Abducens length and vulnerability?

Abstract

Of the three cranial ocular motor nerves, the abducens nerve (sixth nerve) is most commonly affected by increased intracranial pressure. Several authors have attributed the vulnerability of the abducens nerve to its long intracranial course. However, other anatomic factors likely contribute to the apparent vulnerability of the abducens nerve to mass lesions and trauma. In this study, the authors performed a two-part anatomic study of the abducens nerve. In the first part of the study, they compared the length of the abducens with the trochlear nerve (fourth nerve), at the autopsy of 26 pediatric patients. In the second part of the study, the authors used an endoscopic exposure of these two cranial nerves in a preserved human cadaver head. They observed that the abducens nerve was consistently approximately one-third the length of the trochlear nerve at all ages that they studied, suggesting that it is not the length of the intracranial portion of the abducens that is critical to its vulnerability. The endoscopic views revealed the numerous structural and vascular relationships of the abducens nerve, suggesting a high vulnerability of the abducens nerve to compression. The authors conclude from these findings and the literature that abducens nerve vulnerability results from factors other than its intracranial length. They emphasize that the basis pontis slips caudally during transtentorial herniation, which may stretch the nerve.—Valérie Biousse