The differential diagnosis of hypernatraemia in children, with ...

The Diagnosis of Salt Poisoning Leading to Hypematraemia in Children – September 2009
In fact the amount needed will inevitably be more than this, firstly because some of
the administered salt will already have been excreted by the time the initial abnormal
measurement is made, and secondly because (if the salt was administered orally) it is
almost certain that there will still be unabsorbed salt in the GI tract at the time of the initial
measurement. The 1 and 3 teaspoon values actually assume (a) instant total absorption, and
(b) no excretion of any of the administered salt at the time of measurement.
A method of calculating the excess salt load required to raise the serum sodium a given
amount is as follows:
1. Concentration (mmol/l) = Amount (mmol)
Volume (litres)
2. The presumed pre-illness total body amount of Na + in the child’s body is given by:
140 = Amount of Na+ (pre-illness)
Volume (0.65 x wt in kilogram)
. . . total body Na + (pre-illness) = 140 x (0.65 x wt)
3. The total amount of Na + in the child’s body at presentation is similarly given by:
total body Na + (at presentation) = Pres [Na + ] x (0.65 x wt)
4. The excess total body Na + in mmol is given by
Amount (at presentation) – Amount (pre-illness)
This is divided by 17.1 to give the excess Na + in grams (17.1 mmol = 1 gram)
Example
A child presents with a serum [Na + ] of 185mmol/l and a recent wt of 15 kilogram. The
child now weighs 14.6 kilogram
1. Previous total body Na + = 140 x (0.65 x 15) = 1,365 mmol
2. Presenting total body Na + = 185 x (0.65 x 14.6) = 1,756 mmol
3. Excess Na + = 390mmol = 22.8 grams
Evidence statement
When calculating the amount of sodium required to raise the serum sodium by a
specified amount, the total body water (approximately 65% of the child’s weight)
should be used to determine the volume of distribution. [Grade D]
5.3 How well known among the public is the lethal dose of salt?
There is no published information on this question. Accordingly, the RCPCH included a question
about this in a survey of parents undertaken on behalf of the College by BMRB Omnibus.
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