Syndrome of Inappropriate ADH (SIADH) secretion

Related Subjects: |Lung Cancer |Hyponatraemia |SIADH | Sodium Physiology | TURP Hyponatraemia syndrome

The [Na] should only be brought up by 10-12 mmols/24 hrs to avoid Central pontine myelinosis. Urine sodium is elevated and urine is more concentrated than it should be for the serum osmolality

About

• A not uncommon cause of dilutional hyponatraemia
• Severe hyponatraemia is a medical emergency
• Correction of a low sodium should be cautious and controlled

Aetiology

• ADH (arginine Vasopressin) is a hormone produced in the supraoptic and paraventricular nuclei in the hypothalamus
• ADH is released by the posterior pituitary usually primarily in response to a change in serum osmolality. It prevents the loss of water in the collecting ducts. There it is released with any rise in plasma osmolality.
• The action is via G receptors in the corticomedullary collecting ducts by which aquaporins are inserted into the cells lining the lumen making them more permeable to water.
• However in some situations ADH is released in high levels inappropriately. Before one can make the diagnosis one must be sure that adrenal and thyroid function is normal.
• Patients become water intoxicated. The [Na⁺] falls and the patient becomes confused and in extreme circumstances can fit.
• Treatment is water restriction to less than 1000 ml per day if not less and the risk of bringing the [Na⁺] up too quickly is of central pontine myelinolysis

Causes are many and often idiopathic

• Cerebral disease: meningitis, encephalitis, GBS, SAH, Stroke, Bleed, Pituitary surgery: hypopituitary
• Metabolic :Acute intermittent porphyria
• Drugs: SSRIs, TCAs, chlorpropamide, barbiturates, anaesthetics, barbiturates, morphine, diuretics, vincristine, vinblastine, cisplatin, Cyclophosphamide, and melphalan, Opiates and narcotic analgesics
• Malignancies: primary brain tumours, haematologic malignancies, intrathoracic non-pulmonary cancers, skin tumours, gastrointestinal cancers, gynaecological, cancer, breast and prostatic cancer, and sarcomas especially small cell lung cancer
• Respiratory: Positive pressure ventilation, TB, pneumonia, abscess, pleural effusion.

Clinical

• Euvolaemic clinically

Differential

• Cerebral salt wasting seen in SAH
• Renal Sodium losses - diuretics
• Free water overload - TURP syndrome
• Excessive IV dextrose often post-op
• Exclude hypothyroidism (TFTs) and hypoadrenalism (Short synacthen)

Investigations

• Urine Na > 30 mmol/l
• Urine Osmolality > 100 mmol/kg when there is dilute plasma
• Plasma Na < 125 mmol/l due to haemodilution due to free water retention
• Plasma Osmolality < 260 mmol/kg due to free water retention
• Exclude - Heart Failure, Renal Failure, Liver Failure, Hypoadrenalism

Management

• ABC. Stop/remove/treat any causes e.g. drugs
• Avoid bringing up [Na] by more than 12 mmol/24 hours
• Rapid rise in [Na] can cause central pontine demyelination
• Water restriction to < 1000 ml/day
• Consider salt +/- loop diuretic if severe
• Demeclocycline induced Nephrogenic diabetes insipidus
• Hypertonic saline 3% in small volumes e.g. 200 ml can help raise the osmolality in acutely unwell patients.
• New ADH receptor antagonists may be useful e.g. Tolvaptan for short periods