Qualitative MRI as a diagnostic tool in strabismus surgery in severe myopia

Background: This study was conducted to elucidate the etiology of the acquired, restrictive motility disorder in patients with severe myopia by magnetic resonance imaging (MRI) and intraoperative situs and to verify existing theories about strabismus to develop appropriate methods of eye muscle surgery. Methods: Thirty-five patients with unilateral or bilateral high-degree myopia and strabismus, i.e., axial length of the globe averaged 29.4 mm. Multiple coronal, transversal and parasagittal MRI planes were obtained using a Siemens Magnetom (SP 63) 1.5 Tesla MRI scanner (TR = 550 ms, TE = 15 ms; field of view = 21 x 21 cm; pixel matrix = 256 x 512; 3 acquisitions; slice thickness 2 mm; distance factor 0.25). In addition, in a dynamic MRI the patient had to fixate with the less restricted eye for 50 s in different gaze positions. All data were validated by measurements during strabismus surgery. Twenty normal orbits were studied in control MRI scans. Results: The major MRI finding was dislocation of the lateral rectus in the anterior and midorbital region at an average of 3.4 mm into the temporocaudal quadrant in 13 cases with the typical eso- and hypotropia. This dislocation will reduce the abducting torque of the lateral rectus and created depressing and extorting torques. Two anatomical explanations are possible: (1) increasing stretch of the lateral rectus because of temporocranial distension of the globe and inability of intermuscular membranes and pulleys to stabilize the path of the lateral rectus; (2) dehiscence of the lateral levator aponeurosis. The restrictive motility disorder was never caused by contact between the enlarged globe and the bones of the orbital apices. Conclusions: Until now, an abnormal path of recti EOMs has been known only in Duane's syndrome. Prior to strabismus surgery in patients with high-degree myopia, an orbital MRI scan may be useful. If misalignment of the lateral rectus is detected,the most important aim of eye muscle surgery is to normalize the pathological path of the lateral rectus. MRI morphometry in severe myopia may give additional information on the anatomy of the orbit and biomechanical mechanisms of strabismus. Our findings demonstrate the necessity of fixation-controlled MRI scans.