Magnetic Resonance Imaging Of Extraocular Muscles In Convergence And Divergence, And Ocular Motor Nerves

PartⅠ:Magnetic resonance imaging of human extraocularmuscles in convergence and divergenceObjective: The purpose of this study is to analyze magnetic resonance imagingdata of the extraocular muscles in convergence and divergence.Methods: Eight normal subjects, asymmetrical convergence: A total amount of 20~Δbase-out prism was placement in front of the right eye; symmetrical convergence: 10~Δbase-out prism was placement in front of both eyes. Eight normal subjects,asymmetrical divergence: A total amount of 8~Δbase-in prism was placement in frontof the right eye; symmetrical divergence:4~Δbase-in prism was placement in front ofboth eyes. Extraocular muscles (EOMs) paths were evaluated by tri-planar, MRI ofthe orbits during the subject focus to a target at a distance of 5m from the eye for MRimaging in convergence or divergence.Results: In asymmetrical convergence, the converging eye lateral rectus (LR)muscles exhibited significant relaxational thinning, while the other EOMs did notshowed significant contractile changes; the converging eye rotated nasally11.03±1.44°, the aligned eye rotated nasally 1.40±0.46°, and the converging eyeshowed 1.0°intorsion of the rectus pulley array. In symmetrical convergence, the eyerotated nasally 5.26±1.15°; there were no significant contractile changes in the crosssections of the EOMs. In symmetrical divergence, the eye rotated negligibly; therewere no significant contractile changes in the cross sections of the EOMs. Inasymmetrical convergence, the diverging eye rotated temporally 2.31±0.97°, thealigned eye rotated negligibly; neither the diverging eye nor the aligned eye showedsignificant contractile changes.Conclusion: Signficant relaxational thinning of the convergings eye lateral rectusconfirmed the sensitivityof the MRI technigue. The signficant changes of the eyerotation associated with the asymmetrical convergence and symmetrical convergence. Asymmetrical divergence, the diverging eye had a significant temporal rotated.PartⅡ: Magnetic resonance imaging of the ocular motornerves in normal volunteersObjective: To display the detail functional anatomy and relationships withadjacent stractures of ocular motor nerves in the orbits, cavernous sinus and brainstemby high-resolution MRI.Methods: Twenty normal subjects between 24 and 38 years of age, mean age was28.25±4.07 (SD). All subjects had no history of ocular diseases. Imging of the ocularmotor nerves at the brainstem was performed in 0.8-mm thickness image planes usingthe heavily T2-weighted FIESTA sequence; nerves in the cavernous sinus was imagedwith a head coil, the FSE/T2WI was obtained in sagittal and coronal planes; nerves toextraocular muscles (EOMs) in the orbits were imaged with T1 weightingoblique-coronal planes using surface coils, and within 2.0-mm thick planes. A humanorbit was celloidin embedding and serially sectioned at 0.1mm of thickness for HEstain, which would be used to distinguish motor nerves to the EOMs.Results: Oculomotor nerves (CN3) and abducens (CN6) of the cisternal segmentwere well demonstrable in all normal subjects. Trochlear nerves (CN4) were depicted95%. CN3 could be demonstrated consistently in the cavernous sinus, because of theirsmall diameter, CN4 and CN6 not always easily detectable. The superior division ofoculomotor nerve was demonstrated 60%, which reason might due to little fatty tissuesurrounding and completed neighbor structures, such as: small interval with opticnerve, and adjacent to the superior ophthalmic vein and ophthalmic artery. The largerinferior division of CN3 and branches to the target EOMs were demonstrated well inall species. CN6 innervate the lateral rectus muscle from its intraconal surface wereclearly demonstrated in all species. CN4 coursing along the periosteum of paries superior orbitae and levator palpebrae superioris (LPS), showed only 40%.Conclusion: Scan combined with post-procession using 3D-FIESTA Seguencecould precisely show the ocular motor nerves, as well as the relationships withadjacent stractures. The inferior division of CN3 and the CN6 could be well depictedin the orbits. But of the small sizes and complecated neighboring structures in theorbit, the superior division of the CN3 and the CN4 could not be demonstratedconsistently. However, the capabilities of MRI can be fully used only with thoroughknowledge of the complicated topographic relationships in the corresponding region.PartⅢ: Magnetic resonance imaging of ocular motor nervesand extraocular muscles in patients with special forms ofstrabismusObjective: Technical improvements in MRI now afford the opportunity fordetailed study of the functional anatomy of extraocular muscles (EOMs) and cranialnerves (CNs) in the orbits of living subjects, and CNs can be imaged against thesurrounding cerebrospinal fluid as they exit the brainstem. This prospective study wasconducted to examine the utility of imaging for direct confirmation of lesions inspecial forms of strabismus, and for the purpose of disclosing currently unforeseenpathologic mechanisms and provide evidences of imageology for relative basicresearch.Methods: This prospective study of imaging ongoing since May 2006, a total of 15patients with special forms of strabismus underwent high-resolution orbital andintracalvarium MRI under a prospective protocol. Imging of the ocular motor nervesat the brainstem was performed in 0.6-mm or 0.8-mm thickness image planes usingthe heavily T2-weighted FIESTA sequence; nerves in the cavernous sinus was imagedwith a head coil, the FSE/T2WI was obtained in sagittal and coronal planes; nerves to extraocular muscles (EOMs), the EOMs and their associated connective tissues wereimaged with T1 weighting in triplanar scans by dual-phased coils, and within 2.0-mmthick planes.Results: Patients with congenital fibrosis exhibited severe hypoplasia ofoculomotor (CN3), the abducens nerve and the trochlear nerve were also affected.Multiple EOMs were exhibited variable atrophy and abnormal brightness on T1weighted image, particularly severe in superior rectus and levator muscles. Patientswith Duane syndrome (DS) exhibited absence or hypoplasia of CN6 in both orbit andbrainstem regions, often with misdirection of CN3 to the lateral rectus muscle. Patientwith inverse DS showed dual innervation of the medial rectus (MR). Patients withcongenital absence of EOMs display various abnormal embryonic developmental ofCNs and their targets EOMs. A patient with an A-pattern exotropia associate withsynergistic convergence demonstrated an enlarged branch of CN3 to MR and thedoubtful branch of CN3 to inferior rectus.Conclusion: Direct imaging of cranial nerves and EOMs by MRI not only coulddemonstrate pathology of ocular motor nerves directly, but also provided much moreimportant information about the embryonic developmental relations between the CNsand their targets EOMs than we used to expected. Direct imaging of CNs and EOMsby MRI is feasible and useful in differential diagnosis of complex strabismus.