Crouzon syndrome: relationship of rectus muscle pulley location to pattern strabismus

Abstract

Purpose: Investigate the relationship between the extorsion of the rectus muscle pulleys and the V-pattern exotropia and "overelevation in adduction" observed in Crouzon syndrome.

Methods: Twenty children with Crouzon syndrome had assessment of eye alignment. The horizontal and vertical positions of the four rectus muscle pulleys were estimated from coronal CT images. Eye alignment was simulated in Orbit 1.8 software by shifting the corresponding location of the rectus muscle pulley array.

Results: Eleven of the 20 patients had a V-pattern exotropia with displacements of each rectus muscle pulley ranging from 2 to 7 mm. The remaining nine patients were orthotropic with <2 mm displacement of the rectus muscle pulleys. Simulated displacements (>2 mm) of either the horizontal or vertical rectus muscle pulleys produced a similar strabismus pattern. The amount of V-pattern exotropia observed clinically was highly correlated with the amount predicted by pulley displacements in Orbit 1.8 (r(2) = 0.63; P < 0.0001). The displacement of vertical and horizontal rectus muscle pairs was significantly higher for patients having overelevation in adduction.

Conclusions: Rotation of the four rectus muscle pulleys relative to the corresponding rotation planes of the globe changes the direction and magnitude of their active and passive pulling forces in a gaze-dependent manner. Extorsion of the horizontal and vertical rectus muscle pulleys in Orbit 1.8 reproduces the pattern strabismus observed in Crouzon syndrome. The high correlation between the predicted magnitude of the V-pattern exotropia and observed exotropia indicates that extorsion of the rectus muscle pulleys primarily accounts for the pattern strabismus.

Keywords: Crouzon syndrome; craniofacial disorders; rectus muscle pulleys.