Magnetic Resonance Imaging Study Of Extraocular Muscles And Ocular Motor Nerves In Normal Volunteers And Noncomitant Strabismus Patients

Objective:To study the the structural basis and anatomic changes of extraocular muscles and ocular motor nerves in the orbits and brainstem in normal subjects and humans with noncomitant strabismus by high-resolution MRI.Objects and methods:Sixty eight normal subjects and forty patients with noncomitant strabismus were performed high resolution MRI across the orbit and the brain-stem level. MR sequences included:EOMs and ocular motor nerves in the orbits were imaged with T2WI coronal and oblique-coronal planes. The ocular motor nerves at the brainstem were performed using 3D T2-SPACE and 3D-T1 MP RAGE. Ocular motor nerves in the orbits were distinguished to the EOMs in consistently oblique coronal planes and nerves in the brainstem were observed in oblique axial, oblique sagittal and coronal planes acquired with MPR. To compare the value of the two sequences in showing the 3 pairs ocular motor nerves in the brainstem. The vertical and horizontal diameters of EOMs in the maximum planes, CN3 and CN6 in the brainstem were measured on the oblique axial planes.Results:The EOMs of normal Chinese were symmetrical on the two sides, and following a certain rule. The relationships of the short diameters of EOMs in the maximum planes were:IR>MR>SR>LR. That of the long diameters of EOMs were: LR>MR>SR>IR. SPACE sequence had more advantage in showing the ocular motor nerves and adjacent structures than MP RAGE(χ2=109.109, P=0.000). All of 136 CN3 and CN6 of the cisternal segment were well demonstrable in 68 normal subjects (100%). CN4 were depicted 87%. The larger inferior division of CN3 and branches to the target EOMs and CN6 were demonstrated well in all species. The superior division of CN3 was demonstrated 65%, CN4 was showed 47%. The diameters of CN3 and CN4 in the cistern were 2.2mm,1.3mm. Patients with type I DRS (9 cases. 13eyes) exhibited absence of CN6 in both orbit and brainstem regions, normal of CN3. there were no EOMs hypoplasia or LR and SO mild hypoplasia, Apparent branches to the LR were shown in the orbits, which was suggestive of aberrant innervation of LR by CN3. Patients with typeⅡDRS(9 cases,12eyes), CN6 in the affected side was normal, CN3 was normal or mild hypoplasia, the inferior branch of CN3 was close to the LR, the LR had dual-innervation of CN3 and CN6, there were MR normal, LR hypertrophy. Patients with CFEOM (6cases,12eyes), MRI demonstrated severe hypoplasia of CN3, normal or mild hypoplasia of CN6, moderate hypoplasia of the optic nerves, and all EOMs hypoplasia, particularly severe for the SR, LPS and LR. Patients with congenital or chronic CN3 (4 cases, 5eyes) and CN6 (7 cases,11 eyes) palsies had hypoplastic CN3s, CN6s in skull base, with hypoplasia of involved EOMs. Patients with CN4 (2 cases,4eyes) exhibited atrophy or hypoplasia of involved EOMs. Patients with V-exotropia (7 cases,14eyes) had normal ocular motor nerves, exhibited hypertrophy of 10 and atrophy of SO and SR. Patients with DVD (2 cases,4eyes) had normal ocular motor nerves, exhibited hypertrophy of 10. Patients with restrictive strabismus exhibited atrophy of IR, incarceration due to fracture of orbit inferior wall.Conclusions:Sizes of EOMs can be assessed by the symmetry on the two sides and by observing the usual rule of EOMs. SPACE sequence combined with MPR could precisely show the ocular motor nerves, as well as the relationship with adjacent structures in the cistern. MRI can help to the diagnosis and differential diagnosis of noncomitant strabismus, the disinnervation or displasia of the ocular motor nerves is one of the possible mechanism of strabismus.