Each of the muscles that move the eye in a vertical plane about the transverse equatorial axis also renders a torsional movement about the anterior-posterior axis (Fig. 1). An abnormality of one of these muscles is manifest by a combined cyclovertical phoria or tropia.
The nature of eye movement produced by a cyclovertical muscle contraction depends upon the horizontal position of the eye. In the position of abduction, the vertical recti move the eye in the vertical plane and the obliques, in a torsional plane. In adduction, the vertical recti act torsionally and the obliques, vertically. Both the vertical recti and the obliques deliver a combined vertical and torsional action in the primary position; however, the action of vertical recti is somewhat more vertical whereas the action of the obliques is more torsional. Weakness of a single cyclovertical muscle is characterized therefore by vertical and torsional deviation in the primary
PARKS MM. Isolated Cyclovertical Muscle Palsy. AMA Arch Ophthalmol. 1958;60(6):1027-1035. doi:10.1001/archopht.1958.00940081047008