View source for Syndrome of inappropriate anti-diuretic hormone secretion

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===Definition===
Production of inappropriately concentrated urine.
===Epidemiology===
[[image:common.jpg]]The commonest cause of [[hyponatraemia]].
===Pathophysiology===
"Inappropriate" is the problem here. For it to be considered concentrated urine from "inappropriate" ADH secretion, the following four things need to be true:
*Urine osmolarity > Serum osmolarity
*Normal [[renal]] and [[adrenal]] function
*No [[oedema]] or [[hypovolaemia]]
*Persistent concentrated urine secretion with no reason for ADH release
Now there's a whole heap of causes of SIADH so they need to be broadly categorised.
*[[Neoplasia]] - [[lung cancer|lung]] (small cell, [[mesothelioma]]), [[GI cancer]], [[pharyngeal cancer]], [[lymphoma]], [[leukaemia]], thymoma
*[[Respiratory|Lung disease]] - bacterial [[pneumonia]], [[CF]], [[TB]], [[emphysema]], [[pneumothorax]], positive-pressure ventilation
*[[Neurology]] - [[meningitis]], [[encephalitis]], [[head injury]], [[brain tumour]] or [[brain abscess|abscess]], [[brain haemorrhage|haemorrhage]], [[CVA]], [[Guillain-Barre syndrome]], [[MS]]
*[[Drugs]] - [[SSRIs]], [[TCA]]s, [[lithium]], [[tetracyclines]], cytotoxics, carbamazepine, vinicristine, [[MDMA]], [[oxytocin]]
*Others - idiopathic, hereditary, post-op, pain, stress, endurance exercise



===Clinical Features===
Essentially, with the '''[[Hyponatraemia#Clinical Features|symptoms of hyponatraemia]]''' + those of the underlying pathology.

===Investigations===
In terms of investigations, the following needs to be true for a diagnosis of SIADH to be made:
*[[Hyponatraemia]] - serum sodium <135mmol/L
*Plasma osmolality <275mOsm/kg (decreased)
*Urine osmolality >100mOsm/kg (increased)
*Normovolaemia on examination
*Urinary sodium >40mmol/L (increased)
====Bloods====
*[[U+E]] - sodium mainly but also urea and creatinine to look for dehydration and exclude kidney problems
*[[TFTs]]
*Adrenal function (9am cortisol)
====Urine====
*Plasma and urine osmolality comparison
*Urinary sodium

===Management===
===Prognosis===