11 Temporal Lobe Gliomas
11.1 The Importance of Temporal Lobe Gliomas
Temporal lobe glioma surgery is the foundation upon which excellence in glioma surgery elsewhere is built. In other words, if you want to be a great glioma surgeon, you need to start by mastering the temporal lobe.
In part this is because these tumors are common and they are defeated early in your learning curve. More saliently, learning the temporal lobe and how to not only stay safe, but also how to ensure a complete, aggressive resection teaches you a great deal about relationships with the rest of the cerebrum. Mastering a temporal lobe resection is mandatory before working in the insula, as the relationship of these two structures is complex, and necessarily intertwined. Further, knowing the anatomy of the temporal lobe provides you with a firm grasp of its relationships with the inferior frontal lobe, the anterior occipital lobe, and the temporoparietoocipital junction. This chapter is early in the sequence, because it’s challenging to study the rest of this book without a firm handle on this chapter.
11.2 Overview of Temporal Lobe Anatomy
With two exceptions, the IFOF, and uncinate fasciculus, the white matter connections of the temporal lobe structures run posterior and to some extent superiorly in a “C-shape,” which is what one would expect from the brain lobe at the embryologic end of the C-shaped cerebrum. The uncinate largely connects the temporal pole with the orbitofrontal cortex, and both it and the IFOF are largely running into the frontal lobe through the limen insula which is quite anterosuperior to most of the temporal lobe. Thus, most of the temporal lobe fibers are headed backwards to the SLF, the ILF, or the angular or parietal cortices. Thus, a posterior cut in the temporal lobe functionally disconnects most of the temporal lobe.
The most important anatomic observation is that most of key white matter tracts (SLF, IFOF, optic radiations etc) form a lateral system which overlies the atrium and posterior temporal horn. As previous chapters have made clear, the lateral system contains many of the most important connections of the human cerebrum, which comprise the speech, spatiotemporal, and visual networks. Also important is the observation that the medial temporal structures, the amygdala, hippocampus, and parahippocampal gyrus, largely lie in a distinct system medial to the temporal horn. Thus, one can safely follow the medial system using the temporal horn as a landmark, and remove brain structures almost back to the occipital pole, and around the back of the thalamus and brainstem without disturbing the lateral system (Fig. 11.1). Indeed, this concept is the key to temporal lobe glioma surgery.
A few additional gross anatomic relationships are necessary to master for these cases (Fig. 11.2). The STG is purely opercular, meaning that it only overlies the insula and Sylvian fissure. The second is that the insula in normal brains generally extends downward until it parallels the top of MTG. The MTG overlies the temporal horn, which is logical as this is just below the basal ganglia which are deep to the insula. The temporal tip extends well in front of the limen insula and the uncus; thus you are not at the uncus in a temporal resection until you are behind the clivus, and over the tentorial edge, a common point of confusion for beginners.
Most critically, a firm grasp on the medial temporal structures and their relationships are of utmost importance (Fig. 11.3), as they are commonly involved with tumor, are often distorted and grossly enlarged, and are full of traps for the unwary. The uncus/amygdala is anterior, superior and deep to the tip of the hippocampus, and is essentially always spilling over the edge of the tentorium. The hippocampus is the floor of the temporal horn, but it is not the medial bank of the temporal lobe which abuts the ambient cistern, the parahippocampal gyrus is. The hippocampus sits directly inferior to the putamen, which is perhaps the most important anatomic landmark to store in your memory for glioma surgery. While the fornix enters the atrium to course around the thalamus, the PHG bifurcates to join the isthmus of the cingulate gyrus and the lingula.
11.3 The “Define” Stage
11.3.1 Preoperative Planning
First, put the epilepsy book away. Temporal lobe glioma surgery is foundationally based around the temporal lobectomy, which was first mastered in epilepsy, but glioma surgery is not epilepsy surgery any more than meningioma surgery is. Epilepsy surgery aims to disconnect the epileptic focus from the rest of the neural circuit, and the trend has been to cause this lesion in an increasingly targeted way, such as selective amygdalohippocampectomies, radiosurgery, and more recently laser ablation. This trend not only makes sense from a phenomenological perspective, but also from a goals of care perspective: epilepsy patients are usually not going to die of their disease in a short time frame, and thus minimization of mild cognitive and memory problems makes sense with in this mandate.
While targeted methods and avoidance of mild cognitive problems make sense within that framework, I would argue that they are completely illogical in a disease with a fatal natural history, and that sacrificing tumor resection for attempted preservation of subtle functions usually ends in the long-term effect of accomplishing neither. It makes little sense in performing a trans-Sylvian selective amygydalohippocampectomy for a disease that cannot be accurately defined by the naked eye or imaging, but that we know has spread to the surrounding brain. Glioma is an area of the brain, and not a discrete lesion, and it is foolish to ignore the fundamental nature of this disease and allow tumor to recur in easily expendable brain. Further, a selective amygydalohippocampectomy was mastered for a limited resection of the hippocampus, and not for following the hippocampus to the isthmus and behind the thalamus, which is often necessary to completely remove gliomas extensively following the hippocampus. In short, glioma surgery has different goals, and trying to avoid an overwhelming attack on the tumor, often is not doing the patient a favor.
Temporal lobectomy is the best tolerated lobectomy and is highly effective at guaranteeing an excellent resection (there is no chance of leaving residual disease along the temporal floor after a lobectomy for example). Most cuts on the back of the temporal lobe disconnect the most of the temporal lobe circuits from the rest of the brain anyway, so in most cases it is best to just maximize the amount of tumor cells being removed.
11.3.2 Types of Temporal Lobe Gliomas (Fig. 11.4)
While there can be overlap of these subtypes, especially at more advanced stages of the disease, most temporal gliomas can be classified as one of six principle types.
Anterior: These tumors are the most common and are centered within the anterior temporal lobe without major invasion of the SLF. They can spread into the insula via the uncinate fasciculus or into the medial temporal structures.
Hippocampal: These are also common, and involve the medial hippocampal structures. Their preference is to follow the Papez circuitry and extend backwards along the forrix into the ventricle, or into the cingulum and isthmus. They can follow the diagonal band of Broca into the basal forebrain and contralateral amygdala, or the ventral amygdalofugal tract into the hypothalamus. They generally do not spread to the lateral system until late stages.
Inferior: There are uncommon and are based in the fusiform gyrus and preferentially spread along the ILF into the occipital lobe, though they may invade the medial structures as well. The “pull-through” approaches described in Chapter 16 were designed to address these tumors when they become extensive.
Lateral: Thankfully, these tumors are relatively uncommon, but are usually bad news. They invade the lateral system, and preferentially follow the SLF. Many times, very little can be removed due to functional concerns. The hippocampus is generally spared (as we would expect with the lateral vs. medial tract distinction), and I generally try to leave the hippocampus in these cases if it is normal, as the recurrences do not commonly occur there.
Junctional: These are the worst of the bunch. They are fundamentally lateral temporal tumors, extend along the junctional anatomy of Hescl’s and/or supramarginal gyrus, into the posterior insula and abut the internal capsule. The right-sided ones can be resected with patients, but the left sided ones are often impossible to make much progress with if they are small and in the SLF or internal capsule. In essence, they combine all the bad parts of a lateral temporal case, with the risk of injuring the back of the internal capsule. Oh, and the arteries at the back of the insula can also be headed to the motor strip, speech areas, or TPO junction.
Anterior occipital: These are better classified with occipital lobe tumors, but they can invade the medial temporal lobe, but do so largely behind the SLF, instead of in front of it. I discuss this more with occipital lobe tumors, as the anterior occipital cut is a key part of occipital glioma surgery and these cases fit better in that discussion.
11.3.3 The Approach
Unlike frontal gliomas, the decision is relatively simple as there are two major tracts at risk, the temporal ramus of the SLF, and the IFOF. Further, with the exception of lateral and junctional cases (which are a “surrounded on all sides” type cut), the division will be either a posterior temporal or an anterior occipital cut, depending on the tumor’s relationship to the temporal ramus of the SLF. The IFOF is a deeper tract, and while it is critical not to disconnect temporal gyri from this tract during the subcortical work, its exact position does not determine the position of the craniotomy significantly as it is not on the surface.
The craniotomy (Fig. 11.5) is placed over the planned portion of the STG and MTG where I anticipate the SLF and its termini to be based on the DTI. The bone flap is made large enough to be able to move a bit if I find essential parts of the speech network (this movement is almost always anteriorly). I make sure I can reach the “corner of the temporal lobe” (i.e., the part of the STG which intersects with the bridging vein to the sphenoparietal sinus, which is the STG which lies anterior to the limen insula). I also make sure I can reach the temporal floor under the bone flap. There is no need to elevate the entire temporalis muscle and take bone down to the middle fossa floor, as I have never had trouble doing the temporal lobectomy under the bone once the cut is finished.
In anterior, inferior, or hippocampal temporal lobe gliomas, it is almost always smart to drop the tip of the head downwards (Fig. 11.5) as the difficult angles are typically looking from inferior to superior over the patient’s shoulder under the Insula or the lateral system, or upward into the atrium. For hippocampal gliomas which extend so far posterior that they are in the atrium, a slight ipsilateral head rotation can be helpful for looking under the lateral system, but this can be disorienting and it is better to achieve this largely by rotating the bed, as opposed to overly rotating the head position from the ideal.
11.3.4 Cortical Mapping
As always I start the mapping with motor, motor planning, and speech arrest, but it is basically a warm up as these functions are not usually found in this part of the brain. I generally do not expose the supra-Sylvian opercula in these cases. Naming and reading are critical here, especially on the left side. I also perform neglect tasks, on either side, but it usually is mostly right-sided and localizes to the SLF. These are cases in which we generally find something, and the cortical mapping is usually not negative.
11.4 The “Divide” Stage
11.4.1 Posterior Temporal Division (Fig. 11.6)
This is an L-shaped cut which seeks to separate the anterior temporal lobe from the SLF, IFOF and the subinsular region using the temporal horn as a landing site. In general, the further posterior the cut is angled posteromedially under the lateral white matter system into the temporal horn, the easier resecting the medial structures will be, which is especially important in extensive hippocampal gliomas, where an inadequately posterior cut can make reaching under the lateral system to reach involved fornix or isthmus impossible (Fig. 11.7). The challenge with putting the cut very posteriorly is you encounter the semantic networks, which on the left are naming sites and the right are neglect sites. The subcortical work involves balancing the need to push the cut backwards, versus the functional need to push the cut anteriorly.
The first order of business with performing this cut is to find the Sylvian fissure and insula by taking down the STG from front to back. If you have mapped the cortex well, then it is safe to remove the STG anterior to any naming sites found, as the STG’s output is largely headed posteriorly. It is essential to identify the “artery of death” exiting the posteroinferior fissure and try to manuipulate this as little as humanly possible. A stroke of this artery will often injure the entire semantic network and on the left side this is devastating to language. Taking down the STG first teaches you the orientation of the temporal lobe, shows you where the bottom of the insula is, and frees the temporal lobe anteriorly and superiorly for en bloc removal.
One the STG has been removed, I make the posterior cut. I generally use an arm movement and naming double task for the posterior part of this cut. It is proceeds superior to inferior and it is best to try to find the temporal horn as soon as possible as this tells you where the IFOF is running (always above the horn), and the horn is the anchor that the rest of the cut is based around. I then continue the cut inferiorly until I am close to the temporal floor. At that point, I work in the temporal horn from inside out to extend this cut anteriorly below the insula. Once I am roughly anterior to the limen insula, the temporal lobe is disconnected, and the patient is put to sleep as the work on the middle fossa dura is painful and does not need mapping.
These are challenging the sagittal cut runs from t cases. In lateral cases the disconnection is the entire case, as you are potentially disconnecting on the posterior, inferior, anterior and medial surfaces, with the only safe boundary being the pia of the Sylvian fissure. Here, I stimulate often, work slowly, stay subpial as much as possible, and use sulcal boundaries as thoughtfully as possible. At some point, the subcortical mapping, or concern for a tract on DTI will cause you to stop dissecting deeper, and the lower boundary is arbitrarily defined. Occasionally you can make it to the ventricle in these cases.
Junctional disconnections are more complex as they involve two phases (as do all insular disconnections). The first involves exposing the involved deep structures by resecting STG or SMG as much as possible permitted by the mapping. This is slow and deliberate as there are multiple arteries at risk, including the “artery of death.” While the STG is best thought of as a tube sitting on the insula, the SMG is more complex, and can be confusing the first few times you subpially remove it. Essentially it is best thought of as a wide mouthed ice cream cone, with the apex pointing at the back of the insula and the atrium Once you have made the best window into the tumor possible, you should identify the surface of the insula (which may be small this far posteriorly, and if possible enter the atrium, which provides excellent orientation to this area (most of these patients already have field cuts and even if they don’t it is unlikely you will avoid it and still take a tumor like this out). In some cases, a lesser version of a posterior temporal cut, or even a temporal lobectomy may be warranted based on the anatomy. One you reach the deep part of the work, it is wise to switch your mapping to monitoring the leg, as this is what is at greatest risk.
I usually do not stop the subcortical mapping for these cases until the tumor work is done.