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