Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

What is Syndrome of inappropriate antidiuretic hormone secretion?

Syndrome of Inappropriate ADH secretion (SIADH) is the continued ADH secretion in spite of plasma hypotonicity and a normal or expanded plasma volume.

Syndrome of inappropriate antidiuretic hormone secretion; SIADH

Statistics on Syndrome of inappropriate antidiuretic hormone secretion?

Hyponatraemia is relatively common. Its prevalence in hospitalized patients is approximately 2.5% and about one third of these have SIADH.

Risk Factors for Syndrome of inappropriate antidiuretic hormone secretion

Causes:Cancer - Many tumours. Most common is small cell cancer of the lung.Brain - Meningitis, cerebral abscess, head injury, tumour.Lung - Pneumonia, tuberculosis, lung abscess.Metabolic - Porphyria, alcohol withdrawal.Drugs - Opiates, chlorpropramide, carbamezapine, vincristine.

Progression of Syndrome of inappropriate antidiuretic hormone secretion

In most patients the course of this syndrome follows that of the underlying disease. Sometimes, for unknown reasons, the syndrome improves even if the causative disorder deterioates or improves only slightly. This is seen particularly in strokes and other brain injuries.

How is Syndrome of inappropriate antidiuretic hormone secretion Diagnosed?

Urea and electrolytes - low sodium, low plasma osmolality

Urine osmolality and sodium - inappropriatley higher than plasma. Continued urinary sodium excretion. Continued urinary sodium excretion greater than 30mmol/l.

Diagnosis also depends on absence of hypotension and hypovolaemia. (there is also absence of hypokalaemia)

Thyroid function tests, adrenal and renal function - normal: required for diagnosis.

Prognosis of Syndrome of inappropriate antidiuretic hormone secretion

The outcome is related to the underlying disease.

How is Syndrome of inappropriate antidiuretic hormone secretion Treated?

Mild asymptomatic cases need no treatment other than that of the underlying cause.Symptomatic cases - options:Water restriction (500-1000ml in 24 hours)Dimethylchlorotetracycline - inhibits the action of vasopressin on the kidney and may be useful if water restriction is poorly tolerated or ineffective.For severe cases: Hypertonic saline IV slowly - with frusemide to prevent circulatory overload.

Syndrome of inappropriate antidiuretic hormone secretion References

[1] Braunwald, Fauci, Kasper, Hauser, Longo, Jameson. Harrison's Principles of Internal Medicine. 15th Edition. McGraw-Hill. 2001[2] Cotran, Kumar, Collins 6th edition. Robbins Pathologic Basis of Disease. WB Saunders Company. 1999.[3] Hurst JW (Editor-in-chief). Medicine for the practicing physician. 4th edition Appleton and Lange 1996.[4] Kumar P, Clark M. CLINICAL MEDICINE. WB Saunders 2002 Pg 427-430.[5] Longmore M, Wilkinson I, Torok E. OXFORD HANDBOOK OF CLINICAL MEDICINE. Oxford Universtiy Press. 2001