Fusional stamina: An alternative to Sheardâ€™s criterion
Keywords:Fusional stamina, Sheardâ€™s criterion, norms, repeatability, children
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 â€œSheardâ€™s criterionâ€. 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 Sheardâ€™s criterion in children; we have called this â€œFusional Staminaâ€. Heterophorias and positive fusional vergence (PFV) were measured in a group of 82 typical children aged 5â€“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 = Â±5.9 PD) for distance and 12.3 PD (SD = Â±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 Sheardâ€™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 Sheardâ€™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.
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