Coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions.[1] The person may experience respiratory and circulatory problems due to the body's inability to maintain normal bodily functions. People in a coma often require extensive medical care to maintain their health and prevent complications such as pneumonia or blood clots.[2] Coma patients exhibit a complete absence of wakefulness and are unable to consciously feel, speak or move.[3][4] Comas can be derived by natural causes, or can be medically induced.[5]
For other uses, see Coma (disambiguation). Not to be confused with Comma or Colma. "Comas" redirects here. For other uses, see Comas (disambiguation).Coma
Unconsciousness
Can vary from a few days to several years (longest recorded is 42 years)
Clinically, a coma can be defined as the consistent inability to follow a one-step command.[6][7] It can also be defined as a score of ≤ 8 on the Glasgow Coma Scale (GCS) lasting ≥ 6 hours.[8] For a patient to maintain consciousness, the components of wakefulness and awareness must be maintained. Wakefulness describes the quantitative degree of consciousness, whereas awareness relates to the qualitative aspects of the functions mediated by the cortex, including cognitive abilities such as attention, sensory perception, explicit memory, language, the execution of tasks, temporal and spatial orientation and reality judgment.[3][9] From a neurological perspective, consciousness is maintained by the activation of the cerebral cortex—the gray matter that forms the outer layer of the brain—and by the reticular activating system (RAS), a structure located within the brainstem.[10][11]
Etymology[edit]
The term 'coma', from the Greek κῶμα koma, meaning deep sleep, had already been used in the Hippocratic corpus (Epidemica) and later by Galen (second century AD). Subsequently, it was hardly used in the known literature up to the middle of the 17th century. The term is found again in Thomas Willis' (1621–1675) influential De anima brutorum (1672), where lethargy (pathological sleep), 'coma' (heavy sleeping), carus (deprivation of the senses) and apoplexy (into which carus could turn and which he localized in the white matter) are mentioned. The term carus is also derived from Greek, where it can be found in the roots of several words meaning soporific or sleepy. It can still be found in the root of the term 'carotid'. Thomas Sydenham (1624–89) mentioned the term 'coma' in several cases of fever (Sydenham, 1685).[12][13]
General symptoms of a person in a comatose state are:
Causes[edit]
Many types of problems can cause a coma. Forty percent of comatose states result from drug poisoning.[15] Certain drug use under certain conditions can damage or weaken the synaptic functioning in the ascending reticular activating system (ARAS) and keep the system from properly functioning to arouse the brain.[16] Secondary effects of drugs, which include abnormal heart rate and blood pressure, as well as abnormal breathing and sweating, may also indirectly harm the functioning of the ARAS and lead to a coma. Given that drug poisoning is the cause for a large portion of patients in a coma, hospitals first test all comatose patients by observing pupil size and eye movement, through the vestibular-ocular reflex. (See Diagnosis below.)[16]
The second most common cause of coma, which makes up about 25% of cases, is lack of oxygen, generally resulting from cardiac arrest.[15] The Central Nervous System (CNS) requires a great deal of oxygen for its neurons. Oxygen deprivation in the brain, also known as hypoxia, causes sodium and calcium from outside of the neurons to decrease and intracellular calcium to increase, which harms neuron communication.[17] Lack of oxygen in the brain also causes ATP exhaustion and cellular breakdown from cytoskeleton damage and nitric oxide production.
Twenty percent of comatose states result from an ischemic stroke, brain hemorrhage, or brain tumor.[15] During a stroke, blood flow to part of the brain is restricted or blocked. An ischemic stroke, brain hemorrhage, or brain tumor may cause restriction of blood flow. Lack of blood to cells in the brain prevents oxygen from getting to the neurons, and consequently causes cells to become disrupted and die. As brain cells die, brain tissue continues to deteriorate, which may affect the functioning of the ARAS, causing unconsciousness and coma.
Comatose cases can also result from traumatic brain injury, excessive blood loss, malnutrition, hypothermia, hyperthermia, hyperammonemia,[18] abnormal glucose levels, and many other biological disorders. Furthermore, studies show that 1 out of 8 patients with traumatic brain injury experience a comatose state.[19]
Heart-related causes of coma include cardiac arrest, ventricular fibrillation, ventricular tachycardia, atrial fibrillation, myocardial infarction, heart failure, arrhythmia when severe, cardiogenic shock, myocarditis, and pericarditis. Respiratory arrest is the only lung condition to cause coma, but many different lung conditions can cause decreased level of consciousness, but don't reach coma.
Other causes of coma include severe or persistent seizures, kidney failure, liver failure, hyperglycemia, hypoglycemia, and infections involving the brain, like meningitis and encephalitis.
Pathophysiology[edit]
Injury to either or both of the cerebral cortex or the reticular activating system (RAS) is sufficient to cause a person to enter coma.[20]
The cerebral cortex is the outer layer of neural tissue of the cerebrum of the brain.[21] The cerebral cortex is composed of gray matter which consists of the nuclei of neurons, whereas the inner portion of the cerebrum is composed of white matter and is composed of the axons of neuron.[22] White matter is responsible for perception, relay of the sensory input via the thalamic pathway, and many other neurological functions, including complex thinking.
The RAS, on the other hand, is a more primitive structure in the brainstem which includes the reticular formation (RF).[23] The RAS has two tracts, the ascending and descending tract. The ascending tract, or ascending reticular activating system (ARAS), is made up of a system of acetylcholine-producing neurons, and works to arouse and wake up the brain.[24] Arousal of the brain begins from the RF, through the thalamus, and then finally to the cerebral cortex.[16] Any impairment in ARAS functioning, a neuronal dysfunction, along the arousal pathway stated directly above, prevents the body from being aware of its surroundings.[23] Without the arousal and consciousness centers, the body cannot awaken, remaining in a comatose state.[25]
The severity and mode of onset of coma depends on the underlying cause. There are two main subdivisions of a coma: structural and diffuse neuronal.[26] A structural cause, for example, is brought upon by a mechanical force that brings about cellular damage, such as physical pressure or a blockage in neural transmission.[27] While a diffuse cause is limited to aberrations of cellular function, that fall under a metabolic or toxic subgroup. Toxin-induced comas are caused by extrinsic substances, whereas metabolic-induced comas are caused by intrinsic processes, such as body thermoregulation or ionic imbalances (e.g. sodium).[25] For instance, severe hypoglycemia (low blood sugar) or hypercapnia (increased carbon dioxide levels in the blood) are examples of a metabolic diffuse neuronal dysfunction. Hypoglycemia or hypercapnia initially cause mild agitation and confusion, but progress to obtundation, stupor, and finally, complete unconsciousness.[28] In contrast, coma resulting from a severe traumatic brain injury or subarachnoid hemorrhage can be instantaneous. The mode of onset may therefore be indicative of the underlying cause.[1]
Structural and diffuse causes of coma are not isolated from one another, as one can lead to the other in some situations. For instance, coma induced by a diffuse metabolic process, such as hypoglycemia, can result in a structural coma if it is not resolved. Another example is if cerebral edema, a diffuse dysfunction, leads to ischemia of the brainstem, a structural issue, due to the blockage of the circulation in the brain.[25]