Cavernous sinus and abducens nerve in human fetuses near term

A long tortuous course of the abducens nerve (ABN) crossing a highly curved siphon of the internal carotid artery is of interest to neurosurgeons for cavernous sinus surgery. Although a straight intracavernous carotid artery in fetuses can change into an adult-like siphon in infants, there is no information on when or how the unique course of ABN is established. Histological observations of 18 near-term fetuses (12 specimens of frontal sections and 6 specimens of sagittal sections) demonstrated the following: (I) the ABN consistently took a straight course crossing the lateral side of an almost straight intracavernous carotid artery; (II) the straight course was maintained when sympathetic nerves joined; (III) few parasellar veins of the developing cavernous sinus separated the ABN from the ophthalmic nerve; and (IV) immediately before the developing tendinous annulus for a common origin of extraocular recti, the ABN bent laterally to avoid a passage of the thick oculomotor nerve. Since the present observations strongly suggested morphologies at birth and in infants, major angulations of the ABN as well as the well-known course independent of the other nerves in the cavernous sinus seemed to be established during childhood. In the human body, the ABN might be a limited example showing a drastic postnatal change in course. Consequently, it might be important to know the unique course of ABN before performing endovascular interventions and skull base surgery for petroclival and cavernous sinus lesions without causing inadvertent neurovascular injuries to neonates or infants.