Development of crowding in amblyopia depends on how you measure it.

Crowding is extensive in young children and strabismic amblyopia, so clinical assessment is important. Crowding magnitude (logMAR) is calculated from acuities, with closely-separated optotypes thought to enhance amblyopia screening. Test separations are fixed in optotype proportion (FOP), or in arcmin (FAM). We examined impact of test design on crowding development in normal and amblyopic vision. Crowding magnitude (logMAR) was assessed with close-separation FOP and FAM tests. Crowding magnitude (FOP) = “crowded” HOTV-acuity (0.2 optotype-width separation) – “uncrowded” HOTV-acuity (isolated). Crowding magnitude (FAM) = “crowded” Landolt C-acuity (~2.6’ separation) – “uncrowded” Landolt C-acuity (≥35’ separation). Participants for FOP test were N=44 amblyopes and 100 healthy controls: ≤8yr N=21 anisometropic amblyopes (AA), N=20 strabismic amblyopes (SA) and N=59 controls (C). For FAM test there were N=100 amblyopes and 76 controls: ≤8yr N=14 AAs, N=39 SAs and N=36 Cs. Crowding magnitude reduces with age in control eyes (≤8 yr) for FOP (slope±1SD of -0.02±0.01, P<0.05) and FAM (-0.05±0.01, P<0.05) tests. Patterns of development differ in amblyopia. With the FOP test in SA (≤8 yr), crowding magnitudes are parallel (slopes FE -0.01±0.01 and AE -0.01±0.02; magnitudes±1SE are 0.28±0.02 and 0.42±0.03 logMAR, respectively). In AAs, magnitudes are greater (P<0.05) in fellow (0.34±0.03 logMAR) than amblyopic (0.26±0.02 logMAR) eyes but this pattern reverses <5yrs (0.20±0.1 vs 0.34±0.05 logMAR). With the FAM test, in SAs, crowding reduces in fellow eyes (slope -0.06±0.01, P<0.05; 0.17±0.02 logMAR) but arrest-of-development appears for amblyopic eyes (slope=0.0±0.0; 0.31±0.02 logMAR). In AA, magnitudes are greater (P<0.05) in fellow (0.21±0.03 logMAR), than amblyopic (0.12±0.03 logMAR) eyes, but this pattern reverses >8yr (0.04±0.02 vs 0.09±0.03 logMAR). Development of crowding in strabismic and anisometropic amblyopia is different depending on crowded-acuity test used, due to crowding/masking contributions. A crowding-distance test with minimal/no masking may clarify development of crowding in normal and amblyopic vision.