Cerebral Salt Wasting (CSW)

What cerebral salt wasting is

Sodium is the main salt in the blood. A normal blood sodium level is about 135 to 145 mmol/L. Below 135 is called hyponatremia. In cerebral salt wasting, an injured or diseased brain somehow tells the kidneys to excrete far more sodium than the body takes in. Water is lost along with the salt, so the blood volume drops.

The exact mechanism is still debated. The two leading explanations are:

• Natriuretic peptides: the injured brain and heart release hormones such as brain natriuretic peptide (BNP) that make the kidneys dump sodium.
• Reduced nerve signals to the kidneys: brain injury may cut sympathetic nerve input that normally helps the kidneys hold on to salt.

Some experts question whether CSW is truly separate from SIADH, and true CSW is less common than SIADH after brain injury. In practice, doctors still need to sort out which picture they are dealing with, because the treatments clash.

Causes and who gets it

CSW appears almost only in people with an acute brain problem, usually while they are in hospital. Causes include:

• Subarachnoid hemorrhage: bleeding around the brain, usually from a burst aneurysm. This is the most common setting.
• Traumatic brain injury from falls, crashes or assaults.
• Brain surgery, including removal of tumors near the pituitary or hypothalamus.
• Brain tumors, primary or spread from elsewhere.
• Infections such as meningitis (especially tuberculous meningitis) and encephalitis.
• Stroke and other causes of raised pressure inside the skull.

It usually begins within the first 10 days after the brain event.

Symptoms

Many symptoms come from the low sodium itself, which makes brain cells swell. Others come from the loss of fluid.

From low sodium:

• headache, nausea and vomiting
• confusion, slowed thinking or drowsiness
• muscle cramps and weakness
• seizures and reduced consciousness when sodium falls fast or very low

From salt and water loss:

• passing large amounts of urine
• thirst, dry mouth and dry skin
• dizziness or fainting on standing, a fast pulse and low blood pressure
• weight loss over a few days

In someone already unwell from a brain injury, these changes can be subtle and are often picked up on routine blood tests first.

CSW vs SIADH

Both conditions cause low sodium with concentrated urine that contains a lot of sodium. The difference is the body's fluid status.

Getting this wrong matters. Restricting fluid in someone with CSW makes the dehydration worse. After a subarachnoid hemorrhage this can reduce blood flow to the brain and raise the risk of delayed brain damage from narrowed arteries (vasospasm). The SIADH page explains that condition in more detail.

How it is diagnosed

There is no single test. Doctors put together:

1. Blood tests: sodium below 135 mmol/L, low blood osmolality (concentration), and kidney, thyroid and adrenal function to rule out other causes.
2. Urine tests: urine sodium usually above 40 mmol/L and urine that is more concentrated than expected.
3. Volume assessment: heart rate and blood pressure lying and standing, weight trend, skin and mouth dryness, and careful charting of fluid in and out.
4. Response to treatment: if salt and fluid replacement corrects the sodium and the person looks better, CSW is more likely.

Uric acid levels and invasive pressure measurements are sometimes used in intensive care, but the clinical picture of low blood volume with ongoing salt loss is the key.

Treatment

The aim is to replace what the kidneys are losing while raising sodium at a safe speed.

• Intravenous fluids: normal (0.9%) saline to restore blood volume.
• Hypertonic saline: a stronger salt solution given in hospital when sodium is very low or causing symptoms such as seizures.
• Salt tablets: used once the person can eat and drink, sometimes continued for a few weeks after discharge.
• Fludrocortisone: a hormone tablet that helps the kidneys hold on to sodium. It is used in some people, especially after subarachnoid hemorrhage.
• Close monitoring: sodium is checked every few hours at first.

Doctors limit how fast sodium rises. Correcting long-standing low sodium too quickly can damage nerve fibers in the brainstem (osmotic demyelination syndrome), which causes swallowing problems, weakness and sometimes paralysis.

Complications and outlook

Untreated, CSW can lead to seizures, coma, worsening brain swelling and, after subarachnoid hemorrhage, a higher risk of delayed stroke from low blood volume. With recognition and replacement, it is usually temporary. Most people stop losing salt within 3 to 4 weeks, and salt tablets can then be stopped. Recovery depends far more on the underlying brain injury than on the sodium problem.

Frequently asked questions

What is cerebral salt wasting syndrome?

It is low blood sodium caused by the kidneys losing too much salt after a brain injury, bleed, surgery, tumor or infection. Water is lost with the salt, so the person becomes dehydrated as well as low in sodium. It is usually treated in hospital with salt and fluid replacement.

What is the difference between CSW and SIADH?

Both cause low sodium with salty, concentrated urine. In CSW the person is dehydrated, with high urine output and low blood volume. In SIADH the body holds on to water and blood volume is normal. CSW is treated with fluid and salt, while SIADH is usually treated by restricting fluid.

How long does cerebral salt wasting last?

It usually starts within the first 10 days after a brain injury or bleed and settles over about 3 to 4 weeks. Some people go home on salt tablets and have blood tests to check that their sodium stays normal as the kidneys recover.

Is cerebral salt wasting dangerous?

It can be. Very low or fast-falling sodium can cause confusion, seizures and coma, and dehydration can reduce blood flow to an already injured brain. Recognized early and treated with careful fluid and salt replacement, it is usually well controlled and leaves no lasting effects of its own.

Why is fluid restriction harmful in CSW?

People with CSW are already short of fluid because their kidneys are losing salt and water. Restricting fluid makes the blood volume fall further, which can lower blood pressure and reduce brain blood flow. After a subarachnoid hemorrhage this raises the risk of delayed brain injury.