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Hyponatremia (British English: hyponatraemia) is an electrolyte disturbance in which the sodium concentration in the serum is lower than normal. In the vast majority of cases, hyponatremia occurs as a result of excess body water diluting the serum sodium and is not due to sodium deficiency. Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis isvital to the normal physiologic function of cells. Normal serum sodium levels are between 135–145 mEq/L. Hyponatremia is defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L.[1]
Hyponatremia is most often a complication of other medical illnesses in which excess water accumulates in the body at a higher rate than can be excreted (forexample in congestive heart failure, syndrome of inappropriate antidiuretic hormone, SIADH, or polydipsia). Lack of sodium is virtually never the cause of hyponatremia although it can promote hyponatremia indirectly. In particular, sodium loss can lead to a state of volume depletion, with volume depletion serving as signal for the release of ADH (anti-diuretic hormone). As a result ofADH-stimulated water retention, blood sodium becomes diluted and hyponatremia results.
Symptoms of hyponatremia include nausea and vomiting, headache, confusion, lethargy, fatigue, appetite loss, restlessness and irritability, muscle weakness, spasms, or cramps, seizures, and decreased consciousness or coma. The presence and severity of symptoms are associated with the level of serum sodium, with the lowestlevels of serum sodium associated with the more prominent and serious symptoms. However, emerging data suggest that mild hyponatremia (serum sodium levels at 131 mEq/L or above) is associated with numerous complications and undiagnosed symptoms.[2]
Many medical illnesses, such as congestive heart failure, liver failure, renal failure, or pneumonia may be associated with hyponatremia. Thesepatients frequently present because of primary disease symptomatology and are diagnosed after presenting due to manifestations of other medical issues.
Neurological symptoms often show for extremely low levels of sodium. When sodium levels in blood become too low, excess water enters cells and causes the cells to swell. Swelling in the brain is especially dangerous because the brain is confined bythe skull and is unable to expand. Neurological symptoms most often are due to very low serum sodium levels (usually <115 mEq/L), resulting in intracerebral osmotic fluid shifts and brain edema. This neurological symptom complex can lead to tentorial herniation with subsequent brain stem compression and respiratory arrest, resulting in death in the most severe cases. The severity of neurologicalsymptoms correlates with the rapidity and severity of the drop in serum sodium. 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, like a seizure disorder, or non-neurological metabolic abnormalities, also affects the severity of neurologic symptoms.
CausesBased on the above classification, some of the many specific causes of hyponatremia can be listed as follows:
Hypervolemic hyponatremia
congestive heart failure
nephrotic syndrome
massive edema of any cause
Euvolemic hyponatremia
states of severe pain or nausea
in the setting of trauma or other damage to the brain
SIADH (and its many causes)
Hypovolemic hyponatremia
any causeof hypovolemia such as prolonged vomiting, decreased oral intake, severe diarrhea
diuretic use (due to the diuretic causing a volume depleted state and thence ADH release, and not a direct result of diuretic-induced urine sodium loss)
Miscellaneous causes of hyponatremia that are not included under the above classification scheme include:
factitious hyponatremia (due to massive increases in...
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