9 Keyhole Approaches to Tumors of the Cribriform Plate and Orbit

10.1055/b-0035-104221

9 Keyhole Approaches to Tumors of the Cribriform Plate and Orbit

Michael E. Sughrue and Charles Teo

9.1 Introduction

Most skull base surgeons would agree that surgical resection of pathology of the cribriform plate is more straightforward than resecting cranial base pathology in areas such as the petroclival junction or middle fossa. However, the cribriform plate is somewhat resistant to attempts to reduce the craniotomy size, for two principal anatomical reasons. Firstly, as the keyhole analogy tells us, it is very difficult to look through a keyhole and see the back of the door (see Chapter 1). In this case, the olfactory groove is located very anteriorly, and thus is to some extent “behind the door” when using anterolateral approaches such as the eyebrow craniotomy or the mini-pterional. The direct approach passes through the frontal sinus, and is difficult to hide cosmetically without a large bicoronal incision. Secondly, the orbital roof is not flat, but typically slopes downward into a “valley” at the olfactory groove, which is difficult to descend into through a small and lateral craniotomy (see Fig. 9.1 for an example of this). It is therefore hard to address pathology in this area satisfactorily through any mini-craniotomy.

**Fig. 9.1** **a–e** Olfactory groove meningioma removed endonasally. **(a, b)** Preoperative images demonstrating an olfactory groove meningioma with a nice cortical cuff and minimal lateral extension, making it an excellent endonasal case. **(c)** After performing the transcribriform approach, the right olfactory tract can be seen. 1, dura mater; 2, olfactory tract; 3, tumor. **(d)** Once part of the tumor has been resected, the right frontal lobes (RFL) and left frontal lobes (LFL) come into view. Two branches of the A2 component of the anterior cerebral artery (ACA) can be seen here.1, falx cerebri; 2, ACA (A2) branch; 3, tumor. **(e)** The tumor bed (TB) following resection. FL, frontal lobes; IHF, interhemispheric fissure.

There are certainly challenges with these cases and, in many patients, a bifrontal craniotomy with a bicoronal incision and a pericranial flap is the only reasonable option, although the approach can be made smaller than the traditional versions, as described later. However, in many patients, a large opening can be avoided by the careful use of selected keyhole procedures, use of the endoscope, and endonasal endoscopic surgery. The present chapter describes a brief algorithm for approach selection in this area, and also describes some techniques for various tumors, including repair of the endonasal skull base when necessary.

9.2 Approach Selection for Tumors of the Cribriform Plate

In most patients, our general preference is to approach these tumors endonasally whenever possible. The endonasal approach provides direct access at favorable angles of attack, and early tumor devascularization. Importantly, it also provides good access to the depths of the olfactory groove, which is much more difficult when one is working over the orbital roof through a small craniotomy.

The principle anatomical features which steer one away from the endonasal approaches are lateral extension beyond the midpoint of the orbit or involvement of significant cerebral blood vessels visible on imaging (Fig. 9.2). A good rule of thumb for determining whether the latter applies is to look for the presence or absence of a cortical cuff (Fig. 9.2 c), a rim of brain between the tumor and the blood vessels on all imaging slices. The absence of a cortical cuff (Fig. 9.2 e) makes an endonasal approach much less desirable, as it means that pulling on the tumor is much riskier, and the microdissection required to safely remove tumor from blood vessels is usually better done under the microscope via a craniotomy.

**Fig. 9.2** **a–e** Basic radiographic features guiding approach selection for olfactory groove tumors. **(a)** Coronal image of an olfactory groove meningioma. The approximate lateral limit of an endonasal approach is depicted by the red dashed lines, and is roughly located along a plane through the mid orbit. Further lateral extension requires a craniotomy. **(b)** Sagittal image of an olfactory groove meningioma. **(c)** The same image with the anterior cerebral artery (ACA) vessels indicated in red, demonstrating the presence of a cortical cuff between the tumor and the ACA vessels, indicated by the blue shaded area. This suggests that an endonasal approach would be appropriate. **(d)** Sagittal image of a different meningioma. **(e)** Examination of the location of the ACA vessels indicates there is no cortical cuff protecting the vessels, and that a craniotomy is needed. 1. Right anterior cerebral artery (A2 segment); 2. Right and left anterior cerebral arteries covered in arachnoid; 3. Meningioma; EAC, ethmoid air cells; FL frontal lobe; IHF inter-hemispheric fissure; LFL left frontal lobe; RFL right frontal lobe; OGM, olfactory groove meningioma; SS, sphenoid sinus; TB, tip of basilar artery.

What follows is a discussion of decision making and techniques for tumors located in this region, which makes a distinction between tumors which are entirely or mostly above the skull base and tumors with significant endonasal involvement.

9.2.1 Supracribriform Tumors

The goal in these cases is to achieve satisfactory removal of the tumor in a single approach. Tumors above the skull base with a good cortical cuff can usually be removed quickly and easily via the endonasal route, and because this also completely removes the involved dura and bone, it is an excellent option (Fig. 9.1 and Fig. 9.3). Tumors with significant lateral extension, or which lack a cortical cuff, should be removed via a craniotomy. Tumors that are mostly unilateral can be removed via the eyebrow approach; however a truly bilateral tumor is very difficult to reach via the eyebrow approach, since this involves looking directly “behind the door” through the keyhole. A mini-pterional approach can deal with some of these tumors. However, with tumors involving the interhemispheric vessels, it is occasionally necessary to bite the bullet and perform the larger bifrontal craniotomy.

**Fig. 9.3** **a–c** Olfactory groove meningioma removed endonasally. **(a, b)** Preoperative images demonstrating an olfactory groove meningioma with a nice cortical cuff. It is important to note the depth of the olfactory groove in this patient, making the endonasal approach desirable for addressing tumor located in this “valley.” **(c)** Postoperative imaging demonstrating complete resection; note that we did not perform a nasoseptal flap for closure and there were no issues with postoperative cerebrospinal fluid (CSF) leakage.

In open unilateral approaches, the endoscope and angled instruments can be used to visualize and address tumor deep in the olfactory cleft, and usually cauterizing the tumor in this manner is enough (Fig. 9.4). If the tumor recurs in this area, it is usually detached from the brain and is easily addressed with endonasal surgery. However, this is uncommon (between the two of us, we have only had to do this once; see Fig. 9.5).