Abstract: Objective  To investigate the structural basis of three types of congenital fibrosis of extraocular muscles (CFEOM) by high-resolution magnetic resonance (HR-MR) images and try to provide the etiological clue of CFEOM. Design Retrospective case series. Participants Twenty-one CFEOM patients. Methods Twenty-one CFEOM patients were performed MR examination on 1.5T MR unit (Twin Speed, GE). EOMs and the motor nerve branches in the orbits were displayed on T1 weighted image (2.0-mm thick) in triplanar scans by dual-phased coils. Ocular motor nerves were imaged in the cistern on 3D-FIESTA (0.8-mm thick) by head coils. Ocular motor nerves, nerves to EOMs and EOMs were observed. Main Outcome Measures The developmental condition of the ocular motor nerves and EOMs. Results Different degree of hypoplasia of oculomotor nerve was exhibited in patients with three types of CFEOM, and the abducens nerve and the trochlear nerve were also affected. Nearly every EOM was exhibited variable atrophy and abnormal brightness on T1 weighted image, particularly severe in superior rectus and levator muscles. Conclusion HR-MR imaging can directly demonstrate pathology of ocular motor nerves and affected EOM hypoplasia in CFEOM patients，especially the obvious hypoplasia of the ocular motor nerves which suggest that the neuronal hypoplasia is the etiological factor of CFEOM.

Key words: congenital fibrosis of extraocular muscle, ocular motor nerves, magnetic resonance imaging, hypoplasia