12 Keyhole Approaches to Tumors of the Middle Fossa, Cavernous Sinus, Tentorium and Lateral Midbrain

12.1 Introduction

The eyebrow is a very versatile approach. However it cannot do it all, and this chapter is about how to deal with lesions in areas of the middle fossa and surrounding brain when the eyebrow is not the best approach. The subtemporal and pterional approaches have long histories and good track records. While they can easily be made smaller than their original variants, their major downside is that they require a varying degree of division and elevation of the temporalis. Despite techniques designed to minimize the degree of temporalis atrophy with these approaches, there is no technique that entirely eliminates the problem, and it certainly causes more pain than an approach where this is not an issue (such as the eyebrow). Thus, while we do not use the eyebrow approach in an anatomically inferior situation, the tie goes to the eyebrow whenever the merits of the eyebrow and another approach are roughly equal. Fig. 12.1 demonstrates a system for determining when the eyebrow approach is inappropriate, and thus when a middle fossa or mini-pterional approach is needed.

**Fig. 12.1** **a, b** Schematics demonstrating the rough boundaries of the eyebrow approach (a) in the sagittal plane, and (b) from a superior viewpoint. The eyebrow approach boundaries are indicated in blue, while areas in purple should be accessed via the subtemporal or pterional approach.

The endoscope is useful here in many situations. Tumors which straddle the skull base can be followed into the infratemporal fossa using the endoscope. Similarly, the subtemporal approach can be augmented to address extension into the posterior fossa by cutting the tentorium and using the endoscope to look downward (Fig. 12.2).

**Fig. 12.2** Schematic demonstrating the use of the endoscope to follow a middle fossa tumor downward into the prepontine and retroclival space through an incision in the tentorium.

This chapter addresses the application of these approaches and techniques to address tumors below the “eyebrow plane”: tumors of the middle fossa, tentorium, and cavernous sinus.

12.2 Which Approach Is Better for This Region?

Unlike the parasellar region, there is no easy answer to this question. Both the subtemporal and pterional approaches have their merits for specific problems. Our default where there is a tie is the subtemporal approach. It is simple and fast, and requires less temporalis dissection and a smaller incision. Again, we do not try to use it when it is inappropriate, but if a mini-pterional and subtemporal approach are basically equal then we favor the subtemporal approach.

The subtemporal approach is the approach of choice for tumors of the middle fossa floor, tumors located on the middle tentorial incisura (between the peduncular “C” and the lateral collicular border; Fig. 12.3), tumors in the tentorial body anterior to the Labbé vein, tumors located in the inferolateral cavernous sinus, and some tumors which span the upper and middle clivus. Some low-lying tumors limited to the inferior uncus can be removed via a subtemporal approach, although these should be selected carefully. Tumors of the middle clivus alone can often be approached using a retrosigmoid approach (especially if they are at or below the Meckel cave). Tumors of the upper clivus alone can be removed through the eyebrow with the endoscope. Chordomas or other predominantly midline, often extradural pathology are well addressed through an endonasal approach.

**Fig. 12.3** Schematic demonstrating the approaches of choice for addressing midbrain tumors.

The pterional approach becomes more favorable the more superior and anterior the tumor lies in the middle fossa. Sphenoid wing meningiomas (see Video 12.1) and clinoid meningiomas with significant middle fossa components are the best examples of this. Tumors with significant involvement of the middle cerebral artery are inappropriate for the subtemporal approach and need a pterional approach. Additionally, if an extradural approach to the cavernous sinus is planned, only the pterional provides access to the meningoperiorbital band, which needs to be cut to peel open the lateral wall of the cavernous sinus. Tumors with extensive middle fossa involvement with significant superior extension are probably also better managed with a pterional to avoid excessive temporal lobe retraction. The pterional approach is more flexible for addressing a wider range of pathology, and if in doubt, a pterional is more likely to address the entire tumor easily.

12.3 Specific Examples

Several examples of middle fossa pathology addressed through keyhole approaches are provided in this chapter. In most situations, experienced skull base surgeons will find that, with the adoption of keyhole principles outlined in earlier chapters, keyhole surgery to this region is not very different from addressing these patients through the more familiar larger approaches. As with anything in keyhole surgery, thoughtful planning is essential.

12.3.1 Trigeminal Schwannoma

Fig. 12.4, Fig. 12.5, and Fig. 12.6

**Fig. 12.4** **a, b** Giant trigeminal schwannoma removed through a mini-pterional craniotomy. **(a)** Preoperative images demonstrating a massive multicompartmental trigeminal schwannoma. **(b)** Postoperative images following removal via a mini-pterional craniotomy. The pterional approach was selected because of the anterior and superior tumor extension. The endoscope was used to follow this mass back into the posterior fossa.

**Fig. 12.5** **a–e** Trigeminal schwannoma removed through a mini-pterional craniotomy. **(a)** Preoperative images demonstrating an anteriorly and superiorly situated trigeminal schwannoma. Note the significant infratemporal fossa component. **(b, c)** Postoperative images following resection via a pterional craniotomy. The endoscope was used to follow the tumor into the infratemporal fossa. **(d)** The skin incision. **(e)** The final cosmetic result.

**Fig. 12.6** **a–l** Extradural resection of massive recurrent trigeminal schwannoma through a mini-pterional craniotomy. **(a–g)** Preoperative images of a massive recurrent trigeminal schwannoma. **(h–j)** Postoperative images demonstrating resection of this tumor. **(k)** The question-mark incision from a previous operation was ignored and a slight modification was utilized to perform a mini-pterional craniotomy. **(l)** This picture was obtained after exposing the tumor extradurally and partially debulking it. Following this, the tumor was followed into the posterior fossa using the endoscope.

These tumors are hard to deal with as they are in a challenging location and often span multiple compartments, making it difficult to remove the whole tumor in a single operation. We have found that the endoscope provides an excellent opportunity to address multicompartmental trigeminal schwannomas. For example, schwannomas predominantly located in the middle fossa, but which extend significantly into the infratemporal or pterygopalatine fossa, can be addressed via a subtemporal or pterional approach, with the endoscope used to chase the tumor downward through the expanded foramen (Fig. 12.5). Similarly, reasonably minor extensions into the posterior fossa can often be followed and removed using the endoscope (Fig. 12.4). Larger dumbbell tumors can sometimes be addressed in the subtemporal approach by sectioning the dura or tentorium, and looking downward into Meckel cave or posterior fossa using the endoscope (see Video 12.2). In some patients, staged procedures (such as subtemporal followed by retrosigmoid approaches) may be needed, which avoid the morbidity of a single, large skull base approach.

In these patients, it is generally not possible to align the approach along the long axis of the tumor, and the keyhole craniotomy should be placed to aim for the midpoint of the tumor.

It has been suggested that the endonasal approach to these tumors can be aligned with their long axis. While we are certainly advocates of endonasal endoscopic surgery in appropriate circumstances, our personal bias is against this more direct but more complex approach to these tumors. The endoscope-assisted subtemporal craniotomy is rapid, minimally invasive, cosmetically innocuous, and invariably effective in achieving an excellent resection. It does not obey the two-point rule, but neither does the endonasal approach. Furthermore, an endonasal approach increases the potential for cerebrospinal fluid (CSF) fistula formation and is more time consuming and arguably more invasive when one considers the ramifications of extensive mucosal destruction.

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