Silicone-injected, formalin fixed cadaveric heads were dissected to present the microsurgical anatomy of the oculomotor nerve and its topographical relationships (Figure 1). The relationship between the right internal carotid artery aneurysm at the origin of the posterior communicating artery and the right third-nerve is also shown in a 42-year-old woman with an unruptured aneurysm. Intraoperative images, obtained during surgical dissection, through a Zeiss Opmi Pentero Surgery Microscope camera, display the adhesions between the lesion and the entire roof of the cavernous sinus.
In this area, where the supraclinoidal internal carotid artery is situated, three main nerve systems are encountered:
(i)the sympathetic nervous system, (ii) the parasympathetic nervous system, and (iii) the trigeminal nervous system. The sympathetic nerves, which innervate orbital structures, originate from the superior cervical ganglia and take an upward direction, by the side of the internal carotid artery to reach the parasellar region via the internal carotid nerve, which divides into two branches: the lateral branch, which distributes to the internal carotid artery (internal carotid plexus), and the medial branches, which also distributes filaments to the internal carotid artery and, continuing onward, forms the cavernous plexus. Trigeminal nerve fibers are diffusely distributed all over the parasellar structures, together with vessels and dura-mater.24 As the internal carotid artery is surrounded by trigeminal and sympathetic fibers, aneurysm formation with gradual saccular growth may stretch and stimulate the nerve endings and this may cause pain in the periorbital and/or temporal regions. Aneurysmal compression of pain sensory afferent fibers of the ophthalmic division of the trigeminal nerve present around the oculomotor nerve and into parasellar dura is seen by some as cause of orbital pain.25
In conclusion, we reviewed the functional anatomy of circle of Willis, oculomotor nerve and its topographical relationships in order to better understand the pathophysiology linked to pain and third-nerve palsy caused by an expanding ICA-PComA aneurysm.
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