Abstract
Abstract: :
Purpose: We employed high resolution, multipositional MRI to demonstrate directly the extraocular muscles (EOMs) and associated motor nerves in hereditary DRS. Methods: We enrolled four male and three female affected members of two autosomal dominant DRS pedigrees. The DRS phenotypes in both pedigrees map to the DURS2 locus on chromosome 2 (See adjacent abstract). Coronal T1 weighted MRI scans of the orbits were obtained using surface coils in multiple gaze positions at 1.5 – 2 mm thickness (234 – 312 micron resolution), as well as oblique heavily T2 weighted images in the plane of the cranial nerves at 1 mm thickness (390 micron resolution). MRI findings were correlated with motility examinations, and compared with similar images in normal volunteers. Results: All subjects had unilateral or bilateral limitation of abduction, or of both ab– and adduction, with fissure narrowing and retraction in adduction. Orbital motor nerves were typically small, with the abducens nerve uniformly absent or below resolution ipsilateral to abduction deficiency. The lateral rectus (LR) muscle was structurally abnormal (hypoplastic, split, or abnormal bright T1 signal) in 6 cases. In affected orbits, a branch of the oculomotor nerve closely approximated the LR muscle. Corresponding A and V patterns of strabismus in 3 cases, and contractile LR thickening with vertical gaze effort, suggested that the LR was innervated by motor nerves normally supplying vertical rectus EOMs. Four cases had superior oblique, 2 cases superior rectus, and one case had levator EOM hypoplasia. Only the medial and inferior rectus, and inferior oblique EOMs, were spared. Two cases had small oculomotor nerves. Conclusions:DRS linked to the DURS2 locus is associated with bilateral abnormalities of any of the orbital motor nerves, as well as structural abnormalities of all EOMs except those innervated by the inferior division of the oculomotor nerve. The LR may be innervated or co–innervated by oculomotor nerve branches normally destined for any of the other rectus EOMs. Therefore, DURS2–linked DRS is a diffuse congenital cranial dysgenesis disorder not limited to the abducens nerve.
Keywords: extraocular muscles: structure • strabismus: etiology • ocular motor control