3 Anatomy of Eloquent White Matter Tracts
Abstract
Lesions in eloquent areas represent a great challenge for neurosurgeons due to its surgical complexity. While the “eloquent area” is a concept that is more frequently linked to cortical regions alone, the role of deep fiber tracts in the proper functioning of the brain is often underestimated. At the same time, neurosurgical procedures ordinarily require passage through one or multiple fiber tracts to reach the target in intrinsic brain lesions. Ignorance of the functioning fiber bundles and accidental injury may result in postoperative neurological deficits. Hence, the stereoscopic structures of the brain, better than a pure cortical conception, should be taken into account when considering the “eloquence” of the brain. In this chapter, we will introduce the most relevant eloquent white matter tracts and their relationship with surgical approaches. The detailed knowledge of the three-dimensional brain anatomy is the key to avoid impairing brain functions during surgery.
3.1 Introduction
The white matter is made up of bundles of myelinated axons, known as fiber tracts. White matter tracts carry nerve impulses between neurons that resided in gray matter, acting as relays and coordinating communications between different brain areas. Any damage to the white matter tracts may result in impairments of certain brain function.
As axons of neurons, fiber tracts are generally not regarded as “eloquent areas” of the brain. Eloquent brain regions are defined as being essential for carrying out readily identifiable neurological functions, including the sensorimotor, verbal, acoustic, and visual cortex; the thalamus and hypothalamus; the internal capsule; and the brainstem. 1 , 2 If the fiber bundles do not function properly, neurological deficits will present regardless if eloquent cortical areas remain intact. Hence the idea of “eloquent areas” should be expanded to include deep structures rather than a purely cortical concept. And for the same reason, detailed knowledge of the white matter tracts is of paramount importance while dealing with intrinsic lesions in the brain 3 (Fig. 3‑1).
3.2 General Aspects
White matter tracts in the brain are categorized into association, commissural, and projection fibers. Short association fibers, also known as U-fibers, lie immediately beneath the gray matter, and connect adjacent gyri, whereas long association fibers connect the more widely separated gyri in the same hemisphere. The main long association fibers are the uncinate fasciculus (UF), the cingulum, the superior longitudinal fasciculus (SLF), the inferior longitudinal fasciculus (ILF), the arcuate fasciculus (AF), the occipitofrontal fasciculus, and the fornix.
Commissural fibers, which are axons connecting the two hemispheres of the brain, include the corpus callosum, the anterior commissure, the posterior commissure, and the hippocampal commissure. Projection fibers consist of efferent and afferent fibers connecting the cerebral cortex with the brainstem and the spinal cord. The main efferent projection fibers are the corticospinal tracts and the corticopontine fibers, while the principal afferent projection fibers are the optic and acoustic radiations as well as fibers arising from thalamus and reaching different cortical areas through thalamic peduncles. 4
3.3 Relationship of Fiber Tracts and Operative Approaches
3.3.1 Lateral Surface
Among white matter tracts, association fibers are most vulnerable to neurosurgical procedures. In the lateral aspect of the hemisphere, after removing the cortex and the short association U-fibers, three main fiber systems are shown. The SLF/AF system is the longest association fiber system at the lateral surface, connecting the frontal, temporal, parietal, and occipital lobes. In the frontal and temporal region, anteriorly, the UF originated from the frontal lobe displays a C-shape configuration and reaches the temporal pole. In contrast, the occipitofrontal fasciculus courses posteriorly to join the sagittal stratum and terminates in the temporal and occipital cortex (Fig. 3‑2).
In the dominant side, the occipitofrontal fasciculus is considered to be related with language function, while the UF is linked to memory, language, and social–emotional processing. 5 As the SLF/AF system connects the more extensive area of the cerebrum, it is suggested to be related to motor regulation, language, memory, and cognitive functions. Thus, when carrying out operative approaches to deep lesions in the frontal lobe, surgeons should be aware of possible motor or language deficits if the fiber tracts are damaged, especially in the dominant hemisphere. In the meanwhile, surgeons should pay more attention to the psychiatric and neurological illness related to the UF, which is frequently severed during the transsylvian approach to the temporal horn.
In the temporal lobe, the SLF/AF system connects the auditory area to the frontal lobe. As a result, it is considered to be involved in the language pathway. Deep to the occipitofrontal fasciculus and the fibers of the anterior commissure, a projection fiber system named as optic radiation forms the roof of the temporal horn. It carries the visual signal from the lateral geniculate body toward the striate cortex in the occipital lobe. Thus, the language and visual pathways may be at risk during surgical approaches to the temporal lobe. On the other hand, surgeons could also consider the feasibility of different plans, for instance, the transsylvian approach or the supracerebellar transtentorial approach to access the temporal horn, rather than the transsulcal approach, to avoid visual defect postoperatively.
In the frontoparietal region, the pyramidal tract and the thalamocortical pathway correlate to the precentral and the postcentral gyrus, respectively. These are two of the most eloquent projection fiber systems, transmitting motor and sensory signals to and from the spinal cord. However, postoperatively, motor weakness may present despite the intactness of the corticospinal tract if the supplementary motor area (SMA) is disrupted. The SLF is believed to play a role in coordinating the motor area and the SMA. Damage to the SLF in this region may also result in motor weakness.
Meanwhile, the motor and sensory pathways could also be injured in another segment, that is, the internal capsule. In the insular lobe, surgical interventions may cause damage to the internal capsule due to either direct interruption or vascular insults. On the other hand, although the extreme and the external capsule are proposed as long association fiber pathways related to the language circuit (since they connect the Wernicke area and the Broca area), the verbal deficit is not significant when they are severed.
3.3.2 Medial Surface
In the medial surface, the cingulum is the longest association fiber system as well as the main fiber tracts of the limbic lobe, connecting not only the cingulate gyrus and the parahippocampal gyrus, but also the neighboring frontal, parietal, occipital, and temporal lobe. The function of the anterior section of the cingulum is linked to emotion, while the posterior part is more related to cognitive function. Therefore, surgical disruption of the anterior cingulum, known as cingulotomy, is proposed to treat depression and obsessive–compulsive disorder as psychosurgery. Deep brain stimulation in this region is also an option to neuromodulate and treat all these psychiatric disorders (Fig. 3‑3).
The corpus callosum, formed by the strongest commissural fibers called callosal fibers, can also be best inspected at the medial aspect of the cerebrum. It is the broadest, thickest, and largest fiber tract in the human brain, extensively radiating to various parts of different lobes, thus interconnecting each hemisphere. Hence, a corpus callosotomy can be performed to treat patients with refractory epilepsy produced by unilateral epileptogenic foci. In addition, parts of the corpus callosum are frequently opened in order to gain access to the ventricles while dealing with intraventricular pathologies. A well-known adverse effect of callosotomy is the “disconnection syndrome.” Language impairments and memory deficits are also reported, but most complications are transient. 6
Related posts:
1 The Early History of Intraoperative Brain Mapping
2 Anatomy of Eloquent Cortical Brain Regions
9 Awake Craniotomy Operating Room Setup and Surgical Instruments
14 Surgery Around the Command and Control Axis: The Default Mode, Control, and Frontal Aslant Systems
12 Motor Mapping (Rolandic, Pre-Rolandic, and Insular Cortex)
21 Emergence of Deep Learning Methods for Deep Brain Stimulation–Evoked Functional Connectomics
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