Displacement of the medial rectus pulley in superior oblique palsy.

PURPOSE The rectus extraocular muscles pass through fibromuscular connective tissue pulleys that stabilize muscle paths and control the direction of muscle pull. The authors investigated whether abnormal forces associated with superior oblique palsy can cause displacement of pulleys and muscle paths. METHODS Coronal magnetic resonance imaging (MRI) showing significantly reduced superior oblique cross-sectional areas and lack of contractile changes with vertical gaze confirms that seven subjects had superior oblique palsies. Binocular misalignment was quantified using the Hess test. In those seven subjects with palsies and in 18 normal orbits, coronal MRI scans corrected to standardized head position were analyzed digitally to determine muscle paths in primary gaze. Horizontal and vertical coordinates of the pulleys, known histologically to lie just posterior to the equator in primary gaze, were inferred from these muscle paths. RESULTS Normal pulley coordinates were highly uniform. Compared with both normal orbits and fellow orbits, orbits with superior oblique palsies showed a statistically significant 1.1 mm superior displacement of the medial rectus pulley. No other pulley was displaced significantly from normal. Computer simulation using a biomechanical model of ocular statics showed that, in each case, the pulley position shifts alone were insufficient to reproduce the clinical pattern of strabismus. CONCLUSIONS The excyclotorsion of the globe that accompanies superior oblique palsy does not systematically displace the pulleys of all the rectus muscles. The only significant rectus muscle path change is for the medial rectus muscle, and it may arise as a mechanical consequence of the atrophy of the adjacent superior oblique muscle belly. Biomechanical modeling suggests that this displacement of the medial rectus pulley alone does not account for the pattern of strabismus observed in superior oblique palsy.