SIADH (= Syndrome of Inappropriate secretion of AntiDiuretic Hormone) is a condition which results in hyponatraemia.
The Bartter and Schwartz criteria for SIADH is:
- Decreased plasma osmolality (<275 mosm/kg)
- Inappropriately concentrated urine (>100 mosm/kg)
- Euvolaemic
- Elevated urine Na (>20 mEq/L)
- Normal thyroid and adrenal function
- No diuretic use
ADH itself is made in the supraoptic and paracentricular nuclei of the hypothalamus. It is then stored in the posterior pituitary.
ADH is usually released in response to:
- raised plasma osmolality (detected by osmoreceptors in the hypothalamus)
- decreased plasma volume (detected by baroreceptors in the carotids, aorta and left atrium)
- limbic system activation – pain, fear, nausea
It increases synthesis and insertion of aquaporin-2 water channels in the luminal membrane of the collecting ducts, increasing water reabsorption in the kidney
As SIADH causes hyponatraemia, clinically you would expect:
- Sodium <125mEq/l: weakness, headaches, nausea and vomiting
- Sodium <115mEq/l: altered consciousness, seizures, coma
Causes of SIADH include
- Drugs
- Anticonvulsants - Carbamazepine, Sodium valproate
- Antidepressants - SSRIs, MAOIs, TCAs
- Haloperidol
- Amiodarone
- Ciprofloxacin
- Chemo
- Opiates
- MDMA
- Malignancy
- Lung cancer, especially small cell carcinoma
- Pancreatic cancer
- lymphoma
- Head and neck cancers
- Pulmonary disease
- Pneumonia
- TB
- Pneumothorax
- Cystic fibrosis
- Asthma
- CNS
- Stroke
- Infection - encephalitis, meningitis, abscess
- Haemorrhage
- trauma
- MS
- GBS
- Porphyria
Treatment
- Treat cause
- Correct sodium
- Fluid restriction
- ? demeclocycline
- ? hypertonic saline
- ? furosemide
- ? vasopressin receptor antagonists (vaptans)
- ? urea
If acute (48 hrs or less) correction of sodium can be fast
If chronic, correction of sodium should be cautious (around 8meq/day) to avoid osmotic demyelination syndrome.
References:
Esposito, P. et al. The syndrome of inappropriate antidiuresis: pathophysiology, clinical management and new therapeutic options. Nephron Clin Pract. 2011; 119(1):c62-c73.
