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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.[7] The presence and severity of signs and symptoms are associated with the level of plasma sodium (salt level in the blood), with the lowest levels of plasma sodium associated with more severe symptoms (the lower the sodium concentration the more severe the signs/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[8] (e.g., increased falls, altered posture and gait, reduced attention).[9]

Neurological (brain) symptoms typically occur with very low levels of plasma sodium (usually <115 mEq/L).[7] 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 the 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 (stopping breathing), and non-cardiogenic accumulation of fluid in the lungs.[10] 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.

Causes

Based on the above classification, some of the many specific causes of hyponatremia can be listed as follows:

Hypervolemic hyponatremia —Both sodium & water content increase: Increase in sodium content leads to hypervolemia and water content to hyponatremia. Total body water and sodium are regulated independently.[11]

Hyponatremia is corrected slowly in order to lessen the risk of the development of central pontine myelinolysis (CPM), a severe neurological disease involving a breakdown of the myelin sheaths covering parts of nerve cells. In fact, overly rapid correction of hyponatremia is the most common cause of that potentially devastating disorder.[14]