13 Parietal and Occipital Gliomas
In the section on AVMs in his classic text, Yasargil spoke of parietal AVMs with a great deal of respect, noting that in his more experienced years, he had become more cautious with offering surgery for these lesions. When I read that during my training, I was somewhat surprised, as I had never considered the parietal lobe to be as treacherous as the motor strip, the brainstem, or the thalamus. However, experience has taught me that he is probably right on this, as it is quite simple to profoundly affect someone’s quality of life with misadventures in the parietal lobe. The gross anatomy of the parietal lobe is simple, but the microanatomy is exceptionally complex.
I have combined parietal and occipital gliomas into the same chapter, as in many ways the issues with these cases do not substantially differ between the two lobes. Many times these tumors cross the arbitrary boundaries we have made to define these lobes, making such distinctions irrelevant. More importantly, these tumors threaten the same systems and thus the cuts used to remove them follow similar principles. Fundamentally, surgery in the parietal and occipital lobes is surgery around the visual processing system, flanked by the two pathway systems (SLF/IFOF) (speech/praxis or neglect) laterally, and the sensorimotor system anteriorly.
Despite being anatomically simpler than, say, surgery in the insula, parietoccipital lobe surgery involved highly functionally complex brain areas, which integrate fairly abstract aspects of sensation, attention, and cognition. The obvious result of this is that the majority of these cases are mapping cases aimed at identifying and preserving function, with a significantly less demanding anatomic resection than other areas, such as removing the corpus callosum. In other words, this area can only be meaningfully defined in terms of functional anatomy.
13.2 Specific Demands of Parieto-occipital Surgery (Fig. 13.1)
The best friend of a surgeon doing surgery in the parieto-occipital region is the bilateral nature of much of the visual system. While there are neuropsychological syndromes which result from injury to parietal or occipital regions, a large percentage of them require bilateral injury to occur, making them harder to cause. This redundancy results from the richly bilateral networks which doubly represent much of the visual system. Most of these are homotypic connections and connect analogous medially positioned cortices; however, some heterotypic connections exist and also maintain the bilaterally of visual processing and its use.
While this feature gives some parts of the parietal lobe a forgiving nature, it also points out the need to take bilateral connections into your consideration for maintaining the flow of visual information with relevant association areas, such as the semantic networks. For example, a left occipital lobectomy does not on its own cause anomia or alexia; however, damage to the heterotypic connections between the right visual system and the left-sided semantic networks in addition to an occipital lobectomy will. This raises the interesting idea that in the context of an occipital lobectomy, that the corpus callosum becomes eloquent brain tissue worthy of stimulation mapping to preserve the speech network.
It is also critical to have a firm grasp of what parts of the parietal and occipital lobes are not sufficiently expendable and cannot be compensated from the other side. The most obvious lateralized networks are the sensorimotor networks, which are the determinants of the anterior cuts in many of these cases. The SLF connects parts of the parietal lobes involved in visuospatial information to frontal lobe premotor areas which are often critical for useful hand functioning, but equally critically it joins the semantic areas to (SMG, STG, MTG, etc.) to the frontal premotor areas, a critical aspect of the speech, praxis, and neglect networks. These networks are well established to be highly lateralized, and generally less tolerant to unilateral disruption as they generally lack strong homotypic connections from their partner gyri on the other side.
The visual system obviously can be disrupted unilaterally, causing a typical homonymous hemianopia picture or other similar variations on that theme. This is well known to be due to entering the optic radiations. Notably, this may result not only from disruption of the primary fiber from the LGN to the calcarine fissure and its banks, but such injuries likely disrupt communications with the pulvinar and superior colliculus necessary for higher visual processing. What is interesting is that interruptions of later parts of the pathways (dorsal or ventral), can often be compensated for. For example, transecting the left IFOF in the occipital lobe often does not cause a semantic speech loss provided visual information can reach these areas from the other side; however, the subinsular IFOF is not a bilateral tract and cannot be compensated for if cut.
13.3 The “Define” Phase (Fig. 13.2)
The diversity of parieto-occipital tumors that I describe below overestimates their differences. The key questions one needs to ask prior to cutting are always the same, and the classification system thus are different views of the same fundamental problems.
Question 1: Can I save the ipsilateral visual system?
It’s always best, if possible, to try to avoid a field cut, as a hemianopia prevents one from safely driving, among other problems. Sometimes this is not possible. For example, a patient who presents with glioblastoma invading the lateral wall of the atrium is unlikely to avoid a hemianopia in the long term, even if they don’t initially present with one. In a low grade glioma, it may be possible to save the fibers without compromising the oncologic plan. DTIs can be very helpful in this decision making.
Question 2: If the visual system is unilaterally compromised (or will be), how will visual input be transmitted from the other side?
Visual input is a critical component of semantic function. The absence of visual information from one hemisphere mandates that the other side visual information reach there from the other side, which means that fibers crossing in the corpus callosum (specifically the splenium and posterior body) need to be worked into the operative plan. Extension into the splenium can make the goal to save these fibers very challenging.
Question 3: What is the relationship of this tumor to the SLF?
With the exception of tumors in the medial bank of the polar occipital lobe, almost all tumors in the parieto-occipital region have some association with the SLF complex. Performing glioma surgery on these tumors involves dividing the tumor from the SLF on the tract’s posterior or medial border. With tumors in the TPO junction (STG, SMG or angular gyri) the SLF can be involved along a substantial length of the tract.
Question 4: What is the involvement of the motor system?
For some tumors, such as the anterior occipital tumors, the motor system is generally uninvolved. Most parietal tumors have components of the motor system in their anterior aspect, at least parietal association and sensory contributions to the motor network. At the other extreme, TPO Junction tumors can extend into the internal capsule at their deep border.
Question 5: Are the posterior cingulate gyrus or parahippocampal gyri involved?
The presence of tumor in the cingulum network changes the case significantly as it raises issues of risk to the default mode network, and the memory consolidation circuits. Tumor in these gyri can cause functional reorganization of these networks to an extent. They can also require one to follow the cingulate sulcus under the motor strip or down into the medial temporal lobe.
Based on these distinctions, I subclassify parieto-occipital tumors into the following basic groups (Fig. 13.2).
Anterior occipital tumors: These lie in the inferolateral occipital lobe and the principle cut is with the SLF anteriorly. They are principally tumors of the ventral visual stream, and one unique risk (other than the typical risk of causing a field cut and/or speech/neglect problems) is to reading which is uniquely in this part of the brain on the left.
Medial parietal tumors: These tumors lie in the space between the motor system anteriorly, the SLF laterally, and the visual fibers and IFOF inferiorly and posteriorly.
Posterior cingulate tumors: These are often part of a medial parietal tumor case, but can arise independently, in which case I plan the case as a transcortical surgery similar to a medial parietal tumor. They pose specific risks to memory and alertness, and defining the cingulate sulcus and its boundaries is critical for safety, especially when the tumor extends below the motor and sensory areas.
Occipital pole tumors: These tumors involve many of the same basic cuts as a medial parietal tumor, but generally are far less risky. The visual system is ipsilaterally at risk, and in many cases this cannot be saved, but with attention to the contralateral visual contributions, the overall cognitive benefits of maintaining connections between vision and other systems will persist.
Splenial butterfly gliomas: Pure butterfly gliomas are discussed in Chapter 14 with anterior butterfly tumors. It is important to note that many occipital region tumors invade the splenium secondarily and this is a serious threat to overall vision, and it is important to address this when it occurs.
TPO (temporo-parieto-occipital) junction tumors: these are among the most difficult tumors in the cerebrum to remove given that they can involve multiple networks at once and primarily arise in the semantic networks of the brain, in other words compromising those functions that most makes us human, which are centered in the inferior parietal lobule and its connections to the frontal lobe. Being surrounded on all sides, starting with neurologic deficits, working in distorted anatomy, and being forced into limited goals are par for the course in these cases.
These TPO junction cases are discussed in the next chapter on difficult gliomas.
13.3.1 Cortical Mapping
Cortical mapping aims to define the motor network, the attention networks, and the termination points of the SLF. We usually test numerous functional domains up front in these cases, as the redundancy between tests can often double check these tests. For example, testing neglect system function with target cancelation tasks rechecks motor planning.
I check motor planning, speech, and neglect tasks in every case. This is not because I necessarily plan on always finding positive sites for these functions (neglect is usually a right-sided function and speech is usually on the left), but because I always assume the SLF networks might be partially bilateral, and they may need the side I’m operating on.
13.4 The “Divide” Phase
Conceptually, there are really only two basic cuts in these cases: the anterior occipital cut and the medial parietal cut. This does not mean than we always take a resection of a tumor at the occipital pole up to the sensorimotor cortices; however, the more limited resection we perform in these cases is based around the principles based around these cuts.
Both cuts aim to preserve the SLF, and to maintain at least some visual input to the semantic system from either the ventral stream ipsilateral or the contralateral visual system. The principle differences between these disconnections lie in the relationship between the cut and the optic radiations, and the relationship with the sensorimotor network.
13.4.1 Anterior Occipital Disconnection (Fig. 13.3)
This cut principally aims to separate the tumor from the temporal rami of the SLF complex. Similar to a posterior temporal disconnection, once the lateral white matter system is separated from the tumor, the medial temporal lobe can be removed working medial to the ventricle and SLF. Obviously, the optic radiations are at risk, and you are by definition operating within the ventral visual stream; however, the IFOF is usually identifiable and salvageable, maintaining visual connection to the semantic networks. On the left, reading is clearly at risk.
13.4.2 Medial Parietal Cut (Fig. 13.4)
This is a challenging cut. It is L-shaped based anteriorly and laterally and requires a deep cut which will seem aggressive until you have experience doing it. The posterior and medial boundaries are with the core of the visual system (the posterior cut parallels the ventral stream and optic radiations and you are inherently sacrificing at least part of the ipsilateral dorsal stream which can be compensated for), and the falx, respectively.
The lateral cut is with the SLF, and this puts several systems at risk, notably speech and praxis on the left and neglect and the dorsal attention network on the right. I do this cut first almost always because problems with the motor network can make higher testing impossible, even if they are temporary. The cut essentially extends to the roof of the atrium and aims to follow the SLF down to the atrium remaining medial to the tract which similarly protects the optic radiations. Note that the roof of the atrium can have corpus callosum fibers, which include the semantic crossing loop, a small tract linking the bilateral semantic networks.
The anterior cut is a coronal cut which parallels the sensorimotor system anteriorly. It is essential to note that this network can be variably extensive, and you will often note coordination problems when working behind the sensory strip. This is due to manipulation of the parietal contributions to visuomotor coordination, which should be figured into your calculation.
One additional aspect of this cut involves the cingulate gyrus and the corpus callosum. While tumors can invade the posterior cingulate gyrus from the parietal lobe, as with frontal tumors, they often don’t as these white matter systems are quite distinct. If uninvolved, you should try to save the cingulum, using stimulation mapping for concentration tasks, and the cingulate sulcus to define the boundaries and stay out of the cingulum and DMN. This is especially true if you plan on entering the splenium. These should be planned for ahead based on tumor anatomy, the preoperative function and your goals of treatment.