Amblyopia is diagnosed when a patient has a condition known to cause amblyopia and has decreased best-corrected visual acuity that cannot be explained by other diseases of the eye or visual pathways. Vision characteristics alone cannot differentiate amblyopia from other forms of vision loss. The crowding phenomenon, for example, is typical of amblyopia but not pathognomonic or uniformly demonstrable. Subtle afferent pupillary defects may occur in severe amblyopia, but only rarely. Amblyopia sometimes coexists with vision loss that is directly caused by an uncorrectable structural abnormality of the eye such as optic nerve hypoplasia or coloboma. If amblyopia is suspected in such a case, it is appropriate to undertake a trial of amblyopia treatment. Improvement in vision confirms that amblyopia was indeed present.
Multiple assessments of visual acuity are sometimes required to determine the presence and severity of amblyopia. (Assessment of visual acuity is discussed in Chapter 1.) In some cases, the clinician may assume that amblyopia is present and initiate treatment before decreased vision can be demonstrated. For example, if a clinician does not have access to a grating acuity test such as Teller Acuity Cards II, occlusion therapy may be started in a preverbal child in the presence of a high degree of anisometropia or shortly after surgery for a unilateral cataract.
When determining the severity of amblyopia in a young patient, the clinician should remember that both false-positive and false-negative errors may occur with fixation preference testing; a strabismic child may show a strong fixation preference despite having equal visual acuity, whereas an anisometropic child may alternate fixation despite having significant amblyopia. In addition, the young child’s brief attention span frequently results in grating or recognition acuity measurements that fall short of the true limits of acuity; these measurements can mimic those of bilateral amblyopia or mask or falsely suggest a significant interocular difference. Finally, because test-retest variability can be up to a full line of letters in children, it is important for the clinician to evaluate trends when assessing response to treatment.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.