12 Insular Gliomas
12.1 Climbing the Mountain
Insular gliomas have a long learning curve. While I was learning to do these surgeries, I found many of the resources for learning how to take these tumors provided inadequate insight and detail to successfully achieve excellent results every time. I estimate it took me about 40 cases before I had one truly satisfying case (meaning complete resection without neurologic problems).
There are a fair number of facts one must understand about the insula, its anatomy, and techniques for safe and effective resection. Despite these challenges, insular gliomas are a tumor which you can consistently achieve good results if you adhere to a disciplined pattern of attack which builds on the groundwork we have set forth in previous chapters.
What follows is a guide I wished I had had during my learning curve.
12.2 Fundamental Anatomic Relationships of the Insula
What follows is a list of facts about insular anatomy which are essential to success in these cases.
The insula is roughly a right triangle with its right angle under the frontal operculum and its hypotenuse running from the parietal operculum to the Sylvian vallecula (Fig. 12.1). This means it is not rectangular, and it extends more posteriorly on the parietal side than the frontal side.
The insula is completely covered by opercula cortices. You cannot see the insula without either removing them or splitting the Sylvian fissure, mobilizing and retracting them.
The insula extends significantly beyond the immediate boundaries of the Sylvian fissure (Fig. 12.2). The circular sulcus runs roughly parallel to the top of MTG inferiorly, and the top of IFG and SMG superiorly. A significant part of the insula is covered by the face/tongue sensory and motor cortices.
The MCA branches run over the surface of the insula. This means the insula must be removed between the MCA branches.
The insula is bounded by a sulcus on all sides (the circular or marginal sulcus) outside of which the white matter folds over onto the opercular cortices. This means that if you have not seen the circular sulcus, there is still more insular tissue under the operculum in the direction you are looking. The circular sulcus also provides an excellent idea of where the white matter tracts are running.
The putamen is the deep boundary of the insula. It is roughly superior to the hippocampus in the coronal plane as I have pointed out several times in the book. When people ask me how I know how deep to go and when to stop, this is what I tell them.
The SLF and IFOF roughly frame the boundaries of the insula (Fig. 12.3). The SLF dominates the posterior and superior boundaries, while the IFOF runs in the inferior and anterior boundaries.
Deviating out of the insula accidentally in two places can put the motor fibers at risk (Fig. 12.3). One of these places is superiorly above the circular sulcus under the motor opercula. The other is at the posterior aspect of the insula, where the leg portion of the posterior limb of the internal capsule extends laterally at the junction.
12.3 The “Define” Phase
12.3.1 Preoperative Planning
It is important to note that most insular gliomas are not limited to the insula, but are multilobar, as pure insular gliomas are an uncommonly found, early stage of disease. Pluralities of these tumors are primarily temporal, some are primarily frontal, and occasionally they are both. It is critical to understand how these tumors have left the insula, so that these tumor extensions can be addressed in addition to the insula.
Common ways tumors leave the insula are as follows:
Inferior extension into the medial temporal lobe: This is the most common type of spread, this can also be spread to the anterior temporal lobe.
Spread to the orbitofrontal cortex via the uncinate fasciculus: Also common. This is usually best removed during the insular resection by following the tract under the frontal lobe.
Spread upward into the premotor regions: Less common, but it is the worst form of spread.
Spread into parahippocampal gyrus and cingulum: Usually occurs with spread to the medial temporal lobe.
12.3.2 Transopercular versus Transsylvian Approach
I have tried both approaches, and have strongly sided on the side of basing my insular surgeries on the transopercular approach to the insula. The reason for this is a matter of access. The insula extends well beyond the Sylvian cleft under the opercula both frontally and temporally. By limiting to your exposure to that which can be reached by mobilizing the opercula, you are increasing the difficulty of a difficult case by working under lips of brain to see the circular sulcus.
More critically, the insula is bounded by tracts on all side by large white matter tracts. It is difficult or impossible to map the IFOF along its length by reaching under an entire gyrus, and equally difficult to work along the posterosuperior portions of the insula without working blindly near the SLF. By taking off the STG, you get an excellent view of the inferior insula and can map and protect it.
Finally, it is uncommon that a glioma is entirely confined to the insula, and by trying to conceptualize these complex, multilobar tumor merely in terms of the insula, you will tend to underaddress the tumor elsewhere. Insular gliomas are typically a much bigger problem than just the insula. This is human nature: to fixate on the unusual and to ignore the more typical. Most insular gliomas need more than just an insular resection, and the temporal lobe often needs to come out as well, making the transopercular approach necessary for more than just access.
By taking off the operculum, it’s much easier to see what you are doing, to define the boundaries of the insula, and to protect the white matter tracts. Again, gliomas are big tumors and require big resections. Clever and precise approaches make for good videos, but are not always the easiest way to take out a glioma reliably effectively. Perhaps some people can do insular gliomas through a split Sylvian fissure, but you are much more likely to hit the target approaching transopercularly.
12.3.3 Frontal versus Temporal Approach
The key decision to make in the planning phase is which side of the insula to approach the insula by removing the temporal or frontal opercula. This is generally obvious in most cases. For example, if a temporal lobectomy is planned for part of the tumor resection, then the temporal approach is the obvious choice. If frontal operculum or premotor areas are significantly involved, then a frontal approach is needed as it is challenging to just work beneath the frontal operculum. When it is unclear, I approach from the temporal side, as the frontal side can be more limiting due to premotor, motor and speech areas in the frontal operculum. The STG usually can be resected over most of its length in most patients. Massive tumors and tumors encasing the MCA complex might need an approach from both sides.
As could be expected from the positioning described in the previous chapters, a temporal approach should be positioned with the slightly tipped head down, and a frontal approach with the head tipped up to provide a good angle on the insula (Fig. 12.4).
12.3.4 Intraoperative Definition
In an insular glioma case, the aim of cortical mapping is principally to take off the operculum to provide good access to the insula for mapping and resection. This is in addition to the needs of any other parts of the case, such as temporal lobectomy, medial temporal resection, or lateral frontal disconnection. If they need to be done, additional cortical mapping and disconnections need to be added to the insular case as described in other chapters.
What mapping tasks to focus on, obviously depends on the angle of the attack and what else is involved. For example, a left-sided temporal approach is dominated by language considerations, and a right temporal approach by neglect system of the STG. Supra-Sylvian mapping involves language, and motor planning anteriorly. As always, DTI tractography is helpful at predicting the location of the networks to guide your mapping; however, note that the connections between the semantic areas of posterior temporal lobe and the IFOF are often not visible on DTI tractography, as they cross and intermix with the temporal ramus of the SLF. The temporal ramus is usually located in the immediate proximity of the semantic areas though.
12.4 The “Divide” Phase
An insular glioma is a complex two phase resection, which can take some time to complete. Phase one involves removing or mobilizing the opercula in order to visualize the insula, and phase two involves defining and removing the insula.
12.4.1 Phase 1: Exposing the Insula
From the Temporal Side (Fig. 12.5)
A large fraction of insular gliomas co-inhabit the medial and/or anterior temporal lobes, and thus a large percentage of these cases will involve a temporal lobectomy. It is critical to note that I generally do not use the valuable time during mapping to complete the lobectomy. A patient will only map for so long, and you should use this time to focus where you really need to map. Plus, you will bump the dura multiple times taking out the lobe which causes pain, which in turn is distracting.
The initial phase of a temporal approach involves making a posterior temporal cut as described in Chapter 11. This involves first removing the STG from the posterior limit of the cut forward until you reach the anterior corner (i.e., the front of the Sylvian fissure) in front of the limen insula. This shows you where the insula is located, defines the vessels of the inferior Sylvian fissure, and helps locate the IFOF. In this phase, there are two key mistakes to avoid. First, the “artery of death” is usually leaving the Sylvian fissure and heading to the posterior temporal lobe, and this can completely undo a speech mapping case. Second, you want to keep an open mind regarding the short communications between a mapped site (especially language sites) and the IFOF or SLF as DTI tractography may not demonstrate these small fibers well and they can arc gently anterior or just dive directly medially. You should resect the STG as far posteriorly as the mapping will allow, as the posterior insula is both dangerous and easy to miss unless you make a concerted effort to find it. A fair bit of insula is tucked into the temporal junction.
At this point, you should extend the cut inferiorly across MTG towards the middle fossa floor. Once you know where the insula is, then finding the temporal horn is easy. This cut is complete once it extends into the ventricle or to the floor across the entire length of the cut.
Once the temporal lobe is out of the way, the next step is to elevate the frontal opercular cortices. I let the patient rest during this period. I do this by gently retracting the opercula off of the insula bluntly, i.e., by padding the cortex then sweeping to pull apart the arachnoid of the Sylvian fissure. You should be careful to not over retract across the plane of an artery leaving the fissure; however, with care this maneuver is usually safe and effective. You should continue doing this from anterior to posterior until you clearly see the circular sulcus on all sides. If you have not seen the sulcus and its groove where the opercular cortices turn back on themselves to overlie the insula, then there is a very high chance you will miss a fair bit of the insula. Of particular importance, is the need to see the posterior superior and anterior superior portions of the circular sulcus, which are much higher underneath the frontal opercula than you think. If you cannot see them then you will either miss a great deal of tumor, or you will be blindly sweeping near the SLF or corticospinal tract, both of which are suboptimal ways to operate in the insula.
You should be aware that approaching from the temporal side, your attack angle is running slightly parallel to the cortical surface, with a medial angle, which you should take into your calculations as you descend into the insula (Fig. 12.6). The angle is more medial inferiorly and more flat as you progress superiorly.
From the Frontal Side
This is the less desirable approach, as it requires you to tangle with potential speech and motor sites, which adds to the risk. However, if those opercula are involved and are spilling into the insula, then there is no way to adequately address these tumors without a frontal approach. Also tumors which a large superior extent are often very difficult to tackle from the temporal side alone, and sometimes adding a frontal approach is the only way to get a good exposure.
The frontal approach (Fig. 12.7) involves slow, subpialization of the opercula as dictated by the mapping and tumor anatomy. Usually, if a cortex has mapped negative during the define phase, then it can be resected down to the level of the insula. It is also acceptable to resect portions of the subcentral and lower sensorimotor cortices if absolutely needed (they are bilaterally represented and well tolerated). There are numerous arteries intertwined with these cortices, and many of them are going to the motor cortex, speech, motor planning, and other critical sites.
Uninvolved opercula (i.e., ones which are being removed for exposure but not for tumor resection) do not need to be gross totally resected, but instead need to be sequentially trimmed from bottom to top until you can see their pial bank curving into the circular sulcus. A debulked sulcus is often enough to mobilize the pia and arteries and get them out of the way.
An analogous sweeping of the STG off of the insula is then performed with the caveat that the “artery of death” may limit how much you can do this, and you may need to remove some STG as well to get enough room.
Note that while the attack angle of the frontal approach also parallels the insular cortex, it is more consistently parallel than the temporal approach (Fig. 12.8). In other words, you are often looking at the side of the insula, instead of directly medial to lateral as your mind thinks of it.
From Both Sides
Obviously, there are numerous possible permutations depending on the size and anatomy of the tumor and the functional brain anatomy. In most combined cases, I do the temporal side first, because it makes a lot of room quickly, and this shows me where the insula is which assists with the other portions.