Displacement of the rectus muscle pulleys simulating superior oblique palsy

To investigate the structural basis of three cases of apparent superior oblique (SO) palsy caused by extraocular muscle (EOM) pulley heterotopy.

Three subjects were diagnosed as having decompensated idiopathic left SO palsy on the basis of misalignment in diagnostic gaze positions, response to the head tilt test, and results of the Hess screen test. Magnetic resonance imaging of the orbits in coronal planes was used to determine SO muscle size and contractility and to define the rectus EOM pulley locations. Orbit 1.8 computer simulation was performed for each subject by using measured rectus pulley locations. Simulated binocular alignment was compared with the measurements.

The maximal SO cross sections of both eyes of each subject were similar, and exhibited similar contractile thickening from supraduction to infraduction. The superior rectus muscle pulleys in three eyes exhibited significant temporal displacement, while the lateral rectus muscle pulleys in five eyes and the medial rectus muscle pulleys in two eyes were displaced significantly inferiorly compared with published norms. Simulations based on observed pulley position abnormalities alone predicted measured Hess screen data better than did simulations incorporating SO weakness, either alone or combined with other structural abnormalities.

Heterotopy of the rectus EOM pulleys may be associated with cyclovertical strabismus that simulates SO palsy.