Amblyopia: Classification, Severity and PEDIG Treatment Lessons
Last updated: October 2026
Amblyopia is reduced best-corrected vision in one or both eyes from abnormal visual experience in early childhood, with no structural cause that explains it. This page sets out its classification and what the PEDIG trials changed in treatment.
Classification
| Type | Subtypes and notes |
|---|---|
| Strabismic | Constant unilateral squint (esotropia more than exotropia); with or without anisometropia |
| Anisometropic | Unequal refraction: hyperopic difference of 1.5 D or more, astigmatic 1 D or more, myopic 3 D or more |
| Combined (mixed) | Strabismus plus anisometropia: often the densest |
| Ametropic (isoametropic) | Bilateral high uncorrected refractive error (e.g. hyperopia over about 5 D) |
| Meridional | Uncorrected astigmatism in early life |
| Deprivation (amblyopia ex anopsia) | Media opacity or ptosis covering the pupil: congenital cataract, corneal opacity, haemangioma; the most severe and urgent |
| Occlusion (reverse) amblyopia | Iatrogenic, in the patched eye |
| Organic / idiopathic (historical terms) | Older classifications included subtle organic causes and unexplained cases |
Severity bands used in trials
| Band | PEDIG patching trials | Note |
|---|---|---|
| Moderate | 20/40 to 20/80 (6/12 to 6/24) | The first atropine-versus-patching trial (ATS1) used 20/40 to 20/100 |
| Severe | 20/100 to 20/400 (6/30 to 6/120) | 6 hours a day of patching is as good as full-time |
What PEDIG changed
| Finding | Practice point |
|---|---|
| Glasses alone improve vision | Prescribe full correction and wait up to 18 weeks (refractive adaptation) before adding patching |
| 2 hours a day of patching works as well as 6 for moderate amblyopia | Start with 2 hours; 6 hours for severe amblyopia (as good as full-time) |
| Atropine works as well as patching | Weekend atropine is as effective as daily; a true alternative, not a fallback |
| Escalating treatment helps | If vision plateaus on 2 hours, increasing to 6 hours gives more improvement |
| Older children respond | Treat untreated children up to 12, and some up to 17; response falls with age |
| Binocular (dichoptic) treatment | Digital dichoptic therapy is an option, approved in some countries; adherence matters |
Where residents slip
- Starting patching on the day glasses are prescribed: give refractive adaptation time first.
- Defaulting to full-time patching in moderate amblyopia.
- Refusing treatment because the child is "too old".
Key sources
- Pediatric Eye Disease Investigator Group. A randomized trial of patching regimens for treatment of moderate amblyopia in children. Arch Ophthalmol. 2003;121:603-611
- PEDIG. Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol. 2005;123:437-447
- PEDIG. A randomized trial comparing atropine and patching for moderate amblyopia (ATS1). Arch Ophthalmol. 2002;120:268-278
Related: Amblyopia treatment updates, Amblyopia in the classifications page, Glasses in children.
Revise with flip cards
📘 Want the exam-ready deep dive? This topic, and much more is covered in Ophthalmology Explorer (A High-Yield Clinical Reference for Ophthalmology Residents, Fellows & Board Exams) — available on Kindle. See all my books.
Want to know more about treatment? Read about Squint Treatment at Sadbhaav or book a consultation with Dr. Dhaval Patel.