Brain White Matter Tracts: Functional Anatomy and Clinical Relevance
Introduction
Diffusion-weighted imaging (DWI) has proven to be useful in many areas of neuroimaging and is routinely applied in clinical practice. Diffusion tensor imaging (DTI), although increasingly available, is yet to find routine clinical application outside of the research environment. After briefly describing the DTI technique, this review focuses on how DTI can aid in anatomical localization of lesions and subsequently account for the clinical features that arise.
Section snippets
Diffusion Tensor Imaging
In routine DWI, images sensitive to the diffusion of water in brain tissue are acquired with diffusion gradients applied in 3 orthogonal planes; the signals from the 3 separate images are averaged to compensate for the effect that tissue ultrastructure has on water diffusion in any given direction. DTI allows for characterization of this underlying structure by the creation of the “diffusion tensor.” The tensor describes the 3-dimensional probability distribution of water diffusion.1 If
Neuroanatomical Correlation
Standard clinical brain imaging combined with DWI is sensitive in detecting WM pathology, but owing to the poor visualization of specific WM tracts by conventional imaging, it does not always reveal the WM structures that are affected by the pathology detected. The following section reviews which WM structures should be assessed in various different clinical presentations and how DTI can help answer the frequent clinical question: “Does this lesion account for clinical presentation?”
Motor Syndromes
The corticospinal tract (CST) conveys efferent WM projection fibers between the motor cortex and the spinal cord, enabling voluntary movement. The tract predominantly originates from the primary motor cortex (Brodmann area 4) but also includes fibers from the premotor and supplementary motor cortices (area 6), the somatosensory cortex (areas 1, 2, 3a, and 3b), and the superior parietal lobule (area 5).4 These WM fibers converge and descend through the corona radiata, the posterior limb of the
Face, Arm, and Leg Weakness
The somatotopic arrangement of the primary motor cortex is preserved in the CST, which conveys efferent WM projection fibers between the motor cortex and the spinal cord, enabling voluntary movement. Thus, the motor components of cortical middle cerebral artery or anterior cerebral artery territory infarction (face, arm, or leg weakness) can be reproduced by lesions at different points along its course.
Infarcts in the centrum semiovale may be large, presenting with all of the aforementioned
Ataxic Hemiparesis or Dysarthria—Clumsy Hand Syndrome
This presentation should prompt evaluation of the internal capsule (but also thalamic and pontine lesions). As can be seen, the CST runs primarily in the PLIC (Fig. 2), situated in its posterior third quarter. It retains the somatotopic organization seen in the corona radiata or centrum semiovale with the face fibers most anterior, followed by the arm, trunk, and leg being most posterior. The corticobulbar tract runs through the genu of the internal capsule and the corticopontine fibers are in
Hemiparesis With Oculomotor Abnormalities
The association of crossed hemiplegia with third nerve palsy is known as Weber syndrome. The proximity of the third nerve nucleus to the CST in the cerebral peduncle accounts for the association usually encountered in paramedian midbrain infarcts from P1 posterior cerebral artery occlusion. The corticospinal fibers are located centrally within the cerebral crus with face fibers found medially and leg fibers laterally.15 The CST is surrounded by the corticopontine fibers with frontopontine
Crossed Hemiparesis and Facial Symptoms
This association is found in lesions affecting the pontine tegmentum. The involvement of the CST causes the contralateral hemiparesis but involvement of the trigeminal nerve nuclei (motor and sensory) causes ipsilateral (hence crossed) facial motor weakness or numbness. The somatotopic organization of the CST in the pons is of facial fibers anteromedially and foot fibers posterolaterally.18 Other cranial nerve nuclei may be affected by lesions in the pons; the association of hemiplegia with
Hemiparesis With Crossed Tongue Weakness
This combination occurs in anterior-medial medullary syndromes. Owing to the associated sensory involvement, it is discussed in the following sections.
Sensory Syndromes
Sensory pathways ascend from the spinal cord through the brainstem to the thalamus in a variety of tracts, each carrying different modalities of sensory perception. The tract most identifiable on DTI is the medial lemniscus, relaying proprioception and fine touch sensation (Fig. 5). First order sensory neurons ascend the spinal cord within the dorsal columns, terminating in the dorsal column nuclei within the medulla, nucleus gracilis, and nucleus cuneatus. Here, they synapse with second order
Sensory Disturbance With Lower Cranial Nerve Palsies
Medullary lesions (particularly infarcts) may be centered anteriorly, in which case the medial lemniscus and hypoglossal nerve can be affected, leading to clinical features of contralateral decrease in proprioception or vibration (the medial lemnisci are already crossed), ipsilateral weakness of intrinsic muscles of the tongue (and hence unopposed action of the normal tongue muscles, pushing the tongue to the side of the lesion), and contralateral hemiplegia because of involvement of the CST in
Isolated Hemibody Numbness
Anatomically, this can only be explained by a thalamic lesion or a lesion in the pontine tegmentum where the (crossed) medial lemnisci and spinothalamic tracts run in close proximity.
Combined Sensorimotor Stroke
Other than cortical infarcts affecting the precentral and postcentral gyri, the combination of both face and arm weakness with same side multimodal sensory disturbance should prompt inspection of the posterior aspect of the corona radiata where the superior thalamic radiations from the ventral posteromedial or ventral posterolateral thalamic nuclei are found in close proximity to the CST.
Disorders of Language
Assessment of cortical damage to the inferior frontal gyrus or frontal operculum (Broca area) and posterosuperior temporal gyrus or angular gyrus (Wernicke area) is of prime importance in patients presenting with dysphasia or aphasia. However, the WM tracts connecting these areas and other regions of the brain are now recognized to represent a putative “language network” and as such should also be assessed in this clinical scenario. The role of the arcuate fasciculus is covered extensively in
Disorders of Attention or Visuospatial Orientation
DTI is revealing the relevance of the WM structures beneath the typical cortical locations involved in attention—usually, the nondominant parietal lobe. Visual and tactile hemineglect is usually attributed to damage to the inferior parietal lobule, particularly the temporo-parieto-occipital association cortex. However, these broad cortical areas share intimate subcortical WM connections with the underlying SLF, which is described as having 4 subdivisions: SLF I, SLF II, SLF III, and the arcuate
Callosal Disconnection Syndromes
The anatomical arrangement of fibers within the corpus callosum gives rise to its functional organization. Transfer of motor and somatosensory information occurs within the body, auditory information within the isthmus, and visual information within the splenium. Somatotopic organization of the callosal motor fibers within the body of the corpus callosum has been demonstrated using a combination of functional magnetic resonance imaging and DTI.1 Despite being the largest WM tract in the brain (
Other Commissural Tracts
The anterior commissure (Fig. 10) is a compact fiber bundle that crosses the midline at the superior aspect of the lamina terminalis, just anterior to the columns of the fornix. Internally, the commissure comprises 2 limbs: the smaller anterior limb, whose fibers extend within the anterior perforating substance to connect the olfactory bulbs and nuclei, and the main posterior limb, which projects laterally beneath the caudate and lentiform nuclei to reach the middle and inferior temporal gyri,
Frontal Lobe Behavioral Syndromes
Clinical presentation with akinetic mutism and abulia (lack of initiative or motivation) is frequently encountered in patients with damage to the medial frontal lobes (usually anterior cerebral artery territory infarction or post traumatic) and heads of the caudate nuclei. However, the fact that very similar clinical syndromes may be produced by paramedian anterior thalamic infarcts suggests important connections between these 2 structures via the anterior thalamic radiations (Fig. 11).41 These
Psychiatric Disorders
The anterior thalamic radiations and medial forebrain bundles along with the cingulum (described later) have been an area of intense interest for those attempting to find anatomical correlates of both schizophrenia and depression.42
Urinary Incontinence and Pain
Lesions of the anterior cingulate cortex are well known to affect bladder control but recent WM distribution studies have identified damage to the cingulum (Fig. 12) and superior occipitofrontal fasciculus (Fig. 13) to be associated with the type and severity of urinary incontinence.43 The cingulum is readily identifiable on color FA maps, with the largest component running anteroposteriorly in the paramedian subcortical WM of the cingulate gyrus. The superior occipitofrontal fasciculus runs
Surgical Considerations: The Temporal Stem
The WM core connecting the temporal lobe with the frontal lobe is known as the temporal stem (Fig. 14). It contains a number of important association fiber tracts and plays a significant role in the reciprocal spread of tumor, infection, and seizure activity between the temporal lobe and the rest of the brain.7, 24, 25
The precise definition, anatomical boundaries, and contents of the temporal stem vary slightly between authors; however, the most frequently used description is that given by
Conclusion
Although an exhaustive list of potential lesion locations is beyond the scope of this article, it is clear that DTI allows identification of many important WM tracts. Knowledge of the location of these tracts and their respective functions should help the reader identify the clinical relevance of neuroradiological findings and perform directed searches for lesions in the clinical scenarios described.
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