vertical offset-setting (of the horizontal recti)

Vertical offset of the horizontal recti is a strabismus procedure that raises or lowers the insertions of the medial and lateral rectus muscles to treat A and V patterns.¹ The surgeon usually combines the offset with the recession or resection that corrects the deviation in primary position.^1,2

The procedure works because the plane of a horizontal rectus muscle changes as the eye looks up or down.¹ In upgaze the muscle aids elevation, in downgaze it aids depression, and its horizontal action decreases in both positions.¹ Raising or lowering the insertion exaggerates this effect.¹ A lowered medial rectus loses more of its horizontal action in downgaze, and a raised lateral rectus loses more of its horizontal action in upgaze.¹ The surgeon therefore moves each insertion toward the gaze where its horizontal action should be weakest.¹ The medial recti move toward the apex of the pattern, and the lateral recti move away from it.¹

A V pattern is treated by lowering the medial recti or raising the lateral recti, and an A pattern by raising the medial recti or lowering the lateral recti.^1,2 A displacement of 5 to 8 mm is usually sufficient, and lesser amounts are rarely effective.¹ The offset suits patterns without oblique muscle dysfunction.^2,3 Vertical transposition can worsen torsion, and that makes the procedure less suitable for a patient with good stereopsis.³