CLINICAL CASE | Middle Cerebral Artery Occlusion, Right Side Paralysis, and Global Aphasia
A 57-year-old male was brought to the emergency room after being discovered by his wife to be unable to move his right arm or leg. On testing, his right upper limb strength was 0/5 and the lower limb, 1/5. The left limbs had normal strength and spontaneous movements. In addition, there was drooping of the right side of the lower face. Pinch of the nail beds—a mildly noxious stimulus that elicits a withdrawal—revealed withdrawal of the left arm but no response for the right arm. The patient was able to look to the left but not the right; there were no saccadic (rapid, conjugate) eye movements to the right. The patient was unable to speak and only followed simple commands.
Figure 3–1A shows a horizontal MRI. The large white territory corresponds to the infarcted region on the left side of the cerebral hemisphere. Figure 3–18B is a magnetic resonance angiogram (MRA), showing the distribution of arteries with flowing blood. Note the asymmetry in the MRA, with an absence of middle cerebral artery perfusion on the left side.
Answer the following questions based on your readings of the case report and this chapter.
1. The patient's lesion is large. Occlusion of which cerebral artery produced the lesion, and what was the contribution of its deep and superficial branches?
2. Damage to what single key structure could produce the major limb and facial motor signs? Key neurological signs and corresponding damaged brain structures Paralyzed right arm and leg
The corticospinal tract, which is key to controlling the contralateral arm and leg, descends subcortically and then travels in the posterior limb of the internal capsule (see Figure 2–17). This subcortical white matter and the more dorsal parts of the posterior limb of the internal capsule are supplied by deep branches of the middle cerebral artery. The infarction also would have destroyed part of the lateral precentral gyrus, where the corticospinal tract to the arm segments of the spinal cord originates. This is supplied by superficial branches of the middle cerebral artery. By contrast, the infarction spares the leg area of motor cortex (see Figure 10–8). Whereas the descending axons are destroyed when they are in the internal capsule, as we shall see in later chapters sparing of the cortex may help during neurorehabilitation. Right lower facial droop
The corticobulbar tract controls facial muscles. This is the component of the descending cortical motor pathway that controls cranial motor nuclei in the brain stem. (Bulb is an archaic term to describe the lower brain stem.) The tract travels subcortically from lateral part of the precentral gyrus (face area of motor cortex) to the genu and posterior limb of the internal capsule, rostral to the corticospinal tract axons. The subcortical white matter and dorsal parts of the internal capsule are largely supplied by the deep branches of the middle cerebral artery. The face-controlling area of motor cortex is supplied by superficial branches of the middle cerebral artery. Interestingly, control of the lower face—like that of the arms and legs—is strictly contralateral, but upper facial muscle control is bilateral. Damage to the corticobulbar tract on one side thus eliminates control of lower face on the opposite side, resulting in paralysis or major weakness. Upper facial muscles, because they receive control by both sides of the brain, are functional after a unilateral corticobulbar tract lesion. Absence of limb withdrawal to noxious stimulation
Damage to the internal capsule can destroy the ascending thalamocortical projection, carrying somatic sensory information to the postcentral gyrus. This results in the loss of somatic sensation. However, in the case of our patient, there were no spontaneous right arm movements and there was no upper limb strength. Therefore, it is unlikely that he would have been able to move the limb had noxious stimulation been felt. Absence of eye movement to the right
The infarction damaged the cortical regions, and their descending control pathways, for saccadic eye movements. These are the rapid, darting, eye movements we use to shift our gaze from one object of interest to another. Inability to speak and understand language
The cortical centers controlling speech are in the left hemisphere in most right-handed individuals. These areas are Wernicke's area in the superior temporal gyrus, for sensory processing in speech, and Broca's area in the inferior frontal lobule, for producing speech. Both areas are supplied by superficial branches of the middle cerebral artery. Their axonal interconnections are also supplied, in large part, by the middle cerebral artery. In the absence of these structures, there is loss of language, both spoken and understanding.
Brain vasculature disorders constitute a major class of nervous system disease. The principal source of nourishment for the central nervous system is glucose, and because neither glucose nor oxygen is stored in appreciable amounts, when the blood supply of the central nervous system is interrupted, even briefly, brain functions become severely disrupted.
Much of what is known about the arterial supply to the central nervous system derives from three approaches. First, classical studies in normal postmortem tissue used colored dye injected into a blood vessel to identify the areas it supplies. Second, in postmortem tissue or on radiological examination, the portion of the central nervous system supplied by a particular artery can be inferred by observing the extent of damage that occurred after the artery became occluded. Third, radiological techniques, such as cerebral angiography and magnetic resonance angiography, make it possible to view the arterial and venous circulation in the living brain (see Box 3–1). These important clinical tools also permit localization of a vascular obstruction or other pathology.
Box 3–1 Radiological Imaging of Cerebral Vasculature
Cerebral vessels can be observed in vivo using cerebral angiography. First, radiopaque material is injected into either the anterior or the posterior arterial system. Then a series of skull x-ray images are taken in rapid repetition as the material circulates. Images obtained while the radiopaque material is within cerebral arteries are called angiograms or arteriograms. Images can also be obtained later, after the radiopaque substance has reached the cerebral veins or the dural sinuses (venograms). The entire course of the internal carotid artery is shown in cerebral angiograms in Figure 3–9. Images can be obtained from different angles with respect to the cranium. Two views are common—from the front (frontal projection, Figure 3–9A) and from the side (lateral projection, Figure 3–9B). The lateral view shows the C-shape of the anterior cerebral artery (and its branches). The medial-to-lateral course of the middle cerebral artery is revealed in the frontal view.
The rostrocaudal course of the middle cerebral artery, from the point at which it enters the lateral sulcus to the point at which it emerges and distributes over the lateral surface of the cerebral cortex, is revealed in the lateral view (Figure 3–9B). The middle cerebral artery forms loops at the dorsal junction of the insular cortex and the opercular surface of the frontal and parietal lobes (see Figure 3–8). These loops serve as radiological landmarks that aid in estimating the position of the brain in relation to the skull. Figure 3–10 shows the posterior circulation viewed from a lateral perspective. Figure 3–11 shows the two vertebral arteries joining to form the basilar artery and the subsequent bifurcation of the basilar artery into the two posterior cerebral arteries.
Cerebral angiography involves intravascular injection of radiopaque material. The process of injecting this material, and the material itself, can produce neurological complications; therefore, its use is not without risk. Magnetic resonance imaging has also been applied to the study of brain vasculature because it can detect motion of water molecules. This application, termed magnetic resonance angiography (MRA), selectively images blood in motion. The MRA scan in Figure 3–12 is a dorsoventral reconstruction (ie, as if looking up from the bottom). The posterior communicating artery is present only on the left side. This patient does not have a complete circle of Willis. The entire cerebral circulation can be reconstructed from the locations of cerebral arteries or veins at multiple levels.
As discussed in previous chapters, brain vasculature is closely related to the ventricular system and the watery fluid contained within it, the cerebrospinal fluid. This is because most cerebrospinal fluid is produced by active secretion of ions from blood plasma by the choroid plexus. Moreover, to maintain a constant brain volume, cerebrospinal fluid is returned to the blood through valves between the subarachnoid space and the dural sinuses.
This chapter initially focuses on arterial supply because of the importance of distributing oxygenated blood to the brain and spinal cord for normal function, followed by venous drainage. The blood-brain barrier, which isolates the intravascular compartment from the extracellular compartment of the central nervous system, is considered next. Finally, cerebrospinal fluid production and circulation within the different components of the ventricular system is examined.