Detailed MR imaging anatomy of the abducent nerve: evagination of CSF into Dorello canal

Abstract

Background and purpose: The abducent nerve is difficult to identify reliably and consistently with conventional radiologic techniques. In this study, a 3D fast asymmetrical spin-echo MR imaging technique was used to obtain detailed images of the abducent nerve in normal volunteers.

Methods: The 3D fast asymmetrical spin-echo MR protocol was used to image the abducent nerves in 24 normal volunteers by using a 1-mm section thickness in the tilted axial and parasagittal planes. The microanatomy of the abducent nerve within Dorello's canal was also demonstrated in a cadaver study.

Results: In 24 normal volunteers, the anatomy of 47 abducent nerves was clearly depicted on MR images. The length of the cisternal segment of the abducent nerve, extending from the brain stem to its dural foramina, ranged from 6.7 to 19.6 mm (mean, 13.1 mm). The abducent nerves were at an angle of 5 to 90 degrees (mean, 24.5 degrees) to the clivus. CSF evagination was detected in the region of Dorello's canal in 36 (77%) of 47 abducent nerves. The length of CSF evagination varied: 0.9 mm in five nerves, 1.0 to 1.9 mm in 18 nerves, 2.0 to 2.9 mm in eight nerves, and 3.0 mm or more in five nerves. Histologic examination of serial sections of the abducent nerve revealed that the petroclival segment of the nerve was covered by an envelope composed of an arachnoid cell layer.

Conclusion: The course of the abducent nerve was reliably identified using the 3D fast asymmetrical spin-echo MR protocol and a histologically proven arachnoid envelope around the petroclival segment of the nerve was shown as CSF evagination into Dorello's canal by MR imaging.

Fig 1.

MR images of a 42-year-old man were obtained in the parasagittal (A and B) and titled axial (C) planes using a 3D fast asymmetrical spin-echo sequence. Images are displayed in reverse. Axial view images were first examined to identify the cisternal segment of the abducent nerve. Parasagittal (A and B) and tilted axial (C) view images were then obtained parallel to its course. The entire course of the cisternal segment of the abducent nerve could be identified. The nerves on both sides were visualized in the same tilted axial plane.

Fig 2.

MR images of the abducent nerves in a 27-year-old man were obtained by using the 3D fast asymmetrical spin-echo sequence. Black and white reversed images are shown. A and D, tilted axial view images; B and C, parasagittal view images. The right abducent nerve was 30° to the clivus (A and B), and the left abducent nerve was 90° to the clivus (C and D).

Fig 3.

CSF evagination into Dorello canal on tilted axial view images of eight volunteers. Dorello canal can be identified as a CSF-filled evagination of variable length (arrowheads). The CSF evagination ranged from 1.0 to 5.7 mm in these eight volunteers.

Fig 4.

Light micrographs of cross sections of the abducent nerve. A, Cross section obtained at the porus duralis. The abducent nerve at the opening of attenuated connective tissue of the dura mater (dm) was covered by a moderately attenuated connective tissue sheath (cts). The loose connective tissue and fluid-filled space outside the connective tissue sheath represents the arachnoidea and subarachnoid space, respectively. B, Cross section obtained at the midportion of the petroclival segment. The abducent nerve within the Dorello canal was covered by a compact sheath (arrowheads). It was invested in two layers of connective tissue envelope, consisting of an inner loose arachnoidal and an outer attenuated dural extension.