Lumbar puncture - How it is performed

Lumbar punctures are often carried out after somebody is admitted to hospital in an emergency, but they are also often arranged in advance and carried out on a day-case basis.

Preparation

Before having a lumbar puncture, checks will be carried out to make sure the procedure is safe for you. Usually, a CT or MRI scan will have been carried out to ensure that the lumbar puncture is needed and will not cause any harm. It will not be carried out if you have recently taken blood-thinning medication (anticoagulants) such as warfarin.

A doctor or a specially trained nurse will explain why the lumbar puncture is required. You will then be asked to provide written consent for it to be carried out. 

If it is not possible for you to provide informed consent – for example, if you are unconscious or confused – the medical staff may decide to go ahead with the procedure if it is in your best interests. This will normally be discussed with close relatives, but the final decision is made by medical staff.

The procedure

You will usually change into a hospital gown before the procedure. In most cases, you will be asked to lie on one side. You will be asked to curl up with your knees pulled up and your chin tucked in so your spine is curved. This will help separate the bones in your spine, allowing the needle to be inserted more easily.

Sitting while bending forwards is an alternative position that is often used when spinal anaesthetic is administered. However, this position is not always suitable.

An antiseptic solution will be applied to the skin at the base of the spine. A local anaesthetic is then used to numb the area of skin where the lumbar puncture needle will be inserted. 

There is usually little in the way of discomfort beyond an initial sting when the anaesthetic is injected. This is followed by a sensation of pressure when the anaesthetic is injected a little deeper.

If a child needs to have a lumbar puncture, the medication may be given beforehand to help them relax and keep them calm.

The doctor will insert a special spinal needle in between the bones at the base of the spine. The needle is passed into the spinal canal, penetrating the layer that contains the cerebrospinal fluid (CSF).

You may be aware of a pushing sensation at this point and you may experience some pain. Tell your doctor if this becomes too uncomfortable.

Occasionally you may feel a sudden, sharp sensation in one of your legs if the needle tip touches one of the nerves within the spinal canal. This is only a brief pain and will indicate to the doctor that the direction of the needle needs to be adjusted.

Once the needle is in the correct position, the CSF will begin to drip out. Usually the CSF pressure is then measured. To do this, the doctor will attach a length of plastic piping to the needle and note how far up the tube the CSF rises. This is called manometry.

You may be asked to cough or strain down while this is being done, and the doctor may also press gently on one side of your neck to check that the CSF can pass freely between the head and the spinal canal. 

Following manometry, samples of CSF are usually collected in sterile containers. In most cases, only a small volume is needed for laboratory testing, but a larger volume may be removed if the doctor wishes to reduce the pressure within the head.

Once the procedure is complete, the needle will be removed and a small plaster will be put on the skin. The whole procedure usually takes about 30-45 minutes in most cases.

After the procedure

After having a lumbar puncture, you may be asked to lie flat for a period of time. It is thought this may reduce the likelihood of a post-lumbar puncture headache developing.

However, many headaches don't develop for at least several hours, and often not until the following day. 

Most people undergoing non-emergency lumbar puncture on a day-case basis will be allowed to return home as soon as possible, where they can treat any headache with bed rest, fluids and simple painkillers. The skin plaster can be removed the following day. 

When you can return to normal activities depends on how severe any post-lumbar puncture headache is and how long it lasts. 

You can usually return to normal activities, including work or school and driving, once you can get up and about without experiencing a troublesome headache.

However, it is usually best to avoid sports and strenuous activities for at least a week as a precaution.

Getting the results

Your doctor can tell you the results of pressure measurements straight away and explain what they mean. The results of some laboratory tests, such as looking for signs of inflammation or checking for evidence of bleeding into the head, are available quite quickly – in the case of a medical emergency, within a couple of hours. 

Other tests, such as identifying the bacteria that causes meningitis, are more complex and may take at least 48 hours to complete. More specialised tests may take several weeks before the results become available.