Asymmetrical vertical phorias indicating dissociated vertical deviation in subjects with normal binocular vision

We measured the symmetry of phoria angles in six normal subjects. Subjects were selected on the basis of good visual acuity and stereopsis, normal binocular eye alignment and, apart from mild refraction errors, absence of ocular abnormalities. They were instructed to look at a word on a reading chart at 2 m distance. Each measurement consisted of five subsequent intervals of 5 s duration. During these five intervals viewing was binocular, with the right eye only, binocular, with the left eye only, and binocular, respectively. Each experiment consisted of twelve measurements. Eye movements were measured with scleral coils suited for measuring in horizontal, vertical and torsional directions. Five out of six subjects displayed an asymmetrical vertical phoria; one subject showed an alternating hyperphoria; four displayed a left over right vertical phoria that was largest for left eye occlusion. Only one subject showed a symmetrical vertical phoria. Both the size of the vertical phorias and the size of the asymmetries in these vertical phorias were very small: on average 0.16 +/- 0.01 and 0.17 +/- 0.01 degree, respectively. The direction of the vertical phoria asymmetries (the largest left over right was found with left eye occlusion) and the fact that asymmetries were found more often in vertical than horizontal and torsional phorias suggest that these asymmetries are related to dissociated vertical deviation. These results suggest that dissociated vertical deviation, often observed in subjects with a disruption of binocular vision early in life, reflects the enhancement of a phenomenon that is present in normal subjects as well.