It is convenient for a student to divide amblyopia into "mild, moderate, high" by chart line. This is useful for the dose of occlusion — PEDIG does indeed stratify moderate (approximately 20/40–20/80) and severe (20/100–20/400). But it is not a classification of the disease. Two children with 20/100 may need opposite first actions: one needs a cataract removed this week, another needs glasses and a wait for the optical response, a third needs constant esotropia treated as the source of suppression.
The AAO Amblyopia PPP 2022 (Updated 2024) classifies amblyopia by cause:
- refractive: anisometropic; high bilateral (isoametropic);
- strabismic;
- visual deprivation: media opacities; ptosis;
- occlusion (reverse).
This grid is not an academic preference. It coincides with the order of treatment goals that the PPP sets firmly:
- eliminate the cause of deprivation;
- correct refractive errors that produce blur;
- promote use of the amblyopic eye (occlusion, penalisation, filter, digital methods) so as ideally to equalise acuity.
Swapping these items around is a typical error. Patching an eye that contains a dense cataract means treating the cortex with input that does not exist. Operating on strabismus without dealing with refractive error and amblyopia means changing the cosmetics while leaving the suppression. Prescribing occlusion without giving optics a chance is a separate debate that Module 4 will examine through the RCTs; but even there optics does not disappear from the list — it becomes a question of timing, not of necessity.
Mixed forms are the rule, not the exception. The PPP gives observational proportions for unilateral amblyopia: strabismus is associated in 19–50% of cases, refractive error in 46–79%. A third of infants aged 9–14 months with esotropia and more than a quarter of preschool children with esotropia have amblyopia; so do a third of children with 2 D of anisometropia. The odds of amblyopia in these samples: 4.5 times higher with anisometropia of 1–2 D spherical equivalent compared with less than 0.5 D, and 40 times higher with more than 2 D; with strabismus, 2.7–18 times higher. These are odds ratios from observational studies cited by the PPP, not a proven "threshold beyond which amblyopia is inevitable".
The practical conclusion from mixed forms. In clinic you do not look for a "pure" label at any cost. You look for all active factors: is there a tropia, is there anisometropia, is there an opacity, has treatment created reverse amblyopia. The management is built from the sum, but the PPP priority holds: deprivation before refractive error, refractive error before "making the worse eye work".
One more distinction that students confuse with form. Unilateral vs bilateral is about distribution, not cause. Bilateral refractive amblyopia is isoametropic. Bilateral deprivation is both lenses. Bilateral "strabismic" amblyopia in the strict sense almost never occurs: the cortex selects one fixating channel. If both eyes are "amblyopic" in strabismus, look for refractive error or deprivation as well.
The lesson's boundary. Severity (moderate/severe) will be needed when we get to 2 hours versus 6. First — the cause. The module goes on to examine each form as a separate idea, because mixing them in one paragraph is exactly the kind of summary the course forbids.