Chapter 2 Central Nervous System Disorders
Lesions in the two components of the central nervous system (CNS) – the brain and the spinal cord – typically cause combinations of paresis, sensory loss, visual deficits, and neuropsychologic disorders (Box 2-1). Symptoms and signs of CNS disorders must be contrasted to those resulting from peripheral nervous system (PNS) and psychogenic disorders. In practice, neurologists tend to rely on the physical rather than mental status evaluation, thereby honoring the belief that “one Babinski sign is worth a thousand words.”
Box 2-1
Signs of Common Central Nervous System Lesions
Signs of Cerebral Hemisphere Lesions
Of the various signs of cerebral hemisphere injury, contralateral hemiparesis (Box 2-2) – weakness of the lower face, trunk, arm, and leg opposite to the side of the lesion – is usually the most prominent. Damage to the corticospinal tract, also called the pyramidal tract (Fig. 2-1), in the cerebrum or brainstem before (above) the decussation of the pyramids causes contralateral hemiparesis. Damage to this tract after (below) the decussation of the pyramids, when it is in the spinal cord, causes ipsilateral arm and leg or only leg paresis. The extent of the paresis depends on the site of injury.
FIGURE 2-1 Each corticospinal tract originates in the cerebral cortex, passes through the internal capsule, and descends into the brainstem. It crosses in the pyramids, which are long protuberances on the inferior portion of the medulla, to descend in the spinal cord mostly as the lateral corticospinal tract. It terminates by forming a synapse with the anterior horn cells of the spinal cord, which give rise to peripheral nerves. The corticospinal tract is sometimes called the pyramidal tract because it crosses in the pyramids. The extrapyramidal tract, which modulates the corticospinal tract, originates in the basal ganglia and remains within the brain.
During the corticospinal tract’s entire path from the cerebral cortex to the anterior horn cells of the spinal cord, it is considered the upper motor neuron (UMN) (Fig. 2-2). The anterior horn cells, which are part of the PNS, are the beginning of the lower motor neuron (LMN). The division of the motor system into UMNs and LMNs is a basic tenet of clinical neurology.
FIGURE 2-2 A, Normally, when physicians strike the quadriceps tendon with a percussion hammer, it elicits a deep tendon reflex (DTR). In addition, when they stroke the sole of the foot to elicit a plantar reflex, the big toe bends downward (flexes). B, When brain or spinal cord lesions involve the corticospinal tract and cause upper motor neuron (UMN) damage, the DTR is hyperactive: The muscle shows a brisker and more forceful contraction than usual, and adjacent muscle groups often also respond. In addition, with UMN damage, the plantar reflex is extensor (i.e., a Babinski sign is present). C, When peripheral nerve injury causes lower motor neuron (LMN) damage, the DTR is hypoactive and the plantar reflex is absent.
Cerebral lesions that damage the corticospinal tract are characterized by signs of UMN injury (Figs. 2-2 to 2-5):
FIGURE 2-3 This patient with severe right hemiparesis typically shows weakness of the right arm, leg, and lower face. The right-sided facial weakness causes the widened palpebral fissure and flat nasolabial fold; however, the forehead muscles remain normal (see Chapter 4 regarding this discrepancy). The right arm is limp, and the elbow, wrist, and fingers take on a flexed position. The right hemiparesis also causes external rotation of the right leg and flexion of the hip and knee.
FIGURE 2-4 When the patient stands up, his weakened arm retains its flexed posture. His right leg remains externally rotated, but he can walk by swinging it in a circular path. This maneuver is effective but results in circumduction or a hemiparetic gait.
FIGURE 2-5 Mild hemiparesis may not be obvious. To exaggerate a subtle hemiparesis, the physician has asked this patient to extend both arms with his palms held upright, as though each outstretched hand were holding a water glass or both hands were supporting a pizza box (the “pizza test”). After a moment, a weakened arm slowly sinks (drifts), and the palm turns inward (pronates). The imaginary glass in the right hand would spill the water inward and the imaginary pizza would slide to the right. This arm drift and pronation represent a forme fruste of the posture seen with severe paresis (see Fig. 2-3).
Cerebral lesions are not the only cause of hemiparesis. Because the corticospinal tract has such a long course (see Fig. 2-1), lesions in the brainstem and spinal cord as well as the cerebrum may produce hemiparesis and other signs of UMN damage. Signs pointing to injury in various regions of the CNS can help identify the origin of hemiparesis, i.e., localize the lesion.
Another indication of a cerebral lesion is loss of certain sensory modalities over one-half of the body, i.e., hemisensory loss (Fig. 2-6). A patient with a cerebral lesion characteristically loses contralateral position sensation, two-point discrimination, and the ability to identify objects by touch (stereognosis). Loss of those modalities is often called a “cortical” sensory loss.
FIGURE 2-6 Peripheral nerves carry pain and temperature sensations to the spinal cord. After a synapse, these sensations cross and ascend in the contralateral lateral spinothalamic tract (blue) to terminate in the thalamus. From there, tracts relay the sensations to the limbic system, reticular activating system, and other brainstem regions as well as the cerebral cortex. In contrast, the peripheral nerves also carry position sense (tested by movement of the distal finger joint) and stereognosis (tested by tactile identification of common objects) to the ipsilateral fasciculus cuneatus and fasciculus gracilis, which together constitute the spinal cord’s posterior columns (light blue) (see Fig. 2-15). Unlike pain and temperature sensation, these sensations rise in ipsilateral tracts (black). They cross in the decussation of the medial lemniscus, which is in the medulla, synapse in the thalamus, and terminate in the parietal cortex. (To avoid spreading blood-borne illnesses, examiners should avoid using a pin when testing pain.)
Pain sensation, a “primary” sense, is initially received by the thalamus. Because the thalamus is just above the brainstem but below the cerebral cortex, pain perception is usually retained with cerebral lesions. For example, patients with cerebral infarctions may be unable to specify a painful area of the body, but will still feel the pain’s intensity and discomfort. Also, patients in intractable pain did not obtain relief when they underwent experimental surgical resection of the cerebral cortex. The other aspect of the thalamus’ role in sensing pain is seen when patients with thalamic infarctions develop spontaneous, disconcerting, burning pains over the contralateral body (see thalamic pain, Chapter 14).
Visual loss of the same half-field in each eye, homonymous hemianopsia (Fig. 2-7), is a characteristic sign of a contralateral cerebral lesion. Other equally characteristic visual losses are associated with lesions involving the eye, optic nerve, or optic tract (see Chapters 4 and 12). Because they would be situated far from the visual pathway, lesions in the brainstem, cerebellum, or spinal cord do not cause visual field loss.
FIGURE 2-7 In homonymous hemianopsia, the same half of the visual field is lost in each eye. In this case, a right homonymous hemianopsia is attributable to damage to the left cerebral hemisphere. This sketch portrays stippled or crosshatched visual field losses, as is customary, from the patient’s perspective (see Figs 4-1 and 12-9).
Another conspicuous sign of a cerebral hemisphere lesion is partial seizures (see Chapter 10). The major varieties of partial seizures – elementary, complex, and secondarily generalized – result from cerebral lesions. In fact, about 90% of partial complex seizures originate in the temporal lobe.
Signs of Damage of the Dominant, Nondominant, or Both Cerebral Hemispheres
Lesions of the dominant hemisphere may cause language impairment, aphasia, a prominent and frequently occurring neuropsychologic deficit (see Chapter 8). In addition to producing aphasia, dominant-hemisphere lesions typically produce an accompanying right hemiparesis because the corticospinal tract sits adjacent to the language centers (see Fig. 8-1).
When the nondominant parietal lobe is injured, patients often have one or more characteristic neuropsychologic deficits that comprise the “nondominant syndrome” as well as left-sided hemiparesis and homonymous hemianopsia. For example, patients may neglect or ignore left-sided visual and tactile stimuli (hemi-inattention; see Chapter 8). Patients often fail to use their left arm and leg more because they neglect their limbs than because of paresis. When they have left hemiparesis, patients may not even perceive their deficit (anosognosia). Many patients lose their ability to arrange matchsticks into certain patterns or copy simple forms (constructional apraxia; Fig. 2-8).
FIGURE 2-8 With constructional apraxia from a right parietal lobe infarction, a 68-year-old woman was hardly able to complete a circle (top figure). She could not draw a square on request (second highest figure) or even copy one (third highest figure). She spontaneously tried to draw a circle and began to retrace it (bottom figure). Her constructional apraxia consists of the rotation of the forms, perseveration of certain lines, and the incompleteness of the second and lowest figures. In addition, the figures tend toward the right-hand side of the page, which indicates that she has neglect of the left-hand side of the page, i.e., left hemi-inattention.
All signs discussed so far are referable to unilateral cerebral hemisphere damage. Bilateral cerebral hemisphere damage produces several important disturbances. One of them, pseudobulbar palsy, best known for producing emotional lability, results from bilateral corticobulbar tract damage (see Chapter 4). The corticobulbar tract, like its counterpart the corticospinal tract, originates in the motor cortex of the posterior portion of the frontal lobe. It innervates the brainstem motor nuclei that in turn innervate the head and neck muscles. Traumatic brain injury (TBI) and many illnesses, including cerebral infarctions (strokes) and frontotemporal dementia (see Chapter 7), are apt to strike the corticobulbar tract and the surrounding frontal lobes and thereby cause pseudobulbar palsy.
Damage of both cerebral hemispheres – from large or multiple discrete lesions, degenerative diseases, or metabolic abnormalities – also causes dementia (see Chapter 7
