16.1 Introduction

There are several ways to access tumors of the foramen magnum: the suboccipital approach, the far lateral transcondylar approach, the endonasal endoscopic transclival approach, and the transoral approach, among others. In our opinion, the suboccipital approach is the best of the bunch. It is simple and familiar to almost all neurosurgeons, requires less dissection than the other approaches, provides a wide angle of attack to access problems at the foramen magnum, and the risk to the vertebral artery is lower than with the far lateral transcondylar approach. Most relevant in the context of this book, it is easily miniaturized and made less invasive.

Most neurosurgeons would agree that the suboccipital approach is a good approach in general, and that the role of the other approaches to this region is to address situations where the suboccipital approach provides inadequate access, namely when the tumor is located in the ventral foramen magnum.

In addition to providing insight into the use of a keyhole suboccipital approach for midline cerebellar pathology, and our use of the telovelar approach, this chapter provides some examples of how we have utilized the endoscope to get around the problem of inadequate access in many of these patients. Certainly some midline tumors of the lower clivus, notably chordomas, benefit from an endonasal endoscopic approach (see Chapter 7), or sometimes a retrosigmoid approach. In other rare cases, it is absolutely essential to perform a far lateral approach and drill part of the occipital condyle. However an experienced, endoscopically trained skull base surgeon could go years between these cases.

16.2 Intra-Axial Cerebellar Tumors

The principles for planning a keyhole approach to a cerebellar tumor are nearly identical to those for a supratentorial intra-axial tumor. The keyhole should be centered on the exit point of the long axis of the tumor using the two-point rule, and the surface should be exposed. If the two-point rule takes you near or through the transverse or sigmoid sinus, a keyhole retrosigmoid approach can be used (Fig. 16.1, Fig. 16.2, Fig. 16.3, see Video 16.1). Off-midline tumors do not require exposure of both sides, and a unilateral craniectomy is all that is necessary (Fig. 16.4).

Midline trajectories do require craniectomy of the midline keel and division of the occipital sinus, if present. Again, the craniectomy can be small if the surface is uninvolved (see Video 16.2), as the keyhole principle tells us that a wide angle of vision of deep structures is possible if the microscope angle is changed frequently, as required.

16.3 Fourth Ventricular Tumors

Whenever possible, we prefer to approach these tumors using the mini-telovelar approach (Fig. 16.5, Fig. 16.6, Fig. 16.7, Fig. 16.8, Fig. 16.9, see Videos 16.3, 16.4, and 16.5). The details of this approach are explained in Chapter 6; however, most neurosurgeons are very familiar with this approach, as it is a great workhorse approach to this region. By definition, the foramen magnum must be exposed. However, it is not always necessary to completely expose the cerebellar hemisphere, and reducing the size of the craniectomy reduces the amount of lateral muscle dissection required to expose the cerebellar bone. It is best to think of the telovelar approach as a ladder (Fig. 16.5), with the base of the ladder at the foramen magnum, and the bony removal climbs that ladder until it is nearly parallel with the superior extent of the lesion in the fourth ventricle. If the tumor tapers off at the top, then it is possible to just take the craniectomy to the top of the broad part of the tumor. This allows one to safely mobilize the cerebellar hemisphere to a sufficiently high level to reach the whole tumor.

The patient is positioned prone with the head flexed. A keyhole telovelar approach requires craniectomy above and across the rim of the foramen magnum. A full C1 laminectomy is usually unnecessary, but undercutting the upper surface can sometimes be useful. The endoscope allows one to avoid the C1 laminectomy by providing a much better view of the superior fourth ventricle than is possible in the standard telovelar approach without significant cerebellar retraction. We prefer a C-shaped dural opening that crosses the midline, as this obviates the need to remove C1 in order to obtain more lateral exposure. Once the microscope is brought in, the cisternal and tonsillar arachnoid is widely opened to access the tumor.

In some patients, the vermis and superior medullary velum are invaded to such an extent that the long axis of the tumor passes through the vermis. In these patients, we perform a limited midline craniotomy and transvermian approach (Fig. 16.10, see Video 16.6). However, it is important to note if the foramina of Luscka are invaded by this tumor, because in these patients the tonsils should be exposed so they can be mobilized to create space in the foramen. In addition, the endoscope can be a very valuable tool in such cases to deal with tumor in the foramen without excessive tonsillar retraction. As stated in previous chapters, it is important to identify the steps of the specific operation, and expose any structure absolutely necessary to achieve this step. The keyhole concept does not imply that all exposures are minimal, only that nothing should be exposed unnecessarily.

Importantly, fourth ventricular tumors, especially ependymomas, have a tendency to fill the foramen of Luschka. Traditional techniques to reach this difficult area involve resection of the cerebellar tonsil and often a lot of cerebellar retraction. We suggest that the endoscope allows one to work in this area very easily, removing tumor with minimal retraction and providing a better view (Fig. 16.11, see Videos 16.7 and 16.8).