The most common false victory in amblyopia is acuity measured with a single optotype. The child "sees 0.8". In a line with neighbours — 0.3. This is not fatigue. It is crowding: adjacent contours reduce the legibility of a symbol. PPP: in amblyopia, testing with single optotypes is likely to overestimate acuity. An accurate monocular assessment is a line of optotypes; if a single symbol is unavoidable, it is surrounded by crowding bars on all four sides.
How to measure so that numbers are comparable between visits.
- High-contrast black symbols on a white background, standardised conditions.
- Monocularly, with best correction. For the fellow eye, adhesive patch or tape is best; otherwise the child peeks. With nystagmus, including latent nystagmus, opaque occlusion increases the nystagmus: the PPP recommends fogging the fellow eye with a plus lens or a translucent occluder, and additionally recording binocular acuity as a picture of "everyday life".
- First make sure the child knows how to do the test: presentation at near, a matching card for shy children and those with cognitive difficulties.
- A line of five optotypes when possible. To credit a line, most of the symbols must be named correctly. Pointer — show and immediately remove, without covering the neighbours: otherwise you have eliminated the crowding, which is precisely the amblyopic sign.
Which optotypes the PPP considers suitable.
- Preschool children: LEA SYMBOLS — four symbols that blur similarly; HOTV — four letters. Both are easier because the set of responses is closed. The child can point on a card instead of naming.
- Older children: Sloan letters in a logMAR progression with proportional spacing, as in ETDRS. The PPP considers Snellen less desirable: the design is often not standardised, the letters are of unequal legibility, and the spacing does not always meet WHO requirements.
- Do not rely on Allen pictures, Lighthouse symbols or the Kindergarten (Sailboat) chart: the symbols are often not standardised, are presented one at a time, and some are culturally biased. The Tumbling E is conceptually difficult, with a high proportion of "untestable".
Teller Acuity Cards (forced preferential looking) give grating acuity in infants compared with norms. PPP: in amblyopia this method overestimates acuity relative to recognition acuity. Qualitative fixation ("central, steady, maintained", resistance to occlusion of the fellow eye) is a surrogate until recognition testing is possible; as the only screening test for amblyopia, fixation preference tests are inaccurate.
Age-based diagnostic criteria, Table 1 of the Amblyopia PPP. An amblyogenic factor plus a deficit is required.
Unilateral:
- asymmetric resistance to monocular occlusion;
- fixation preference: does not initiate / does not maintain fixation, strong preference for one eye;
- preferential looking: interocular difference of ≥2 octaves (for the full Teller set this is 4 cards);
- BCVA: interocular difference of two or more lines, with the better eye within the normal range.
Bilateral, BCVA of each eye:
- 3–<4 years: worse than 20/50 in both;
- 4–<5 years: worse than 20/40 in both;
- ≥5 years: worse than 20/30 in both.
Asymmetry is permitted in the bilateral form.
The ATS visual acuity testing protocol (Holmes et al., Arch Ophthalmol 2001) was created precisely so that preschool children in multicentre amblyopia trials would be tested in the same way: HOTV, logMAR, crowded (an isolated symbol with surround), a reproducible algorithm. This is not "the only correct chart in the world". It is an example of why, without a protocol, lines from different clinics cannot be added together into "a trend".
Distance and near. PPP: routinely both distance (3–6 m) and near (35–40 cm). Refractive and deprivation pictures sit differently at these two distances; "distance only" in a preschool child with hyperopia can easily mislead.
Boundary. The course does not teach projector calibration and does not review all commercial charts. It teaches one thing: in amblyopia, acuity without crowding is not the acuity on which decisions are made.