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Why is a lumbar puncture used in dementia?

Dr Catherine Mummery

Sometimes doctors take samples of spinal fluid using a lumbar puncture, to help make a diagnosis of a form of dementia like Alzheimer’s disease. Here, Dr Cath Mummery aims to answer your questions about lumbar punctures, what the procedure involves and why it is used.

What is cerebrospinal fluid?

Cerebrospinal fluid (CSF) is a clear, colourless fluid that fills the spaces that surround our brains and spinal cord.

It has three main functions:

  • It protects our brain from trauma.
  • It supplies nerve cells with nutrients.
  • It clears waste away from the brain.

CSF contains molecules that can act as markers of normal or abnormal biological processes, or flag that someone may have a particular condition or disease. Because of this, these molecules are called biomarkers.

Researchers have shown that levels of these biomarkers in CSF can mirror biochemical changes occurring in the brain. This is really helpful for diagnosis and research in neurodegenerative diseases, like Alzheimer’s disease. Because these changes could indicate harmful changes happening in the brain that could be causing someone to experience symptoms of dementia.

Using CSF to help diagnose dementia

An accurate diagnosis of dementia is important. It gives people the opportunity to access treatments, care and support that allows them to plan for the future while they are still able to make important decisions. It also enables people to take part in dementia research via initiatives like Join Dementia Research.

If you are worried about memory problems, the doctor will ask about your symptoms and medical history. They will ask you to do some memory and thinking tests. Other tests, like brain scans, blood tests and a lumbar puncture, may help the doctor to identify the cause of your symptoms.

There is no one diagnostic test or ‘magic marker’ for the diseases that cause dementia. Putting the results of several different investigations together helps doctors to give an accurate diagnosis.

In June 2018, the guidelines for diagnosing dementia were updated by the National Institute for Health and Care Excellence (NICE) to include assessment of CSF by lumbar puncture where appropriate.

In recent years, research has shown that CSF tests can be a valuable tool for the detection of hallmark proteins that are involved in diseases that cause dementia. For example, amyloid and tau protein build up in Alzheimer’s disease, and their levels can be measured in CSF.

Using CSF in research

Accurate diagnosis of a disease, like Alzheimer’s, that causes dementia using diagnostic tests is vital so that people can access the right care and treatments. The ability to pick up diseases that cause dementia accurately and early is also a key goal for research, as we must to be able to test potential new treatments in the right people.

In dementia research studies, lumbar punctures may be used as part of the screening process, to see if somebody is eligible for the study. Or they may be used during a study to monitor the effects of taking part in a drug trial.

By assessing a CSF sample, researchers can monitor important biomarkers over time to see if the study drug has had any beneficial effect on reducing signs of damage in the brain. In trials, it is likely that the volunteer will also have brain scans done to assess if there are any signs that brain shrinkage is slowing down too.

Without correct screening, measurement and assessment of research volunteers, scientists are unable to properly test if new medicines are effective.

Having a lumbar puncture

Getting a sample of cerebrospinal fluid is a bit like getting a sample of blood for a blood test in that a needle is used to access the fluid and a sample is taken.

A needle is inserted in a specific place between two of the bones in the lower back. The doctor will ask you to be in a position that allows the space between these bones (or vertebrae) to be as wide as possible. This may be curled up on your side or sitting up and leaning forward over a pillow.

After the area is numbed using a local anaesthetic, a needle is put through the skin and a protective membrane called the dural sac. After this, it enters the space containing the CSF. When the needle is in place the fluid will gradually flow out and be collected using a syringe.

The process takes around 30 minutes in total. When collection is complete, you will typically lie down and rest at the hospital for an hour or so. During this time nurses will check that you’re feeling okay.

The film below is voiced by Prof Nick Fox who explains the process of a lumbar puncture. Please note the film contains footage of needles and injections.

Debunking myths on lumbar punctures

However, they are a standard medical procedure used for a wide range of medical purposes. It’s not common to experience side-effects after a well-conducted lumbar puncture.

When there are side effects, these are generally limited to headaches that can last for several days. Despite this there are some common myths and misconceptions about the procedure that prevent some people from having one done.

  • Pain

People believe that lumbar punctures are painful. In fact, most people report that the painful or uncomfortable part of the procedure is the sting they feel when the anaesthetic is injected. Discomfort associated with a lumbar puncture seems to vary from person to person and the doctor will make sure that you are feeling comfortable throughout the procedure.

  • Infections

There is a misconception that people can develop meningitis from a lumbar puncture.

This myth may have come about because bacterial meningitis, where bacteria makes its way into the spinal canal, is diagnosed by using a lumbar puncture to collect spinal fluid for testing. There is virtually no risk of meningitis from a lumbar puncture.

  • Paralysis

Some people we speak to are reluctant to get a lumbar puncture because they have heard it can cause paralysis. Paralysis can be caused by spinal cord injury.

The spinal cord ends about five inches above the spot where the lumbar puncture needle is inserted, so is not affected by the procedure. This means that there is almost no chance of paralysis.

Nerves do branch from the spinal cord and dangle down through the lower part of the spine. Sometimes the needle may brush against one of these nerves. This can feel like an electric shock or a twinge down one of the legs.

The feeling usually goes away very quickly, but if it does not the doctor will re-adjust the needle before proceeding any further. The nerves are surrounded by spinal fluid so will be pushed aside by the needle, which is blunt to prevent any damage.

 

It is important that people are aware of how to get involved in dementia research and have clear information about procedures like lumbar punctures.

If you have questions about dementia research or want to find out more about how to get involved, contact our Dementia Research Infoline on 0300 111 5 111 or email infoline@alzheimersresearchuk.org

If you want further information on lumbar puncture procedure please see the NHS webpage here https://www.nhs.uk/conditions/lumbar-puncture/

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