The plasma sodium concentration is a good indication of the water content of body fluids. It is not usually a guide to sodium or salt loading.
Although it is possible to administer too much sodium and cause this, water depletion is a much more common cause. The main causes of water depletion are:
1. Reduced water intake (e.g. coma, dysphagia, extreme depression). Because hypernatraemia is an extremely strong stimulus to thirst, reduced water intake is almost always involuntary.
2. And increased losses of hypotonic fluid. Usually both are present, though either alone can be sufficient.
- increased loss via gut, skin or respiratory tract. Cholera syndromes (likely to be sodium depleted too); severe sweating, etc
- increased loss in urine caused by impaired ability to concentrate urine (diabetes insipidus, central, nephrogenic or drug-induced)
If circulating volume is reduced hypernatraemia is exacerbated by reabsorption of sodium due to aldosterone secretion.
Clinical features include thirst, oliguria and concentrated urine. More severe cases may result in confusion and weakness, and possibly tachycardia, and finally hypotension and coma. Plasma urea is usually increased. These findings, along with a urine osmolality of over 600 mosm/kg confirm water depletion.
Treatment is by oral replacement of water in mild cases; 5% dextrose (i.v.) in moderate cases; and a combination of 5% dextrose and 0.9% (150mmol/l) saline (i.v.) if dehydration is severe, as volume expansion requires salt as well as water.
This usually indicates an increase in the relative proportion of water to sodium in plasma, rather than a reduced sodium content. The main causes of hyponatraemia are:-
- Increased total body water alone (e.g. due to the syndrome of inappropriate antidiuretic hormone secretion, SIADH; though secretion of ADH is also triggered by understandable physiological stimuli)
- Increased body water and sodium; excess water exceeds excess sodium. Often seen in cardiac, liver or renal failure.
- Reduced body sodium: water depletion accompanied by excessive Na+ depletion – e.g. large electrolyte losses replaced largely by water drinking. Seen in Addison’s disease, for example, when kidneys fail to retain sodium.
Confusion, drowsiness and sometimes seizures are features of hyponatraemia. Other manifestations depend on cause; increased body fluid, or dehydration.
It is dangerous to correct [Na] too quickly. Central pontine myelinolysis can be a consequence, causing severe permanent brain damage. Hypertonic saline solutions should not be used, and the maximum rate of correction should be less than 10mmol/l per 24h. For dehydrated patients, administer 0.9% (isotonic) saline (150 mmol/l NaCl) to replenish volume.
Management of fluid overloaded patients involves restricting water and fluid intake. ADH receptor antagonists are becoming available as an alternative approach.
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