Earlier today we posted an article about Deep Brain Stimulation inspired by a 38-year old patient that regained consciousness. Below is an excerpt from Plum and Posner’s Diagnosis of Stupor and Coma 4th edition, to help you further understand how miraculous Deep Brain Stimulation is.
Consciousness is the state of full awareness of the self and one’s relationship to the environment. Clinically, the level of consciousness of a patient is defined operationally at the bedside by the responses of the patient to the examiner. It is clear from this definition that it is possible for a patient to be conscious yet not responsive to the examiner, for example, if the patient lacks sensory inputs, is paralyzed, or for psychologic reason decides not to respond. Thus, the determination of the state of consciousness can be a technically challenging exercise. In the definitions that follow, we assume that the patient is not unresponsive due to sensory or motor impairment or psychiatric disease.
Consciousness has two major components: content and arousal. The content of consciousness represents the sum of all functions mediated at a cerebral cortical level, including both cognitive and affective responses. These functions are subserved by unique networks of cortical neurons, and it is possible for a lesion that is strategically placed to disrupt one of the networks, causing a fractional loss of consciousness. Such patients may have preserved awareness of most stimuli, but having suffered the loss of a critical population of neurons (e.g., for recognizing language symbol content, differences between colors or faces, or the presence of the left side of space), the patient literally becomes unconscious of that class of stimuli. Patients with these deficits are often characterized as “confused” by inexperienced examiners, because they do not respond as expected to behavioral stimuli. More experienced clinicians recognize the focal cognitive deficits and that the alteration of consciousness is confined to one class of stimuli. Occasionally, patients with right parietotemporal lesions may be sufficiently inattentive as to appear to be globally confused, but they are not sleepy and are, in fact usually agitated.
Thus, unless the damage to cortical networks is diffuse or very widespread, the level of consciousness is not reduced. For example, patients with advanced Alzheimer’s disease may lose memory and other cognitive functions, but remain awake and alert until the damage is so extensive and severe that response to stimuli is reduced as well. Hence, a reduced level of consciousness is not due to focal impairments of cognitive function, but rather to a global reduction in the level of behavioral responsiveness. In addition to being caused by widespread cortical impairment, a reduced level of consciousness can result from injury to a specific set of brainstem and diencephalic pathways that regulate the overall level of cortical function, and hence consciousness. The normal activity of this arousal system is linked behaviorally to the appearance of wakefulness. It should be apparent that cognition is not possible without a reasonable degree of arousal.
Sleep is a recurrent, physiologic, but not pathologic, form of reduced consciousness in which the responsiveness of brain systems responsible for cognitive function is globally reduced, so that the brain does not respond readily to environmental stimuli. Pathologic alteration of the relationships between the brain systems that are responsible for wakefulness and sleep can impair consciousness. The systems subserving normal sleep and wakefulness are reviewed later in this chapter. A key difference between sleep and coma is that sleep is intrinsically reversible: sufficient stimulation will return the individual to a normal waking state. In contrast, if patients with pathologic alterations of consciousness can be awakened at all, they rapidly fall back into a sleep-like state when stimulation ceases.
Patients who have a sleep-like appearance and remain behaviorally unresponsive to all external stimuli are unconscious by and definition. However, continuous sleep-like coma as a result of brain injury rarely lasts more than 2 to 4 weeks.