Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Definition

Inappropriate secretion of ADH leading to water retention and hyponatraemia (normal 135 mmol/L)

Presentation

Plasma sodium <>

Plasma sodium <>

No oedema

Aetiology

Tumours – small cell lung carcinoma, prostate, thymus, pancreas, lymphomas

Pulmonary lesions – pneumonia, tuberculosis, lung abscess

CNS causes – meningitis, tumours, head injury, subdural haematoma, cerebral haematoma, SLE vasculitis

Metabolic causes – alcohol withdrawal, porphyria

Drugs – chlorpropamide, carbamazepine, cyclophosphamide, vincristine, phenothiazines

Diagnosis

Dilutional hyponatraemia due to excessive water retention

Low plasma osmolality with urine osmolality higher than that of plasma

Continued urinary sodium eexcretion > 30 mmol/L

No hypokalaemia (or hypotension)

Normal renal, adrenal and thyroid function

Treatment

Treat underlying cause where possible

Symptomatic relief:

1. Restrict fluid intake to 500-1000ml daily

2. Frequent measurement of plasma sodium and osmolality and bodyweight

3. If water restriction poorly tolerated or ineffective, demeclocycline may be given to inhibit action of ADH on kidney causing reversible nephrogenic diabetes insipidus

4. When syndrome very severe, rarely hypertonic saline (300 mmol/L slow i.v.) is given with furosemide – dangerous and used with extreme caution

Source: Kumar and Clark