Teaching case. A seven-year-old, strabismus never noticed, doing well at school; at the school screening "one eye can't read". Cycloplegia: OD +1.25 D, OS +4.75 D. With correction OS 0.3, OD 1.0. The family asks: "How could we not have seen it?" The answer is unpleasantly simple: the fellow eye was seeing for two, and there was no one to complain.
Anisometropic amblyopia is a unilateral form: different refractive errors make the image on one retina less well focused than on the other. It occurs with and without strabismus. PPP: the mechanism combines the direct effect of blur and interocular competition or inhibition, similar but not necessarily identical to the strabismic kind. The greater the anisometropia or astigmatism, the higher the risk and severity (the PPP refers to the Pediatric Eye Evaluations PPP).
Anisometropia thresholds without strabismus at which the Pediatric Eye Evaluations PPP recommends correction (minimum difference between the eyes):
| <1 year | 1–<2 | 2–<3 | 3–<4 | |
|---|---|---|---|---|
| Myopic anisometropia | ≥4.00 D | ≥3.00 D | ≥3.00 D | ≥2.50 D |
| Hyperopic | ≥2.50 D | ≥2.00 D | ≥1.50 D | ≥1.50 D |
| Astigmatic | ≥2.50 D | ≥2.00 D | ≥2.00 D | ≥1.50 D |
The meaning of the asymmetry of thresholds. Hyperopic anisometropia is more amblyogenic than myopic anisometropia of the same magnitude: accommodation is driven by the fixating (less hyperopic) eye, and the other remains in constant defocus. The myopic eye can at least receive a relatively sharp image at near. That is why a 1.5 D hyperopic difference in a preschool child is already a reason for glasses, whereas a 1.5 D myopic difference is not yet the same conversation. This is not a "law of nature from an RCT"; it is the logic of the input plus the consensus thresholds of the PPP table.
Observational odds ratios worth remembering not as a calculator but as a scale of concern (PPP): 1–2 D of spherical anisometropia — odds of amblyopia 4.5 times higher than with <0.5 D; more than 2 D — 40 times. A third of children with 2 D of anisometropia in the cited studies already have amblyopia. This does not mean that 1 D can always be ignored: the correction table for hyperopia in a preschool child starts at a 1.50 D difference.
The clinical signature of the form.
- Often no manifest strabismus — hence late detection.
- Fixation may be central, the cover test clean; the Brückner test sometimes shows reflex asymmetry because of the refractive error.
- The child does not squint "with one eye" the way a myope squints.
- Optics by itself treats a significant share: in the spectacle phase of ATS5 (PEDIG, 84 children aged 3–<7 years with previously untreated anisometropic amblyopia, follow-up up to 30 weeks), amblyopic-eye acuity improved by more than 2 lines in 77%, and amblyopia resolved in 27% with correction alone. This is a specific population and a specific design, not a promise to every third child to "remove the diagnosis with glasses".
Mixing with strabismus. Anisometropia may be accompanied by a microtropia that is easy to miss. If both are present, the label is "mixed", and the competition is stronger than with pure defocus. Do not argue about the purity of the form — look for both factors.
Boundary. The course does not claim that contact lenses are always better than glasses in anisometropia: aniseikonia matters, but an RCT of "lenses versus glasses as treatment of amblyopia" does not stand among the foundations of the PPP as a mandatory substitution. First, full correction in the form the child will actually wear. Refractive surgery in children is mentioned in the PPP as a possible option in selected situations — not as a standard of this module.