Sheard postulated that symptoms from heterophoria can be avoided if the fusional reserves in the opposite direction are at least twice the size of the phoria itself; this is known as “Sheard’s criterion”. Although later studies have provided supporting evidence for this postulate, dichotomous criteria of this type are subject to a lack of sensitivity since small differences in measurements can result in a change in classification. The main purpose of this study was to provide a reliable continuous alternative measure to Sheard’s criterion in children; we have called this “Fusional Stamina”. Heterophorias and positive fusional vergence (PFV) were measured in a group of 82 typical children aged 5–11 years. The suggested new variable, fusional stamina, was calculated by dividing PFV by two and then subtracting the (hetero-) phoria. Repeatability at near was checked after one year for a subgroup of 40 children assumed to be at greater risk for binocular vision deficits. Mean fusional stamina was 8.1 PD (Prism Dioptres base out) (SD = ±5.9 PD) for distance and 12.3 PD (SD = ±8.2 PD) for near. Repeatability at near was r = 0.62, p < 0.001, compared to r = 0.38, p = 0.02 for PFV break-value. Five children passed Sheard’s criterion on only one of two repeated tests, but showed low fusional stamina on both tests. Normative values of fusional stamina for a group of typical children are presented. Repeatability for near is high and better than for PFV. Results demonstrate that there is a risk of missing binocular problems based only on passing Sheard’s criterion. The continuous alternative of fusional stamina can be useful for researchers as well as clinicians in quantifying binocular vision problems and to monitor the effects of treatment.

SIGNIFICANCEThe development of visual acuity has often been looked upon as a function of age. This study considers whether cognition might also be a predictor of acuity in children. The results indicate that cognition is a predictor of acuity and therefore should play a role in vision evaluations and developmental research.PURPOSEPrior studies have shown that changes in visual acuity in typically developing children occur beyond primary school age. However, these studies almost exclusively use chronological age as the sole predictor for visual development. Because many of the tasks used to measure acuity have a cognitive demand, it is possible that age is not the best predictor for changes in this function. The aims of this study were to explore the effect of cognition on the development of visual acuity and to compare this predictor with age.METHODSThe predictive ability of chronological age and cognition on acuity was assessed in a group of 81 typical children between 5 and 11 years old.RESULTSAnalysis of resulting trajectories showed that, although age indeed was a good predictor, development of visual acuity was equally well predicted by cognition. Moreover, partial correlations showed a strong correlation between cognition and acuity when controlling for age but no significant correlation between age and acuity when controlling for cognition.CONCLUSIONSThese results suggest that age alone is not the optimal determinant for the development of visual acuity in typical school-aged children, as cognition was also found to be an important predictor.