A state of deep and persistent unconsciousness; it is explicitly not a form of deep SLEEP but something pathological. Sleep, especially REM SLEEP, involves significant and widespread neural activity; coma does not. Coma is also discriminated from STUPOR, a state of extreme unconsciousness, but one from which it is possible to rouse individuals by sensory stimulation: patients in coma are not responsive to sensory stimulation. Even REFLEX activation is lost in coma, which is a significant discriminator between this and PERSISTENT VEGETATIVE STATE.
Coma is caused by damage or dysfunction in the CENTRAL NERVOUS SYSTEM. These can be divided into three broad groups: the first two are defined by their relationship to the TENTORIUM, a landmark close by the CEREBELLUM: (1) subtentorial lesions—that is, any LESION in the BRAINSTEM, typically in the PONS, (2) supratentorial lesions -typically damage to the vascular system in and around the MENINGES, (3) disorders of METABOLISM, such as HYPOGLYCAEMIA, which have widespread effects on brain. Coma is rated using any of a variety of neuropsychological instruments: the best-known is the GLASGOW COMA SCALE. For a discussion of the medical and legal issues raised by states such as coma, see PERSISTENT VEGETATIVE STATE.
