Книги по МРТ КТ на английском языке / PLUM AND POSNER S DIAGNOSIS OF STUPOR AND COM-1

66

A

Brainstem intact (metabolic encephalopathy)

B

Right lateral pontine lesion (gaze paralysis)

C

MLF lesion

(bilateral internuclear ophthalmoplegia)

D

Right paramedian pontine lesion

(1 1/2 syndrome)

E

Midbrain lesion (bilateral)

eyes toward the side of cold water stimulation. Any activation of the anterior canal (which activates the ipsilateral superior rectus and the contralateral inferior oblique muscles) and the posterior canal (which activates the ipsilateral superior oblique and contralateral inferior rectus muscles) by caloric stimulation cancel each other out.

When caloric stimulation is done in an awake patient who is trying to maintain fixation (e.g., in the vestibular testing laboratory), cool water (about 308C) causes a slow drift toward the side of stimulation, with a compensatory rapid saccade back to the midline (the direction of nystagmus is the direction of the fast component). Warm stimulation (about 448C) induces the opposite response. The traditional mnemonic for remembering these movements is ‘‘COWS’’ (cold opposite, warm same), which refers to the direction of nystagmus in an awake patient. This mnemonic can be confusing for inexperienced examiners, as the responses seen in a comatose patient with an intact brainstem are the opposite: cold water induces only tonic deviation (there is no little or no corrective nystagmus), so the eyes deviate toward the ear that is irrigated. The presence of typical vestibular nystagmus in a patient who is unresponsive indicates a psychogenic cause of unresponsiveness (i.e., the patient is actually awake). The absence of a response to caloric stimulation does not always imply brainstem dysfunction. Bilateral vestibular failure occurs with phenytoin or tricyclic antidepressant toxicity. Aminoglycoside vestibular toxicity may obliterate the vestibular response, but oculocephalic responses may persist, the neck muscles supplying the afferent information.112

On the other hand, because the oculomotor pathways are spatially so close to those involved in producing wakefulness, it is rare for a patient to have acute damage to the oculomotor control system without a change in consciousness.

Patient 2–1

A 56-year-old man with a 20-year history of poorly controlled hypertension came to the emergency department with a complaint of sudden onset of severe dizziness. On examination, he was fully

awake and conversant. Pupils were 2.5 mm and constricted to 2.0 mm with light in either eye. The patient could not follow a moving light to either side or up or down. Hearing was intact, as were facial, oropharyngeal, and tongue motor and sensory responses. Motor and sensory examination was also normal, tendon reflexes were symmetric, and toes were downgoing.

The patient was sent for computed tomography (CT) scan, which showed a hemorrhage into the periventricular gray matter in the floor of the fourth ventricle at a pontine level, which tracked rostrally into the midbrain. During the CT scan the patient lapsed into coma. At that point, the pupils were pinpoint and the patient was unresponsive with flaccid limbs. He subsequently died, but autopsy was not permitted.

Comment. The sudden onset of bilateral impairment of eye movements on the background of clear consciousness is rare, and raised the possibility of a brainstem injury even without unconsciousness. Although the CT scan demonstrated a focal hemorrhage selectively destroying the abducens nuclei and medial longitudinal fasciculi, the proximity of these structures to the ascending arousal system was demonstrated by the loss of consciousness over the next few minutes.

Finally, if there has been head trauma, one or more eye muscles may become trapped by a blowout fracture of the orbit. It is important to distinguish this cause of abnormal eye movements from damage to neural structures, either peripherally or centrally. This is generally done by an ophthalmologist, who applies topical anesthetics to the globe and uses a fine, toothed forceps to tug on the sclera to attempt to move the globe (forced duction). Inability to move the globe through a full range of movements may indicate a trapped muscle and requires evaluation for orbital fracture.

Interpretation of Abnormal

Ocular Movements

A wide range of eye movements may be seen, both at rest and during vestibular stimulation. Each presents clues about the nature of the insult that is causing the impairment of consciousness.

RESTING AND SPONTANEOUS

EYE MOVEMENTS

A great deal of information may be gained by carefully noting the position of the eyes and any movements that occur without stimulation. Table 2–3 lists some of the spontaneous eye movements that may be observed in unconscious patients. Detailed descriptions are given in the paragraphs below. Most individuals have a mild degree of exophoria when drowsy and not maintaining active fixation. However, other individuals have varying types of strabismus, which may worsen as they become less responsive and no longer attempt to maintain conjugate gaze. Hence, it is very difficult to determine the meaning of dysconjugate gaze in a stuporous or comatose patient if nothing is known about the presence of baseline strabismus.

On the other hand, certain types of dysconjugate eye movements raise suspicion of brainstem injury that may require further examination for confirmation. For example, injury to the oculomotor nucleus or nerve produces exodeviation of the involved eye. Unilateral abducens injury causes the involved eye to deviate inward. In skew deviation,114 in which one eye is deviated upward and the other

downward, there typically is an injury to the brainstem (see below).

CONJUGATE LATERAL DEVIATION OF THE EYES

This is typically seen with destructive or irritative lesions, as compressive or metabolic disorders generally do not affect the supranuclear ocular motor pathways asymmetrically. A destructive lesion involving the frontal eye fields causes the eyes to deviate toward the side of the lesion (away from the side of the associated hemiparesis). This typically lasts for a few days after the onset of the lesion. An irritative lesion may cause deviation of the eyes away from the side of the lesion. These eye movements represent seizure activity, and often there is some evidence of quick, nystagmoid jerks toward the side of eye deviation indicative of continuing seizure activity. If seizure activity abates, there may be a Todd’s paralysis of gaze for several hours, causing lateral gaze deviation toward the side of the affected cortex (i.e., opposite to the direction caused by the seizures). Hemorrhage into the thalamus

may also produce ‘‘wrong-way eyes,’’ which deviate away from the side of the lesion.115,116

This may be due to interruption of descending corticobulbar pathways for gaze control, which pass through the thalamic internal medullary lamina, rather than the internal capsule. Damage to the lateral pons, on the other hand, may cause loss of eye movements toward that side (gaze palsy, Figure 2–9). The lateral gaze deviation in such patients cannot be overcome by vestibular stimulation, whereas vigorous oculocephalic or caloric stimulation usually overcomes lateral gaze deviation due to a cortical gaze paresis.

CONJUGATE VERTICAL DEVIATION OF THE EYES

Pressure on the tectal plate, such as occurs with a pineal mass or sometimes with a thalamic hemorrhage, may cause conjugate downward deviation of the eyes.117,118 Oculogyric crises may cause conjugate upward deviation. The classical cause of oculogyric crises was postencephalitic parkinsonism.119 Few of these patients still survive, but a similar condition is frequently seen with dystonic crises in patients exposed to neuroleptics120 and occasionally in patients with acute bilateral injury of the basal ganglia.

NONCONJUGATE EYE DEVIATION

Whereas nonconjugate eye position may be due to an old baseline strabismus, failure of one eye to follow its mate during spontaneous or evoked eye movements is typically highly informative. Absence of abduction of a single eye suggests injury to the abducens nerve either within the brainstem or along its course to the orbit. However, either increased intracranial pressure or decreased pressure, as occurs with cerebral spinal fluid leaks,121 can cause either a unilateral or bilateral abducens palsy, so the presence of an isolated abducens palsy may be misleading. Isolated loss of adduction of the eye contralateral to the head movement implies an injury to the medial longitudinal fasciculus (i.e., near the midline tegmentum) on that side between the abducens and oculomotor nuclei (Figure 2–9). Bilateral lesions of the medial longitudinal fasciculus impair adduction of both eyes as well as vertical oculocephalic and vestibulo-ocular eye movements, a condition that is distinguished from bilateral oculomotor nucleus or nerve injury in the comatose patient by preservation of the pupil-

lary light responses. (Voluntary vergence and vertical eye movements remain intact, but require wakeful cooperation.)

Combined loss of adduction and vertical movements in one eye indicates an oculomotor nerve impairment. Typically, there may also be severe ptosis on that side (so that if the patient is awake, he or she may not be aware of diplopia). In rare cases with a lesion of the oculomotor nucleus, the weakness of the superior rectus will be on the side opposite the other third nerve muscles (as these fibers are crossed) and ptosis will be bilateral (but not very severe). Occasionally, oculomotor palsy may spare the pupillary fibers. This occurs most often when the paresis is due to ischemia of the oculomotor nerve (the smaller pupilloconstrictor fibers are more resistant to ischemia), such as in diabetic occlusion of the vasa nervorum. Such patients are also typically awake and alert, whereas third nerve paresis due to brainstem injury or compression of the oculomotor nerve by uncal herniation results in impairment of consciousness and early pupillodilation.

Trochlear nerve impairment causes a hyperopia of the involved eye, often with some exodeviation. If awake, the patient typically attempts to compensate by tilting the head toward that shoulder. Because the trochlear nerve is crossed, a trochlear palsy in a comatose patient suggests damage to the trochlear nucleus on the opposite side of the brainstem.

SKEW DEVIATION

Skew deviation refers to vertical dysconjugate gaze, with one eye displaced downward compared to the other. In some cases, the eye that is elevated may alternate from side to side de-

pending on whether the patient is looking to the left or the right.95,122 Skew deviation is due

either to a lesion in the lateral rostral medulla or lower pons, vestibular system, or vestibulo-

cerebellum on the side of the inferior eye, or in the MLF on the side of the superior eye.123–125

ROVING EYE MOVEMENTS

These are slow, random deviations of eye position that are similar to the eye movements seen in normal individuals during light sleep. As in sleeping individuals who typically have some degree of exophoria, the eye positions may not be quite conjugate, but the ocular

excursions should be conjugate. Most roving eye movements are predominantly horizontal, although some vertical movements may also occur. Most patients with roving eye movements have a metabolic encephalopathy, and oculocephalic and caloric vestibulo-ocular responses are typically preserved or even hyperactive. The roving eye movements may disappear as the coma deepens, although they may persist in quite severe hepatic coma. Roving eye movements cannot be duplicated by patients who are awake, and hence their presence indicates that unresponsiveness is not psychogenic. A variant of roving eye movements is periodic alternating or ‘‘ping-pong’’ gaze,126 in which repetitive, rhythmic, and conjugate horizontal eye movements occur in a comatose or stuporous patient. The eyes move conjugately to the extremes of gaze, hold the position for 2 to 3 seconds, and then rotate back again. The episodic movements of the eyes may continue uninterrupted for several hours to days. Periodic alternating eye movements have been reported in patients with a variety of structural injuries to the brainstem or even bilateral cerebral infarcts that leave the oculomotor system largely intact, but are most common during metabolic encephalopathies.

NYSTAGMUS

Nystagmus refers to repetitive rapid (saccadic) eye movements, often alternating with a slow drift in the opposite direction. Spontaneous nystagmus is uncommon in coma because the quick, saccadic phase is generally a corrective movement generated by the voluntary saccade system when the visual image drifts from the point of intended fixation. However, continuous seizure activity with versive eye movements may give the appearance of nystagmus. In addition, several unusual forms of nystagmoid eye movement do occur in comatose patients.

Retractory nystagmus consists of irregular jerks of both globes back into the orbit, sometimes occurring spontaneously but other times on attempted upgaze. Electromyography during retractory nystagmus shows that the retractions consist of simultaneous contractions of all six extraocular muscles.127 Retractory nystagmus is typically seen with dorsal midbrain compression or destructive lesions117 and is thought to be due to impairment of

descending inputs that relax the opposing eye muscles when a movement is made, so that all six muscles contract when attempts are made to activate any one of them.

Convergence nystagmus often accompanies retractory nystagmus and also is typically seen in patients with dorsal midbrain lesions.128 The eyes diverge slowly, and this is followed by a quick convergent jerk.

OCULAR BOBBING AND DIPPING

Fischer129 first described movements in which the eyes make a brisk, conjugate downward movement, then ‘‘bob’’ back up more slowly to primary position. The patients were comatose and the movements were not affected by caloric vestibular stimulation. The initially described patients had caudal pontine injuries or compression, although later reports described similar eye movements in patients with obstructive hydrocephalus, uncal herniation, or even metabolic encephalopathy. A variety of related eye movements have been described including inverse bobbing (rapid elevation of the eyes, with bobbing downward back to primary position) and both dipping (downward slow movements with rapid and smooth return to primary position) and inverse dipping

(slow upward movements with rapid return to primary position).130,131 The implications of

these unusual eye movements are similar to those of ocular bobbing: a lower brainstem injury or compression of normal vestibulo-ocular inputs.

Seesaw nystagmus describes a rapid, pendular, disjunctive movement of the eyes in which one eye rises and intorts while the other descends and extorts.132 This is followed by reversal of the movements. It is most commonly seen during visual fixation in an awake patient who has severe visual field defects or impairment of visual acuity, and hence is not in a coma. Seesaw nystagmus appears to be due in most cases to lesions near the rostral end of the periaqueductal gray matter, perhaps involving the rostral interstitial nucleus of Cajal.133 It may occasionally be seen also in comatose patients, sometimes accompanied by ocular bobbing, and in such a setting may indicate severe, diffuse brainstem damage.134

Nystagmoid jerks of a single eye may occur in a lateral, vertical, or rotational direction in patients with pontine injury. It may be associated

with skew deviation and if bilateral, the eyes may rotate in the opposite direction.

MOTOR RESPONSES

The motor examination in a stuporous or comatose patient is, of necessity, quite different from the patient who is awake and cooperative. Rather than testing power in specific muscles, it is focused on assessing the overall responsiveness of the patient (as measured by motor response), the motor tone, and reflexes, and identifying abnormal motor patterns, such as hemiplegia or abnormal posturing.

Motor Tone

Assessment of motor tone is of greatest value in patients who are drowsy but responsive to voice. It may be assessed by gently grasping the patient’s hand as if you were shaking hands and lifting the arm while intermittently turning the wrist back and forth. Tone can also be assessed in the neck by gently grasping the head with two hands and moving it back and forth or up and down, and in the lower extremities by grasping each leg at the knee and gently lifting it from the bed or shaking it from side to side. Normal muscle tone provides mild resistance that is constant or nearly so throughout the movement arc and of similar intensity regardless of the initial position of the body part. Spastic rigidity, on the other hand, increases with more rapid movements and generally has a clasp-knife quality or a spastic catch, so that the movement is slowed to a near stop by the resistance, at which point the resistance collapses and the movement proceeds again. Parkinsonian rigidity remains equally intense despite the movement of the examiner (lead-pipe rigidity), but is usually diminished when the patient is asleep or there is impairment of consciousness. In contrast, during diffuse metabolic encephalopathies, many otherwise normal patients develop paratonic rigidity, also called gegenhalten. Paratonic rigidity is characterized by irregular resistance to passive movement that increases in intensity as the speed of the movement increases, as if the patient were willfully resisting the examiner. If the patient is drowsy but responsive to voice, urging him or her to ‘‘relax’’ may result in increased tone.

Paratonia is often seen in patients with dementia and is normally found in infants between the second and eighth weeks of life, suggesting that it represents a state of disinhibition of forebrain control as the level of consciousness becomes depressed. As patients become more deeply stuporous, muscle tone tends to decrease and these pathologic forms of rigidity are less apparent.

Motor Reflexes

Muscle stretch reflexes (sometimes erroneously referred to as ‘‘deep tendon reflexes’’) may be brisk or hyperactive in patients who are drowsy or confused and have increased motor tone. As the level of consciousness becomes further depressed, however, the muscle stretch reflexes tend to diminish in activity, until in patients who are deeply comatose they may be unobtainable.

Cutaneous reflexes such as the abdominal or cremasteric reflex typically become depressed as the level of consciousness wanes. On the other hand, in patients who are drowsy or confused, some abnormal cutaneous reflexes may be released. These may include extensor plantar responses. If the extensor plantar response is bilateral, this may signify nothing more than a depressed level of consciousness, but if it is asymmetric or unilateral, this implies injury to the descending corticospinal tract.

Prefrontal cutaneous reflexes, sometimes called ‘‘frontal release reflexes’’ or primitive reflexes,135 may also emerge in drowsy patients with diffuse forebrain impairment. Rooting, glabellar, snout, palmomental, and other reflexes are often seen in such patients. However, these responses become increasingly common with advancing age in patients without cognitive impairment, so they are of limited value in elderly individuals.136 On the other hand, the grasp reflex is generally seen only in patients who have some degree of bilateral prefrontal impairment.137 It is elicited by gently stroking the palm of the patient with the examiner’s fingers. The patient may grasp the examiner’s fingers, as if grasping a branch of a tree. The pull reflex is a variant in which the examiner curls his or her fingers under the patient’s as the patient attempts to grasp. The grasp is often so strong that it is possible to pull the patient from the bed. Many elderly

patients with normal cognitive function will have a mild tendency to grasp the first time the reflex is attempted, but a request not to grasp the examiner quickly abolishes the response. Patients who are unable to inhibit the reflex invariably have prefrontal pathology. The grasp reflex may be asymmetric if the prefrontal injury is greater on one side, but probably requires some impairment of both hemispheres, as small, unilateral lesions rarely cause grasping.137 Grasping disappears when the lesion involves the motor cortex and causes hemiparesis. It is of greatest value in a sleepy patient who can cooperate with the exam; it disappears as the patient becomes more drowsy.

A Metabolic encephalopathy

B Upper midbrain damage

C Upper pontine damage

Like paratonia, prefrontal reflexes are normally present in young infants, but disappear as the forebrain matures.135

Motor Responses

After assessing muscle tone, the examiner next tests the patient for best motor response to sensory stimulation (Figure 2–10). If the patient does not respond to voice or gentle shaking, arousability and motor responses are tested by painful stimuli. The maneuvers used to provide adequate stimuli without inducing actual tissue damage are shown in Figure 2–1.

Responses are graded as appropriate, inappropriate, or no response. An appropriate response is one that attempts to escape the stimulus, such as pushing the stimulus away or attempting to avoid the stimulus. The motor response may be accompanied by a facial grimace or generalized increase in movement. It is necessary to distinguish an attempt to avoid the stimulus, which indicates intact sensory and motor connections within the spinal cord and brainstem, from a stereotyped withdrawal response, such as a triple flexion withdrawal of the lower extremity or flexion at the fingers, wrist, and elbow. The stereotyped withdrawal response is not responsive to the nature of the stimulus (e.g., if the pain is supplied over the dorsum of the toe, the foot will withdraw into, rather than away from, the stimulus) and thus is not appropriate to the stimulus that is applied. These spinal level motor patterns may occur in patients with severe brain injuries or even brain death. It is also important to assess asymmetries of response. Failure to withdraw on one side may indicate either a sensory or a motor impairment, but if there is evidence of facial grimacing, an increase in blood pressure or pupillary dilation, or movement of the contralateral side, the defect is motor. Failure to withdraw on both sides, accompanied by facial grimacing, may indicate bilateral motor impairment below the level of the pons.

Posturing responses include several stereotyped postures of the trunk and extremities. Most appear only in response to noxious stimuli or are greatly exaggerated by such stimuli. Seemingly spontaneous posturing most often represents the response to endogenous stimuli, ranging from meningeal irritation to an occult bodily injury to an overdistended bladder. The nature of the posturing ranges from flexor spasms to extensor spasms to rigidity, and may vary according to the site and severity of the brain injury and the site at which the noxious stimulation is applied. In addition, the two sides of the body may show different patterns of response, reflecting the distribution of injury to the brain.

Clinical tradition has transferred the terms decorticate rigidity and decerebrate rigidity from experimental physiology to certain patterns of motor abnormality seen in humans. This custom is unfortunate for two reasons. First, these terms imply more than we really know about the site of the underlying neuro-

logic impairment. Even in experimental animals, these patterns of motor response may be produced by brain lesions of several different kinds and locations and the patterns of motor response in an individual to any one of these lesions may vary across time. In humans, both types of responses can be produced by supratentorial lesions, although they imply at least incipient brainstem injury. There is a tendency for lesions that cause decorticate rigidity to be more rostral and less severe than those causing decerebrate rigidity. In general, there is much greater agreement among observers if they simply describe the movements that are seen rather than attempt to fit them to complex patterns.

Flexor posturing of the upper extremities and extension of the lower extremities corresponds to the pattern of movement also called decorticate posturing. The fully developed response consists of a relatively slow (as opposed to quick withdrawal) flexion of the arm, wrist, and fingers with adduction in the upper extremity and extension, internal rotation, and vigorous plantar flexion of the lower extremity. However, decorticate posturing is often fragmentary or asymmetric, and it may consist of as little as flexion posturing of one arm. Such fragmentary patterns have the same localizing significance as the fully developed postural change, but often reflect either a less irritating or smaller central lesion.

The decorticate pattern is generally produced by extensive lesions involving dysfunction of the forebrain down to the level of the rostral midbrain. Such patients typically have normal ocular motility. A similar pattern of motor response may be seen in patients with a variety of metabolic disorders or intoxications.138 However, the presence of decorticate posturing in cases of brain injury is ominous. For example, in the series of Jennett and Teasdale, after head trauma only 37% of comatose patients with decorticate posturing recovered.139

Even more ominous is the presence of extensor posturing of both the upper and lower extremities, often called decerebrate posturing. The arms are held in adduction and extension with the wrists fully pronated. Some patients assume an opisthotonic posture, with teeth clenched and arching of the spine. Tonic neck reflexes (rotation of the head causes hyperextension of the arm on the side toward

which the nose is turned and flexion of the other arm; extension of the head may cause extension of the arms and relaxation of the legs, while flexion of the head leads to the opposite response) can usually be elicited. As with decorticate posturing, fragments of decerebrate posturing are sometimes seen. These tend to indicate a lesser degree of injury, but in the same anatomic distribution as the full pattern. It may also be asymmetric, indicating the asymmetry of dysfunction of the brainstem.

Although decerebrate posturing usually is seen with noxious stimulation, in some patients it may occur spontaneously, often associated with waves of shivering and hyperpnea. Decerebrate posturing in experimental animals usually results from a transecting lesion at the

level between the superior and inferior colliculi.140 It is believed to be due to the release of

vestibulospinal postural reflexes from forebrain control. The level of brainstem dysfunction that produces this response in humans may be similar, as in most cases decerebrate posturing is associated with disturbances of ocular motility. However, electrophysiologic, radiologic, or even postmortem examination sometimes reveals pathology that is largely confined to the forebrain and diencephalon. Thus, decerebrate rigidity is a clinical finding that probably represents dysfunction, although not necessarily destruction extending into the upper brainstem. Nevertheless, it represents a more severe finding than decorticate posturing; for example, in the Jennett and Teasdale series, only 10% of comatose patients with head injury who demonstrated decerebrate posturing recovered.139 Most patients with decerebrate rigidity have either massive and bilateral forebrain lesions causing rostrocaudal deterioration of the brainstem as diencephalic dysfunction evolves into midbrain dysfunction (see Chapter 3), or a posterior fossa lesion that compresses or damages the midbrain and rostral pons. However, the same pattern may occasionally be seen in patients with diffuse, but fully reversible, metabolic disorders, such as hepatic coma, hypoglycemia, or sedative drug

ingestion.138,141,142

Extensor posturing of the arms with flaccid or weak flexor responses in the legs is typically seen in patients with injury to the lower brainstem, at roughly the level of the vestibular nuclei. This pattern was described in the 1972 edition of this monograph, and has since been

repeatedly confirmed. The physiologic basis of this motor pattern is not understood, but it may represent the transition from the extensor posturing seen with lower midbrain and high pontine injuries to the spinal shock (flaccidity) or even flexor responses seen from stimulating the isolated spinal cord.

FALSE LOCALIZING SIGNS IN PATIENTS WITH METABOLIC COMA

The main purpose of the foregoing review of the examination of a comatose patient is to distinguish patients with structural lesions of the brain from those with metabolic lesions. Most patients with structural lesions require urgent imaging. Patients with metabolic lesions often require an extensive laboratory evaluation to define the cause. When focal neurologic findings are observed, it becomes imperative to determine whether there is a destructive or compressive process that may become life threatening or irreversibly damage the brain within a matter of minutes. On the other hand, even when there is no focal or lateralizing finding to suggest a structural lesion, it is important to know which signs point to specific metabolic causes, such as hypoglycemia or sepsis, that must be sought urgently. Therefore, the physician should become familiar with the few focal neurologic findings that are seen in patients with diffuse metabolic causes of coma, and understand their implications for the diagnosis of the metabolic problem.

Respiratory Responses

The range of normal respiratory responses includes the Cheyne-Stokes pattern of breathing, which is seen in many cognitively normal people with cardiac or respiratory disorders, particularly during sleep.43–45 Sleep apnea must also be distinguished from pathologic breathing patterns. Patients with severe sleep apnea may stop breathing for 10 seconds or so every minute or two. Their color may become dusky during the oxygen desaturation that accompanies each period of apnea.

Kussmaul breathing, in which there are deep but slow rhythmic breaths, is seen in