Hyponatremia in the Critically Ill Child by A. Moscatelli, A. Olszewski | OPENPediatrics

Learn about the pathophysiology and types of hyponatremia, and how to properly diagnose and correct hyponatremia through clinical case studies.

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My name is Andrea Moscatelli. I'm the director of the Neonatal and Pediatric Intensive Care Unit at the Gaslini Children's Hospital in Genova, Italy. In the next few minutes, we'll talk about the treatment and prevention of hyponatremia in the critically ill child. We will start with the definition and pathophysiology of hyponatremia, then we will go through some case presentation, and we'll focus on the treatment and prevention of hyponatremia.

Pathophysiology of Hyponatremia.

You have hyponatremia when there is a relative excess of free water with an underlying condition that impairs the kidney's ability to excrete free water. This might happen in the case of excessive intake of water, if there is a marked reduction in the glomerular filtration rate, or if there is a renal hypoperfusion, either if there is an ADH excess.

This condition is affecting 25% of the critically ill children. It is considered moderate if sodium is below 130, and this condition is affecting more than 1% of hospitalized children.

Serum sodium is strictly regulated by some interplay mechanism. The first one is the incretion of vasopressin. Vasopressin is increted by the posterior hypophysis in response to stimuli, like a reduction in the circulating volume or an increase in plasma osmolarity, while the renin-angiotension-aldosterone system is able to enhance the reabsorption of sodium and water in case of hypovolemia.

The sympathetic nervous system has similar effects to the renin-angiotension-aldosterone system.

Atrial natriuretic peptide has opposite effects. So if you think about the cell, the main intracellular ion is potassium, while the main extracellular ion is sodium, and urea is able to move freely across the cell membrane.

So to measure the plasma osmolarity, you can go through this formula, which takes into account sodium, glucose, and the BUN. But if you think about the driving force for the movement of water across the cell membrane, you should think about tonicity. So you wouldn't take into account urea, which is able to move freely across the cell membrane. And you just take into account sodium and glucose.

By using the above formulas for osmolarity and tonicity, or by measuring a patient's osmolarity as a lab value, you can assess whether they are hypo-osmolar or hypotonic, serum osmolarity of less than 280, isotonic serum osmolarity of 280-295, or hypertonic serum osmolarity of greater than 295. A patient's osmolarity and tonicity are important in assessing the causes of hyponatremia.