Impairment in Consciousness

Global changes in CNS function, best described in terms of impairment in consciousness, are generally referred to as encephalopathy or altered mental status. An acute change in the level of consciousness undoubtedly is the most common neurologic complication that occurs after ICU admission. Consciousness is defined as a state of awareness (arousal or wakefulness) and the ability to respond appropriately to changes in environment.⁴² For consciousness to be impaired, global hemispheric dysfunction or dysfunction of the brainstem reticular activating system must be present.⁴³ Altered consciousness may result in a sleeplike state (coma) or a state characterized by confusion and agitation (delirium). States of acutely altered consciousness seen in the critically ill are listed in Table 1-2.

TABLE1-2 States of Acutely Altered Consciousness

When an acute change in consciousness is noted, the patient should be evaluated keeping in mind the patient’s age, presence or absence of coexisting organ system dysfunction, metabolic status and medication list, and presence or absence of infection. In patients with a primary CNS disorder, deterioration in the level of consciousness (e.g., from stupor to coma) frequently represents the development of brain edema, increasing intracranial pressure, new or worsening intracranial hemorrhage, hydrocephalus, CNS infection, or cerebral vasospasm. In patients without a primary CNS diagnosis, an acute change in consciousness is often due to the development of infectious complications (i.e., sepsis-associated encephalopathy), drug toxicities, or the development or exacerbation of organ system failure. Nonconvulsive status epilepticus is increasingly being recognized as a cause of impaired consciousness in critically ill patients (Box 1-1).^44–53

States of altered consciousness manifesting as impairment in wakefulness or arousal (i.e., coma and stupor) and their causes are well defined.^42,43,54,55 Much confusion remains, however, regarding the diagnosis and management of delirium, perhaps the most common state of impaired CNS functioning in critically ill patients at large. When dedicated instruments are used, delirium can be diagnosed in more than 80% of critically ill patients, making this condition the most common neurologic complication of critical illness.^56–58 Much of the difficulty in establishing the diagnosis of delirium stems from the belief that delirium is a state characterized mainly by confusion and agitation and that such states are expected consequences of the unique environmental factors and sleep deprivation that characterize the ICU experience. Terms previously used to describe delirium in critically ill patients include ICU psychosis, acute confusional state, encephalopathy, and postoperative psychosis. It is now recognized that ICU psychosis is a misnomer; delirium is a more accurate term.⁵⁹

Currently accepted criteria for the diagnosis of delirium include abrupt onset of impaired consciousness, disturbed cognitive function, fluctuating course, and presence of a medical condition that could impair brain function.⁶⁰ Subtypes of delirium include hyperactive (agitated) delirium and the more common hypoactive or quiet delirium.⁵⁸ Impaired consciousness may be apparent as a reduction in awareness, psychomotor retardation, agitation, or impairment in attention (increased distractibility or vigilance). Cognitive impairment can include disorientation, impaired memory, and perceptual aberrations (hallucinations or illusions).⁶¹ Autonomic hyperactivity and sleep disturbances may be features of delirium in some patients (e.g., those with drug withdrawal syndromes, delirium tremens). Delirium in critically ill patients is associated with increased morbidity, mortality, and ICU length of stay.^62–64 In general, sepsis and medications should be the primary etiologic considerations in critically ill patients who develop delirium.

As has been noted, nonconvulsive status epilepticus is increasingly recognized as an important cause of impaired consciousness in critically ill patients. Although the general term can encompasses other entities, such as absence and partial complex seizures, in critically ill patients, nonconvulsive status epilepticus is often referred to as status epilepticus of epileptic encephalopathy.⁵³ It is characterized by alteration in consciousness or behavior associated with electroencephalographic evidence of continuous or periodic epileptiform activity without overt motor manifestations of seizures. In one study of comatose patients without overt seizure activity, nonconvulsive status epilepticus was evident in 8%.⁵¹ Nonconvulsive status epilepticus can precede or follow an episode of generalized convulsive status epilepticus; it can also occur in patients with traumatic brain injury, subarachnoid hemorrhage, global brain ischemia or anoxia, sepsis, and multiple organ failure. Despite the general consensus that nonconvulsive status epilepticus is a unique entity responsible for impaired consciousness in some critically ill patients, there is no general consensus on the electroencephalographic criteria for its diagnosis or the optimal approach to treatment.⁶⁵