Файл: PLUM AND POSNER S DIAGNOSIS OF STUPOR AND COM-1.pdf

of the leg or complaints of unilateral sensory symptoms or diplopia, suggests a cerebral or brainstem mass lesion.

GENERAL PHYSICAL

EXAMINATION

The general physical examination is an important source of clues as to the cause of unconsciousness. After stabilizing the patient (Chapter 7), one should search for signs of head trauma. Bilateral symmetric black eyes suggest basal skull fracture, as does blood behind the tympanic membrane or under the skin overlying the mastoid bone (Battle’s sign). Examine the neck with care; if there is a possibility of trauma, the neck should be immobilized until cervical spine instability has been excluded by imaging. Resistance to neck flexion in the presence of easy lateral movement suggests meningeal inflammation such as meningitis or subarachnoid hemorrhage. Flexion of the legs upon flexing the neck (Brudzinski’s sign) confirms meningismus. Examination of the skin is also useful. Needle marks suggest drug ingestion. Petechiae may suggest meningitis or intravascular coagulation. Pressure sores or bullae indicate that the patient has been unconscious and lying in a single position for an extended period of time, and are especially frequent in patients with barbiturate overdosage.1

LEVEL OF CONSCIOUSNESS

After conducting the brief history and examination as outlined above and stabilizing the patients’ vital functions, the examiner should

conduct a formal coma evaluation. In assessing the level of consciousness of the patient, it is necessary to determine the intensity of stimulation necessary to arouse a response and the quality of the response that is achieved. When the patient does not respond to voice or vigorous shaking, the examiner next provides a source of pain to arouse the patient. Several methods for providing a sufficiently painful stimulus to arouse the patient without causing tissue damage are illustrated in Figure 2–1. It is best to begin with a modest, lateralized stimulus, such as compression of the nail beds, the supraorbital ridge, or the temporomandibular joint. These give information about the lateralization of motor response (see below), but must be repeated on each side in case there is a focal lesion of the pain pathways on one side of the brain or spinal cord. If there is no response to the stimulus, a more vigorous midline stimulus may be given by the sternal rub. By vigorously pressing the examiner’s knuckles into the patient’s sternum and rubbing up and down the chest, it is possible to create a sufficiently painful stimulus to arouse any subject who is not deeply comatose.

The response of the patient is noted and graded. The types of motor responses seen are considered in the section on motor responses (page 73). However, the level of response is important to the initial consideration of the depth of impairment of consciousness. In descending order of arousability, a sleepy patient who responds to being addressed verbally or light shaking, or one who responds verbally to more intense mechanical stimulation, is said to be lethargic or obtunded. A patient whose best response to deep pain is to attempt to push the examiner’s arm away is considered to be stuporous, with localizing responses. Patients who

make only nonspecific motor responses (wincing, restlessness, withdrawal reflexes) without a directed attempt to defend against the stimulus are considered to have a nonlocalizing response and are comatose. Patients who fail to respond at all are in the deepest stage of coma.

This rough grading system, from verbal responsiveness, to localizing responses, to nonlocalizing responses, to no response, is all that is needed for an initial assessment of the depth of unresponsiveness that can be used to follow the progress of the patient. If the initial evaluation of the level of consciousness demonstrates impairment, it is essential to progress through the next steps of the coma examina-

tion as rapidly as possible to safeguard that patient’s life. More elaborate coma scales are described in Box 2–1, but many of these depend upon the results of later stages in the examination, and it is never justified to delay attending to the basics of airway, breathing, and circulation while performing a more elaborate scoring evaluation.

ABC: AIRWAY, BREATHING, CIRCULATION

It is critical to ensure that the patient’s airway is maintained, that he or she is breathing ad-

equately, and that there is sufficient arterial perfusion pressure. The first goal must be to correct any of these conditions if they are found inadequate (Chapter 7). In addition, blood pressure, heart rate, and respiration may provide valuable clues to the cause of coma.

Circulation

It is critical first to ensure that the brain is receiving adequate blood flow. Cerebral perfusion pressure is the systemic blood pressure minus the intracranial pressure. The physician can measure blood pressure but in the initial examination can only estimate intracranial pressure. Over a wide range of blood pressures, cerebral perfusion remains stable because the brain autoregulates its blood flow by mechanisms described in the paragraphs below and illustrated in Figure 2–2. If the blood pressure falls too low or becomes too high, autoregulation fails and cerebral perfusion follows perfusion pressure passively; that is, it falls as the blood pressure falls and rises as the blood pressure rises. In this situation, both too low (ischemia) and too high (hypertensive encephalopathy; see Chapter 5) a blood pressure can damage the brain. To ensure adequate

brain perfusion, the physician should attempt to maintain the blood pressure at a level normal for the individual patient. For example, a patient with chronic hypertension autoregulates at a higher level than a normotensive patient. Lowering the blood pressure to a ‘‘normal level’’ may deprive the brain of an adequate blood supply (see Figure 2–2). Conversely, the cerebral blood flow (CBF) in children and pregnant women, who normally run low blood pressures, is regulated at lower levels and may develop excessive perfusion if the blood pressure is raised (e.g., pre-eclampsia).

The perfusion pressure of the brain may be influenced by the position of the head. In a normal individual, as the head is raised, the systemic arterial pressure is maintained by blood pressure reflexes. At the same time, the arterial perfusion pressure to the head is reduced by the distance the head is raised above the heart, but the intracranial pressure is also reduced because of the improved venous and cerebrospinal fluid (CSF) drainage. The net effect is that there is very little change in brain perfusion pressure or CBF. On the other hand, in a patient with stenosis of a carotid or vertebral artery, the perfusion pressure for that vessel may be much lower than systemic arterial pressure. If the head of the bed is raised,

perfusion pressure may fall below the threshold for autoregulation, and blood flow may be diminished below the level needed to support neurologic function. Such patients may show improvement in neurologic function when the head of the bed is flat. Conversely, in cases of head trauma where there is increased intracranial pressure, it may be important to raise the head of the bed 15 to 30 degrees to improve venous drainage to maximize cerebral perfusion pressure.6 Similarly, it is necessary to remove tight neckwear and ensure that a cervical spine collar is not applied too tightly to a victim of head injury to avoid diminishing venous outflow from the brain.

In a patient with impaired consciousness, the blood pressure can give important clues to the level of the nervous system that has been damaged. Damage to the descending sympathetic pathways that support blood pressure may result in a fall to levels seen after spinal transaction (mean arterial pressure about 60 to 70 mm Hg). Blood pressure is supported by a descending sympathoexcitatory pathway from the rostral ventrolateral medulla to the spinal cord, and so damage along the course of this pathway can result in spinal levels of blood pressure. The hypothalamus in turn provides a descending sympathoexcitatory input to the medulla and the spinal cord.7,8 As a consequence, bilateral diencephalic lesions result in a fall in sympathetic tone, including meiotic pupils (see below), decreased sweating responses, and a generally low level of arterial pressure.9

However, persistent hypotension below these levels in a comatose patient is almost never caused by an acute neurologic injury. One of the most common mistakes seen in evaluation of a comatose patient with a mean arterial pressure below 60 mm Hg is the assumption that a neurologic event may have caused the hypotension. This is almost never the case. A mean arterial pressure at or above 60 mm Hg is generally sufficient in a supine patient to support cerebral and systemic function. On the other hand, acute hypotension, due to cardiogenic or vasomotor shock, is a common cause of loss of consciousness and a threat to the patient’s life. Thus, the initial evaluation of a comatose patient with low blood pressure should focus on identifying the cause of and correcting the hypotension.

On the other hand, lesions that result in stimulation of the sympathoexcitatory system may cause an increase in blood pressure. For example, pain is a major ascending sympathoexcitatory stimulus, which acts via direct collaterals from the ascending spinothalamic tract into the rostral ventrolateral medulla. The elevation of blood pressure in response to a painful stimulus applied to the body (pinch of skin, ster-

nal rub) is evidence of intact medullospinal connections.10,11 In a patient who is still semi-

wakeful after subarachnoid hemorrhage, blood pressure may be elevated as a response to headache pain. Each of these conditions is associated with a rise in heart rate as well.

Direct pressure to the floor of the medulla can activate the Cushing reflex, an increase in blood pressure and a decrease in heart rate.12 In children, the Cushing reflex may be seen when there is a generalized increased intracranial pressure, even above the tentorium. However, the more rigid compartmentalization of intracranial contents in adults usually prevents this phenomenon unless the expansile mass is in the posterior fossa.

Activation of descending sympathoexcitatory pathways from the forebrain may also elevate blood pressure. Irritative lesions of the hypothalamus, such as occur with subarachnoid hemorrhage, may result in an excess hypothalamic input to the sympathetic and parasympathetic control systems.13 This condition can trigger virtually any type of cardiac arrhythmia, from sinus pause to supraventricular tachycardia to ventricular fibrillation.14 However, the most common finding in subarachnoid hemorrhage is a pattern of subendocardial ischemia. Such patients may in fact have enzyme evidence of myocardial infarction, and at autopsy demonstrate contraction band necrosis of the myocardium.15

Sympathoexcitation is also seen in patients who are delirious. The infralimbic and insular cortex and the central nucleus of the amygdala provide important inputs to sympathoexcitatory areas of the hypothalamus and the medulla.8 Activation of these areas due to misperception of stimuli in the environment causing emotional responses such as fear or anger may result in hypertension, tachycardia, and enlarged pupils.

Stokes-Adams attacks are periods of brief loss of consciousness due to lack of adequate

cerebral perfusion. These almost always occur in an upright position. In recumbent positions, when the head is at the same height as the heart, it takes a much steeper fall in blood pressure (below 60 to 70 mm Hg mean pressure) to cause loss of consciousness. The fall in blood pressure during a Stokes-Adams attack may reflect a failure of the baroreceptor reflex arc on assuming an upright posture (in which case it can be reproduced by testing orthostatic responses). Alternatively, hyperactivity of the baroreceptor reflex nerves may occasionally cause hypotension (e.g., in patients with carotid sinus hypersensitivity or glossopharyngeal neuralgia, where brief bursts of activity in barore-

ceptor nerves trigger a rapid fall in heart rate and blood pressure).16,17 In other patients, the

fall in blood pressure may be caused by an intermittent failure of the pump (i.e., cardiac arrhythmia). Thus, careful cardiologic evaluation is required if a neurologic cause is not identified.

PATHOPHYSIOLOGY

The brain ordinarily tightly controls the circulation to provide an adequate level of cerebral perfusion. It does this in two ways. First,

across a wide range of arterial blood pressures, it autoregulates its own blood flow.18–21 The

mechanism for this remarkable stability of blood flow is not entirely understood, although it appears to be due to intrinsic innervation of the cerebral blood vessels and may also be regulated by local metabolism.20,22 In general, local increases in CBF correspond to increases in local metabolic rate, allowing the use of blood flow (in positron emission tomography [PET] imaging) or local blood volume (in functional magnetic resonance imaging [MRI]) to approximate neuronal activity. However, there are also neuronal networks that regulate cerebral perfusion distinct from metabolic need. The two systems normally act in concert to ensure sufficient blood supply to allow normal cerebral function over a wide range of blood pressures but are dysregulated following some brain injuries.

Second, the brain acts through the autonomic nervous system to acutely adjust systemic arterial pressure in order to maintain a pressure head that is within the range that allows cerebral autoregulation. Blood pressure is the product of the cardiac output times the total vascular peripheral resistance. Cardiac

output in turn is the product of heart rate and stroke volume. Both heart rate and stroke volume are increased by beta-1 adrenergic stimulation from sympathetic nerves (or adrenal catechols), which play a key role in regulating cardiac output. Heart rate is slowed by muscarinic cholinergic action of the vagus nerve, and hence, increased vagal tone decreases cardiac output. Peripheral resistance is regulated mainly by the level of alpha-1 adrenergic tone in small arterioles, the most important resistance vessels. Therefore, the blood pressure is regulated by the balance of sympathetic tone, which increases both cardiac output and vasoconstrictor tone, versus parasympathetic tone, which slows heart rate and therefore decreases cardiac output. The cardiac vagal tone is maintained by the nucleus ambiguus in the medulla, which contains most of the cardiac parasympathetic preganglionic neurons.23 Sympathetic vascular and cardiac sympathetic tone is set by neurons in the rostral ventrolateral medulla that provide a tonic activating input to the sympathetic preganglionic neurons in the thoracic spinal cord.24

When in a lying position, the brain is at the same level as the heart, but as one rises, the brain elevates to a position 20 to 30 cm above the heart. This drop in perfusion pressure (arterial pressure minus intracranial pressure) is equivalent to 15 to 23 mm Hg, and it may be sufficient to cause a drop in cerebral perfusion pressure that would make it difficult to maintain CBF necessary to allow conscious brain function.

To defend against such a precipitous fall in perfusion pressure, the brain maintains reflex mechanisms to compensate for the hydrodynamic consequences of gravity. The level of arterial pressure is measured at two sites, the aortic arch (by the aortic depressor nerve, a branch of the vagus nerve) and the carotid bifurcation (by the carotid sinus nerve, a branch of the glossopharyngeal nerve). These two nerves terminate in the brain in the nucleus of the solitary tract, which is the main relay for all visceral sensory information in the brain.25,26 The nucleus of the solitary tract then provides an excitatory input to the caudal ventrolateral medulla.27

The caudal ventrolateral medulla in turn provides an ascending inhibitory input to the tonic vasomotor neurons in the rostral ventrolateral