48 year old with severe hyponatraemia, at risk of seizures
FBC normal
Severe hyponatraemia (112), requires investigation
severe hypochloraemia – ?salt losing (no history of vomiting), dilutional
Normal renal function and potassium
Osmolality 227.3 (glucose not given) – low osmolality
Urine osmolality high
therefore, severe hyponatraemia with low plasma osmolality and high urine osmolality – SIADH
Elevated GGT, ALP and ALT with normal bilirubin – obstructive picture (gallstones, tumours, no history of alcohol use given)
Interpretation – 48 year old man presents with severe hypoNa (low plasma osmolality, high urine osmolality) likely to be due to SIADH. He requires fluid restriction (daily weight), observation for seizures, consider ivi Na to increase Na to 120 (beware CPM). Further investigation for cause of SIADH (tumours -lung, liver, pancreas, drugs- SSRI, NSAIDs, TCA, infection – pneumonia, TB, meningitis/encephalitis).
Other causes of high urine osmolality – dehydration (may be contributing factor), adrenal insufficiency, glycosuria, hypernatraemia, high protein diet.
His spot urine Na concentration was 9mmol/l which was not helpful in terms of SIADH.
CXR showed wedge shaped opacity directed inferolaterally from hilum with blunting of rt costophrenic angle. This was interpreted as likely to be infective rather than neoplastic.
The patient respiratory status deteriorated in the ward, he required ICU admission and intubation. He subsequently had a bronchoscopy and biopsy of a mass in the right lower lobe which was small cell carcinoma on histology.
SIADH
Diagnostic criteria:
1. Hyponatraemia
2. clinical euvolaemia
3. low plasma osmolality
4. high urine osmolality
5. urine Na concentration >20mmol/l
6. Normal TFT, renal and adrenal function
Causes:
Tumours – small cell Ca, bronchogenic ca, pancreatic ca, Hodgkins Lymphoma
Pulmonary – pneumonia, lung abscess, TB
Medication – SSRI, Carbamazepine, TCA, NSAID
CNS – tumour, encephlitis, SAH, trauma
AIDS
Ventilation
Treatment:
1. water restriction
2. salt restriction
3. daily weight
4. loop diuretic – not carbonic anhydrase inhibitors
5. Urea (rarely used)
6. Treat the cause