AIM:To observe the structural basis of ocular motility abnormalities in patients with congenital fibrosis of the extraocular muscles type Ⅰ(CFEOM Ⅰ)due to missense mutations in the developmental kinesin KIF21A using high-resolution magnetic resonance imaging(MRI).

METHODS: Totally 11 affected individuals reported KIF21A mutations were correlated with MRI studies demonstrating extraocular muscles(EOMs)size, location, contractility, and innervation. EOMs and the motor nerve in the orbits were imaged with T1 weighted in a triplanar scan using a dual-phased coils with 2.0mm thick. Motor nerves were imaged at the brainstem using head coils and 3D-FIESTA with 0.6-mm thick.

RESULTS: Patients with CFEOM Ⅰ exhibited different degrees of hypoplasia of oculomotor nerve, the abducens nerve and the trochlear nerve were also affected, of which 8 cases of orbital section could see the signal of abnormal nerve dominated by oculomotor nerve to lateral rectus. The both sides of six EOMS in all patients exhibited variable atrophy and abnormal bright internal signal on T1 imaging, particularly severe for the superior rectus and levator muscles.

CONCLUSION: High-resolution MRI can directly demonstrate pathology of motor nerves,affected EOMs, and ‘Pulley' hypoplasia caused by CFEOM Ⅰ due to mutations in KIF21A,and these findings suggest that the neuronal hypoplasia is the etiological factor of CFEOM.