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MCQs - Hyponatremia

Updated on September 16, 2015

Hyponatremia is the low level of serum sodium


What is Hyponatremia?

Hyponatremia is defined as low level of sodium in the blood serum. The normal range of serum soium value is 135 – 145 meq/l. Why are the neurology patients at risk of developing hyponatremia? Most of the neurology patients will have an altered level of consciousness and sensorium which may impair their normal oral intake of food and fluids. It may also be because of the conditions such as syndrome of inappropriate antidiuretic hormone and cerebral salt wasting syndrome.

Hyponatremia Explained Clearly

Classification of Hyponatremia in a neurology patient

Mild hyponatremia is classified as a serum sodium level between 125 – 135 meq/l and severe hyponatremia is serum sodium less than 125 meq/l.

What are the consequences of Hyponatremia?

  • Before discussing the consequences of hyponatremia, it is important to understand what osmotic pressure is.

  • Osmotic pressure is the pressure exerted by the solutes in any solution.

  • It is rather the pulling force for water from a compartment of low solute concentration to a compartment of high solute concentration.

  • In hyponatremia, as the level of serum sodium decreases, there will be a loss in the osmotic pressure of the intravenous fluid.

  • This lack of osmotic pressure in the intravascular compartment will result in the movement of fluid out of the intravascular compartment to the interstitial and intracellular spaces.

  • When this happens in the brain, it causes osmotic brain edema.

Causes of Hyponatremia

Mild hyponatraemia
Patients with Intracranial disorders
Profuse Sweating
Syndrome of inappropriate ADH secretion (SIADH)
Hypotonic fluids
Cerebral salt wasting syndrome
Diarrhoea and Vomitting
Use of diuretics

What are the bad effects of Hyponatremia?

Sodium is essential in the propagation of nerve impulses. Hyponatremia can cause lethargy, drowsiness, confusion, weakness and tremors. When the serum sodium level dangerously falls below the level of 120 meq/l, seizures can occur or even the person may go into coma.

Syndrome of inappropriate ADH secretion (SIADH)

In SIADH, there is persistently high level of antidiuretic hormone (ADH). Normally ADH is secreted when the osmolality of the plasma increases. This will prevent excessive urine output and help in conserving the water to increase the blood volume. In SIADH, even in the presence of fluid overload and hypo osmolality, ADH is secreted. This will result in the conservation of water and lead to a state called dilutional Hyponatremia.

Cerebral salt wasting syndrome

This is another reason for hyponatremia in neuro patients. It differs from SIADH in that in cerebral salt wasting syndrome, there is renal excretion of sodium, high urine sodium, hypovolumia and polyuria.


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