Syndrome of inappropriate anti-diuretic hormone secretion

Definition

Production of inappropriately concentrated urine.

Epidemiology

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 (small cell, mesothelioma), GI cancer, pharyngeal cancer, lymphoma, leukaemia, thymoma
• Lung disease - bacterial pneumonia, CF, TB, emphysema, pneumothorax, positive-pressure ventilation
• Neurology - meningitis, encephalitis, head injury, brain tumour or abscess, haemorrhage, CVA, Guillain-Barre syndrome, MS
• Drugs - SSRIs, TCAs, lithium, tetracyclines, cytotoxics, carbamazepine, vinicristine, MDMA, oxytocin
• Others - idiopathic, hereditary, post-op, pain, stress, endurance exercise

Clinical Features

Essentially, with the 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