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Hyponatremia (American English) or hyponatraemia (British English) is low sodium concentration in the blood. Normal serum sodium levels are between approximately 135 and 145 mEq/liter (135 - 145 mmol/L). Hyponatremia is generally defined as a serum sodium level of less than 135 mEq/L and is considered severe when the serum sodium level is below 125 mEq/L.〔(【引用サイトリンク】title=Hyponatremia )〕〔(【引用サイトリンク】title=Hyponatremia )〕 Too little sodium in the diet alone is very rarely the cause of hyponatremia, although it can promote hyponatremia indirectly and has been associated with MDMA-induced hyponatremia. Sodium loss can lead to a state of low blood volume, which signals the release of anti-diuretic hormone (ADH). ADH release leads to water retention and dilution of the blood resulting in a low sodium concentration. Many conditions including congestive heart failure, liver failure, kidney failure and pneumonia are commonly associated with a low sodium concentration in the blood. This state can also be caused by overhydration from drinking too much water because of excess thirst (polydipsia). Exercise-associated hyponatremia (EAH) is common in marathon runners and participants of other endurance events. 13% of the athletes who finished the 2002 Boston Marathon were in a hyponatremic state, that is, the salt levels in their blood had fallen below usual levels. Sodium is the primary positively charged ion in the environment outside of the cell and cannot freely cross from the interstitial space into the cell. Charged sodium ions attract up to 25 water molecules around them thereby creating a large polar structure that is too large to pass through the cell membrane. == Signs and symptoms == Signs and symptoms of hyponatremia include nausea and vomiting, headache, short-term memory loss, confusion, lethargy, fatigue, loss of appetite, irritability, muscle weakness, spasms or cramps, seizures, and decreased consciousness or coma. The presence and severity of signs and symptoms are related to the level of salt in the blood, with lower levels of plasma sodium associated with more severe symptoms. However, emerging data suggest that mild hyponatremia (plasma sodium levels at 131 mEq/L or above) is associated with numerous complications or subtle, presently unrecognized symptoms〔 (e.g., increased falls, altered posture and gait, reduced attention). Neurological symptoms typically occur with very low levels of plasma sodium (usually <115 mEq/L).〔 When sodium levels in the blood become very low, water enters the brain cells and causes them to swell. This results in increased pressure in the skull and causes ''hyponatremic encephalopathy''. As pressure increases in the skull, herniation of the brain can occur, which is a squeezing of the brain across the internal structures of the skull. This can lead to headache, nausea, vomiting, confusion, seizures, brain stem compression and respiratory arrest, and non-cardiogenic accumulation of fluid in the lungs. This is usually fatal if not immediately treated. Symptom severity depends on how fast and how severe the drop in blood salt level. A gradual drop, even to very low levels, may be tolerated well if it occurs over several days or weeks, because of neuronal adaptation. The presence of underlying neurological disease such as a seizure disorder or non-neurological metabolic abnormalities, also affects the severity of neurologic symptoms. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hyponatremia」の詳細全文を読む スポンサード リンク
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