Vertical imbalance induced by prism-ballasted soft toric contact lenses fitted unilaterally.

Fitting a prism-ballasted soft toric contact lens unilaterally may potentially induce a vergence error that the subject cannot compensate. In the present study, a prism-ballasted (1.5Delta base down) soft toric contact lens was fitted in the right eye only of 10 subjects with normal binocular vision. The effect on binocular vision was evaluated by measuring vertical phoria adaptation and vertical fixation disparity adaptation. The results show that the vertical vergence error induced was well compensated and fully adapted to. However, care might be needed when fitting these lenses unilaterally in subjects with vertical phoria-related problems.