Published on 02/03/2015 by admin
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Last modified 02/03/2015
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Clinicians assessing a patient with hyponatraemia should ask themselves several questions.
Am I dealing with dangerous (life-threatening) hyponatraemia?
Am I dealing with water retention or sodium loss?
How should I treat this patient?
To answer these questions, they must use the patient’s history, the findings from clinical examination, and the results of laboratory investigations. Each of these may provide valuable clues.
In assessing the risk of serious morbidity or mortality in the patient with hyponatraemia, several pieces of information should be used:
the presence of signs or symptoms attributable to hyponatraemia
evidence of sodium depletion
the serum sodium concentration
how quickly the sodium concentration has fallen from normal to its current level.
The serum sodium concentration itself gives some indication of dangerous or life-threatening hyponatraemia. Many experienced clinicians use a concentration of 120 mmol/L as a threshold in trying to assess risk (the risk declines at concentrations significantly greater than 120 mmol/L, and rises steeply at concentrations less than 120 mmol/L). However, this arbitrary cut-off should be applied with caution, particularly if it is not known how quickly the sodium concentration has fallen from normal to its current level. A patient whose serum sodium falls from 145 to 125 mmol/L in 24 hours may be at great risk.
Often, the clinician must rely exclusively on history and, especially, clinical examination to assess the risk to the patient. Symptoms due to hyponatraemia reflect neurological dysfunction resulting from cerebral overhydration induced by hypo-osmolality. They are non-specific and include nausea, malaise, headache, lethargy and a reduced level of consciousness. Seizures, coma and focal neurological signs are not usually seen until the sodium concentration is less than about 115 mmol/L.
If there is clinical evidence of sodium depletion (see below), there is a high risk of mortality if treatment is not instituted quickly.
History
Fluid loss, e.g. from gut or kidney, should always be sought as a possible pointer towards primary sodium loss. Even if there is no readily identifiable source of loss, the patient should be asked about symptoms that may reflect sodium depletion, such as dizziness, weakness and light-headedness.
If there is no history of fluid loss, water retention is likely. Many patients will not give a history of water retention as such; history taking should instead be aimed at identifying possible causes of the SIAD. For example, rigors may point towards infection, or weight loss towards malignancy.
The clinical signs characteristic of ECF and blood volume depletion are shown in Figure 9.1
Clinical Biochemistry An Illustrated Colour Text-5E
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