Syndrome of Inappropriate ADH (SIADH)

Pathology

Pathology is characterized by the abnormal secretion of the anti-diuretic hormone, leading to an excessive buildup of water in the body and a resulting relative hyponatraemia.

Aetiology

• Idiopathic
• Endocrine: Hypothyroidism, Addison's Disease
• Ectopic Production: Small cell lung cancer, pneumonia, abscess, tuberculosis
• Malignancy: Lung, pancreas, ovary, lymphoma, thymoma
• CNS: Tumour, trauma, infection, stroke, multiple sclerosis, Guillain-Barre Syndrome, meningitis, encephalitis, subarachnoid haemorrhage
• Drugs: Selective Serotonin Reuptake Inhibitors (SSRIs), Tricyclic Antidepressants (TCAs), opiates, haloperidol, Monoamine Oxidase Inhibitors (MAOIs), diuretics, Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

Symptoms

The symptoms of this pathology generally include fatigue, confusion, falls, seizures, inappropriate thirst, headache, and nausea.

Signs

The signs that are displayed in someone who has contracted this ailment are weakness, coma, euvolaemia, papilloedema, myoclonus, ataxia, and tremor.

Investigations

• Bloods: U&E – hyponatraemia, serum osmolarity, Thyroid Function Tests (TFTs), and Cortisol level measurement
• MSU: Urinary sodium level (raised) and osmolarity
• Imaging: Computed Tomography (CT) head scan to assess for malignancy

Treatment

The most common treatments for this pathology involve restricting fluid intake and taking Demeclocycline.

Complications

Rapid corrections can lead to severe complications, such as central pontine myelinolysis. As such, a sodium correction rate of 0.5-1mmol/hour should be used in order to prevent this.

Prognosis

For cases arising from idiopathic or drug-related causes, the prognosis is usually good.