Wolfgang Draf

General Operative Techniques

The transsphenoidal approach was originally developed as a primary extradural approach for the removal of pituitary neoplasms. The historical aspects were previously discussed (see p. 274). Later this approach came to be used for the removal of clivus tumors (Decker and Malis 1970; Derome 1979; Hardy 1977; Rougerie et al. 1967). Transsphenoidal exposure of the clivus is indicated for tumors that involve the clivus as well as the adjacent paranasal sinuses and pharynx. The superior limit of this approach is the diaphragma sellae, and the inferior limit is the foramen magnum. Dissection can be carried laterally as far as the bony limit of the middle skull base (carotid canal, petrous apex, etc.). Besides pituitary tumors, interosseous meningiomas, fibrous dysplasia, and other bone tumors can be approached through this route.

Surgery of the Internal Auditory Canal and Cerebellopontine Angle

The internal auditory canal and the structures of the cerebellopontine angle are accessible through three surgical approaches (Krmpotic et al. 1985): n n1) n nThe transtemporal extradural approach (Fisch 1976; House 1964). n n n n n2) n nThe transmastoid translabyrinthine approach (Fisch 1976). n n n n n3) n nThe lateral suboccipital (retrosigmoid) approach.

Surgery of Space-Occupying Lesions of the Anterior Skull Base

Tumors of the anterior skull base can arise from the frontal sinus, the ethmoid cells, the sphenoid sinus, and even from the maxillary sinus, from which they can spread toward the inside of the skull. Invasion may occur through the posterior wall or floor of the frontal sinus, the ethmoid roof, the cribriform plate, the planum sphenoidale, or the tuberculum sellae.

Surgery of Malformations of the Anterior Skull Base

In discussing the surgical treatment of malformations of the anterior skull base, we must recognize two major groups: n n1) n nClosure defects of the skull and brain-the fistulas, cysts, and celes (frontobasal dysraphic anomalies). n n n n n2) n nThe various types of craniofacial synostosis and dysostosis.

Anatomy of the Jugular Foramen

In adults the lateral rim of the jugular foramen lies 14.1 (8–19) mm from the inferior margin of the porus acusticus on the right side and 15.5 (9–21) mm on the left side. Hence: n n1) n nThe mean distance is shorter on the right side, probably due to the fact that the jugular foramen is better developed on that side. n n n n n2) n nThis distance shows a very large range of individual variation.

Anatomy of the Clivus

Various surgical approaches are available for operations in the region of the clivus (e.g., for chordomas or chondromas): subtemporal transtentorial, transcervical, subglossal, and rhinoseptal. Hakuba (1985) reported on the transoral approach to this region. Calcaterra et al. (1973) emphasized that tumors of the nasopharynx (and maxillary sinus, ethmoid cells, palate and facial region) can grow through the foramina of the skull base. They found that these lesions can spread by the perineural route without seeding metastases to regional lymph nodes or other organs. Gabrielsen and Pingman (1964) noted that the skull base tumors known as “chondromas” in the German literature are equivalent to the “ osteochondromas” of English and American authors. These tumors arise from cartilage remains of the primordial skull base and initially grow extradurally, although they can penetrate the dura. Their most common sites of occurrence are parasellar, and occasional lesions are found in the middle fossa, posterior fossa, the CPA region, about the petrous apex, and rarely in the area of the petrooccipital suture. Figure SA 75 shows how the position of the notochord changes at the craniocervical junction and in the clivus region during development and indicates the directions in which chordomas (derived from the notochord) may spread (see Lang 1986 for details). Luschka (1856) was probably the first to describe such tumors, which Muller (1858) then related to the notochord. The routes by which chordomas can spread to the posterior fossa, nasopharynx, sphenoid sinus, and the pituitary region of the middle fossa are indicated in the figure. Evidently the course of the notochord is variable in the area of its rostral end. Some researchers describe it as terminating in the area of the dorsum sellae, others at the posterior wall of the pituitary fossa (on which a “sellar spine” is occasionally seen). It appears that we were the first to describe a sellar spine, which we observed in a 23-year-old male (Lang 1977). Since then there have been numerous descriptions of a spiny process projecting into the hypophysis (e.g., Dietemann et al. 1981).

Surgery of Traumatic Lesions of the Middle Skull Base

Trauma to the medial and lateral portions of the middle skull base with injuries to bone, dura, brain, and neurovascular structures can be an indication for operative treatment. Otologic aspects of these injuries have been reviewed in comprehensive survey works by H. B. Boenninghaus (1979), F. Escher (1973), W. Kley (1968, 1976) and other authors. Current neurosurgical guidelines on the treatment of lateral skull base injuries have been published by Becker and Ward (1982), Gillingham et al. (1971), Guleke (1950), Kessel (1969), Metzel (1973, 1980), and Samii and Brihaye (1983). These works are particularly instructive with regard to clinical presentation and evaluation. Here we shall focus on the indications for operative treatment and corresponding techniques that have evolved from years of cooperative experience between the neurosurgical and otorhinolaryngologic specialties.

Anatomy of Optic Nerve Decompression

The optic canal transmits the optic nerve with its dural and arachnoid coverings and also the ophthalmic artery from the orbit to the cranial cavity. In our material we determined a mean length of 9.8 mm for the upper wall of the optic canal and 4.63 mm for the lower wall. The medial wall of the canal is 11.4 (8–16) mm long, the lateral 10.79 (8.5–15) mm long (Lang and Oehmann 1976; Lang and Reiter 1985). The posterior superior wall of the optic canal is overlapped by a dural layer 2.58 (0.5–8.0) mm long composed of transverse fibers. Bony substance is lost from this area in postnatal life, leaving only the dura mater. We found in our material that the ophthalmic artery, with an outer diameter of 1.47 (0.9–2.1) mm (Lang and Kageyama, in press), can emerge in the subarachnoid space (approx. 47% of cases), at the junction between the dural opening of the internal carotid artery and subdural space (33.3%), or even in the cavernous sinus (approx. 18%) (Engel 1975).

Dura Mater of the Anterior Cranial Fossa

The dura mater of the cranium is composed of an outer fibrous layer and an inner fibrous layer. The outer layer acts as the inner periosteum of the adjacent cranial bones. In the anterior fossa the fibers of the outer layer radiate from the frontal tuber, passing downward and medially. The dura is thinned in the area of the olfactory fossa, and the dura on the planum sphenoidale consists of transverse fibers grouped into thicker bundles. The inner layer of the dura mater is relatively thin. Between the two layers are the larger branches of the dural vessels, which give off small branches to the cranium and to the dura itself. The major vessels of the cerebral dura mater (and of the bones of the anterior fossa) are the ethmoidal arteries and the frontal branch of the middle meningeal. The ethmoidal arteries anastomose with various nasal arteries, while the frontal branch of the middle meningeal anastomoses with branches of the ethmoidal arteries and also with branches of the ophthalmic artery. In our material we occasionally found a branch of the internal carotid artery entering the most posterior portion of the floor of the anterior cranial fossa (Yasargil et al. 1984 identified this branch consistently). In the frontal midline approach to the skull base, attention must be given to the superior sagittal sinus, which extends only part way to the skull base, and to the falx cerebri. The falx carries a relatively large branch of the anterior ethmoidal artery called the anterior falceal artery. All the dural arteries interanastomose with one another and with the contralateral homonymous vessels. Venous drainage from the dura mater generally follows the course of the dural arteries.

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 Schafer 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.