Anatomy of the Cavernous Sinus
Parkinson (1965) described an approach to the cavernous part of the internal carotid artery through a triangular area (“Parkinson’s triangle”) bounded superiorly by the trochlear nerve, posteriorly by the rim of the clivus (posterior petroclinoid fold), and inferiorly by the upper border of the ophthalmic nerve. It is noteworthy that the trigeminal ganglion is frequently placed above the upper border of this nerve in our material. The length of artery accessible through Parkinson’s triangle will vary somewhat with the individual vascular anatomy. Using the definition of Parkinson’s triangle given by Harris and Rhoton (1976), we were able to identify the feature in only 31.4% of our material. Assuming that a safe approach to the internal carotid artery through Parkinson’s triangle requires a minimum width of 4 mm between the ophthalmic nerve and trochlear nerve (to avoid damage to adjacent nerves), we found that this criterion was satisfied in 35% of cases on the right side and in 24% on the left side. However, it must be considered that the abducent nerve runs lateral to the internal carotid artery. This nerve passes above the superior pole of the trigeminal ganglion, and thus through the route of surgical approach to the internal carotid artery, in 16% of cases on the right side and in 23% on the left side. In 13.5% of the specimens examined, nerves IV and V1 and the ophthalmic nerve ran so close together that it appeared impossible to reach the internal carotid artery either below the trochlear nerve or between that nerve and the oculomotor. It should be noted that the position of the cranial nerves may be altered in the presence of an AV shunt or aneurysm in the cavernous sinus. Moreover, attention need not be given to the aforementioned cranial nerves if they are already damaged, and the internal carotid artery may be approached without concern. In cases where the carotid arteries take a relatively straight course in the cavernous sinus (14.7%), Parkinson’s triangle may lie well above the internal carotid artery. Finally it should be noted that the internal carotid gives off 2–6 small arteries that may be ruptured in skull trauma (Lang and Schäfer 1976). However, the vessel that we identified as the posterior caroticocavernous trunk usually arises from the posterior cavernous curvature of the internal carotid artery and gives off the pituitary fossa branches and other branches in addition to the inferior pituitary artery (see Fig. SA 19). The caroticocavernous trunk lies in the path of the surgical approach through Parkinson’s triangle. Koos et al. (1985) point out that 14% of “giant aneurysms” (which are rare) involve the internal carotid artery. They described an anterior approach to the anterior sinus genu of the internal carotid artery that involves removing the anterior clinoid process and opening the cavernous sinus in that area.
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