Purpose: The vergence position of rest (phoria) has been found to vary over time. We asked whether the vergence position of rest is correlated with discomfort on reading.
Methods: We examined 20 subjects who reported strain on prolonged reading ("asthenopes") and 20 subjects who did not ("non-asthenopes"). The eyes were normal in all subjects. Associated phoria (AP) was determined by asking the subjects to align monocular markers in the Mallett Near Vision Unit. In addition, subjects were asked to adjust Risley prisms so that viewing appeared most relaxing while they were looking at fully fusionable pictures (self-selected prism=SSP). Both AP and SSP were determined at near. Each subject participated in 6 sessions. Before each session, the subjects read a text for 30 minutes. Immediately afterwards they indicated on a visual analogue scale the discomfort they had experienced during the 30 minutes.
Results: In asthenopic individuals, AP fluctuated up to 19Δ, SSP up to 20Δ. In non-asthenopic individuals, AP fluctuated up to 17Δ, SSP up to 20Δ. The correlation between AP and SSP was r=0.73 (p<0.001) in the non-asthenopes and r=0.81 (p<0.001) in the asthenopes. Neither AP nor SSP were significantly larger in the asthenopes than in the non-asthenopes. Intra-individually, among the 6 sessions, the magnitude of discomfort was not significantly correlated to AP or SSP, neither in the asthenopes nor in the non-asthenopes. Asthenopes who rated their discomfort higher did not have a larger heterophoria.
Conclusion: The lack of a significant correlation between discomfort and (horizontal) heterophoria (determined as AP or SSP), especially intra-individually among repeated measurements, does not support a causal relationship.
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