ANSWER 98
The blood results show hyponatraemia and the chest X-ray shows a mass overlying the
right hilum. This degree of hyponatraemia might be expected to cause some cerebral
changes. A lower level might be associated with fits. Above 120–125 mmol/L the effects
are likely to be non-specific tiredness.
Possible causes for the hyponatraemia in this man are:
- inappropriate argenine vasopressin (AVP, antidiuretic hormone) secretion found in
association with the respiratory disorders of undifferentiated small-cell carcinoma of
the lung or, occasionally, with pneumonia or tuberculosis - Addison’s disease (adrenocortical failure) which would be expected to produce a high
potassium level and postural hypotension. Addison’s disease might be linked with
respiratory problems through adrenal involvement by metastases or tuberculosis.
Other causes such as diuretic treatment, inappropriate AVP from drug therapy (e.g. carba-
mazepine, phenothiazines, amitriptyline), cerebrovascular events, salt-losing nephropathies
or overhydration from intravenous fluids or overdrinking are not likely from the story given
here. He has been treated with antidepressants, but not for the last 18 months. In view of the
chest X-ray the most likely diagnosis is inappropriate AVP secretion with a small-cell
undifferentiated carcinoma of the lung. This can be confirmed by measurement of serum
and urine osmolarities to show serum dilution while the urine is concentrated. Levels of AVP
can be measured.
In this case, the osmolarities confirmed the syndrome of inappropriate anti-diuretic hor-
mone (SIADH) secretion; bronchial biopsies at fibreoptic bronchoscopy showed a small-cell
undifferentiated carcinoma. Extension to the carina and computed tomography (CT)
appearances showed it to be not resectable. Fluid restriction to 750 mL daily produced an
increase in serum sodium to 128 mmol/L with improvement in the confusion and weakness.
If this fails to produce adequate results demeclocycline can be used. This derivative of
tetracycline antibiotics interferes with the action of ADH in the renal tubules.
Chemotherapy was started for the lung tumour. Such treatment often produces a response
in terms of shrinkage of the tumour, improved quality of life and increased survival. It
may also help the ectopic hormone secretion. Unfortunately, cure is still infrequent.
Small-cell undifferentiated carcinomas of the lung are fast-growing tumours, usually
unresectable at presentation.
- Change of character may have a metabolic explanation.
- The commonest cause of hyponatraemia is diuretic therapy.
- Measurement of serum and urine osmolarities can help to determine the cause of
hyponatraemia.