Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
Table of Contents
What is SIADH?
The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterised by the excessive secretion of antidiuretic hormone (ADH) from the posterior pituitary gland, or another source. ADH controls water reabsorption via its action on the kidney nephrons, causing the retention of water (but not the retention of solutes). Therefore by increasing water retention, ADH causes dilution of the blood and in turn decreases the concentration of solutes, such as sodium. In fact, SIADH is one of the most common causes of hyponatraemia.
1. ADH (also known as Vasopressin) is produced by the hypothalamus in response to increased serum osmolality.
2. ADH is transported from the hypothalamus to the posterior pituitary gland.
3. Next, ADH is released into the circulatory system via the posterior pituitary gland.
4. ADH then travels to the kidneys, where it binds to ADH receptors on the distal convoluted tubules.
5. The binding of ADH to its receptors causes Aquaporin-2 channels to move from the cytoplasm, into the apical membrane of the tubules:
These aquaporin-2 channels allow water to be reabsorbed out of the collecting ducts and back into the bloodstream.
This results in both a decrease in volume and an increase in osmolality (concentration) of the urine excreted.
6. The extra water that has been reabsorbed re-enters the circulatory system, reducing the serum osmolality.
7. This reduction in serum osmolality is detected by the hypothalamus and results in decreased production of ADH.
Deranged physiology in SIADH
The important difference between normal physiology and what occurs in SIADH is the lack of an effective negative feedback mechanism. This results in an inability to reduce or stop ADH production – as a result, ADH is continually produced, regardless of what the serum osmolality is. This ultimately leads to abnormally low levels of serum sodium and relatively high levels of urinary sodium, giving rise to the characteristic symptoms and signs associated with SIADH.
What causes SIADH?
There are lots of potential causes of SIADH including:
Primary brain injury – e.g. meningitis. subarachnoid haemorrhage (SAH)
Malignancy – e.g. small-cell lung cancer
Drugs – e.g. carbamazepine, SSRIs, amitriptyline
Infectious – e.g. atypical pneumonia, cerebral abscess
Signs and Symptoms
Symptoms vary greatly depending on the time taken for the hyponatraemia to develop
Mild hyponatraemia may cause significant symptoms if the drop in sodium is acute, whereas chronically hyponatraemic patients may have very low serum sodium concentrations and yet be completely asymptomatic. This is thought to be due to a compensatory process known as cerebral adaptation, in which brain cells adapt their metabolism to cope with abnormal sodium levels. Cerebral adaptation can only take place if the change in sodium concentration is gradual, explaining the more severe symptoms seen in acute hyponatraemia and the potentially less severe symptoms seen in patients who are chronically hyponatraemic.
It is difficult to give a generic step-by-step management strategy for SIADH as it can result from so many different causes. Therefore, the management strategy should be targeted to treat the underlying cause, in order to achieve long-term correction of sodium metabolism. Specialist endocrine input should be sought before commencing any of these management strategies.
This is a common management strategy used to increase serum sodium concentrations, at least temporarily, whilst the underlying cause is sought and treated. Usually, fluid restriction is between 1-1.5 litres per day. However, this strategy is greatly dependent on patients co-operating with the treatment plan.
Another general management strategy for treating hyponatraemia is to replace sodium either orally or via IV fluid. However, this must be done with great care – if the sodium concentration is corrected too rapidly, it can result in a devastating complication known as central pontine myelinolysis. Central pontine myelinolysis is characterised by permanent damage to the myelin sheath in the brain stem, causing acute paralysis, dysphagia, dysarthria, diplopia and decreased level of consciousness. As a result, this treatment strategy has to be carried out with extreme caution, and with the recommendation of not correcting serum sodium levels more than 10 mmol/L/24h.
Examples of treating the underlying cause
Pneumonia – antibiotics
Carbamazepine – consider switching to alternative anti-epileptic