Background Although neuroradiological findings of M(o)bius syndrome have been reported as a result of brain and brainstem abnormalities,magnetic resonance imaging (MRI) now permits the direct imaging of the cranial nerve (CN) and branches in the orbits.This study presents the MRI findings in patients with sporadic M(o)bius syndrome.Methods Prospectively,CNs were imaged in the cist using head coils and three dimensional fast imaging employing steady-state acquisition (3D-FIESTA),yielding a 0.5 mm2 resolution in planes of 0.8 mm thickness in seven patients with sporadic M(o)bius syndrome.The cavous and intraorbital segment of the CN and the extraocular muscles (EOMs) were imaged with T1 weighting in all patients.The cavous segment was imaged in coronal planes,while the intraorbit in quasicoronal planes were imaged using surface coils.Intraorbital resolution was 0.16 mm2 within 2.0 mm thick planes.Results In the seven patients,the CN were absent or showed hypoplasia in the cist,cavous sinus,and orbit.Abducens (CN Ⅵ) and facial (CN Ⅶ) nerves were absent on the affected sides.Unilateral CN IX (glossopharyngeal nerve) in two cases displayed dysplasia.Branches from the inferior division of CN Ⅲ were observed to innervate the lateral rectus (LR) bilaterally in three cases and unilaterally in one case,and had intimate continuity with the LR muscle in two cases bilaterally and two cases unilaterally.Hypoplasia of EOMs was shown in five cases.Dysplasia of the medulla on the left side was found in one patient.Conclusions Direct imaging of CNs and EOMs by MRI is useful in diagnosis of M(o)bius syndrome.It can directly demonstrate the abnormalities of the CN and orbital structures.The absence or hypoplasia of CN Ⅵ and CN Ⅶ may be the most common radiologic features in sporadic M(o)bius syndrome,and hypoplasia of CN IX may be an associated feature.The abnormality of EOMs and aberrant innervations in the orbit should be observed,and may be important for the study of the etiology.