[Significance of anatomic variants of bony surroundings of the internal carotid artery and their significance for lateral surgical approaches to the cavernous sinus]

Robert Narolewski
Annales Academiae Medicae Stetinensis 2003, 49: 205-29

OBJECTIVE: The central skull base region and the cavernous sinus in particular have traditionally been thought inaccessible to surgical manipulations. Despite significant progress in anatomy, physiology, pathology, imaging and surgical techniques, it is still challenging to explore this part of the skull. The main risk is injury to the internal carotid artery running through the area surrounded by bony structures e.g.: anterior, middle and posterior clinoid processes, optic strut, and petrous bone (Fig. 1, 2). Access to this area requires removal of bony structures but at the same time represents the greatest surgical risk. This microsurgical anatomic study was performed to give surgeons more details on these structures and their relation to the internal carotid artery and venous neighborhood, thus making the operative procedure safer.

METHODS: 40 central skull base area specimens were harvested from 20 fresh human cadavers during regular anatomopathologic autopsy. Almost immediately after excision, arterial and venous systems were cannulated and injected with colored acrylic. Microsurgical dissections were performed using a surgical microscope, along with morphometric measurements and photographic documentation of anatomical variants of structures studied.

RESULTS: The anterior clinoid process is a structure of complex architecture. In most cases (70%), at least one bony spicula arises from the tip of anterior clinoid and not infrequently passes in a fold of the dura directly under the internal carotid. Bony bridges were observed joining spiculae with similar protrusions of the middle and/or posterior clinoid thought to be especially dangerous during anterior clinoidectomy (Tab. 2, Fig. 2). This observation is in agreement with the literature. 60% of anterior clinoids were pierced by narrow venous canals arising from the anterior cavernous sinus and running through the clinoid space (Fig. 12). They are considered a source of bleeding usually encountered at the very end of anterior clinoid removal. The optic strut seems to be a structure of stable and relatively simple architecture. In 20% of specimens, optic struts were pierced by narrow venous canals arising from the anterior cavernous sinus and running through the clinoid space. The anterior clinoid process and the optic strut happen to be pneumatized. In this case, the pneumatized area is covered with a mucous membrane and opens to the sphenoid sinus or posterior ethmoidal cells (Tab. 2). It was quite surprising that the carotid canal roof was incompletely closed in all samples, although the length of opening was highly variable (Tab. 12, Fig. 1). A relatively thick connective tissue layer covered the opening, usually occupying the area under the Gasserian ganglion i.e. the most medial part of the canal. The greater superficial petrosal nerve was a good landmark for identification of position of the carotid canal in the petrous bone. The canal ran almost directly under the nerve. The hiatus of greater superficial petrosal nerve poorly demarks the area of bone removal lateral to the posterior loop of the internal carotid (Tab. 12). Numerous morphometric measurements of anterior clinoid, optic strut, carotid canal and their anatomical variants give an even greater insight into the area (Tab. 3-13, Fig. 3-11). It is obvious that preoperative CT scanning should be done using thinnest slicing available taking into consideration the scale of measurements presented in this paper.

CONCLUSIONS: More details were revealed concerning an area already explored by many researchers. New observations are presented on venous canals passing through the clinoid space. A new insight has been obtained into the area around the roof of the internal carotid canal. The present rich morphometric and photographic documentation should be helpful when dealing with vascular, neoplastic or traumatic lesions of the central skull base. It can also be useful for preoperative planning or training at a neuroanatomy lab.

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