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Summary Table

Category	Details
Risk Factors	Children with asymmetric ocular growth, unilateral congenital cataract, monocular aphakia or pseudophakia, post-refractive surgery, ocular trauma, high myopia or hyperopia in one eye, family history of refractive error
Etiology	Unequal axial length between the two eyes (most common), asymmetric corneal curvature, lenticular power differences, acquired causes such as cataract or post-surgical change
Patient Presentation	Adults: asthenopia, frontal headache after prolonged reading, blurred vision, intermittent diplopia after new glasses. Children: often asymptomatic, referred from failed school vision screening, squinting, head tilt, or visible strabismus
Classic Physical Exam	Refractive difference of $\geq 1.00$ D between the two eyes, reduced best-corrected visual acuity in the more ametropic eye, possible esotropia or exotropia, reduced stereoacuity, normal anterior and posterior segment unless there is an underlying cause
Key Diagnostic Results	Cycloplegic refraction documents unequal refractive error, Snellen chart shows acuity disparity, stereoacuity testing (Titmus, TNO) is reduced or absent, cover test may reveal heterotropia
Management	Optical correction is first line. Spectacles for low degrees, contact lenses for moderate to high degrees to reduce aniseikonia, occlusion or atropine penalization therapy for amblyopia in children, refractive surgery in selected adults, treat underlying cause (e.g., cataract extraction)
Keywords	"Unequal refractive error", "one eye much weaker than the other", "child failed vision screening", "aniseikonia", "image size mismatch", "monocular amblyopia with normal fellow eye"

1. Pathophysiology

Anisometropia is defined as a difference in refractive power between the two eyes of at least 1.00 diopter. Although a small difference of less than 1.00 D is present in most of the population and is physiologically tolerated, clinically meaningful anisometropia begins when the brain can no longer fuse the two retinal images comfortably.

The underlying mechanism almost always traces back to asymmetric emmetropization. During the first years of life the eye actively grows toward emmetropia through coordinated changes in axial length, corneal curvature, and lens power. When one eye fails to keep pace, the result is a power difference between the two eyes. Each millimeter of axial length disparity corresponds to roughly 2.5 to 3.0 diopters of refractive difference, which is why even a small anatomical asymmetry produces a visually relevant deficit.

The clinical consequences arise from two phenomena. First, the more ametropic eye projects a chronically blurred retinal image, and during the critical period of visual development (birth to roughly 7 to 8 years) the visual cortex suppresses this input, producing anisometropic amblyopia. This is the classic mechanism of amblyopia in a child whose two eyes look anatomically normal and who has no strabismus. Second, when corrective spectacles are placed in front of unequal eyes, the lenses produce unequal magnification or minification of the retinal images, a state called aniseikonia. Each diopter of spectacle anisometropia generates about 1 to 2 percent aniseikonia, and most patients tolerate up to 5 to 7 percent before fusion breaks down. This explains why an adult given new glasses for newly recognized anisometropia complains of headache, dizziness, and diplopia despite each eye seeing clearly on its own.

A third pathway relevant to children is prismatic imbalance on downgaze, where the two lenses induce unequal vertical prism, contributing to asthenopia during reading.

2. Classification of Anisometropia

Type	Description	Common Setting
Simple anisometropia	One eye emmetropic, the fellow eye ametropic	Mild congenital asymmetry
Compound anisometropia	Both eyes have the same type of refractive error (both myopic or both hyperopic) but unequal in magnitude	Most common pattern
Mixed anisometropia (antimetropia)	One eye myopic and the fellow eye hyperopic	Highest risk of amblyopia and aniseikonia
Simple astigmatic anisometropia	One eye emmetropic, the other has only astigmatism	Usually mild
Compound astigmatic anisometropia	Both eyes astigmatic but to different degrees	Often missed without cycloplegic refraction
By severity	Low: $1.00$ to $2.00$ D. Moderate: $2.00$ to $6.00$ D. High: $> 6.00$ D	Severity guides choice of correction
Anisomyopia / Anisohyperopia / Anisoastigmatism	Subtype based on dominant error	Hyperopic anisometropia of $\geq 1.00$ D and myopic anisometropia of $\geq 3.00$ D in children carry the highest amblyogenic risk

Hyperopic anisometropia is considered far more amblyogenic than myopic anisometropia at any given degree, because the more hyperopic eye is blurred at all distances, while a myopic eye still sees clearly at near.

3. Diagnostic Workup

Test	Role	Finding
Visual acuity (uncorrected and best-corrected)	Initial screening	Disparity in acuity between eyes
Cycloplegic refraction	Gold standard for diagnosis	Documents true refractive difference of $\geq 1.00$ D between eyes
Cover and cover-uncover test	Detects associated strabismus	Heterotropia or heterophoria
Stereoacuity testing (Titmus, TNO, Lang)	Functional impact	Reduced or absent stereopsis
Slit lamp and fundus examination	Identify underlying cause	Cataract, corneal scar, retinal pathology
Ocular biometry (axial length, keratometry)	Anatomical correlation	Asymmetric axial length or corneal power
Aniseikonia testing (New Aniseikonia Test)	When patient is symptomatic with glasses	Quantifies image size disparity

The workup begins with uncorrected and best-corrected visual acuity in each eye separately. A difference of two or more Snellen lines, or any failure to reach age-appropriate acuity in the more ametropic eye, is a red flag.

The single most important investigation is cycloplegic refraction, performed with cyclopentolate 1 percent or atropine in young children. This is the most accurate test because it eliminates accommodation, which can mask or distort manifest refraction, particularly in hyperopic children who can accommodate to compensate for their refractive error. Manifest refraction alone routinely underestimates hyperopic anisometropia and is therefore inadequate for diagnosis in pediatric cases.

A cover test is mandatory because anisometropia and strabismus frequently coexist and the management changes when both are present. Stereoacuity is the most sensitive functional measure; it is often reduced even when each eye reads well in isolation, because the two retinal images cannot be fused. Slit lamp and dilated fundus examination identify treatable causes such as cataract or corneal pathology, and biometry helps localize whether the asymmetry is axial or refractive in origin. Aniseikonia testing is reserved for adults who are symptomatic with their corrected glasses, to decide whether contact lenses or iseikonic spectacles are needed.

4. Management and Treatment

Step	Intervention	Indication
1	Full optical correction with spectacles	Low anisometropia ($< 3.00$ D), all initial cases
2	Contact lenses (rigid gas permeable or soft)	Moderate to high anisometropia, intolerance to glasses, aniseikonia
3	Amblyopia therapy (occlusion or atropine penalization)	Children with anisometropic amblyopia after refractive correction
4	Refractive surgery (LASIK, PRK, phakic IOL)	Stable adults intolerant of glasses or contacts
5	Treat underlying cause	Cataract extraction with intraocular lens for unilateral cataract
6	Iseikonic spectacle lenses	Persistent aniseikonia despite contacts

The first principle is full optical correction based on cycloplegic refraction. In children, prescribing the full hyperopic correction is essential even if it seems strong, because partial correction perpetuates the amblyogenic blur. Spectacles are the default starting point for low-degree anisometropia (under roughly 3.00 D) because they are cheap, safe, and easy to monitor.

For moderate to high anisometropia, contact lenses are preferred. The reason is optical: a contact lens sits at the corneal plane rather than at the spectacle plane (about 12 mm anterior to the cornea), and at this position it produces almost no aniseikonia. This is the next best step when a patient cannot tolerate the image size mismatch from spectacles. Children as young as infancy can wear contact lenses safely under supervision, and this is standard care after pediatric cataract surgery in the aphakic eye.

In children, optical correction alone is not enough once amblyopia has set in. After a refractive adaptation period of approximately 16 to 18 weeks wearing the new correction (during which up to one third of children show acuity recovery from glasses alone), amblyopia therapy is added if the visual acuity gap persists. The mainstays are part-time occlusion of the better eye with an adhesive patch, typically 2 to 6 hours per day depending on severity, or atropine 1 percent penalization of the better eye one drop daily or on weekends. Both are effective; atropine is favored when compliance with patching is poor. Treatment continues until acuities equalize or plateau, with monitoring every 6 to 12 weeks.

In adults, the goal shifts from amblyopia reversal (the visual cortex has matured) to comfortable binocular function. Refractive surgery such as laser ablation or implantation of a phakic intraocular lens can equalize the eyes optically and is the definitive treatment for the patient who cannot tolerate glasses or contacts. When the cause is an acquired cataract, cataract extraction with an intraocular lens calculated to match the fellow eye restores both clarity and equality of refractive power in a single step.

Contraindications worth noting: refractive surgery is avoided in unstable refraction (under age 18 or until refraction has been stable for at least one year), in keratoconus or thin corneas, in active ocular surface disease, and during pregnancy and lactation due to refractive fluctuation. Atropine penalization is used cautiously in children with light irides or systemic atropine sensitivity.

5. Differential Diagnosis and Distractors

Differential	Why it looks similar	Key Discriminator
Strabismic amblyopia	Both cause monocular reduced vision in a child	Manifest tropia is present from early age, cycloplegic refraction shows similar refraction in both eyes
Deprivation amblyopia	Monocular reduced vision unresponsive to glasses	Visible obstruction such as congenital cataract, ptosis, or corneal opacity blocking the visual axis
Refractive error without anisometropia	Asthenopia and headache after reading	Both eyes have similar refraction; symptoms relieved by ordinary glasses without aniseikonia complaints
Accommodative spasm or convergence insufficiency	Headache and diplopia at near	Refraction is symmetric, near point of convergence is receded, symptoms reproduce on near point testing
Aniseikonia from other causes	Image size mismatch and asthenopia	Refraction is equal between the two eyes, the size disparity arises from retinal pathology such as epiretinal membrane
Unilateral cataract	Reduced vision in one eye	Lens opacity on slit lamp examination, although it can also produce acquired anisometropic myopia ("second sight")
Functional (non-organic) visual loss	Asymmetric acuity with normal examination	Inconsistent responses, normal stereoacuity, no refractive difference on cycloplegic refraction

6. Traps and High-Yield Pearls

The most common error is treating anisometropia as a purely optical problem in a child and missing the amblyopia. A vignette will describe a 6-year-old who failed school vision screening, has normal-appearing eyes, no strabismus, and a hyperopic refractive difference of 2.50 D between the eyes. Students often pick "prescribe glasses" as the final answer, but the complete answer is full cycloplegic correction followed by amblyopia therapy if acuity does not equalize within the refractive adaptation period. The competency being tested is recognizing that anisometropia is the leading cause of amblyopia in a child without strabismus or media opacity.

A second trap is the adult who returns complaining about new glasses. The temptation is to label the patient non-compliant or to weaken the prescription. The correct reasoning is to recognize aniseikonia as the cause of the symptoms and to switch the patient to contact lenses, which neutralize the magnification difference at the corneal plane. Reducing the prescription only perpetuates undercorrection.

A third pearl involves the "second sight" phenomenon, where an elderly patient develops new acquired myopia in one eye due to a nuclear sclerotic cataract and now reads without glasses on that side, producing acquired anisometropia. The lesson is that any sudden refractive shift in one eye in an adult demands a slit lamp examination before a new prescription is dispensed.

The cleanest summary: always confirm with cycloplegic refraction, always correct fully, always check for amblyopia in children, and always think contact lenses when the difference is large or when aniseikonia is symptomatic.

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