5 White Matter Anatomy of the Cerebrum
White matter anatomy is not a new hobby for neurosurgeons, but at the same time it is in a period of great change as new techniques and technologies allow us ask and answer questions which were not realistically possible to answer previously. Notably, with improvements in technology such as DTI and DSI, we can now ask what parts of the brain are connected by various tracts, and the relationship of different tracts to each other in 3-dimensional space. Thus, this raises the possibility of a gross anatomic understanding of functional brain networks on a macroscale, with the obviously profound implications for cerebral surgery.
This chapter is a repository of cerebral white matter anatomy. Some of the information in this chapter is described in other work, while much of this is from my own studies of smaller local white matter connections. Functional implications of these connections are discussed in other chapters, especially as it relates to tumor surgery.
5.2 Major Cortico-cortical Tracts
We start with a description of the major cortico-cortical tracts both because of their major importance to functional outcome, but also because a deep understanding of these tracts provides a reference point for smaller tracts, and local subgyral white matter anatomy, which often either joins these tracts, or at least can be explained in reference to them.
A helpful way others have classified the major tracts is in reference to the corona radiata (Fig. 5.1), and basal ganglia which divides the major tracts into lateral tracts, which generally run anterior to posterior, and medial tracts, which are similarly oriented in the anterior-posterior plane, but also include the callosum fibers which run bi-hemispherically. The central core fibers of the corona radiata run mainly rostral-caudal and include the corticospinal tracts, the fibers of the thalamic peduncle, among others.
Beyond just providing a classification system, this way of mentally classifying the tracts forces us to think differently about the transgressing the cerebrum as it shows us that the lateral tracts form a wall which generally blocks direct lateral approaches to deep targets such as the ventricle. This forces us to take what are counter-intuitive and longer approaches to the lateral ventricle (Fig. 5.2), which avoid these large association tracts. Alternately, Fig. 5.3 demonstrates a case which shows that in an emergency, taking the easiest route to a clot is often the wrong answer for long term functional recovery, as in this case when a transinsular approach avoided the tracts in the absence of image guidance and saved the patients function in the long term.
5.2.1 Lateral Tracts
Superior Longitudinal Fasciculus (SLF)
We start with the SLF as it is the dominant lateral tract, and it dominates our thinking about functional networks and glioma surgery. It is a massive tract, which covers a great deal of ground, and its preservation is integral to leaving the patient as a functional person. There are a large number of networks whose primary hubs require communication via the SLF, including the speech, neglect, praxis, and attention networks, and this means that there are few better ways to hurt someone than cutting through the core of the SLF complex.
The SLF has been described as being comprised of 4 layers with the arcuate fasciculus being the lateral part of this tract, the arcuate fasciculus, and the SLF I, SLF II, and SLF III. In the description of functional networks, it is quite helpful to know these distinctions, as they help to reduce the mental complexity of this extensive tract to usable subunits. In surgery, these distances between systems are generally not observable or actionable, and thus distinctions are academic, and you should view the SLF as a single large tract.
It is easy to mistakenly view this as a c-shaped tract, as the bulk of it mirrors the c-shape of the hemisphere; however, by viewing it as a modified-Y shape, you remind yourself of the parietal ramus, which is sizeable and essential for networks which engage the inferior parietal lobule, such as praxis. This ramus is the lateral margin of most medial parietal cuts, so it’s critical to know it.
The most obvious enpoints of the SLF complex are the supramarginal gyrus, the inferior parietal lobule, and the middle frontal and triangularis/orbitalis parts of the inferior frontal gyri. The fact that this tract links the major language centers of the left hemisphere should be obvious to most neurosurgeons, and this is how I view the left SLF, as a highway for phonemic and syntactic aspects of speech. It is critical to note that the SLF is fundamentally more complex that this, and its carrying a great deal more information than this.
We have long ignored the SLF at our own peril. During evacuations of basal ganglia hemorrhages, it was not at all rare to see someone pick the shortest path (usually through part of the SLF) and transgress that to evacuate the clot. We then would chalk up a bad outcome to the initial bleed. Careful review of white matter anatomy makes the foolishness of this approach obvious. We now think a better approach is through a more anteromedial corridor.
The SLF creates a virtual wall over the lateral hemisphere, and often makes the direct lateral approach to deep targets challenging or impossible.
Anatomy of the SLF in Normal Patients
It is critical to note that all of the following descriptions of normal tract position and positions come from anatomic studies of patients without gliomas, and that we have seen a wide variation from this in our glioma patients with tracts in unusual places, and often with major rami not present from either destruction or re-organization. Thus, the images in Fig. 5.4 provide a basic understanding of what the SLF typically looks like, with the caveat that it is probably not exactly what your patient’s SLF (or any other tract) is going to look like.
The SLF generally has an anterior-posterior core running under the sensory and motor cortices paralleling the lateral ventricle, and a genu which bends just deep to the supramarginal gyrus. There are three principle rami which contain the main termination of the tract: The temporal ramus, which terminates in the supramarginal, middle temporal, and superior temporal gyri, the frontal ramus which terminates mostly in the middle and inferior frontal gyrus, and the parietal ramus which largely ends in the inferior parietal lobule and the banks of the intraparietal sulcus.
The SLF (and most other tracts for that matter) are best viewed as a bus route rather than an airplane flight. Our methods, be they physical dissection of imaging based methods, are limited in their ability to reliably define small contributions from overlying gyri. Thus, it is likely that small contributions from many of the overlying gyri also enter the SLF at several points along its course, so it’s to consider the SLF as a bus route more than an airplane flight, as there are probably multiple points of entry and exit.
One thing worth noting is that the majority of the SLF runs in the white matter core which lies deep to the depths of the overlying sulci. Only at its rami, when fibers turn towards the cortical surface does it present itself superficially. Thus, I generally do not start to look for the tract until I have reached the bottom of the relevant sulci.
Additionally, the SLF and several other tracts cross just lateral to the posterior most point of the internal capsule just posterior to the insula, and deep to the temporo-parieto-occipital junction, making this area one of the most functionally treacherous parts of the cerebrum.
Inferior Fronto-occipital Fasciculus
This tract is quite long (Fig. 5.5). It runs from anterior to posterior connecting the inferior occipital and frontal lobes. Again, it’s best to think of this tract like a bus route than a single connection. Its most notable feature is that it runs at the base of the insula, just above the temporal stem, and hooks anterior at the limen insula before fanning out to the frontal lobe. This fact dominates insular glioma surgery.
The occipital origins of this tract are principally in the lingula and cuneus and after hooking superiorly around the pre-occipital notch before diverging from the ILF and crossing at the TPO junction as a largely compact bundle. After bending anterosuperiorly at the limen insula, the fibers fan out. The bulk of these fibers are connected to the SFG where they interact with a large length of the gyrus. There are also communications with the IFG, and a lesser communication with the MFG.
The IFOF is similarly located at the crossing fibers of TPO junction and runs just deep to the genu and temporal ramus of the SLF and superficial to the optic radiations. Together with the SLF it frames the insula, anterior and inferior (IFOF) versus posterior and superior (SLF) and working in the insula safely working within that “frame” (Fig. 5.6).
Superior Frontal Occipital Fasciculus
This fiber bundle, which was claimed to run just lateral to the cingulum was described in fiber dissections long ago and has had numerous names. Yasargil first questioned its existence stating that it was an artefact of dissection techniques. Our work with fiber tracking supports this, as we have found no evidence that there is a tract running in this region connecting the frontal and occipital lobes. Most of the communications between the processed outputs of the visual system and frontal planning areas run via the SLF, which highlights the parietal lobe’s important role of translating visual information into a form useable by the motor systems.
These are probably one of the better known white matter tracts to neurosurgeons so I will not belabor the point (Fig. 5.7). Probably the most surgically important aspect of this anatomy is that the optic radiations comprise form the majority of the lateral wall of the atrium of the lateral ventricle, a fact which radically alters the ideal approach to the atrium and similarly positioned structures.
It is also of relevance to note that the visual system probably works in large part by communications with the pulvinar which are critical in serial visual processing. These fibers generally run in the same bundle as the optic radiations. There are numerous examples of this fact worth noting demonstrated in this chapter.
Inferior Longitudinal Fasciculus (ILF)
The ILF is an anterior to posterior tract running from the occipital pole to the temporal pole running just deep to the fusiform gyrus on the inferior temporal/occipital surface (Fig. 5.8). This tract does not typically block a lateral approach to a deeper target in the way other lateral tracts do; however, it is a common path for spread of gliomas.
It primarily connects the inferior temporal gyrus to the lingula, though some fibers extend superiorly into the temporal pole.
Most of us are aware that this tract is running in the limen insula and connects the frontal and occipital lobes. Specifically, it connects the inferior frontal gyrus, pars triangularis to the anterior temporal lobe (Fig. 5.9).
It is laterally positioned relative to a similar bend in the IFOF, and this has caused the functions of these two tracts to become confused. Its most clinically relevant attribute is as a pathway of spread of gliomas from the temporal to the frontal lobes through the insula.
About the Superior Fronto-occipital Fasciculus (SFOF)
The IFOF was so named because early fiber tract dissections suggested a small tract running from occipital to frontal lobes. Subsequent dissections raised questions about its existence, and our own work has failed to demonstrate any evidence that such a long tract exists. We believe that what was previously called “SFOF” is in reality parts of the medial SLF components, or an artefact of the U-fiber anatomy. It is important to note that there is not a great deal of information directly carrying early visual processing information to the motor planning areas (as an SFOF would be expected to do).
Because of its familiarity to us, we call the more definitively present IFOF by its historic name.
5.2.2 Medial Tracts
This tract is best known for its inclusion in the Papez circuit, completing the loop by connecting the cingulate cortex to the parahippocampal gyrus. While this is certainly one of the functions of the cingulum, I would argue that its role in attention networks, such as the default mode network, is of equal or greater importance when operating near the midline.
The cingulum is a c-shaped tract which runs in the center of the cingulate gyrus, the isthmus, and the parahippocampal gyrus (Fig. 5.10). Its primary branches are to the superior frontal and subparietal gyri (probably part of the default mode network).
This well-known tract is really a collection of numerous of c-shaped bihemispheric connections (Fig. 5.11). The basic parts (from anterior to posterior) are the rostrum, genu, body, and splenium. The majority of the callosal fibers connect homologous contralateral brain regions. For example, large connections exist between homologous parts of the superior frontal gyrus, the superior parietal lobule, the medial occipital lobe, etc. There are homologous connections between more lateral cortices (middle frontal and inferior frontal gyri for example), and there are some connections between nonhomologous brain regions (as with what I term here the crossed FAT tract below which connects the middle frontal and inferior frontal gyri with the contralateral superior frontal gyrus).
The connections of the callosum are easily remembered by dividing the supra-rostral parts of the callosum into fifths. The genu and anterior fifth connects medial prefrontal cortices, the second fifth connects motor and pre-motor cortices, the third fifth connect parietal cortices, and the posterior two-fifths connects occipital cortices (Fig. 5.11). Equally important is the relationship between the corpus callosal fibers and the cingulate gyrus and cingulum. The corpus callosum wraps inferiorly, then laterally, then superiorly around the cingulate gyrus and cingulum towards its target (Fig. 5.11). Because the cingulum and corpus callosum run roughly perpendicular to each other, they do not cross, there is only occasional cross talk between their relevant gyri (they are also different types of cortex), and they can be dissected free from each other without too much difficulty in cadaveric fiber dissections. This relationship is critical for understanding how to safely remove butterfly gliomas.