Introduction to Spontaneous Intracranial Hypotension

What is SIH?

Spontaneous intracranial hypotension (SIH) happens when fluid that cushions the brain and spinal cord (cerebrospinal fluid, or CSF) leaks from the covering of the spine. The most common symptom is a sudden, ongoing headache that is worse when upright and better when lying flat. You may also notice neck stiffness, nausea, light sensitivity, ringing in the ears, reduced hearing, dizziness, or tiredness. Some people describe “brain fog”.

SIH can be caused by tiny tears in the spinal lining, small outpouchings that rupture, or a CSF-venous fistula where CSF drains abnormally into nearby veins. Finding the exact leak can be difficult. Brain scans can look normal and a lumbar puncture pressure can be normal. Special spinal imaging is often needed.

How common is it?

SIH is more common than once thought. Around 4 to 5 people in every 100,000 develop it each year. It is seen more often in women and in people with connective tissue conditions such as Marfan syndrome, Ehlers–Danlos syndrome, or generalised joint hypermobility.

Is it serious?

Yes, because symptoms can be very disabling. Untreated SIH can lead to complications in a small number of people. The good news is that most people improve with the right treatment. Early diagnosis and treatment give the best chance of full recovery.

What causes it?

SIH is due to a CSF leak from the spine. Common patterns include:

• A tear on the front of the dura from a sharp disc or bony spur.
• A tear on the side of the dura due to thin or fragile tissue.
• A CSF-venous fistula where CSF drains into a spinal vein, often near a nerve root.
• A small leak from a nerve root sleeve into nearby soft tissues.

Who is more at risk?

Women, and people with conditions that make connective tissues more stretchy or fragile, such as Marfan syndrome, Ehlers–Danlos syndrome, or general joint hypermobility.