8.1 Introduction

If one wants to learn keyhole techniques for tumors, one must first master the keyhole glioma. —Michael E. Sughrue

This is true not only because gliomas and other intra-axial brain tumors comprise the bulk of most tumor surgeons’ practices, but also because learning the keyhole techniques for gliomas enables one to address and master the basic concepts of keyhole surgery planning and execution in a setting where it is uncommon to work down long narrow corridors, and this allows one to build confidence in keyhole techniques. For this reason, this is arguably the most important chapter in the book to understand and master, and is therefore the longest.

In no other type of tumor surgery will the surgeon discern a more immediate and widespread change in their practice and its outcomes than with glioma surgery. Simply put, changing to a keyhole paradigm will change everything. Decision making regarding repeat surgery is radically altered when problems with irradiated wounds are less of an issue, and when repeat surgery involves a day in the hospital with minimal pain and morbidity. Opening the dura is less of an issue when you only expose what you need (i.e., cortex you plan on removing completely). Radiotherapy can be started the day after surgery if desired (we have not noticed an issue to date with this), as the wounds are short and linear. If the bone flap becomes infected, then the cosmetic consequences of losing it are minimal instead of devastating. Importantly, most of the learning in these operations is cognitive, and thus a fully keyhole treatment algorithm can be rapidly enacted, which is not necessarily the case with other tumors.

Technical aspects of glioma surgery have been relatively ignored compared to the effort dedicated to approaching and removing cranial base tumors. This is unfortunate, as it often leads surgeons to think of these cases as simple operations, and to neglect the possibility that an improvement in technique could yield an improved extent of resection, less collateral damage to the surrounding brain, and less approach-related morbidity. Typical glioma surgery is performed though a large incision, with a large bone flap, elevation of most of the temporalis muscle when appropriate, and a wide dural opening exposing most of the brain under the bone flap. We would strongly argue that it is much easier to perform a repeat surgery following a keyhole craniotomy than after a traditional opening, as far less cortex is likely to be stuck to the dura when minimal cortex was exposed in the first place.

8.2 Keyhole Surgery

As outlined in Chapters 1 and 2, keyhole surgery does not refer to a specific approach or set of approaches, but rather a concept that allows surgeons to reduce the size of their openings without sacrificing efficacy or safety (when performed correctly). More specifically, it utilizes the wide range of viewing angles which can be obtained through a small hole if the viewing angle is adjusted frequently. In other words, most of a room can be visualized through the keyhole if one changes angle as needed. In addition, image guidance is critical in these patients to place the small bone flap in the ideal location. Patient positioning is also critical. In both cases, it is important to note that the keyhole approach leaves no room for error, and thus while these issues are always important in glioma surgery, they are even more critical in keyhole surgery.

It is often questioned whether it is in fact better to make a smaller skin and bone opening, when the brain surgery itself is clearly the most important part. Given that it is possible to see everything necessary to perform glioma surgery through a small opening placed in the correct location, then the issue comes down to whether a small craniotomy is better than a large craniotomy. This is a difficult question for evidence-based approaches to address definitively. However, there are many reasons to argue that a smaller craniotomy probably is better, over and above the fact that patients prefer minimally invasive surgery whenever they are given this option. Some of the potential benefits of a smaller incision and opening are listed below:

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  Less disruption to the cutaneous innervation of the scalp

  
  
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  Less vascular compromise of the scalp

  
  
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  Less postoperative pain

  
  
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  Fewer cosmetic consequences if the wound becomes infected and the bone flap is lost

  
  
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  Less mobilization of the temporalis and therefore less atrophy and better cosmesis

  
  
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  Less exposed brain and therefore less potential for heat and pressure damage to exposed cortex

  
  
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  More rapid opening and closing of the wound

  
  
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  Less dissection of the extradural space and therefore less chance for postoperative extradural hematomata

Keyhole surgeries nearly always employ small linear incisions, which inherently have a better blood supply and are more likely to heal correctly than the large curved scalp flaps which are in most patients required to perform large bone flaps. Irradiated scalp flaps often break down at their apex, where the blood supply is most tenuous, and this is often the first step in a long series of wound crises for these patients.

As gliomas tend to recur at or near the initial resection site, the surgical approach should be chosen for compatibility with a possible reoperation. Smaller incisions are less likely to result in suboptimal incision choices in future operations, such as T-shaped incisions, right- or acute-angled flaps, etc., than are curved incisions which cover large distances. More importantly, the most difficult and dangerous part of a repeat glioma surgery is often opening the dura, which is frequently stuck to cortices and arteries and therefore challenging to open fully without causing injury to these structures. Making a small dural opening means that less surface area of brain is exposed, resulting in the formation of fewer adhesions, as the dural opening is often directly over previously resected cortex. Thus, if the recurrence is near the previous surgery, a keyhole approach inherently interferes less with the repeat surgery than an approach which exposes large areas of normal brain.

Finally, patients generally feel better when they have had less brain exposed and manipulated. This is our own observation after performing many of these operations: there is less pain, less nausea, and patients usually wish to go home the next day, which, in our experience, is rare when performing a large craniotomy. There is never likely to be a randomized control trial which demonstrates this, and thus a leap of faith is required. However, you should ask yourself how many of your bur hole biopsy patients have spent 3 to 5 days in hospital on IV pain medication, as most keyhole glioma surgeries are similar in size and invasiveness.

8.3 How to Plan a Keyhole Approach to an Intra-Axial Brain Tumor

In general, the extracranial part of the procedure is straightforward: the generic keyhole construct is a linear skin incision, maximally utilized through subgaleal dissection (see Fig. 8.1 , and Chapter 6), followed by a 1.5- to 2-cm bone flap, and a small curvilinear or cruciate dural incision. Thus, keyhole surgery is largely a matter of appropriate planning of the location of that approach, and judging how and when to modify this approach slightly. It is also important to note that, as with any glioma surgery, one must work with what one is given: in other words, if a patient has had previous glioma surgery, the ideal trajectory may not be available, and some modification may be necessary to utilize the previous incision. However, it should be noted that we do not hesitate to perform a keyhole approach overly compromising from the previous incision, if using the old approach would force us to be unnecessarily invasive. Fig. 8.1 demonstrates a gallery of tricks we have used successfully in order to avoid the need to open a much larger previous wound. Because of the short incision length, we often find it is possible to break the “incision rules” many of us were taught.

For intra-axial tumors in particular, the degree of malignancy suspected before surgery determines whether the tumor should be subtotally or totally resected, and thus how much of it will have to be exposed. More specifically, gliomas are infiltrative lesions, and thus whenever possible these should be treated with anatomical resections. In other words, one defines the involved anatomy (such as the right frontal lobe, the posterior insula, the occipital pole, etc.) and plans an approach which removes this anatomy. Obviously there are limits to the application of this concept. For example, planning an anatomical resection that involves the internal capsule and midbrain would be unwise. However, anatomical resections are desirable whenever possible, as they are less likely to leave small fingers of gross tumor infiltration than a resection that attempts to conform to the margins. In many patients, these very large and extensive infiltrating tumors may be removed via a minimal keyhole construct. Occasionally the underlying pathology dictates a larger craniotomy, although it is unlikely that the largest craniotomy you will perform using the keyhole paradigm will ever be bigger than your standard craniotomy is now.

Focal lesions, such as metastatic tumors, WHO grade 1 gliomas (including pilocytic astrocytomas and pleomorphic xanthoastrocytomas), cavernous malformations, and other well defined, noninfiltrative lesions are more amenable to surgery, and thus are also more amenable to limited approaches which require small openings, minimal trajectories, and utilize sulci to enter the lesion, which is then removed from within. Thus, minimal openings are the rule in these cases, as it is rare to have extensive surface involvement which requires a large opening. Some small or eloquent gliomas are probably best removed as a lesionectomy, others as an anatomical resection.

What follows are seven basic guidelines for planning a keyhole approach to a supratentorial glioma.

8.4 Guideline 1: Determine the Size of the Keyhole Needed

One caveat of the keyhole concept is that a small opening provides many more angles of freedom for deep targets than it does for superficial ones (Fig. 8.2 ). This is analogous to the difficulty of seeing the back of a door through the keyhole. This problem is demonstrated in Fig. 8.3 . Thus, while a deep-seated tumor can easily be addressed through the generic keyhole, tumors with a significant amount of superficial cortex involved generally need the entire superficial portion of the tumor exposed, or at least all the cortices that one intends to remove. In general, the keyhole will need to be widened if the depth is less than approximately 2 cm. Thus, if the most superficial portion of the tumor is 2 cm below the cortical surface, even if it is very large, then the standard keyhole approach will likely be appropriate. Tumor-involved brain superficial to this depth usually requires exposure at the bone level to be adequately addressed.

8.5 Guideline 2: The Two-Point Rule

The underlying phenomenon that makes the two-point rule so important is the fact that as you internally debulk a tumor, the brain surrounding the tumor will tend to fold downward with gravity. By placing the primary trajectory of approach down the longest axis of the tumor, one can minimize the number of working angles needed to remove the tumor. Furthermore, by orienting the long axis of the tumor directly up and down, one can utilize gravity to pull the brain away from the working field in much the same way as banana peel will fall downward away from a banana held vertically as it is peeled. In this way, precious space is not taken up by fixed blade retraction, and visualization of the bulk of the tumor is simple.

The two-point rule is the principal consideration in keyhole approaches to gliomas. Its application is depicted in Fig. 8.4 . In short, the ideal keyhole craniotomy position is determined by drawing a line though two points along the longest single axis of the tumor, which is then traced outward to the bone. The ideal keyhole is placed over this point, and the patient is positioned so this two-point line is aligned as near to the vertical as possible.

The two-point rule provides us with the ideal keyhole location, and often this is the only consideration which needs to be made prior to planning the approach. However, there are exceptions that force the surgeon to use a less than ideal location for the keyhole, and these are discussed below.

8.6 Guideline 3: View the Ideal Keyhole in the Context of Eloquent Brain Regions

In many patients, the ideal trajectory is based on an approach passing through eloquent or presumed eloquent brain regions. Obviously, in these situations the keyhole must be moved in order to avoid this. In these patients, we typically determine a slightly less than ideal trajectory which avoids eloquent brain, and then utilize the endoscope to remove tumor that is not easily visualized by looking along this corridor with the microscope. This concept is depicted in Fig. 8.5 .

8.7 Guideline 4: View the Ideal Keyhole in the Context of Fissures

In some patients, the two-point rule will lead one down a transcortical route to a brain region that lies within a large, easily navigable fissure. It is always preferable to work within the subarachnoid space for as long as possible, to minimize injury to normal brain, and thus in some cases the keyhole is best placed over a part of the fissure that takes you nearest to the ideal trajectory. The endoscope can be utilized in these patients to address tumor portions which lie far outside the trajectory (see Video 8.1). These considerations most commonly apply to insular tumors, cingulate tumors, and tumors lying within diencephalic structures such as the thalamus (Fig. 8.6 ).

8.8 Guideline 5: View the Ideal Keyhole in the Context of Surgical Intentions

The best treatment for most gliomas is radical resection. Often this is not possible or advisable, either in a single stage or at all. One such example is shown in Fig. 8.7 . In this case, complete resection was not considered possible in one stage, and thus the tumor was viewed as three separate portions. For each stage, a keyhole approach was planned for one portion of the tumor using a two-point rule axis drawn for that part. The tumor was then successfully removed in three stages.

8.9 Guideline 6: Stick to Simple and Familiar Approaches Whenever Possible

Anyone who has even a casual familiarity with cranial neurosurgery will recognize that it is inappropriate to apply the two-point rule in certain parts of the head. Two-point trajectories which take one to the face, across the zygomatic arch, or through the superior sagittal sinus are inappropriate, and other minimally invasive approaches such as the supraorbital eyebrow, the mini-pterional, and the mini-interhemispheric-transcallosal, respectively, are better suited, despite being indirect. These approaches are outlined in Fig. 8.8 . They utilize anatomy which should be familiar to most neurosurgeons, are simple and flexible, and have minimal approach-related morbidity in experienced hands.

8.10 Guideline 7: Orient Incisions Parallel to Nerves

Incisions oriented across the direction of travel of subcutaneous nerves increase the number of fibers which are transected, increasing the risk for postoperative pain and numbness. Thus, whenever possible, incisions should be made parallel to these nerves. The location of these nerves and the ideal orientation of keyhole incisions for craniotomies placed in different locations on the head are outlined in Fig. 8.9 .

8.11 Guideline 8: Cheat Whenever Possible

The principal tenet we emphasize in this book is that when the tumor runs along the surface of the brain, or in other words the area immediately below the bone flap, then it is necessary to fashion a bone flap which exposes the involved surface tissue. While this is generally true, there are situations when one can avoid exposing the entire surface, with advantageous consequences. We have termed this technique “cheating.” The act of “cheating” in keyhole surgery acknowledges that while the tumor involvement may extend far along the surface of the brain, in many cases work can be safely performed underneath the skull without subjecting the patient to unnecessary risk. This allows us to dramatically reduce the size of the incision and bone flap without sacrificing efficacy and safety. The most common situations where we “cheat” are gliomas when we plan on performing a lobectomy as part of the operation. We would argue that it is entirely unnecessary to expose the whole surface extent of the occipital tip in order to completely remove a tumor filling the occipital lobe. Instead, we create a keyhole craniotomy overlying the “action zone,” where the tumor meets brain we plan on leaving behind, and simply fold the tip in and remove it. Similar techniques for performing frontal and temporal lobectomies can dramatically limit the size of the opening, as a result of acknowledging that there is no point in being delicate with brain that is going to be removed, and that we do not need perfect exposure for steps in the operation which do not require delicate microdissection.

8.12 Keyhole Craniotomies in Specific Locations

The principles outlined above represent critical concepts for planning keyhole surgery in general, and specifically for intra-axial tumors. Experienced surgeons attempting to apply these techniques will quickly note that this paradigm has some limitations, or more specifically that there are parts of the head where the application of these techniques is not immediately obvious. The rest of this chapter is dedicated to a detailed description of how to apply keyhole craniotomies to intra-axial brain tumors throughout the cerebrum.

8.13 Specific Examples

8.13.1 Simple Convexity Tumor, No Opercular Involvement

Fig. 8.10

As seen in this patient, the lesion is small, with minimal surface and no opercular involvement. Planning the keyhole is simple for these patients: the incision is linear and oriented anterior–posterior. The craniotomy is minimal, and the approach is transcortical or possible transsulcal in some cases.

8.13.2 Deep Frontal Tumor

Fig. 8.11

This patient presented with hemiplegia caused by his large tumor. Despite the tumor’s large size, there was only modest superficial cortical involvement. The bone flap was made slightly larger than in the previous case, but not significantly, as the deeper tumor was easily accessible using keyhole maneuvers.

Related posts:

2 Cognitive Principles of Planning Keyhole Approaches 3 Technical Principles of Operating in Keyhole Craniotomies 12 Keyhole Approaches to Tumors of the Middle Fossa, Cavernous Sinus, Tentorium and Lateral Midbrain 15 Keyhole Surgery of the Tectum and Pineal Region 13 The Cerebellopontine Angle 5 An Atlas of Intracranial Endoscopy

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