Published studies of amblyopia include only patients with visual acuity (VA) worse than 20/40 in one or both eyes. The purpose of this study is to evaluate patients diagnosed and treated as amblyopic despite not meeting traditional VA criteria for amblyopia.

Retrospective clinical cohort study.

Setting: Institutional practice. Patient population: All patients diagnosed with amblyopia at Boston Children's Hospital between 2010 and 2014. Inclusion criteria: VA better than 20/40 but not correctable to 20/20 in one or both eyes; age 2 to 12 years. Observations: Demographics, VA, baseline characteristics. Outcome measures: Resolution, defined as VA 20/20 in both eyes; stereopsis at the last follow-up.

Of 2311 patients reviewed, 464 (20.1%) had subthreshold amblyopia. A majority (61.7%) had an amblyogenic factor, most commonly anisometropia (32.8%). Patients were followed for a median of 3.1 years; nearly all (97.5%) were treated. Of 318 patients who returned for follow-up, 47.8% achieved resolution, including 55.7% of treatment-naïve patients, and 62.5% (5 of 8 patients) offered observation alone. Median stereopsis improved by 0.4 log units in those who achieved resolution, with no change in those with persistent amblyopia. In the multivariate analysis, a longer length of follow-up was significantly associated with resolution of subthreshold amblyopia (odds ratio: 1.38; 95% confidence interval: 1.22-1.57, P < .001).

Patients with subthreshold amblyopia represent a sizeable cohort in real-world amblyopia practice. When offered treatment, half achieved 20/20 vision in both eyes with improved stereopsis as well. Further studies are needed to assess whether observation alone would result in similar outcomes.