Crowding distance beats acuity and crowded acuity in detecting strabismic amblyopia

Aim: Amblyopia (“lazy eye”) is a developmental condition that if untreated, leads to permanent vision loss. Children are screened with crowded acuity tests (letters surrounded by clutter). However, acuity (a size limit) and crowding (a spacing limit) have different developmental time courses and neural substrates. Crowding limits normal peripheral vision and central vision in strabismic amblyopia; in anisometropic amblyopia, blur limits acuity and crowding seems to be normal. Could specific measures of crowding distance during child screening improve detection of strabismic amblyopia in children? Methods: Crowding distance (centre-to-centre optotype spacing in deg) can be measured centrally with a skinny font (Pelli, Waugh, Martelli et al., 2016). We measured crowding distance, isolated Sloan letter acuity and clinical crowded acuity (using Sonksen logMAR Test, SLT, the gold-standard at Cambridge University’s Addenbrooke’s Hospital). For crowding distance and isolated Sloan acuity measures, QUEST estimated threshold in 20 trials. Three approximately age-matched groups participated: healthy controls (n=24; age=7.4±2.0 yrs), anisometropic amblyopes (n=22; age=6.2±1.3yrs) and strabismic amblyopes (n=21; age=7.2±1.6 yrs). Results: SLT acuity (letter size in deg) for control children was 0.07±0.003deg, with anisometropic amblyopic eyes (AAEs) being ×1.9±0.01 worse and strabismic amblyopic eyes (SAEs), ×3.1±0.03 worse. Isolated Sloan acuities were similar to SLT acuities in control eyes (0.07±0.01deg), with AAEs being ×2.1±0.01 worse and SAEs ×3.7±0.07 worse. Crowding distance was 0.09±0.01deg in control eyes and was slightly larger for AAEs (at 0.20±0.02deg or ×2.3±0.02 controls). However, they were significantly larger for SAEs (0.58±0.15deg), so ×6.10±0.15 larger than controls. Interocular differences (IODs) in crowding distance for strabismic amblyopes were also very significantly higher (F(1,64)=20.17; p=0.00003) at 0.40±0.08deg, than IODs in isolated (0.17±0.04deg) or SLT (0.14± 0.02deg) acuities. Conclusion: In children with strabismic amblyopia, crowding distance reveals significantly enhanced interocular differences compared to isolated, or crowded acuities, so could improve screening outcomes.