CASE

A 76 yo woman is sent in from the nursing home with myoclonic jerks. She has a past medical history of slight dementia and seizures. Her GCS is 13-14/15. On arrival she has constant myoclonic jerks. Initial Blood tests reveal the following:

Na 165
K 3.6
Cl 112
U 12
Cr 112

You make the diagnosis of hypernatraemia. What causes it? How do we correct it?

Hypernatraemia is defined as Na>145mmol/L.
Na>180 mmol/L are associated with high mortality.

Causes of Hypernatraemia

Lets think of this simply; its either too much water out or not enough water in. In more detail:

  1. Decrease in water intake: This can be for a variety of reasons, from not having water to not being able to communicate thirst, to disordered thirst perception.
  2. Increased water loss, more than salt loss
    1. Losses from GIT or Skin
    2. Renal Disease affecting concentrating ability
    3. Diabetes insipidus
  3. Inadequate salt intake
    1. Mineralcorticoid: Primary Hyperaldosteronism
    2. Glucocorticoid: Cushings

How The Patient Presents

The hypernatraemic patient presents with a range of symptoms that include:

  • Lethargy
  • Tremor/Twitching
  • Ataxia
  • Seizures
  • Coma

How to correct the Hypernatraemia.

The answer is SLOWLY over 24 hours, with a maximum drop of 12mmol/L over the 24 hours. Rapid correction can result in cerebral oedema and seizures.

  1. Calculate total body water: Weight x (0.5 for males) or (0.45 for females)
  2. Calculate how much fluid will need to lower the Na
    1. (Na content in fluid -Patient Na)/(TBW + 1)
    2. Divide this into the number of mmol/L  we need to reduce in 24 hours.

For example:

The Na was 165 and we wish to use 5% Dextrose which has zero sodium in it, on our 60kg woman

TBW = 0.45 x x60 =27L

(0-165)/(27+1)= 5.9L

Let’s reduce the Na by 10 mmol/L over 24 hours
Amount of 5% needed is 10/5.9L= 1.7 L- add to this any other expected losses.

Sodium Content of common IV fluids

Fluid Type0.9% NaCl0.45% NaCl/ 4% Glucose0.18% NaCl/4% Gluc5% GlucoseHartmann'sRinger's Lactate
Na Content mmol/L15477310131130