Syndrome of inappropriate antidiuretic hormone release

From Ganfyd

Introduction

Syndrome of inappropriate anti-diuretic hormone release (SIADH) is a common cause of hyponatraemia and is a final pathway of many pathologies that produce hyponatraemia. Many patients present with a mixed aetiology so may not meet the following prerequisities. A mistake often made in acute hospital medicine is to label and even manage a patient as SIADH when the most important issue is water intoxication, which would be clear from inspection of a fluid balance chart, recent prostate surgery or even an oral history of recent fluid intake.

Prerequisites

• Urine sample

1. [Na⁺] >20 mmol/l
2. Urine osmolality > 500 mmol/kg

when

• Blood test

1. [Na⁺] < 125 mmol/l
2. Plasma osmolality < 260 mmol/kg

Causes

Drugs

A common cause and the mechanism of production of hyponatraemia of most drugs, for example anti-depressants and anti-epileptic agents. It must be remembered that drugs such as amiloride, acetazolamide, thiazide diuretics, ACE and Angiotensin-II inhibitors act on the kidney directly to produce hyponatraemia.

Malignancy

• Lung cancer
• Prostate cancer
• Pancreatic cancer
• Leukaemia
• Lymphoma

Chest disorders

• Pneumonia

CNS lesions

• Raised Intracranial Pressure from any cause eg Brain abscess
• Guillain-Barré syndrome

Metabolic causes

Management

1. Treat cause
2. Water restriction < 1200 mls/day
3. Induce Diabetes insipidus using demeclocycline