IN NORMAL eyes with good fusion heterophoria is commoner than orthophoria. When the eyes are dissociated by the cover test, fusion is interrupted for a short time, and the eye under cover will move into the dissociated position. The dissociated position is influenced by the anatomical factors determining the position of the eyes, such as orbital assymmetry, size of eyeballs, interpupillary distance, check ligaments, and insertion of muscles. A good discussion of these factors can be found in "Worth's Squint" by Chavasse.1 The dissociated position is also influenced by innervational factors, such as convergent and divergent impulses and accommodative, vestibular, cerebellar, and other reflexes.
When normal eyes are dissociated for a period of time, as in the Marlow prolonged-occlusion test, vertical deviations are found to be just as prevalent as lateral deviations. Marlow2 was a pioneer in working for a better understanding of the dissociated position of the
URIST MJ. HORIZONTAL SQUINT WITH SECONDARY VERTICAL DEVIATIONS. AMA Arch Ophthalmol. 1951;46(3):245-267. doi:10.1001/archopht.1951.01700020252002