Ophthalmic prisms. Deviant behavior at near.

Unpredictability in strabismus surgery results may be due in part to errors in the measurement of strabismic deviations. Whenever measuring strabismic deviations with a fixation target at near, the distance from the eye to the prism must be taken into account to measure the true deviation. For example, when a prism is held 4 cm from the cornea of an eye deviating 50 prism diopters from a fixation target at 33 cm, the measured deviation is 62 prism diopters. The required prism power needed to neutralize a deviation with fixation at near will be increased, the further the prism is held from the cornea. This may lead to surgical overcorrections if the surgery is based on the near measurement. The deviation neutralized by a prism held in any angular position, the effect of prism measurements at near, and the measurement of deviations over Fresnel prisms can be calculated and the appropriate corrections made.