The Effect of Prism Presentation Order on Near Vertical Fusional Vergence Ranges of Normal Young Adults

Fusional vergence ranges (FVR) quantify the oculomotor system's ability to overcome heterophoria, playing a critical role in diagnosis and treatment. This study investigated the effect of prism order on near vertical FVR using the smooth and step methods.

Normal participants were randomly assigned to either the smooth or step testing method and to Base-Up (BU, infravergence) first or Base Down (BD, supravergence) first prism type. After an hour, they crossed over to the alternative testing method and prism-type. The mean of two consecutive measurements of break and recovery values for one eye in each of the 16 experimental conditions were compared using the Friedman test with post-hoc Bonferroni correction, and non-parametric Bland and Altman analysis.

The mean break/recovery values of 27 participants (mean age: 22.5±2.0, range: 20-29, 20 female) when BU or BD were measured first were BU: 4.20±1.15^Δ/3.07±1.04^Δ and BD: 4.20±1.21^Δ/3.00±0.96^Δ for step, and BU: 4.31±0.98^Δ/2.97±0.85^Δ and BD: 4.15±1.14^Δ/2.70±0.87^Δ for smooth methods, respectively. When measured second, they were BU: 3.74±1.02^Δ/2.63±0.93^Δ and BD: 3.85±1.09^Δ/2.64±1.06^Δ for step, BU: 3.91±0.99^Δ/2.73±0.93^Δ and BD: 3.81±1.04^Δ/2.54±1.04^Δ for smooth, respectively. The Friedman test found a significant effect of prism order on break values of the smooth (p<0.001) and step (p<0.02) methods, and recovery values of the step method (p<0.005), but post-hoc tests showed no significant differences. Mean differences were below 0.50^Δ indicating clinical insignificance.

Unlike horizontal FVR, prism order does not affect near vertical FVR using the smooth and step. This simplifies clinical assessment and suggests that horizontal and vertical fusion systems may be treated as separate entities.