Chapter 17 - Depression of consciousness

Definitions

We should begin with some definitions because medical and lay jargon for the various levels of depression of consciousness are not consistently applied. Careful observation of the subject's behavior (or absence of behavior) is most informative.

Coma

Coma is a pathologic state of unconsciousness from which a person cannot be aroused to make any purposeful responses. As a rule light coma is present when reflex motor response (i.e., decorticate and decerebrate posturing) can be elicited by noxious stimulation. In deep coma there is no response.

Stupor

Stupor is a state of pathologic reduced consciousness from which the patient can be aroused to purposeful response only with intense or persistent stimulation. This includes the broad range from persistent drowsiness (from which the patient can be briefly aroused to produce purposeful responses by stimulation) to deep stupor, from which the patient can only be aroused to produce poorly directed defense against intense, noxious stimuli.

Sleep

 

Sleep, by contrast, is a nonpathologic depression of consciousness from which the subject can be aroused to persistent alert wakefulness with appropriate non-noxious stimuli. Sleep is an active and reversible state of consciousness.

Coma-like States

Hysterical coma or stupor is feigned or subconsciously assumed depression of consciousness. This is differentiated clinically from true coma or stupor by a normal and alert electroencephalogram, the presence of nystagmus on caloric irrigation of the external auditory canal (see Chapter 6), and the absence of abnormal neurologic signs.

There are some neurologic conditions in which a person has lost most if not all ability to move without impairment of consciousness. For example, blockade or damage of neuromuscular junction (as with severe myasthenia gravis or certain drugs and medications) and conditions that diffusely affect peripheral nerves (such as Guillain-Barre syndrome or porphyria) can prevent a patient from responding, although s/he may be perfectly awake and alert (though on a ventilator). However, these patients are not likely to be considered comatose since the history is usually obvious.

On the other hand, there are conditions that damage the base of the pons that may not be so obvious. When the base of the pons is severely damaged, usually by hemorrhage, infarction or acute destruction of myelin (such as by central pontine myelinolysis), the patient becomes acutely unresponsive (despite being awake and aware). Such patients are unable to move except for vertical and convergent eye movement and eye-opening systems (these functions are located in the midbrain). The auditory system, lying laterally along the brain stem, usually is not affected by the damage which often arises from involvement of the paramedian arteries. Thus the patient hears and sees. Unless the clinician asks them to look up or down, the patient may be thought to be comatose. They are said to be "locked in", a term aptly applied to this state by Plum and Posner.

Hysterical coma or stupor is feigned or subconsciously assumed depression of consciousness differentiated clinically from true coma or stupor by a normal and alert electroencephalogram, the presence of nystagmus on caloric irrigation of the external auditory canal (see Chapter 6) and the absence of abnormal neurologic signs.

Table 17-1 lists examples of the more common processes leading to pathologic depression of consciousness.

Substrates of consciousness

Before considering the conditions affecting consciousness, it is worth considering the brain structures that are necessary to maintaining it. Consciousness requires varying amounts of normal activity of the cerebral cortex since the hemispheres are the substrate for awareness of self and environment and embody the sentient functions that define human intellectual existence. Therefore, anything that diffusely depresses the activity of cerebral cortical neurons will produce stupor and coma. In general, this can either be due to actual destruction of cortical neurons or can be due to conditions that suppress the activity of the cortex, which is generically called "encephalopathy". It is important to note that normal activity of the cerebral cortex is maintained by activity in the reticular formation of the rostral brain stem extending from the rostral pons and caudal midbrain through the diencephalon (Figure 17-1), termed the reticular activating system.

The reticular formation can function normally even after destruction of the cerebral cortex (such as following diffuse cerebral anoxia from cardiac arrest). In general, the cerebral cortex is more sensitive to metabolic or toxic damage. In the case of the patient with diffuse cerebral cortical damage, the reticular formation and brain stem may be capable of supporting a crude sleep-waking vegetative state. In contrast, the cerebral hemispheres cannot function in the absence of reticular activation. Bilateral loss of the reticular formation at the midbrain level (for example from severe ischemic or hemorrhagic damage of the upper brain stem), is likely to terminate all meaningful cerebral activity.

In this chapter we concern ourselves with the two processes that depress consciousness, those being by diffuse suppression of cerebral cortical activity or by damaging the reticular formation. Some conditions do both. We will consider these two potential substrates for stupor and coma and how they are approach and evaluated.

Cerebral Hemispheres

Stupor and coma that is due to diffuse depression of cerebral cortical function is almost invariably the result of metabolic abnormality or toxic effect on the cortex and is termed "encephalopathy". Depending on the type and degree of metabolic insult, the reticular formation may also be depressed. However, the reticular formation is substantially more resistant to metabolic and toxic influences than the cerebral cortex, so brain stem functions are relatively preserved.

In general, metabolic depression of brain function (encephalopathy) is the most common cause of stupor and coma and may manifest itself in a number of ways. The various manifestations of metabolic encephalopathy are discussed in greater detail in these case examples. Table 17-1 lists the basic categories of metabolic dysfunction associated with depression of consciousness that must be considered.

Reticular Formation

As described above, the midbrain reticular formation (reticular activating system) is necessary for maintenance of consciousness. The reticular formation can be suppressed by the same processes that suppress cerebral cortical function, although the reticular formation is somewhat more resistant than the cortex. However, the reticular formation is also susceptible to direct damage because of the fact that it is contained in a relatively small area of the brain stem. Several types of lesions commonly directly damage the reticular formation, the most common being ischemia/infarction, hemorrhage, neoplasm, abscess, or direct trauma. In addition to these primary injuries, the reticular formation can also be damaged by compression. This usually results from a space-occupying lesion that forces part of the temporal lobe (usually the uncus) through the tentorial notch. Recall that the midbrain passes through this notch. This process, termed "transtentorial herniation" (or "uncal herniation") occurs due to space occupying lesions that affect the hemispheres. These include neoplasms/tumors, hemorrhage (intracerebral, subdural, or epidural hematomas), abscesses, or conditions causing brain edema.

In the case of herniation, the initial lesion is usually unilateral and many cases present with signs and symptoms of damage to that side of the brain (e.g., contralateral hemiparesis, hemihypoesthesia, dysphasia) that precede secondary brain stem compression. As the pressure builds, the medial temporal lobes shift away from the lesion compressing the midbrain reticular formation (fig. 17-2). This results in the depression of consciousness. As can be seen in the figure, brain stem compression occurs because the vectors of force from an expanding mass in the hemispheres are directed toward the tentorial notch, which is the only significant exit from the otherwise closed, rigid-walled, supratentorial space. This progression of transtentorial herniation results in what has been termed “rostrocaudal deterioration” in function. Progressively, the diencephalon, mesencephalon, and finally the pons and medulla are compromised. Remember, the expanding mass that causes this can be anything that occupies space, including swollen (edematous) brain tissue.

Evaluation

From the above discussion, you should be aware that depressed consciousness might result from either bilateral cerebral cortical depression (often with later suppression of reticular function) or from damage to the brain stem reticular activating system. The neurologic exam of the comatose patient is primarily focussed on discriminating these two types of causes of depressed consciousness. This is because the conditions that produce diffuse cortical suppression are very different from the conditions that damage the reticular activating system and require totally different further evaluation and management. Therefore, once the immediately life-threatening issues are being brought under control (hemorrhage, respiratory, or cardiovascular arrest) the evaluation of the comatose patient focuses on beginning the evaluation to differentiate these two broad types of coma. This permits the immediate and correct selection of further diagnostic tests and institution of appropriate therapeutic management. Table 17-2 lists the major steps taken by the physician from the time the patient enters the emergency department.

The neurologic evaluation of the comatose patient can be a relatively rapid and efficient procedure and should enable the examiner to differentiate between bilateral cerebral cortical depression (encephalopathy) versus damage to the brain stem reticular formation. Of course, it is not possible to examine all elements of the nervous system in the patient with depressed consciousness. However, careful observation of five categories of neurologic function is adequate for these purposes in most cases: (1) level of consciousness, (2) respiratory rate and pattern, (3) pupillary function, (4) oculomotor-vestibular function, and (5) motor function. The evaluation of these levels of functioning can help to differentiate encephalopathy from brain stem damage. Also, serial observation of these variables can detect progression of the condition. An excellent 12 minute video on this part of the exam can be found here.

1. Level of consciousness

The degree of impairment of consciousness is assessed, ranging from awake and alert to stuporous and comatose based on the degree of response to stimulation. Of course, the patient who is awake and alert responds appropriately and normally to questions and commands. The patient who is drowsy opens their eyes to verbal stimulation and responds appropriately, but drifts back to a condition with their eyes closed when not stimulated. A deeper stupor requires more vigorous stimulation in order to get the patient to open their eyes and attend to stimuli. And finally, in a deep coma, there is no response to even noxious stimulation (such as tickling the nostrils or squeezing the fingernail). Motor responses range from being able to follow commands, through localization of noxious stimuli, to simple withdrawal from noxious stimuli to decorticate posturing and decerebrate posturing as progressively lower parts of the nervous system are involved. Note that the patient showing a best motor response of withdrawal or posturing would not be conscious. This level of motor response should be recorded so that it can be determined whether the patient is deteriorating as time passes.

2. Respiration (see Figure 1-1).

The inspiratory and expiratory centers are located in the medullary reticular formation. These areas are under control from higher levels of the neuraxis and different patterns of respiration will occur as progressive levels of the nervous system are suppressed or damaged. These respiratory patterns have a general, although not perfectly reliable, relationship to involvement of different levels of the brain stem.

Obviously, the changes between these types of respirations can attend changes in the patient's condition and progression down the list (from Cheyne-Stokes to central neurogenic hyperventilation, for example) is a sign of a worsening state.

3. Pupils.

Pupil size and reactivity are mediated through variations in the equilibrium between sympathetic dilation and parasympathetic constriction (see Chapter 4).

4. Oculomotor-vestibular function (see Chapter 6).

Vestibulo-ocular function is one of the most important evaluations that can be made in the comatose patient. This is because the reflexes involved in eye movements to vestibular stimulation traverse most of the core of the brain stem (adjacent to the reticular activating system). Additionally, in the patient who is awake and alert, there are competing reflexes generated by the cerebral cortex that produce a distinctive pattern of eye movements called nystagmus. This video shows a normal response to infusion of ice water into the left ear canal in an awake and alert subject. If this person had an intact brain stem reticular activating system but was in a coma due to suppression of the cerebral cortex, the eyes would have drifted toward the side of the ice water and stayed there for minutes after the infusion, but would not have had the jerks of nystagmus. Therefore, this single assessment can determine whether the brain stem reticular formation is intact and also how alert the cerebral cortex is.

5. Motor function (see Chapter 8).

Rostral-caudal deterioration

Expanding masses are a model for discussion of the effects of damage to the various levels of the nervous system. Masses can include tumors, abscesses or hemorrhage but also can include edema. Expanding masses result in a somewhat predictable rostral-caudal deterioration of neural function which is likely to be fatal unless the process can be stopped. In these patients, cortical function is affected first (usually with some lateralizing signs), followed by dysfunction of the diencephalon, then midbrain. This is often accompanied by herniation of the uncus of the medial temporal lobe, called uncal herniation or transtentorial herniation. Later the pons and medulla are likely to be affected. Figure 17-2 depicts an expanding right intracerebral hemorrhage and its effect on the rostral brain stem. The sequential effects of such a lesion on functions in the 5 categories (consciousness, respirations, pupils, oculomotor/vestibular function and motor response) are listed in Table 17-3.

The first effect of such an expanding lesion would be due to its local effects on sensory and motor function (for example, in the figure 17-2 of a patient with a mass lesion on the right side, there may be left hemiparesis, loss of sensation on the left or a left visual field deficit). However, when the mass expands to the level of pressing on the upper brain stem (i.e., diencephalon), characteristic changes in the 5 variables (i.e., consciousness, respiration, pupils, vestibulo-ocular function and motor response) will begin to occur. The deterioration of these variables usually follows a predictable sequence, although steps can be skipped, particularly in rapidly evolving lesions. For example, consciousness will progress from stupor to coma; respirations will progress from eupnea through Cheyne-Stokes respirations, central neurogenic hyperventilation, ataxic breathing and, finally apnea; pupils will progress from normal reactive, through small reactive, to midposition/fixed; the vestibulo-ocular response will go from normal nystagmus through loss of the fast component, to dyconjugate response and then loss of the tonic phase of the reflex (usually accompanied by loss of the corneal reflex); and the motor response will deteriorate from normal mobility, to localizing noxious stimuli, to withdrawal responses, then decorticate posturing and decerebrate responses before losing all motor response other than simple spinal reflexes. The chart usually allows you to determine the level of preserved function (Table 17-3).

It is a useful exercise to understand these levels of function. However, for practical purposes, the determination of progressive levels of rostrocaudal deterioration has prognostic but little therapeutic value once the process has extended beyond the midbrain. In acute compressive injuries, severe loss of midbrain function is often accompanied by secondary hemorrhages (Duret hemorrhages) in the core of the midbrain and pons. This is associated with permanent cessation of reticular activating system function (i.e., irreversible coma). These hemorrhages are presumed to occur due to tearing of the tiny penetrating blood vessels. Because of the irreversibility of the condition once it has progressed, it is extremely important to recognize rostrocaudal deterioration and institute therapy early in order to prevent progression.

Metabolic encephalopathy

The most common causes of stupor and coma fall into the category of toxic/metabolic encephalopathy (this includes sedative drug-related stupor and coma). The cerebral cortex is very susceptible to toxic, metabolic or drug-related suppression of activity, much more susceptible is the brain stem. Therefore, toxic and metabolic causes of coma initially spare brain stem reflexes such as the vestibular, respiratory and pupillary responses. The pupillary light reflex is usually the last reflex to be detected as the brain stem is being suppressed by the toxic, metabolic or drug related insult. A very bright light may be necessary to detect the pupillary response.

Once the assessment has been made that the patient has a toxic/metabolic or drug-related encephalopathy, you must consider the large array of potential causes. many of the most common are included under #2 in Table 17-1. Please see this web page for some supplemental specific case exercises illustrating the principles of evaluation and management of patients with depression of consciousness.

Dementia vs. encephalopathy

We have already discussed dementia (Chapter 2 and Chapter 16), which results from bilateral degeneration of the cerebral hemispheres. Of course, dementia causes varying degrees of loss of cognitive and emotional function. However, if dementia is very severe, there may be so much loss of cortical function as to produce depressed consciousness. However, more often, depressed consciousness in a patient with dementia is due to the presence of one or more metabolic dysfunctions. Patients with dementia are very susceptible to conditions (and drugs) that depress remaining cortical function. Conditions that usually don’t result in major change in consciousness in healthy individuals (such as mild to moderate infections, renal insufficiency, simple dehydration or malnutrition, or mild sedative drugs), may result in coma in a patient with dementia (such as Alzheimer’s disease).

Reference

Questions

Define the following terms:

stupor, coma, delerium, encephalopathy, reticular activating system, decorticate posture, decerebrate posture, "locked-in", Cheyne-Stokes respirations, central neurogenic hyperventilation, ataxic respiration, vestibulo-ocular reflex, diencephalic pupils.
Stupor is a nonsleep depression of consciousness where normal reactions to the environment are blunted.
Coma is a nonsleep loss of consciousness where normal reactions to the environment are lost.
Delerium is a nonsleep depression of consciousness where normal reactions to the environment are blunted and replaced by agitated responses.
Encephalopathy is diffuse suppression of normal cerebral cortical function that often results in stupor or coma.
The reticular activating system is the reticular system connecting the rostral pontine and midbrain through the thalamus to the cerebral cortex.
Decorticate posture is a posture in which the lower limbs are extended and the upper limbs flexed in response to noxious stimuli.
Decerebrate posture is a posture in which the lower and upper limbs are extended in response to noxious stimuli.
"Locked-in" refers to damage to the base of the pons with preservation of consciousness and vertical eye movements, but loss of all other voluntary movements.
Cheyne-Stokes respiration is a pattern of breathing characterized by waxing and waning amplitude of respiration with preserved respriatory frequency.
Central neurogenic hyperventilation typically occurs with pontine lesions, with increased depth of respiration.
Ataxic respiration is a pattern of respiration with irregular depth and frequency of respirations with pauses.
The vestibulo-ocular reflex is the reflex that keeps eyes directed on a target during head movements. It can be elicited by head movments or caloric tests.
Diencephalic pupils refer to bilaterally small pupils with lesions of the thalamus.

17-1. What are the two potential causes of coma?

Answer 17-1. Coma may result from diffuse dysfunction of cerebral hemispheres or from damage to reticular activating system in brain stem (especially midbrain).

17-2. What are the causes of diffuse cerebral cortical suppression?

Answer 17-2. This may be due to direct cerebral effects of sedative drugs, systemic electrolyte disturbances, various severe metabolic upsets, trauma, diffuse ischemic damage,may be observed in the period after seizure (postictal). In these cases of toxic or metabolic encephalopathy, brainstem function is usually preserved until last - may see Cheyne-Stokes respirations.

17-3. What physical findings would indicate that coma was due to diffuse cerebral cortical dysfunction rather that to brain stem damage?

Answer 17-3. With diffuse cerebral cortical dysfunction you would expect to find normal dysinhibited oculovestibulor reflex eye movement to caloric testing. Motor findings and responses would be the same on both sides of the body (symmetrical). It is critical to be sure that there is no structural damage to the reticular formation. This is accomplished by determining whether eye movements are affected (extraocular nuclei are close to reticular formation).

17-4. What are key physical exam findings in patients with coma?

Answer 17-4. The vestibuloocular reflexes (oculocephalic testing or caloric testing) is critical to this assessment. Pupillary reactions may also be important.

17-5. What is transtentorial herniation?

Answer 17-5. Transtentorial herniation occurs with lateralized, supratentorial masses with displacement of the brain away from the expanding lesion. This produces stupor and coma by damaging the midbrain and reticular activating system. The uncus of the temporal lobe is usually the structure that herniates.

17-6. What are common symptoms of transtentorial herniation?

Answer 17-6. The third cranial nerve is often involved early in transtentorial herniation (with pupillary constrictor fibers usually damaged first). This is usually ipsilateral to the side of expanding lesion. The corticospinal tract is often involved next with contralateral weakness. Occasionally, with large shifts of the brain stem, this may be reversed (false localizing sign: Kernohan's notch).

17-7. What is the "locked-in" syndrome?

Answer 17-7. Locked-in syndrome usually results from damage at the level of the pons. Consciousness is preserved.

17-8. How can you recognize "locked-in" syndrome?

Answer 17-8. In the "locked-in" syndrome, vertical gaze and convergence is usually preserved. Eye opening may be preserved (eye closure is passive only). Other voluntary motions (including horizontal gaze included) are abolished.
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