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A.J. Toole, G.L. Mitchell, M. Kulp, M. Earley, CITT Group; Symptoms and Their Relationship to Positive Fusional Vergence and the Positive Fusional Vergence/Phoria Ratio: The CITT Pilot Study . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3143.
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Sheard proposed that the magnitude of fusional reserve required for comfortable binocular vision is dependent upon the magnitude of the dissociated phoria. However, normative fusional vergence ranges typically exceed what would be required by Sheard’s criteria and symptoms may occur in persons with normal phorias and restricted fusional vergence ranges. Therefore, it is hypothesized that comfortable binocular vision often requires a minimum quantity of fusional vergence reserve when a small phoria is present and a greater fusional vergence reserve when a higher phoria is present, with the magnitude depending upon the size of the phoria. The purpose of this study is to test this hypothesis.
38 symptomatic children aged 9 to 18 years with convergence insufficiency were enrolled in the Convergence Insufficiency Treatment Trial (CITT) Pilot Study and randomized to 1 of 3 groups (12 weeks of office–based vision therapy/orthoptics (OBVT), placebo OBVT, or pencil push–up therapy). After treatment, symptoms were measured with the Convergence Insufficiency Symptom Survey (CISS). The near phoria (high neutral [HN] and low neutral [LN] and positive fusional vergence (PFV) ranges at near were measured with the alternate cover test and prism bar according to standard protocols. Spearman correlations were calculated between CISS and PFV and between CISS and a PFV/phoria ratio using various phoria cut points.
For HN phorias of ≤7Δ (n=18), symptoms correlated better with PFV than with PFV/HN phoria ratio (R=–0.46, p=0.055 and R=–0.18, p=0.48, respectively); however for HN phorias of >7Δ (n=11), symptoms were significantly correlated with PFV/HN phoria ratio but not with PFV (R=–0.65, p=0.029 and R=–0.49, p=0.13, respectively). Similarly, for LN phorias of ≤4Δ (n=13), symptoms were significantly correlated with PFV but not with PFV/LN phoria ratio (R=–0.69, p=0.009 and R=–0.54, p=0.058, respectively); whereas for LN phorias of >4Δ (n=19), symptoms were significantly correlated with PFV/LN phoria ratio but not with PFV (R=–0.51, p=0.024 and R=–0.38, p=0.11, respectively).
These results support the hypothesis that comfortable binocular vision often requires a minimum quantity of fusional vergence reserve when a normal phoria is present and a greater fusional vergence reserve (with the magnitude depending upon the size of the phoria) when a higher phoria is present. The magnitude of phoria at which this change occurs should be investigated further.
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