Episode 14: A Hypernatremic Child
Author: Dr. Suneet Sood
Editor: Dr. Suneet Sood
Narrators: Thong Yi Kun, Alan Koay
I’ve just remembered a fascinating clinical problem we encountered a few weeks ago.
What was that?
This three week baby was admitted to the paediatrics ward with a history of not feeding for two days. The mother said the baby was sleeping excessively, and couldn’t be woken up for feeds. The baby passed urine once in the last 24hrs, and didn’t pass stool for 3days. This is her first baby, and she was careful to follow the advice everybody gives: breast feed only! So the baby was exclusively breast fed. At birth the baby had weighed 3.5 kg, and had been fine except for a physiological jaundice. At admission we found that the baby’s weight had dropped to 2.5 kg.
Isn’t a little loss of weight usual in the first few days of life?
It is. Babies lose up to 10% of their body weight in the first 5 days of life due to physiological diuresis. They regain the birth weight by day 14. This child has lost about a third of its weight. Any loss of weight of more than 10% is pathological and should be investigated.
On examination except mild jaundice and absence of fat in the buttocks area no abnormality was found. The baby’s blood tests were normal except for two changes. The bilirubin was 135 micromoles/liter, consistent with physiological jaundice that’s elevated by dehydration. The second change was in the electrolytes. The baby’s serum sodium level was 165 millimoles/L.
That’s a significant hypernatremia!
That’s right; for a baby the normal values are 135-140
So what was the cause?
Simply that the mother was producing insufficient breast milk! She refused to provide supplementation, so the baby started getting dehydrated.
That doesn’t make sense. How can a baby get dehydrated so quickly? And I know that milk is a low-sodium fluid, so insufficient feeding would simply worsen the hyponatremia, not cause hypernatremia.
Good questions. Let’s break these into two parts. One, why does dehydration occur? And two, how does hypernatremia occur? The answers are simple. One, babies lose a lot of water by insensible loss. In addition to the water lost during respiration, a lot of water is lost through the skin, so they get dehydrated quickly. Two, insensible water loss is pure water loss. That’s why hypernatremia occurs.
Two more questions. Why do babies lose so much water? Isn’t there sodium loss during insensible fluid loss?
In response to question one, newborns lose a lot of water through insensible losses because they have a high surface area in proportion to their weight. Adults have a skin surface area of 250 square cm/kg. In comparison, infants have a skin surface are of seven HUNDRED square cm/kg. Compared to adults, that’s an enormously increased surface area per kg.
In response to question two, no, the sodium loss during insensible losses is low. In such a setting the best replacement for a neonate is milk. For an older child or an adult dehydrated due to excessive insensible losses the best replacement is water or intravenous dextrose.
What happened to the baby?
We just fed the baby with formula milk via a nasogastric tube. The baby recovered quickly. I’ve brought up this case because there are several important messages here. One, though breast is best, it sometimes just isn’t enough. Two, babies have a very high surface area in relation to their weight, and, consequently, can dehydrate surprisingly fast. Three, insensible losses should be considered to be water losses, not water and electrolyte losses.
Myers RS. Pediatric fluid and electrolyte therapy. J Pediatr Pharmacol Ther 2009 Oct-Dec; 14(4): 204–211