To determine the sensitivity and specificity of several classification rules for stability and instability of angle in childhood esotropia.
We conducted 10 000 Monte Carlo simulations of participants with no actual change in angle of esotropia during follow-up, where “observed” changes in ocular alignment were sampled from a distribution of measurement errors for the prism and alternate cover test. Additional simulations were conducted for a range of “true” changes (1.0, 2.5, 4.2, 5.0, 7.5, and 10.0 prism diopters [PD] per visit) with up to 10 follow-up visits. We then estimated sensitivities and specificities for specific rules for retrospectively classifying stability (all measurements within 0, 5, 10, or 15 PD) and instability (≥2 measurements differing by ≥10 PD, etc) across a fixed number of visits. Results were extended to classifying ocular alignment stability and instability prospectively based on a varying number of measurements.
For a series of 4 measurements, the rules that optimized sensitivity and specificity were “all measurements within 5 PD” for stability and “at least 2 measurements differing by 15 PD or more” for instability. For a series of 3 measurements, all 3 measurements needed to be identical to confirm stability.
We derived definitions of stability and instability in childhood esotropia using estimates of actual measurement error that may be useful for clinical practice and for future clinical studies of esotropia.