Fundamentals
Muscle Weakness and Other Motor Disturbances
Sensory Disturbances
Disturbances of Consciousness
Dysfunction of Specific Areas of the Brain
Fundamentals
The neurological deficits produced by a lesion in any given area of the nervous system are characteristic of the area involved and relatively independent of the type of lesion. Thus, the clinical manifestationsof neurological disease are determined above all by the site of the lesion. A thorough knowledge of these relationships is essential in clinical practice. The first step in diagnostic assessment is always the localization of the disease process in the nervous system. This can usually be done with great precision based on the information obtained in the clinical interview and neurological examination. The etiology is then sought in a second step with the aid of further information (course of the disease over time, any accompanying nonneurological manifestations, results of ancillary tests).
In this chapter, we will show how the clinical manifestations of neurological disease can be used to make inferences about the site of the lesion and its possible etiologies. We will first describe the typical findings of lesions affecting individual functional systems (the motor and somatosensory systems) and then those of lesions in particular areas of the brain. The manifestations of diseases affecting the spinal cord and peripheral nerves will be discussed in the relevant, later chapters.
Muscle Weakness and Other Motor Disturbances
Anatomical Substrate of Motor Function
It is a useful simplification to consider the motor system as consisting of the following components (Fig. 5.1):
First (central) motor neuron (neurons in the precentral gyrus). The axons travel in the corticobulbar and corticospinal tracts through the internal capsule and cerebral peduncle and terminate either in the cranial nerve nuclei of the pons and medulla (corticobulbar pathway) or on the anterior horn cells of the spinal cord (pyramidal pathway). Lesions of the first motor neuron in the precentral gyrus, or at any other site, produce the following deficits:
spastic weakness (elevated muscle tone, diminished raw strength, and impaired fine motor control);
increased intrinsic muscle reflexes, spreading of reflex zones, and pathological reflexes (Babinski, Oppenheim, and Gordon, pathologically brisk Hoffmann sign and Trömner reflex, inextinguishable or asymmetrically persistent clonus); diminished or absent extrinsic muscle reflexes (e. g., abdominal skin reflex);
no muscle atrophy (though there may be mild atrophy of disuse in the later course of disease);
asymmetry of the reflexes if the lesion is unilateral.
The second (peripheral) motor neuron originates in one of the motor relay stations mentioned above (the motor cranial nerve nuclei or the anterior horn cells of the spinal cord). It consists of a cell body (ganglion cell) and an axon that travels by way of a spinal nerve root, plexus, and peripheral nerve to the skeletal muscle. Each ganglion cell, together with its axon and the muscle fibers that it innervates (there may be many, or only a few), comprises a single motor unit. The following deficits are associated with a lesion of the peripheral motor neuron:
flaccid weakness (diminished muscle tone and raw strength);
diminished or absent intrinsic muscle reflexes;
muscle atrophy becoming evident about three weeks after injury and progressing thereafter.
Motor end plate and muscle. In addition to the first and second motor neurons, normal motor function requires effective impulse transmission from the peripheral nerve to the muscle fiber, followed by fiber contraction. A lesion or functional disturbance of either or both of these elements causes flaccid weakness usually accompanied by atrophy and diminished reflexes (p. 275).
Because every movement, as we have seen, is the product of a complex interaction of many different ana tomical structures, motor processes are subject to a wide range of pathological disturbances. Typical findings of lesions of individual components of the motor system are listed in Table 5.1. The following table, in contrast, begins with certain typical constellations of motor deficits, then lists the likely site(s) of the lesion producing each, and finally some of the possible etiologies. Table 5.2 thus reflects the “classic” threefold paradigm of clinical thinking, from the physical findings to the site of the lesion to the diagnosis.
Fig. 5.1 Anatomical substrate of movement (modified from Liebsch, R.: Intensivkurs Neurologie. Urban & Schwarzenberg, Munich, 1996, and Mumenthaler, M.: Neurologische Differenzialdiagnose, 4th edn, Thieme, Stuttgart, 1992).
Motor Regulatory Systems
The smooth, precise, and economical execution of a movement requires a properly functioning “regulatory system” in addition to the effector components discussed above. The regulatory system must do the following:
integrate proprioceptive input from the peripheral nerves, posterior columns (fasciculus gracilis and fasciculus cuneatus), thalamus, and thalamocortical pathways, along with further input from the vestibular apparatus and the visual system, and use this “feedback” data to optimize each phase of the movement at every moment;
plan the force and amplitude of the movement (extrapyramidal system and cerebellum);
coordinate the activity of all of the muscles taking part in a movement and, in particular, ensure the effective complementary functioning of agonist and antagonist muscles (extrapyramidal system, cerebellum, and spinal cord).
A loss of function of one or more components of this regulatory system impairs the execution and coordination of movement. Disturbances of this type are typically manifest as:
ataxia,
hypokinesia,
involuntary movements.
Ataxia is an impairment of the smooth performance of goal-directed movement (repeated deviation from the ideal line of a movement). The different types of ataxia have specific clinical features, depending on the nature and location of the underlying lesion:
Cerebellar ataxia is characterized by irregularity of the entire course of a movement. A lesion in a cerebellar hemisphere produces ataxia in the ipsilateral limbs, while a vermian lesion mainly produces truncal ataxia (ataxia of stance and/or gait). On the other hand, involvement of the dentate nucleus or its efferent fibers causes intention tremor: in targeted movements, the deviation from the ideal line of approach increases as the limb approaches the target (Fig. 3.20, p. 29).
Central sensory ataxia results from impaired position sense due to lesions of the somatosensory cortex, the thalamus, or the thalamocortical pathways.
Posterior column ataxia is produced by lesions of the afferent somatosensory pathways in the dorsal portion of the spinal cord (fasciculus gracilis and fasciculus cuneatus—also known as the columns of Goll and Burdach). It is most apparent when the patient walks; it is regularly accompanied by impaired proprioception and position sense. Patients can compensate for posterior column ataxia to some extent with visual cues; this form of ataxia is thus appreciably worse in the dark, or when the patient's eyes are closed, than in a well-lit room with the patient's eyes open.
The distinguishing characteristic of spinal, as opposed to cerebellar, ataxia is that it is mainly evident when visual input is removed.
Peripheral sensory ataxia is caused by disease processes affecting the peripheral sensory nerves, e. g., polyneuropathy, and is associated with loss of reflexes and impaired epicritic sensation.
Other types of ataxia. Frontal lobe lesions sometimes cause contralateral ataxia; motor weakness can also impair motor coordination, causing ataxia. Psychogenic ataxia is typified by its irregularity and by the lack of constant, objectifiable neurological deficits. Patients with psychogenic ataxia do not fall.
Hypokinesia is generalized slowing of all types of movement.
It is typically found in (hypokinetic) Parkinson disease (p. 128). Spontaneous movements are sparse or absent, automatic accessory movements cease (e. g., arm movements during walking), and all voluntary movements are slowed. The muscles are rigid and the cogwheel phenomenon is usually demonstrable (p. 30).
It is also found in depression as a sign of generally diminished drive; in such cases, it is not accompanied by any other neurological deficit.
Involuntary movements come in many varieties, the more important of which are listed in Table 5.3. The phenomenology and localizing significance of each type of involuntary movement are described.
Table 5.3 Involuntary movements and movement disorders
|
Designation |
Manifestations |
Localization; Remarks |
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Spontaneous muscle activity not producing movement |
||
|
|
phasic contractions of individual muscle fibers, not visible to the naked eye, only demonstrable by EMG |
due to contractions of individual muscle fibers; pathological at rest or as prolonged insertional activity in the EMG (p. 58) |
|
|
brief, irregular contractions of individual groups of muscle fibers, visible to the naked eye |
due to contractions of individual motor units; always pathological; especially typical of chronic lesions affecting the anterior horn ganglion cells |
|
|
visible waves of contraction passing across many different fiber bundles in a muscle or group of muscles |
unknown |
|
Hyperkinetic phenomena |
||
|
|
rhythmic twitching in a muscle group (always the same one) producing movement; frequency usually 1–3 Hz |
central nervous system |
|
|
nonrhythmic, rapid, large-amplitude, sometimes very intense twitching of one or more muscles, producing visible movement |
cerebral cortex, cerebellum; seen physiologically in persons who are falling asleep (hypnagogic myoclonus) |
|
|
rhythmic oscillation (usually fine, sometimes coarse), at a frequency that remains roughly constant for the affected individual, of more or less constant localization; may be observed at rest (rest tremor) or with action (action tremor; e. g., postural tremor, kinetic tremor, intention tremor) |
central nervous system (mainly cerebellum, extrapyramidal system) |
|
|
brief and relatively rapid, shooting muscle contractions, mainly distal, nonrhythmic, irregular, of varying localization, sometimes putting the joints into extreme positions for a brief period of time |
basal ganglia/striatum |
|
|
like chorea, but slower, writhing movements with longer-lasting hyperflexion or hyperextension of the joints |
basal ganglia |
|
|
brief, shooting muscle contractions, mainly proximal and therefore causing pronounced movement (flinging movements of the limbs, jactation) |
subthalamic nucleus |
|
|
involuntary, longer-lasting muscle contraction that slowly overcomes the resistance of the antagonist muscles, usually leading to turning movements and bizarre postures of individual parts of the body (trunk, limbs, head) |
basal ganglia |
|
|
irregular muscle contraction limited to certain parts of the body, rapid, but not lightninglike |
psychogenic |
|
Other |
||
|
|
muscle contractions of variable frequency and intensity, occurring at irregular intervals, occasionally painful; two examples are |
facial nerve lesion, extrapyramidal disorder (a type of dystonic movement disorder); very rarely psychogenic |
|
|
long-lasting, tonic contractions of individual muscles or muscle groups, fixed position of the joints, usually painful, often in the calf |
of muscular origin |
Sensory Disturbances
Anatomical Substrate of Sensation
It is another useful simplification to consider the somatosensory system as consisting of the following components (Fig. 5.2, Table 5.4):
The peripheral part of the somatosensory system contains sensory (afferent) nerves and receptors that are specialized for the perception of the individual modalities of somatic sensation.
Sensory receptors in the periphery are classified into three principal types. Exteroceptive receptors (exteroceptors) transduce physical stimuli from the external environment (e.g., mechanoreceptors, thermoreceptors). Proprioceptive receptors (proprioceptors) inform the nervous system about head and body posture, the positions of the joints, and tension in muscles and tendons (muscle spindles and Golgi tendon organs). Finally, the nociceptors, which subserve pain, occupy an intermediate position between the extero- and proprioceptors. The density of somatosensory receptors is greatest in the skin, but they are also found in most other tissues of the body, including the viscera (but not in the brain or spinal cord!).
Table 5.4 The somatosensory system
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peripheral portion |
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exteroceptors (mechano- and thermoreceptors) proprioceptors (body posture, joint position, tension in muscles and tendons) nociceptors |
|
|
peripheral nerves, plexuses, posterior roots |
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|
central portion |
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posterior columns anterolateral columns spinocerebellar tracts |
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posterior column fibers terminate in synaptic relay stations in the medulla (nucleus gracilis, nucleus cuneatus); the efferent fibers of these nuclei ascend in the brainstem as the medial lemniscus and terminate in the thalamus |
|
|
|
thalamus thalamocortical tracts somatosensory cortex |
Afferent sensory nerve fibers run in the peripheral nerves, plexuses, and posterior spinal nerve roots. These are the axons of the first somatosensory neurons, whose cell bodies lie in the spinal ganglia (dorsal root ganglia). All other sensory neurons have their cell bodies within the central nervous system.
The central part of the somatosensory system comprises all of the somatosensory pathways and nuclei of the spinal cord, brainstem, and cerebral hemispheres. These can be classified, according to their function, as follows:
Posterior column system. The centripetal processes of the pseudounipolar spinal ganglion cells (first sensory neuron) that subserve epicritic sensation carry information from both exteroceptors (tactile sense, stereognosis and vibration) and proprioceptors (position sense). They travel by way of the posterior columns to the nucleus gracilis and nucleus cuneatus of the medulla, without any intervening relay in the spinal cord. These medullary nuclei contain the second sensory neurons, whose axons, in turn, form the medial lemniscus, which travels onward to the thalamus.
Lesions affecting the posterior column system impair all of the “high-resolution” somatosensory modalities:
diminished ability to recognize objects by touch (astereognosia) and impaired two-point discrimination;
impaired vibration sense (pallhypesthesia or pallanesthesia) and impaired position sense and kinesthesia;
Fig. 5.2 Anatomical substrate of somatic sensation.
unsteady stance and gait (spinal ataxia, see below) due to the lack of proprioceptive feedback regarding the posture and movements of the head, trunk, and limbs.
Anterolateral column system. The centripetal processes of the pseudounipolar spinal ganglion cells (first sensory neuron) that subserve protopathic sensation (pain, temperature sensation, coarse touch and pressure) form synapses onto second sensory neurons in the posterior horn of the spinal cord. The axons of these cells cross the midline in the anterior spinal commissure and then ascend in the spinal cord and through the brainstem to terminate in the thalamus. Fibers related to pain and temperature sensation travel in the lateral spinothalamic tract, fibers related to coarse touch and pressure in the anterior spinothalamic tract.
Lesions affecting the lateral spinothalamic tract in the spinal cord or brainstem, or the corresponding thalamic nuclei, produce a dissociated sensory deficit: pain and temperature sensation are impaired below the level of the lesion, while touch remains intact. The deficit is contralateral to the lesion because the lateral spinothalamic tract is crossed.
Thalamocortical system. The axons of the second neurons of both the posterior column system and the anterolateral column system terminate in the thalamic nuclei that contain the third neurons of the somatosensory system. These neurons, in turn, send their axons by way of the posterior limb of the internal capsule to the primary somatosensory cortex (postcentral gyrus) and the neighboring association areas. The third neurons thus belong to the so-called thalamocortical system. Lesions of this system produce a contralateral hemisensory deficit, which usually affects all of the somatosensory modalities, though sometimes to different extents.
The spinocerebellar system conveys information regarding tension and stretch of muscles and tendons from the muscle spindles and Golgi tendon organs to the paleocerebellum. The main spinal pathways used by this system are the posterior spinocerebellar tract (which exclusively carries information from the ipsilateral half of the body) and the anterior spinocerebellar tract (which carries information from both sides of the body). The paleocerebellum, in turn, gives off multiple efferent pathways, which influence muscle tone to ensure the smooth cooperative functioning of agonist and antagonist muscle groups in standing and walking. The paleocerebellum thus plays an important role in the regulation of balance, though its activity is wholly unconscious. Lesions of the spinocerebellar pathways and paleocerebellum cause ataxia of stance and gait (see above).
Table 5.5 is analogous to Table 5.2; it provides an overview of the typical constellations of somatosensory deficits and their pathoanatomical basis. We have not mentioned any specific diagnoses in this table in order to keep it perspicuous. Some of the typical clinical findings are illustrated in Fig. 5.3.
Tabelle 5.5 Patterns of distribution of somatosensory deficits
|
Pattern of distribution of deficit |
Sensory qualities affected |
Anatomical substrate; remarks |
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sharply delimited, unilateral, focal, asymmetrical |
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less sharply delimited, unilateral, segmental, asymmetrical |
|
|
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gradually increasing from proximal to distal, bilaterally symmetrical (stocking-and-glove distribution) |
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|
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segmental, bilaterally symmetrical |
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|
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unilateral below a given spinal cord level |
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|
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bilateral below a given spinal cord level |
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unilateral, including the face |
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|
|
|
|
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unilateral, sparing the face |
|
|
Fig. 5.3 Typical patterns of distribution of somatosensory deficits. a Peripheral nerve lesion: meralgia paraesthetica due to a lesion of the lateral femoral cutaneous n. b Radicular lesion: typical sensory deficit in L5 radiculopathy. c Polyneuropathy: distal, stocking-and-glove sensory deficit. d Central lesion: contralateral hemisensory deficit. e Spinal cord lesion at the T6 level: hemihypesthesia below the level of the lesion.
Disturbances of Consciousness
Anatomical substrate. Intact consciousness requires normal functioning of the cortex of both cerebral hemispheres. The “driving force” of cortical activity, however, is located in a lower center, i. e., in a collective of neurons in the brainstem called the reticular formation, which sends impulses toward the cerebral cortex by way of the intralaminar thalamic nuclei. The reticular formation and its ascending projections are known collectively as the ascending reticular activating system.
Table 5.6 Causes of impaired consciousness
|
Clinical situation |
Possible causes |
|
Impaired consciousness or coma without focal signs or meningism (purely toxic/metabolic or anoxic coma) |
various metabolic disorders |
|
Impaired consciousness or coma without focal signs, with meningism |
meningitis subarachnoid hemorrhage |
|
Impaired consciousness or coma with focal signs (structural lesion) |
supratentorial lesions (acute: infarct, trauma, intracranial hemorrhage, subdural/epidural hematoma; subacute/chronically progressive: infections, tumors); impairment of consciousness is often due to edema in the brain tissue around the lesion and the resulting intracranial hypertension and capillary hypoxia, sometimes accompanied by midline shift and herniation, with secondary brainstem damage |
|
Transient impairment of consciousness, possibly accompanied by involuntary motor phenomena |
generalized epileptic seizure |
Impairments of consciousness may thus be caused either by the simultaneous impairment of function of both cerebral hemispheres, or by damage to the reticular formation in the brainstem, and/or to its ascending projections (uncoupling of the cortex from the activating input of the reticular formation). Depending on their severity, impairments of consciousness are termed somnolence, stupor, or coma(Table 3.9, p. 40). Coma, the most severe impairment of consciousness, can also be more finely graded, according to any of several semiquantitative schemes that have been proposed. The best known of these is the Glasgow Coma Scale (GCS, Table 6.6, p. 88).
Causes. Consciousness may be impaired either by a structural lesion of brain tissue, or else indirectly by a systemic disturbance of some kind (metabolic, toxic, or anoxic coma; see below). If a structural lesion is the cause, there are often accompanying focal neurological deficits whose presence enables the clinician to infer the probable site of the lesion. The direct cause of the impairment of consciousness is often not the lesion itself, but rather the cerebral edema surrounding it (cf. Table 5.6). Focal neurological signs are usually absent in purely metabolic, toxic, or anoxic coma.
Bilateral cortical dysfunction can also be the result of an epileptic seizure or an infectious/inflammatory process such as meningitis or encephalitis (in which case meningism is usually present). Finally, there are also purely psychogenic states (psychogenic stupor) that may superficially resemble an organic impairment of consciousness. The more important causes of impaired consciousness are listed in Table 5.6. Here we have classified all impairments of consciousness into four basic clinical situations and listed the common etiologies for each.
Differential diagnosis. We will now briefly describe three other types of disturbance of consciousness that must be distinguished from coma.
Apallic syndrome/coma vigil (“persistent vegetative state”). This condition is usually due to severe and extensive brain damage. It is characterized by a complete uncoupling of midbrain and diencephalic activity from cortical activity and thus by a complete dissociation of wakefulness and consciousness. (The term “apallic” signifies “without cerebral cortex.”) The vegetative functions (breathing, cardiovascular regulation, sleep-wake cycle) are preserved, though possibly abnormal to some extent. Cognitive or goal-directed motor activity is entirely lacking. Unlike the comatose patient, the apallic patient lies in bed with eyes open, staring blankly into the distance, not fixating the gaze on anything, and not responding to verbal or noxious stimuli. At other times, the patient is in a sleeplike state, with eyes closed. Muscle tone is elevated, and automatisms and primitive reflexes are sometimes observed in the perioral area. Common types of autonomic dysfunction seen in apallic patients include tachycardia, excessive sweating, and rapid breathing.
Akinetic mutism is often due to extensive, bilateral frontal lobe damage or to a lesion affecting the projections of the ascending reticular activating system to the frontal lobes, e. g., a bilateral thalamic or midbrain lesion.Swallowing and extrinsic muscle reflexes are intact and the patient's eye movements are usually normal, yet spontaneous verbal or motor expressions are lacking. The patient still appears to be awake and can sometimes be induced to speak or move with intensive prompting.
Locked-in syndrome is not a disturbance of consciousness, though it can be mistaken for one. The patient is awake and alert, but can express him- or herself only through vertical eye movements and eyelid movements, because all four limbs are paralyzed, as well as all of the muscles innervated by the lower cranial nerves. The unwary clinician may have the false impression of a comatose patient who does not respond to external stimuli. A detailed description of the locked-in syndrome and its causes is found in the next section.
Dysfunction of Specific Areas of the Brain
Up to this point, we have described the characteristic neurological deficits produced by lesions of individual functional components of the nervous system. “Normally,” however, more than one functional component is affected. There is often simultaneous impairment of motor function, cooordination, sensation, and possibly consciousness. The individual clinical signs and symptoms described above often appear together in particular constellations (= syndromes) that are characteristically associated with the region of the nervous system in which the lesion is located and largely independent of the nature of the lesion itself.
We will now describe the major syndromes of individual regions of the brain.
Syndromes of the Individual Lobes of the Cerebral Hemispheres
Frontal lobe syndrome is characterized by the following manifestations, in variable severity, depending on the extent and precise location of the causative lesion:
abnormalities of personality and behavior (loss of drive and initiative, apathy, indifference; if only the orbitofrontal cortex is affected, there may be disinhibition, absent-mindedness, and socially inappropriate behavior);
primitive reflexes, e.g., grasp reflex and brisk palmomental reflex;
motor phenomena, e.g., spontaneous, compulsive grasping of objects, copying of other people's gestures (echopraxia), motor perseveration, and sometimes contralateral gaze paresis;
lateralized deficits: motor aphasia in lesions of the language-dominant hemisphere, anosognosia (nonrecognition of one's own illness, e.g., hemiparesis) and contralateral apraxia (p. 42) in lesions of the nondominant hemisphere;
akinetic mutism, usually caused by extensive, bilateral lesions (the patient is awake, but does not respond to environmental stimuli and does not speak; see above):
in lesions affecting the frontal eye fields: déviation conjuguée to the side of the lesion, because voluntary gaze to the opposite side is impossible;
irritative signs: adversive seizures (epileptic seizures in which the head and trunk are involuntarily turned to the side opposite the lesion; the contralateral arm is sometimes raised as well).
Syndrome of lesions of the precentral and postcentral gyri. Each of these gyri contains a somatotopic cortical representation of the entire body, as described in detail 50 years ago by the neurosurgeon Wilder Penfield (Fig. 5.4shows the classic “Penfield homunculus”). Lesions involving these paracentral gyri thus impair the function of specific parts of the body, with the specific site and extent of bodily dysfunction depending on the site and extent of the brain lesion. This can be most impressively observed in lesions of the precentral gyrus:
There are focal motor deficits, e. g., monoplegia of a limb; if the lesion is restricted to the precentral gyrus itself, the weakness may be flaccid, but this is rarely the case. Simultaneous dysfunction of the premotor cortex usually causes spastic weakness.
Sensory deficits are less frequently observed in such patients and cannot be clinically distinguished from those caused by thalamic lesions.
The intrinsic muscle reflexes are generally increased on the contralateral side of the body and there are accompanying pyramidal tract signs.
Irritative phenomena may appear in the form of Jacksonian epilepsy of focal onset (motor and/or somatosensory) or Kozhevnikov's epilepsia partialis continua (p. 166).
Temporal lobe syndrome takes different forms depending on the precise location of the lesion:
impairment of memory (e.g., in lesions affecting the hippocampus on both sides);
sensory aphasia (Wernicke aphasia, p. 41) in lesions involving the language-dominant (usually left) hemisphere;
Fig. 5.4 The cortical representation of different parts of the body in the primary somatosensory cortex of the postcentral gyrus (left) and the primary motor cortex of the precentral gyrus (right) in the human being. (After Penfield, W., H. Jasper: Epilepsy and the Functional Anatomy of the Human Brain. Little, Brown, Boston 1954.)
possible disturbance of spatial orientation in lesions involving the non-language-dominant (usually right) hemisphere;
in deep-seated lesions, a visual disturbance taking the form of contralateral homonymous upper quadrantanopsia;
irritative phenomena: complex partial seizures (temporal lobe seizures, p. 166), sometimes with ictal olfactory or gustatory hallucinations (uncinate fits)—these are usually reported as unpleasant;
mental abnormalities: irritability, depression.
Parietal lobe syndrome manifests itself in somatosensory deficits and a variety of neuropsychological abnormalities:
The most prominent sign is usually a hemisensory deficit.
Lesions of the language-dominant hemisphere (usually left) can cause left/right confusion, finger agnosia, acalculia, and agraphia (Gerstmann syndrome), and/or astereognosia.
Lesions of the nondominant hemisphere (usually right) can cause anosognosia (see above).
With regard to motor function, there are often poorly coordinated, ataxic hand and foot movements on the side opposite the lesion.
With regard to somatic sensation, there may be neglect for the contralateral half of the body (so-called extinction phenomenon: raw sensation is intact bilaterally, but if the examiner touches the patient simultaneously and equally intensely at mirror image sites on the two sides, the patient will report having felt something on one side only).
Deep-seated lesions may produce contralateral homonymous lower quadrantanopsia or hemianopsia, orelse only visual neglect for the contralateral hemifield.
Occipital lobe syndrome is mainly characterized by:
a contralateral visual field defect (homonymous hemianopsia; cf. Fig. 3.6, p. 19);
possible cortical blindness (in the case of bilateral occipital lobe lesions), in which elementary or formed visual hallucinations, or seeing gray, may be present; patients often deny being blind (anosognosia);
visual agnosia, i. e., the inability to recognize colors or shapes, despite normal visual acuity;
irritative phenomena: visual hallucinations, perhaps as the initial symptom of an epileptic seizure.
Syndromes of the Extrapyramidal Motor System
Function. The extrapyramidal motor system plays an important role in the smooth and purposeful execution of all motor processes, both voluntary and involuntary. One of its functions is to efficiently combine individual motor components into complex patterns of movement and to enable their largely automatic execution. Further ones are to give the signals for the initiation and termination of a movement and to regulate muscle tone.
Anatomical substrate. The main nuclei of the extrapyramidal motor system are the basalganglia (caudate nucleus, putamen, and globus pallidus). Further components are the subthalamic nucleus (in the diencephalon) as well as the substantia nigra and the red nucleus (both in the midbrain). Extensive fiber connections link these nuclei to each other and to higher motor cortical areas (by way of the thalamus). They influence the activity of spinal motor neurons through a number of afferent and efferent spinal pathways.
Deficits. Lesions of individual components of the extrapyramidal motor system produce various types of disturbance, corresponding to the precise location of the lesion. Because the functions of the extrapyramidal system are essentially as described above, functional impairments can manifest themselves as an excess or deficiency of movement-initiating impulses, automatic movement, and/or muscle tone:
There may be diminished spontaneity of movement, i.e., hypokinesia (e.g., in Parkinson disease) usually combined with elevated muscle tone, i. e., rigidity → hypertonic–hyperkinetic syndrome, p. 127.
On the other hand, there may be hyperkinesia of a wide variety of types, which may be thought of as the uncontrolled expression of complex motor programs resulting from a removal of their normal inhibition by the extrapyramidal motor system. These involuntary, repetitive movements include chorea, athetosis, ballism, and dystonia, all of which are described in detail on p. 131. Choreatic syndromes are often associated with diminished muscle tone → hypotonic–hyperkinetic syndrome.
Acute basal ganglionic lesions can also cause transient hemiparesis.
Thalamic Syndromes
Function. The thalamus is the synaptic relay station for many somatosensory and special sensory pathways; it transmits afferent impulses from peripheral extero- and proprioceptors, as well as from the higher sensory organs (eye, ear), to higher centers. In the thalamus, impulses pertaining to the body's various senses are integrated, affectively colored, and then passed on to the cortex (conscious perception appears to be possible only if the impulses reach the cortex). The thalamus also receives neural input from the extrapyramidal motor system and participates in the regulation of attention and drive as a component of the ascending reticular activating system (see below). Finally, certain components of the thalamus play a role in memory.
Deficits. Because the functions of the thalamus are as we have just described, lesions affecting it can produce the following deficits:
Somatosensory deficits: these mainly consist of impaired proprioception on the side opposite the lesion. There may also be painful, burning sensationsthat either arise spontaneously (dysesthesia) or are induced by, and outlast, a tactile stimulus delivered to the skin (hyperpathia).
Deficits of movement and coordination: there may be contralateral hemiparesis (which is usually transient) or hemiataxia.
Contralateral hemianopsia may be present.
Abnormal posture, particularly of the hands, may be present. In the “thalamic hand,” the metacarpophalangeal joints are flexed, while the interphalangeal joints are hyperextended.
Brainstem Syndromes
Function. The brainstem is a “throughway” for many fiber pathways of the nervous system, which lie adjacent to one another here in a very tightly confined space. All of the motor and somatosensory projections to and from the periphery pass through the brainstem; some of them cross here (decussate) to the other side and some undergo a synaptic relay. In addition, the brainstem contains many nuclei: all of the somatic and visceral motor and sensory nuclei of cranial nerves III through XII are located within it. Two brainstem nuclei, the red nucleus and substantia nigra, belong to the extrapyramidal motor system. Finally, among the nuclei of the reticular formation are found the vital autonomic regulatory centers controlling cardiovascular and respiratory function, as well as nuclei of the ascending reticular activating system that send activating impulses to the cerebral cortex and are essential for the maintenance of consciousness.
Deficits. As one would expect from the very large number of important neural structures located within the brainstem and fiber tracts passing through it, a correspondingly wide variety of deficits can be produced by lesions of different sizes and at different locations in the brainstem. The pattern of clinical manifestations usually enables the clinician to localize the level of the lesion to one of the three brainstem segments (midbrain, pons, or medulla). One can also clinically distinguish focal lesions from partial or complete cross-sectional lesions of the brainstem:
Unilateral focal lesions are usually of vascular origin (lacunar infarct). The typical clinical picture is the socalled alternating hemiplegia syndrome, in which a cranial nerve deficit on the side of the lesion appears together with a motor and/or sensory deficit on the contralateral half of the body. There are different alternating hemiplegia syndromes depending on the level of the lesion; some of these are described further in Table 6.4, p. 86.
Focal diencephalic lesions can produce diabetes insipidus as well as disturbances of thermoregulation, the sleep–wake cycle, eating behavior, and other instinctual behaviors.
Bilateral partial cross-sectional lesions of the brainstem. The classic example of a disturbance produced by this type of lesion is the locked-in syndrome, which is due to an extensive lesion of the ventral portion of the pons (e.g., an infarct secondary to thrombosis of the basilar a.). The corticobulbar and corticospinal pathways of the basis pontis are totally interrupted and part of the pontine reticular formation may be as well. All four limbs are paralyzed (quadriplegia), and the caudal cranial nerves are dysfunctional: the patient cannot swallow, speak, or, usually, produce facial expressions. Vertical eye movements and lid closure, both of which are midbrain functions, are preserved, but horizontal eye movements, which are a function of the pons, are abolished. Consciousness remains intact because the reticular formation is largely spared. The patient can communicate through vertical eye movements and lid closure.
Bulbar palsy and pseudobulbar palsy are two further syndromes caused by bilateral partial cross-sectional lesions of the brainstem. (True) bulbar palsy is produced by system atrophy of the motor cranial nerve nuclei of the medulla and therefore manifests itself as bulbar dysarthria, dysphagia, and tongue atrophy, with fasciculations. In pseudobulbar palsy, the causative lesion does not involve the cranial nerve nuclei themselves, but rather their innervating corticonuclear pathways bilaterally, or else the cortical areas from which these pathways arise. The clinical picture resembles that of bulbar palsy, but tongue atrophy and fasciculations are absent because the peripheral motor neuron is intact.
Complete cross-sectional lesions of the brainstem (brainstem transection) are due either to a pathological process in the posterior fossa or the brainstem itself (infratentorial lesion), or to acute intracranial hypertension in the supratentorial compartment, with secondary herniation and brainstem compression. Systemic processes (prolonged hypoxia or cardiorespiratory arrest; see above) can also cause extensive damage to the brainstem, as well as to the cerebral hemispheres. Mid-brain lesions cause severe impairment of consciousness, ranging to deep coma, and characteristic motor and oculomotor signs. The same is true of pontine lesions. The most prominent sign of medullary transection is loss of all autonomic function. The level of brainstem injury can almost always be correctly deduced from the pattern of clinical deficits and the findings of a few special tests (particularly of the brain stem reflexes), as described in Table 5.7. A patient who survives acute, extensive damage to the midbrain will probably be quadriplegic and suffer from akinetic mutism (see above).
Cerebellar Syndromes
Function. The tasks of the cerebellum are to optimize the amplitude, speed, and precision of voluntary movement and simultaneously to regulate the motor control of balance and adapt muscle tone to the demands placed on the body's movement apparatus. The cerebellum also plays a role in the regulation of gaze-related movements of the eyes and in ensuring the smooth complementary functioning of agonist and antagonist muscle groups.
In order to perform these coordinating tasks, the cerebellum requires information from various different parts of the nervous system. These different types of information are processed separately in three parts of the cerebellum that are distinct from one another both functionally and phylogenetically:
Impulses from the cerebral cortex for the initiation and planning of voluntary movement travel in the corticopontocerebellar pathway, by way of the brachium pontis (middle cerebellar peduncle), to the neocerebellum (located in the cerebellar hemispheres). This phylogenetically youngest part of the cerebellum is mainly responsible for the fine control of very precise movements, particularly of the limbs (especially the hands and fingers) and of the motor apparatus of speech.
Information regarding joint position and muscle tone from peripheral proprioceptors (muscle spindles and Golgi tendon organs) travels, by way of the anterior and posterior spinocerebellar tracts, through the restiform body and brachium conjunctivum (inferior and superior cerebellar peduncles) to the paleocerebellum (located in part of the vermis and the paraflocculus). This part of the cerebellum is mainly responsible for the smooth, synergistic functioning of the muscles when the individual stands or walks (see above).
Impulses from the vestibular system travel by way of the restiform body (inferior cerebellar peduncle) to the archicerebellum (nodulus and flocculus). This phylogenetically oldest part of the cerebellum mainly serves to keep the upright body in balance during standing and walking.
The cerebellum integrates the various types of afferent impulses it receives and then influences the motor regulatory functions of the brain and spinal cord in the manner of a feedback system. Efferent impulses travel:
from the cerebellar cortex to the dentate nucleus, where further processing takes place, and then through the superior cerebellar peduncle to the lateral nucleus of the thalamus, and onward to the cerebral cortex (Fig. 5.5),
while other efferent impulses travel from the dentate nucleus via the red nucleus to the thalamus, or via the red nucleus to the olive, and then back to the cerebellum. These two neuronal loops give off descending fibers to the rubrospinal and reticulospinal tracts, which terminate in the motor nuclei of the spinal cord (Fig. 5.5).
The integration of the cerebellum in the complex functional system controlling voluntary movement is shown in Fig. 5.6.
Deficits. In accordance with the functions of the cerebellum described above, cerebellar lesions produce disturbances of muscle tone and movement:
basal lesions near the midline (mainly affecting the archicerebellum) produce disturbances of truncal posture and the maintenance of balance, which are particularly evident when the patient tries to sit;
vermian lesions (mainly affecting the paleocerebellum) produce impaired coordination of stance and gait;
lesions of the cerebellar hemispheres (mainly affecting the neocerebellum) produce impaired coordination of (fine) movements of the limbs on the side of the lesion.
A detailed list of clinical manifestations of cerebellar disease is provided in Table 5.8.
Fig. 5.5 Anatomical connections of the cerebellum. The connections to the cerebral cortex, brainstem, vestibular system, and spinal cord are illustrated. For details, see text.
Fig. 5.6 Functional relations of the cerebellum to other motor centers. To keep the diagram simple, the sensory feedback to the cerebellum and basal ganglia is not shown (modified from Ellen and Tsukahara 1974).
Table 5.8 Clinical manifestations of cerebellar disease
|
Clinical manifestation |
Definition/description |
Remarks |
|
Diminished muscle tone |
can be felt by the examiner during repeated passive movement, e. g., pronation and supination of the forearm |
|
|
Dyssynergia |
lack of coordination of the various muscle groups participating in a single movement |
e. g., when walking on all fours, lack of precise alternation of limbs (each arm with opposite leg) |
|
Dysmetria |
poor control of the force, speed, and amplitude of voluntary movement |
e. g., opening fingers too wide when trying to grasp a small object |
|
Intention tremor |
alternating, progressively severe deviation from the ideal course of a directed movement as the limb approaches the target |
cf. Fig. 3.20 |
|
Pathological rebound phenomenon |
when a muscle is actively contracted against resistance and the resistance is suddenly released, the antagonist muscles fail to contract within a normally brief interval after the release |
cf. Fig. 3.21 |
|
Dysdiadochokinesia |
the alternating contraction of agonists and antagonists cannot be performed as rapidly and smoothly as normal |
cf. Fig. 3.17 |
|
Sinking of a limb in postural testing |
the tonic muscle contraction needed to keep the limb in a particular antigravity posture cannot be sustained as long as normal on the affected side |
the sinking limb is ipsilateral to the cerebellar lesion |
|
Truncal ataxia |
the patient is unable to stay sitting up |
indicates a vermian lesion |
|
Unsteady stance |
observable in the Romberg test |
cf. Fig. 3.1e |
|
Cerebellar gait |
wide-based, unsteady, ataxic gait |
indicates involvement of the vermis |
|
Past-pointing in the Bárány pointing test |
slowly lowering the extended arm onto a previously demonstrated target, with eyes closed; deviation to the side of the affected cerebellar hemisphere |
also positive in ipsilateral vestibular lesions, cf. p. 26 |
|
Nystagmus |
coarse nystagmus toward the side of the lesion, increasing with gaze toward the side of the lesion, decreasing on closure of the eyes |
cf. Fig. 11.1 |
|
Pathological nystagmus suppression test |
the patient stands up, stretches out his or her arms forwards, stares at his or her own extended thumbs, and keeps on doing so while the examiner rapidly rotates the patient around the bodily axis; staring at the thumbs completely suppresses the induced vestibular nystagmus in normal persons, but not in persons with cerebellar disease |
cf. Fig. 11.5 |
|
Cerebellar dysarthria |
choppy, explosive speech (“scanning dysarthria”) |
patients with degenerative cerebellar diseases are said to develop a “lion's voice” |