intorsion (incyclodeviation)

Intorsion is a rotation of the eye around its anteroposterior axis that turns the upper pole of the cornea nasalward. Rotations around this axis are called cycloductions, and intorsion is also called incycloduction; the opposite rotation, which turns the upper pole templeward, is excycloduction.¹

The superior oblique and superior rectus form the muscle pair that intorts the eye, and the inferior oblique and inferior rectus form the pair that extorts it.² The vertical recti originate at the nasally displaced orbital apex and pull on the globe at a small angle, so the superior rectus adds only a modest intorsional effect.³ An imbalance between the two pairs causes most cyclodeviations, and such deviations almost always accompany a paretic or paralytic cyclovertical muscle problem, particularly one involving the obliques.²

Torsional misalignment is invisible in primary position. The clinician detects it from skewed deviations in other gaze directions or from an incyclorotated or excyclorotated fundus on fundoscopy.³ Incyclotorsion is often attributed to superior oblique overaction, because the torted muscle insertions produce a vertical deviation in adduction that looks like overaction.³