Timothy C. Hain, MD, Most recent update: October 7, 2017
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:
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.
Associations of other eye findings with amblyopia:
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.