[Neurology of consciousness and of consciousness disorders]

The two physiologic components of conscious behaviour, namely arousal (vigilance) and content of consciousness (presence of mind), may be affected differently, depending on the type and distribution of the underlying brain disease. Disturbance of arousal primarily affects wakefulness and awareness and leads to obtundation, stupor and coma. States of reduced arousal are caused by bilateral lesions of the so-called ascending reticular activating system (ARAS), which is situated in the upper brainstem and the paramedian diencephalon. If, on the other hand, cognitive and mnemic function are degraded, the contents of consciousness are disordered; depending on the extent of the disturbance, confusion, lethargy and, finally, a vegetative state ensues. The chronic vegetative state (coma vigil) describes a condition of total mental loss with preserved vegetative functions and arousal. Degraded cognitive and mnenic functions results from either toxic-metabolic or extensive structural disorders of the cerebral cortex, where the limbic and mesial frontal areas play a dominant role for conscious behaviour. If the function of extensive areas of both hemispheres is suddenly depressed, temporarily reduced arousal also results. In order to differentiate between toxic-metabolic and structural brain diseases, the motor reactions, the ocular and the pupillary reflexes must be examined apart from responsiveness.