Brain function in physiologically, pharmacologically, and pathologically altered states of consciousness
Boveroux P, Bonhomme V, Boly M, Vanhaudenhuyse A, Maquet P, Laureys S.; Int Anesthesiol Clin. 2008 Summer;46(3):131-46.
Altered Conscious States
Three main types of altered states of consciousness can be defined: physiologically, pharmacologically, and pathologically altered conscious states. Normal sleep, encompassing light (stage II) and deep (stage III and IV) slow wave sleep stages, and dreaming [mostly occurring during rapid eye movement (REM) sleep] can be considered as the main physiologically altered conscious states. Pharmacologically induced unconscious states are commonly named deep sedation or general anesthesia. Sedative anesthetic agents can be separated into 3 main categories: the classic anesthetic agents, the dissociative anesthetic agents, and the minimally sedative agents. Examples of classic anesthetic agents are benzodiazepines, barbiturates, propofol, and halogenated compounds such as desflurane or sevoflurane. They are able to induce graded states of sedation and combine alterations of wakefulness and awareness. Dissociative anesthetic agents, such as ketamine or nitrous oxide, are able to blunt out conscious processes while maintaining signs of wakefulness. Finally, minimally sedative agents induce graded alterations of wakefulness while preserving several cognitive brain functions. Their main representatives are the a2-adrenergic agonists, such as clonidine or dexmedetomidine.12 The last category of altered states of consciousness refers to pathologic altered conscious states, which are the result of severe brain insults of various origins, and encompass comatose, vegetative, and minimally conscious states.13 Comatose patients have no signs of vigilance or awareness, whereas patients in the vegetative state look awake, meaning that their eyes are open, but are not aware of their environment, only exhibiting reflex behavior. Minimally conscious patients show (minimal) signs of voluntary behavior, but are not able to show interactive communication.14 As illustrated above, the different types of altered states of consciousness associate varying combinations in the alteration of vigilance or awareness. Studying global, regional, and cerebral connectivity changes between conscious and unconscious states in the presence or absence of external stimulation is, therefore, of particular interest for the quest for the neural correlates of consciousness. This will be detailed hereafter.
Consciousness and Global Decreases in Cerebral Metabolism
Is awareness lost when overall cortical activity falls bellow a certain threshold? Using fluorine-18-labeled deoxyglucose positron emission Brain Function in Altered States of Consciousness ’ 133 tomography (PET), it was shown that in slow wave sleep,15,16 overall brain metabolism decreases to approximately 60% of normal waking values, whereas in REM sleep, metabolism returns to nearly normal waking values.17 When different anesthetics, such as propofol,18 halothane,19 or isoflurane,20 are titrated to the point of unresponsiveness, global cerebral metabolism decreases to about 40% to 50% of normal values. In propofol anesthesia, global decreases in brain metabolism have been reported down to 28% of normal waking values.18 In pathologic coma (of either traumatic or hypoxic origin), gray matter metabolism is, on average, 50%to 70%of normal values.21–24 Patients who ‘‘awaken’’ from coma to a vegetative state, classically show a massive cerebral metabolic decrease to about 40% of normal values (Fig. 1).22,25–28
Web Resource: www.coma.ulg.ac.be