Background and rationale
Amblyopia is a developmental disorder that causes reduced vision and can significantly reduce quality of life (QoL).1 The condition results from abnormal visual experiences in early life due to strabismus (turned eye), anisometropia (unequal refractive error between the eyes), visual deprivation or a combination of these factors.2 3 The abnormal visual experience alters neurodevelopment of visual brain areas, which results in visual impairment. The primary clinical indicator of amblyopia is reduced visual acuity (VA) in the affected eye that cannot be explained by pathology. However, other visual functions such as contrast sensitivity,4 spatial localisation,5 fixation stability6 and asymmetries in pursuits7 might also be impaired in individuals with amblyopia. Worldwide, the estimated total number of people with amblyopia was 99.2 million in 2019 and the total number of people with amblyopia is estimated to increase to 175.2 million in 2030.8
Conventional amblyopia treatment modalities for children include occlusion or penalisation of the non-amblyopic fellow eye using an eye patch, atropine or optical penalisation.9 10 Appropriate optical correction alone can also improve vision in children with amblyopia (mean VA improvement of approximately three lines)9 11–13 and is recommended as the first step in treatment.
While effective in children, current treatments are typically not offered to youths or adults because it is assumed that no substantial improvement in visual functions are possible beyond the ‘critical period’ of visual development.3 14 15 Clinicians often consider the upper age limit for successful treatment to be around 9 or 10 years of age, and there is evidence for a slightly reduced response to treatment after the age of 6 or 7.14 16 However, standard amblyopia therapy is still effective in older children and teenagers (up to 17 years old) with amblyopia17 and adults can also benefit from strategies like monocular perceptual learning to improve their visual function (for review, see Levi and Li18). Furthermore, vision in adults with amblyopia can improve with optical correction alone,19 with the BRAVO clinical trial20 reporting a 13% improvement in VA from optical correction alone in their adult participants with amblyopia during a 16-week prerandomisation spectacle adaptation period. Despite this, clinical trial evidence for adult amblyopia treatment is lacking and high-quality evidence is needed to guide clinical decisions for evidence-based practice.
The SPECTRA study aims to assess changes in the best-corrected VA (BCVA) and other monocular and binocular visual functions before and after 24 weeks of wearing appropriate optical treatment in adults with amblyopia. The SPECTRA study is the first prospective interventional trial of optical treatment of amblyopia in adults, which aims to enhance VA in the amblyopic eye beyond the level achieved through immediate optical correction of refractive error. Optical treatment differs from optical correction in that for optical correction, vision is improved immediately following correction of the refractive error with appropriate lenses. In the case of optical treatment, the improvement in vision requires neuroplastic changes to occur within the visual cortex and is a gradual process that ultimately enables improved processing of visual information from the amblyopic eye, thereby improving vision. Evidence from existing studies12 13 lend support to this premise with data showing optical treatments generally taking effect within the first 16–18 weeks but with some continuing to show improvements up to 45 weeks. These studies, however, were completed in populations of children with amblyopia, which assumes a relatively early (childhood) detection of the condition. If detected late, amblyopia in adults is often left untreated due to the belief that the brain no longer has the capacity to alter cortical processing in response to corrected visual input.21 22 24 weeks was selected on the basis that a previous clinical trial17 observed that this duration was sufficient for 23%–25% of 7–17-year-old patients with mixed treatment history to show a significant improvement in VA following optical correction. The results of this trial will provide valuable evidence on the efficacy of optical correction for the treatment of amblyopia in adults. If successful, this treatment could provide a simple, non-invasive and cost-effective option for improving the vision of adults with amblyopia. The study’s findings may also lead to a better understanding of the neuroplasticity of the adult brain and its potential for improvement in response to changes in sensory input.
This post was originally published on https://bmjopen.bmj.com