Relationship Between Amblyopia and Refractive Errors Focusing on Anisometropia

[Purpose] Unilateral amblyopia is categorized as strabismic amblyopia, anisometropic amblyopia, and deprivation amblyopia. Reportedly, refractive error, anisometropia and strabismus are the causes for amblyopia. As indicated in the AAO guidelines for eyeglass prescription to minimize the risk of amblyopia, clinically it is important to recognize amblyopia as a type of refractive errors. However, the complex onset of amblyopia seems to involve the emmetropization process, anisometropia and strabismus. Clinically, many cases of amblyopia are combined with anisometropia or strabismus, and it can be difficult to distinguish amblyopia with anisometropia from amblyopia with strabismus. This study aimed to investigate refractive errors in amblyopia with a focus on anisometropia as well as the characteristics of amblyopia combined with anisometropia or strabismus.<BR>[Subjects and Methods] Subjects were 139 children with amblyopia who visited Okayama University Hospital between January 2008 and March 2012. The subjects were divided into three groups according to the type of their amblyopia: Group A (n=52) with strabismus, Group B (n=45) with combined strabismus and anisometorpia, and Group C (n=39) with anisometropia. Pretreatment refractive errors, anisometropia and visual acuity (VA) were compared among these three groups.<BR>[Results] 1. The amblyopic and fellow eyes had the median spherical equivalent of +3.25 D and +1.62 D and the median cylinder power of 1.00 D and 0.75 D, respectively. The refractive errors of the amblyopic eyes were significantly greater than those of the fellow eyes (P < 0.01). 2. The amblyopic eyes with greater refractive errors also showed severer anisometropia. 3. The respective median refractive errors of the fellow eyes in Group A, B, and C were +1.88 D, +1.62 D, and +1.38 D. No significant differences were observed among these groups. The respective median refractive errors of the amblyopic eyes in Group A, B, and C were +2.12 D, +4.75 D and +3.62 D. Group B had a significantly greater median refractive error than Group C (P < 0.01). 4. The amblyopic eyes in Group A, B, and C had VA (logMAR)of 0.43, 0.70 and 0.30, respectively. Group B had significantly lower VA than Group A and C (P < 0.05, P < 0.01). Furthermore, anisometropia significantly correlated with VA in Group B and C (P < 0.01). <BR>[Conclusion] Amblyopia was strongly associated with refractive errors, astigmatism, anisometropia and particularly with hypermetropia. In amblyopic eyes with combined anisometropia and strabismus, the more hyperopic the amblyopic eye was, the more likely the eye would suffer from severe amblyopia.