Do I need a brain scan?

A National Migraine Centre factsheet

Why CT and MRI scans are rarely used to diagnose migraine

Brain scans for migraines

When it comes to brain scans for headaches, there are two types of scans used most frequently.

The first is the computed tomography (CT) scan, which uses x-rays. The other is the magnetic resonance imaging (MRI) scan, which uses magnetism and radio waves.

Both types of scans use complex computer programmes to build a cross-sectional picture of the body or brain.

What does a brain scan for migraine show?

A scan shows the physical structure of the brain, but it doesn’t show how the brain works. To use a computing analogy, the scan checks the hardware of the brain, not the software.

Almost all headaches are what is known as primary headache. Using the computing analogy again, this is a software problem so does not show up when the hardware is checked. So, a brain scan for headache is rarely necessary.

Pain is important, warning us of possible injury or disease: it is like the body’s alarm signal. For people with migraine, it’s like a faulty fire alarm. The longer the warning goes on with nothing else bad actually happening, the less likelihood there is of a serious underlying cause – and the longer you have headache, the less chance of the cause showing up on a scan.

Which headache patients need a brain scan?

If there are other symptoms, such as epileptic attacks, paralysis, or a change in brain function, then those symptoms may independently warrant a brain scan.

There are three main situations where headache alone requires a brain scan.

  1. So-called ‘thunderclap headache’, which comes on very rapidly, building in intensity in under few minutes to become the most painful headache imaginable. Having this type of headache means you should go straight to emergency care and have a CT scan on the day that the headache begins. It can be a warning of bleeding in or around the brain.
  2. If you have developed new headaches and have had cancer of a type that can spread to the brain, then a brain scan is a wise precaution. These cancers include lung, breast, thyroid, kidney bowel cancers and melanoma

If your pain is brought on by coughing, exercise, or change in posture, this can mean that the fluid around the brain and spine is partly blocked, which shows up on an MRI scan.

Could it be a brain tumour?

You may have heard someone tell you they knew someone diagnosed with migraine who then died of a brain tumour. Migraine is a common disease, affecting one in seven people, so obviously one in seven people diagnosed with a brain tumour will have a history of migraine. It doesn’t mean the diagnosis of migraine was incorrect.

Why not be safe and scan everyone with a headache?

Headache is so common that it isn’t practical, necessary or cost-effective to scan everyone.

If everyone were scanned, how often should the scan be repeated if headache continues? The millions of people with migraine would inevitably prevent those with critical conditions, such as bleeding on the brain, accessing scans.

What are the risks of having a brain scan?

CT scans use x-rays, which can increase the risk of cancer. Children who have a CT scan have a 20 per cent increased risk of cancer, and it is likely adults have a similarly raised risk. It’s an important reason why an unnecessary brain CT scan for headache should be avoided.

Some significant abnormalities found on brain MRI scans for headache cannot be seen on CT, so there is a small chance of false reassurance from a regular CT scan.

MRI scanning takes longer than CT and normally requires the patient to lie still in a noisy tube, however it does avoid the risk of radiation.

MRI scans often find abnormalities that are unrelated to headache, entirely by chance. The risk of finding a minor abnormality of no medical significance is around one in five. The risk of a chance abnormality that might potentially need treatment is about 1.4 per cent.

Once abnormalities have been found, patients can find it more difficult to obtain travel and other insurance and there may be a temptation to repeat the scan time and time again to check that the abnormality is not changing.

Listen to our Heads Up podcast episode 3 in series 1 on brain scans for migraine to find out more.

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