What is Anisometropia
Anisometropia is a difference in refractive error between the two eyes that results in a relatively clear retinal image in one eye and a relatively blurred retinal image in the other eye. 1
Definition and Clinical Significance
Anisometropia represents an asymmetric refractive condition where the optical power differs between the two eyes, creating unequal image clarity on each retina. 1 This condition is distinct from bilateral refractive errors and occurs when parallel light rays entering each eye focus at different points relative to their respective retinas. 1
Prevalence Patterns
- General population: Approximately 20% prevalence for inter-ocular differences of 0.5D or greater in spherical equivalent refraction, decreasing to 2-3% for differences of 3D or above 2
- Age-related patterns: Prevalence is relatively high in newborns, increases during teenage years with myopia onset, and rises most notably in older adults after presbyopia begins 2
- Relationship to ametropia: Anisometropia prevalence is higher in highly ametropic groups, suggesting similar emmetropization failures 2
Clinical Thresholds for Correction
The American Academy of Ophthalmology provides age-specific guidelines for when anisometropia warrants refractive correction (without strabismus): 1
- Age <1 year: Myopia ≥4.00D difference, hyperopia ≥2.50D difference, astigmatism ≥2.50D difference
- Age 1 to <2 years: Myopia ≥3.00D difference, hyperopia ≥2.00D difference, astigmatism ≥2.00D difference
- Age 2 to <3 years: Myopia ≥3.00D difference, hyperopia ≥1.50D difference, astigmatism ≥2.00D difference
- Age 3 to <4 years: Myopia ≥2.50D difference, hyperopia ≥1.50D difference, astigmatism ≥1.50D difference
These thresholds should be lower if the child has strabismus or amblyopia. 1
Association with Amblyopia
- Strong correlation: At least two-thirds of patients with amblyopia have anisometropia, either alone or combined with strabismus 3, 2
- Increased risk: The odds of having amblyopia are 6.5 to 26 times greater when anisometropia is present 1
- Causation complexity: While anisometropia can cause amblyopia (as demonstrated in animal models), amblyopia secondary to deprivation or strabismus can also lead to subsequent anisometropia development 2
- Age-dependent changes: In children younger than 6 years, anisometropia tends to decrease with age, while it increases in older children 4
Optical Consequences and Symptoms
Aniseikonia (difference in perceived image size between eyes) commonly accompanies anisometropia and can arise from both the refractive error itself and its optical correction. 3 Image size differences above 1-3% can produce clinical symptoms including: 3
- Asthenopia (eye strain)
- Headache
- Diplopia in vertical gaze
- Impaired binocular summation and stereopsis (at differences ≥3%)
- Binocular inhibition or suppression (at differences ≥5%)
Management Considerations for Eyeglass Correction
The majority of adults can tolerate up to 3.00D of difference in eyeglass refractive correction between the two eyes, though some individuals may tolerate more. 1 Management strategies include: 1
- Reducing symptomatic aniseikonia by undercorrecting (at the expense of acuity) or modifying lens base curve or thickness to alter relative image size
- Addressing vertical prism-induced diplopia in presbyopic patients wearing bifocals through slabbing-off or slabbing-on the bifocal segment, using dissimilar segment types, or prescribing separate reading glasses
- Considering contact lenses as an alternative, which minimize aniseikonia compared to spectacles 3
Critical Timing for Pediatric Intervention
Visual deprivation during the first 3 postnatal months can produce profound and permanent visual acuity reductions, making early detection and correction of significant anisometropia critical. 5, 6 The American Academy of Ophthalmology emphasizes examining children with amblyopia risk factors (including anisometropia) at the earliest possible age and treating promptly to prevent lifelong visual disability. 5