Hyponatremia overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Saeedeh Kowsarnia M.D.[2]
Overview
Historical Perspective
In 1858, Claude Bernard, French physiologist first proposed a direct relationship between the central nervous system and renal excretion of osmotically active solutes. In 1913, Jungmann and Meyer in Germany induced polyuria and increased urinary salt excretion in animals through medullary lesion. In 1950, Peters, Welt, and co-workers described few patients with encephalitis, hypertensive intracranial hemorrhage, and bulbar poliomyelitis who presented with severe dehydration and hyponatremia.
Classification
Hyponatremia (serum sodium less than 135 mEq/L) may be classified based upon serum ADH level, duration of hyponatremia, serum osmolality and volume status. The various classification systems enable accurate identification of the cause of hyponatremia and hence translate into optimal management based on the condition of the patient.
Pathophysiology
Hyponatremia is defined as serum sodium less than 135 mEq/L (mmol/L). Sodium is the major electrolyte which determines serum osmolality. Hyponatremia is a water balance disorder in which the ratio between sodium and water is disturbed. Water homeostasis is regulated mainly by two organs: hypothalamus by ADH secretion and thirst, kidney by water reabsorption or excretion. ADH is secreted due to alteration in serum osmolality or intravascular volume. Mechanisms in which different disorders cause hyponatremia involve ADH (secretion or action) and kidney function ( absorption or excretion). ADH secretion is increased by increased osmolality of serum or decreased effective intravascular volume.
Causes
Hyponatremia is caused by either increase ADH action/ secretion or kidney function impairment. SIAD is the most common cause of euvolemic hyponatremia. After SIAD, polydipsia, drugs and clinical disorders are the most encountered etiologies in clinical practice.
Differentiating Hyponatremia
Different disorders which cause hyponatremia are differentiated based on volume status, clinical presentation, serum and urine osmolality.
Epidemiology and Demographics
Hyponatremia is the most common electrolyte disorder. Its frequency is higher in females, elderly, and the patients who are hospitalized. The incidence of hyponatremia depends largely on the patient population which is a dependent of underlying cause. A hospital incidence of 15–30% is common. Age over 30, female gender and lower body weights are risk factors for developing complications associated with hyponatremia.
Risk Factors
Hyponatremia, the most common electrolyte abnormality, is more common in patients with chronic underlying diseases. Certain drugs, low body weight and previous history of hyponatremia are the most prominent risk factors for developing hyponatremia.
Screening
Hyponatremia is the most common electrolyte disturbances which is common with certain medical conditions and drugs. Screening the hyponatremia is necessary for preventing further decrease in serum sodium and complications of treatment.
Natural History, Complications, and Prognosis
Brain adaptive mechanisms to hyponatremia are developed over hours. Shifting of water to brain cells causes brain edema and increased intracranial pressure. Excretion of osmole from brain cells decrease osmotic gradient and brain edema. Impairment of adaptive mechanisms and acute onset of hyponatremia cause encephalopathy and brain herniation. Rapid treatment of hyponatremia will not allow adaptive mechanisms to develop and may cause in osmotic demyelination syndrome, also called central pontine demyelination.