reviewed. Monocular occlusion methods including patching and atropine penalisation will be assessed, along with newer binocular treatment methods and perceptual learning tasks. The primary aim of treatment is to improve VA in the AE and potentially match it to the VA of the good eye (GE), thus restoring binocularity. Hence, when evaluating the efficacy of a treatment, I will look for statistically significant improvements (SS) in VA. The type and severity of amblyopia used in studies are important factors. I will look for selection bias, adverse events and details of researcher methodology.
Neurophysiology of Amblyopia
(Wiesel and Hubel, 1963) conducted experiments to determine the neurophysiological mechanism behind amblyopia. Understanding these mechanisms is of paramount importance when developing treatment strategies. The results show that amblyopia occurs due to anatomical changes in the primary visual cortex (V1). This is the location where information from both eyes is first combined in the brain. Evidence of a reduction in binocularly driven neurones in V1, compared to a non-amblyopic subject was found (Joly and Franko, 2014). Further findings show that the neuronal input from the AE is weak, contributing only low spatial frequency detection and only in monocular sites (Kiorpes and McKee, 1999). Thus, the GE dominates the cortex and the input from the AE is supressed. The key mechanism behind amblyopia is therefore suppressive in nature.
These anatomical changes often termed ‘cortical rewiring’ occur during the developmental stages of life known as the critical period (CP) (Hensch, 2005). During this CP, the brain demonstrates neural plasticity and it is particularly sensitive to the effects of reduced/abnormal binocular stimulation (Harwerth et al., 1986). Hence, visual function is modifiable in this period. It was thought until recently, that all amblyopic subjects must be treated before the CP begins. After this period the ‘’neuronal circuits stabilize’’ and no further change will occur beyond this age (7 years) (Khan, 2015). However, studies have been conducted which prove that visual performance can be improved in amblyopic subjects in their late teens and adulthood for example (Scheiman et al., 2005) and (Li et al., 2008).
Refractive Adaptation
Evidence has proven that in some cases, prescribing a refractive correction alone to an amblyopic subject can significantly improve VA (Maconachie and Gottlob, 2015). This could reduce the need to introduce further treatment methods. (Stewart et al., 2004) conducted a fantastic study (MOTAS), which evaluated the effects of a refractive adaptation period on three types of amblyopes. There was a total of 65 participants in the study, anisometropic (18), strabismic (16) and mixed (31). The mean age was 5.1 years old and these children had never been treated for amblyopia prior to the study. The study showed a mean improvement in logMAR VA of 0.24. This is a SS improvement and shows that the refractive correction is effective at improving acuity. Another interesting finding, is that the study showed that this improvement was independent of the type of amblyopia and age.