Clinically, a coma can be defined as the inability to follow a command at a consistent level.  It can also be defined as a score of ≤ 8 on the Glasgow Coma Scale (GCS) ≥ 6 hours. To maintain a patient`s awareness, the components of alertness and consciousness must be preserved. Wakefulness describes the quantitative degree of consciousness, while consciousness refers to the qualitative aspects of functions transmitted by the cortex, including cognitive abilities such as attention, sensory perception, explicit memory, language, task execution, temporal and spatial orientation, and judgment of reality.   From a neurological point of view, consciousness is maintained by the activation of the cerebral cortex – the gray matter that forms the outer layer of the brain, and the reticular activation system (SAR), a structure that is found inside the brainstem.   The assessment of posture and body is the next step. It includes a general observation on the patient`s positioning. There are often two stereotypical attitudes seen in Comatian patients. Decorative posture is a stereotypical posture in which the patient has bent his arms at the elbow, and his arms towards the body, with both legs elongated. The posture of decerebrate is a stereotypical posture, in which the legs extend similarly (stretched), but the arms are also stretched (stretched at the elbow). The posture is critical because it indicates where the damage is in the central nervous system.
A decorative posture indicates a lesion (a damage point) on or above the red core, while a deciraction position indicates a lesion on or under the red core. In other words, a decorative lesion is closer to the cortex, unlike a deformation posture that indicates that the lesion is closer to the brainstem. The second most common cause of coma, which accounts for about 25% of cases, is lack of oxygen, which is usually caused by cardiac arrest.  The central nervous system (ZNS) needs a lot of oxygen for its neurons. Lack of oxygen in the brain, also known as hypoxia, reduces sodium and calcium from outside neurons and increases intracellular calcium, which interferes with neuronal communication.  Lack of oxygen in the brain also causes ATP depletion and cell degradation due to cytoskeleton damage and nitrogen oxide production. The coma can last from several days to several weeks. In more severe cases, a coma can last more than five weeks, while some have lasted as long as several years. After this period, some patients gradually emerge from coma, some enter a vegetative state and others die. Some patients who have entered a vegetative state become aware and, in some cases, remain in the vegetative state for years or even decades (the longest recorded period is 42 years).   A coma can be classified as (1) supratentorial (via Tentorium cerebelli), (2) subtentorial (under Tentorium cerebelli), (3) metabolic or (4) diffuse.  This classification depends only on the position of the initial damage that caused the coma and is not correlated with severity or prognosis.