Factors associated with base-in prism treatment outcomes for convergence insufficiency in symptomatic presbyopes.
PURPOSE The aim was to determine whether baseline measurements of binocular vision are associated with symptoms of convergence insufficiency (CI) both before and after CI treatment with base-in prism in symptomatic presbyopes. We further determined the factors that predicted CI treatment outcomes. METHODS A total of 29 symptomatic CI subjects aged 45 to 68 years were examined and the following binocular vision measurements were obtained at the baseline: near heterophoria by cover test, associated phoria at near (AP), near point of convergence (NPC) and positive fusional vergence at near (PFV). Each subject wore each of two pairs of progressive addition spectacles for a three-week period, one with prism and one with the same prescription but no prism. The wear order was randomised across subjects. Subjects completed the CI symptom survey (CISS) before and at the end of the three weeks, thus baseline, placebo and treatment CISS scores were tabulated. The reduction in symptoms (CISS score improvement) was calculated as the difference between the placebo and treatment CISS scores. RESULTS Neither baseline CISS score nor treatment CISS score had statistically significant correlations with any of the binocular vision measurements. CISS score improvement was statistically significantly associated with near heterophoria, AP and NPC break but not with PFV. CISS score improvement was significantly predicted by both near heterophoria and NPC break but not by AP and PFV. With other variables held constant, the CISS score improved by 1.57 for each additional prism dioptre of exophoria at near and by 0.81 for each additional centimetre that the NPC break was receded. CONCLUSION There was no significant association between the CISS score with any of the baseline binocular vision measurements before or after prism treatment. Our findings suggest that a greater near heterophoria and/or more receded NPC before treatment are associated with more reduction in CI symptoms by prism treatment.