The Syndrome of Inappropriate ADH Secretion (SIADH)

Contents

• Introduction
• Pathology and clinical features
• Investigations
• Treatment

Introduction

The syndrome of inappropriate ADH secretion is a condition in which there are persistently elevated levels of antidiuretic hormone. There are numerous causes which may be grouped as anything in the head, anything in the chest, extracranial tumours and others.

Intracranial Causes

• Meningitis
• Encephalitis
• Intracranial abscess
• Intracranial tumour
• Intracranial haemorrhage

Thoracic Causes

• Pneumonia
• Tuberculosis
• Lung abscess
• Bronchial small cell carcinoma
• Thymic carcinoma

Extracranial, Extrathoracic Tumours

• Pancreatic carcinoma
• Prostatic carcinoma
• Lymphoma

Other Causes

• Hypothyroidism
• Systemic lupus erythematosus
• Alcohol withdrawal
• Porphyria
• Chlorpromazine
• Chlorpropamide
• Carbamazepine
• Cyclophosphamide
• Vincristine

Pathology and Clinical Features

The inappropriate secretion of ADH causes the body to retain water, without solutes, even though the plasma osmolality is low. This produces hyponatraemia and an increase in the volume of the extracellular fluid. Hypertension does not occur.

The retention of water by the kidney results in concentrated urine.

The clinical features reflect they hyponatraemia and in severe cases cerebral oedema.

Investigations

Evaluation of blood levels of ADH is not necessary. Instead, plasma osmolality can be measured (and may also be calculated from the urea and electrolytes, which will also reveal the hyponatraemia). The detection of a high urine osmolality despite a low plasma osmolality is effectively definitive.

It is prudent to exclude other causes of hyponatraemia such as renal disease or Addison's disease.

Treatment

The underlying cause of the SIADH should be treated, if possible.

The plasma osomolality is corrected by fluid restriction to an intake of 500 to 1000ml per day. Frequent monitoring of the plasma osmolality is essential

Demethylchlortetracycline is an antagonist of ADH and can be used if fluid restriction is not possible.

If patients have symptoms of severe neurological dysfunction, such as seizures or coma, intravenous hypertonic normal saline can be given with extreme caution and very close monitoring. If an osmolality imbalance is corrected too rapidly central pontine myelinolysis can result which may be fatal or produce the locked in syndrome.