Fusional stamina: An alternative to Sheard’s criterion

Abstract

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.

Prevalence of heterophoria and its association with near fusional vergence ranges and refractive errors

“…Vergence measures have been suggested to vary with race, ethnicity and age. 19 Specifically for race, the mean findings of heterophoria in the present study are comparable to those from other African populations 10,13 (Table 5) 11,10,12,31,8,24,42,26,14,27,23,21,4,22,15,25,16,20,19 ; the slight differences may be because of those studies 11,13 that enrolled younger schoolchildren. Furthermore, different phoria dissociation methods were used in some studies.…”

“…Aligned to heterophoria and fusional vergences is Sheard's criterion, which is a good indicator of symptoms. 46 The 6.5% failure rate of Sheard's criterion in this study is lower than the 12.5% found by Myklebust et al 42 in Norwegian schoolchildren, while the 4.6 mean convergence ratio found is comparable to 4.45 pd by Lanca and Rowe in Portugal. 23 The findings on the association of phorias and refractive errors from previous studies are diverse, 37,38,39,40,41,42 which may be related to different study designs and criteria applied to define variables.…”

“…5 In addition, phoria and fusional vergence are variables applied to classify vergence anomalies and a guideline to predict if a patient may be symptomatic. 6,7 Studies have investigated the distribution of heterophoria 3,4,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42 and associations with fusional vergences 3,4,8,9,10,11,12,13, 14,15,16,17,18,19,20,21,22 ,23,24,25,26,27,28,29,30,31,32,33,34,35,36 and refractive errors 37,38,39,40,41,42 in various populations. Consistent findings across the studies show that data from heterophoria are not normally distributed; orthophoria is the dominant phoria state at distance while exophoria dominates at near fi...…”

Sensitivity and Specificity of Sheard and Percival’s Criteria for the Diagnosis of Young People with Near-heterophoria

“…Regarding the opposite fusion vergence NFV, the mean difference to blur, break and recovery points was p = 0.025, p = 0.022, and p = 0.026, respectively, which was statistically significant. Myklebust and Riddell reported that there is a risk of missing decompensated heterophoria based only on passing Sheard's criterion and recommended that a continuous fusion persistence alternative may be useful for clinicians in quantifying binocular vision problems and monitoring treatment effects [12]. However, Moon et al [8] stated that Sheard's criterion is a useful tool for screening convergence insufficiency with exophoria associated with near vision activities.…”

“…The second intrasubject method was introduced by Sheard this technique related the heterophoria to its opposing fusional vergence blur point. It is stated that the opposing fusional reserve to the blur point should be at least twice the size of the phoria [1], [11], [12]. Sheard assumed that symptoms from heterophoria (latent strabismus) can be avoided if the fusional reserves in the opposite direction are at least twice the size of the phoria [1], [4], [8].…”