Timothy C. Hain, MD, Most recent update: February 19, 2018
Amblyopia is a term for low visual acuity in one eye. The term literally means "dullness of vision". It indicates lack of vision, not complete blindness, but it does not specify the cause. Although vaguely defined, the term is generally used to unilateral reduction of vision found in persons with either strabismus or anisometropia (different refraction in each eye). The reduction of vision is attributed to developmental factors. For example, in strabismus, while the optics of both eyes are identify, "amblyopia" may develop in children who preferentially view from one eye. It is currently felt that the likely site of the reduced visual acuity, in amblyopic eyes with no pathology of the eye itself, is likely the brain. This amblyopia is a neurological problem.
The prevalence (i.e. frequency in the population) of amblyopia is high, and a reasonable estimate is that it ranges between 1-4%. The figures vary according to population and the criteria adopted for how much visual acuity must be reduced.
Classification of Amblyopia Term(s) Mechanism Strabismic Associated with misalignment of eyes Anisometropic Difference in refraction between the two eyes, such as uncorrected hypermetropia or astigmatism Visual deprivation Lack of visual input -- such as from cataract Organic amblyopia A variety of disorders mainly involving the retina
Amblyopia generally is considered when there is a difference of at least 2 lines on a visual acuity chart between the eyes. Ideally, one should use several visual acuity tests.
In strabismic amblyopia, eccentric fixation is common. These patients do not assume central fixation when the fellow eye is covered, but the amblyopic eye remains more or less deviated. The literature suggests that this occurs in roughly 70% of strabismic amblyopes (Van Noorden, 1980). Eccentric fixation is rare in nonstrabismic amblyopes.
Anomalous correspondance is a shift of the subjective visual direction of the nonfixating eye, with respect to those of the fixating eye. Presumably AC is caused by an adaptation of the sensory visual system to the abnormal motor condition of the eyes, in an attempt to restore some binocularity.
Testing for amblyopia:
All testing for the sensory relationship between the two retinae are necessarily subjective tests. There are a myriad of fascinating devices, largely depending on providing different visual input to each retina, or afterimages. The Lancaster red-green test, polarized projection methods are among them.
Westall and Schor (1984, 1985) wrote about adaptation of the vestibular system in persons with amblyopia. They reported that persons stimulated for 2 minutes (this is not very long), measurable asymmetry in the VOR gain could be produced. This suggests that in persons with amblyopia faced with a need to adapt their vestibular system (perhaps new glasses), might develop an asymmetrical (i.e. incorrect) adjustment of their VOR, and develop oscillopsia (illusions of movement with head movement). We would like to see a similar experiment using telescopic lenses, which adapts the VOR, and see whether or not it adapts visual parameters.
Treatment of amblyopic risk factors, such as anisometropic or high refractive error, strabismus, cataract, and ptosis should be managed early (Koo et al, 2017).
Forced viewing from the amblyopic eye, called occlusion therapy, is often advocated for amblyopia. Yazdani et al (2017) suggest that 6 hours is the optimal time. There is a large literature about this that we will not review as it is beyond the scope of this page.
There is a substantial recent literature about using video games, with each eye viewing images separately (Dichoptic viewing). Foss et al (2017) reported that work done with video games as treatment should be considered "exploratory rather than definitive". They also stated that "Dichoptic stimulation is a viable treatment option for the treatment of amblyopia. " Ziak et al (2017), and Vedamurthy et al (2015) have recently reported success using dichoptic games (2017). Wang et al (2016) reported that use of liquid crystal lenses (a method of Dichoptic viewing), is "not inferior" to adhesive patching. There are also several suggestions that this treatment might lead to video game addiction. We hope to see more research on this in the future.
Gore eta al (2016) reviewed medical (drug) treatments of amblyopia. They comment that "treatment of older children and adults is usually unsuccessful due to closure of a window of cortical brain plasticity. " They review " the acetylcholinesterase inhibitor donezepil, serotonin receptor inhibitor fluoxetine, dopamine precursors carbidopa-levodopa, and catecholamine modulator citicoline." It is our thought that perhaps a gaba antagonist, such as flumazenil, might be helpful (none of the drugs above are gaba antagonists).