Hyponatremia overview

# 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 on the underlying cause. A hospital incidence of 1530% 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 are 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 decreases 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.

# Diagnosis

## Diagnostic study of choice
Best diagnostic test to measure hyponatremia, serum sodium < 135 mEq/L, is direction-specific electrode potentiometry. Other tests are associated with false results in certain conditions. Different etiologies of hyponatremia are differentiated based on serum osmolality, urine osmolality, and urine sodium.

## History and Symptoms
Symptoms associated with hyponatremia are caused mostly by impairment of brain function. There is a spectrum of signs from no detectable presentation to death. To evaluate the causes of hyponatremia, careful history has to be taken. Drug history and past medical history can lead to the most common causes of hyponatremia.

## Physical Examination
Hyponatremia by itself has the signs of CNS function impairment and the other signs which can be detected in the physical exam are caused by the etiologies of hyponatremia. Depending on the severity of hyponatremia, signs vary from subtle cognitive impairment to brain death. Patients who present with hyponatremia, depending on the underlying causes, may present with different signs in clinical evaluation.

## Laboratory Findings
In hyponatremia, depending on the causes, different laboratory abnormalities can be found. Check for

## Electrocardiogram

## X-ray

## Ultrasound

## CT scan

## MRI

## Other Imaging Findings

## Other Diagnostic Studies

# Treatment

## Medical Therapy
Hyponatremia, serum sodium < 135 mEq/L, is the most common electrolyte disturbances in the clinical encounter. Treatment of hyponatremia based on the etiologies is the best approach because in most cases, hyponatremia resolves with the treatment of underlying causes. The rate of correction for hyponatremia is very important to prevent the syndrome of osmotic demyelination. Hyponatremia must be corrected slowly in order to lessen the chance of the development of Osmotic demyelination syndrome or central pontine myelinolysis (CPM), a severe neurological disease. In fact, overly rapid correction of hyponatremia is the most common cause of that potentially devastating disorder. During treatment of hyponatremia, the serum sodium should not be allowed to rise by more than 8  mmol/l over 24 hours (i.e. 0.33  mmol/l/h rate of rising). In practice, rapid correction of hyponatremia and then CPM is most likely to occur during the treatment of hypovolemic hyponatremia. In particular, once the hypovolemic state has been corrected, the signal for ADH release disappears. At that point, there will be an abrupt water diuresis (since there is no longer any ADH acting to retain the water). A rapid and profound rise in serum sodium can then occur. Should the rate of rising of serum sodium exceed 0.33  mmol/l/h over several hours, vasopressin may be administered to prevent ongoing rapid water diuresis.

## Surgery

## Primary Prevention
In patients at risk of developing hyponatremia, preventing approaches has to be done to eliminate the aggravation of hyponatremia.

## Secondary Prevention
The rate of correction for hyponatremia is very crucial for preventing the complication of treatment like osmotic demyelination syndrome.