Background: In traditional descriptions of the intracranial course of the trochlear nerve, the tentorial segment of this nerve has not been described. This segment assumes importance as it is at risk of injury during tentorial sectioning in skull base surgery. In the present study, the tentorial segment of the trochlear nerve was studied.
Methods: 30 cadaver sides were studied and the following parameters were measured: the parts and lengths of the tentorial segments of the fourth nerve; the distances between the point where the third nerve came into contact with the tentorial edge and (a) the point where fourth nerve first touched the tentorium and (b) the point where fourth nerve pierced the tentorial dura; and the transverse separation between the fourth nerve and the gasserian ganglion.
Results: The tentorial part of the fourth nerve was found to have two segments: segment 1-from the point where the fourth nerve first came into contact with the tentorial edge to the point where it pierced the tentorium; segment 2-from the point of tentorial piercing to the point where the fourth nerve entered the cavernous sinus. The mean distance between the third nerve and the point of piercing along the tentorial edge was 9.9 ± 2.7 mm (5.29-15.32).
Conclusions: The most consistent and reliable anatomical landmark for avoiding injury to the fourth nerve was the point of contact between the third nerve and the tentorial edge. An incision 15 mm posterior to this point along the tentorial edge would avoid injury to the fourth nerve.