Approach to the Patient with Fluid & Electrolyte Imbalance – Part One

Urine osmolality is a measure of the concentration of urine. Diabetes insipidus produces large volumes of urine that have low concentrations of solutes - they have low urine osmolality.

How do changes help with diagnosis?

When these patients are deprived of water, the urine does not concentrate (as you would expect from a healthy individual). The cause can either be central (when the body isn't producing/releasing ADH - anti-diuretic hormone) or in the kidneys themselves (nephrogenic, where the kidneys do not respond to ADH).

If the patient is then given desmopressin (a synthetic version of ADH), the urine will start to concentrate (osmolality will increase). This means the cause of the DI is central/cranial as the kidneys can still respond to ADH.

Vomiting and diarrhoea can lead to dehydration; the kidneys will concentrate the urine to compensate for the loss of fluid and therefore the urine osmolality will increase. As explained above, diabetes insipidus will make the urine less concentrated, and excess 5% dextrose can do the same: once the dextrose (glucose) is absorbed into the cells, the fluid will remain in the patient and need to be excreted in the urine.

If hyponatraemia is corrected too rapidly, there can be fatal demyelination of nerves, especially in the pons of the brainstem (see here). Details about emergency management of severe hyponatraemia are available from the Society of Endocrinology.

Hypervolaemic hypotonic states suggest heart, liver and kidney failure. The patient may have signs of pleural or peripheral oedema.

The isovolaemic hypotonic hyponatraemic patient is most likely due to SIADH or thyroid deficiency.

The hypovolaemic hypotonic hyponatraemic patient is more likely to have GI, third space loss or diuresis. The patient will likely appear dehydrated with dry skin and mucous membranes, and tachycardia.

ADH is synthesised in the hypothalamus and released via the posterior pituitary gland. It helps to increase blood pressure and water in the body by stimulating the kidneys to reabsorb more water. When released in excess, as with SIADH (syndrome of inappropriate ADH), it causes hyponatraemia due to too much water being reabsorbed, but the urine will be more concentrated. I.e. they will be euvolaemic, hyponatraemic but with increased urine osmolality.