Definitions and Initial Assessment
Consciousness is awareness of oneself and one’s environment; it requires both arousal and mental content. The anatomic substrate for arousal is the reticular activating system of the brain stem and thalamus; the anatomic substrate for mental content is the cerebrum. Impaired consciousness thus follows either small brain stem lesions or large bilateral cerebral lesions. Temporary loss of consciousness can follow sudden severe lesions within a single cerebral hemisphere, for example, intracerebral hemorrhage. The mechanism is a poorly understood transsynaptic functional suppression of the opposite hemisphere referred to as diaschisis and reflected in decreased blood flow, oxygen uptake, and glucose metabolism. Prolonged or permanent loss of consciousness, however, indicates damage—structural or metabolic—either to the cerebral hemispheres bilaterally or to the reticular activating system.
Coma is a state of unconsciousness that clinically differs from syncope in being sustained and from sleep in being less readily reversed. Whereas cerebral oxygen uptake is reduced in coma, it is normal during sleep and actually increases during the rapid eye movement (REM) phase. Coma, moreover, reflects inhibition or destruction of brain circuits, whereas sleep involves activation of circuits that are normally suppressed during wakefulness.
“A 50-year-old woman begins having attacks of daytime sleepiness.”
“A 52-year-old man begins having nocturnal insomnia, impotence, and loss of libido.”
Clinically, coma is defined by the neurological examination, particularly by responses to external stimuli. As noted in Chapter 2, terms such as stupor, obtundation, and lethargy indicate different points along a continuum from coma to alertness, but such terms are not precisely defined, and it is therefore appropriate to record the minimal stimulus required to elicit a response and the nature of that response (eg, “The patient responds to her name being shouted by briefly opening her eyes and mumbling incomprehensibly, but she does not look at the examiner or follow commands.”). The actual level of consciousness is sometimes difficult to determine, for example, in patients with catatonia, severe depression, or akinesia plus aphasia. A patient receiving a neuromuscular blocking drug such as curare might be fully alert, but to an examiner, the alertness would be masked by total paralysis.
Delirium refers to severe inattentiveness, often with abnormal mental content and agitation. It can presage or alternate with obtundation, stupor, or coma.
Assessment of a comatose patient begins with the identification and treatment of any immediately life-threatening condition such as hemorrhage, shock, cardiac arrhythmia, airway obstruction, or apnea. If the diagnosis is uncertain, blood is drawn for glucose determination and glucose is administered intravenously, accompanied by thiamine. (Thiamine is a cofactor for a number of enzymes involved in glucose metabolism; in a thiamine-deficient patient—usually an alcoholic—glucose administration can precipitate Wernicke-Korsakoff encephalopathy by depleting critical thiamine stores ...