Vertical yoked prism--patient acceptance and postural adjustment.

Vertical yoked prism is sometimes incorporated into multifocal lenses to obtain a thinner lens, or prescribed for oculomotor deficiencies, or occurs if the vertical placement of spectacle lenses before the eyes is inappropriate. Patient acceptance of and postural and perceptual adaptation to such prism were studied. Twenty-three subjects wore spectacles with no vertical prism and also with bilateral vertical yoked prism. The subject population could not significantly differentiate between 2 delta base down and 0 delta. There was a nearly unanimous rejection of 4 delta base down. Postural changes were significant during 4 delta wear but not during 2 delta wear. The results suggest that 2 delta may be accepted by most patients, but 4 delta will not.