A lumbar puncture, also called a spinal tap, is a common medical procedure that involves the insertion of a needle into the spinal canal to obtain a sample of cerebrospinal fluid (CSF) for examination. CSF is the fluid which surrounds the brain and spinal cord.
During a lumbar puncture, a needle is inserted into the spinal canal through the lower back or lumbar region to collect samples of CSF and to measure the pressure in the fluid. The samples are studied for colour and clarity, blood cell counts and other substances such as protein and glucose. Some of the CSF fluid samples may be put into a special culture cup to see if bacteria or fungi grow, indicating the presence of infection.
Lumbar Puncture Technique
Lumbar puncture is performed in the spaces between the lumbar vertebrae located in the lower back, usually at the L4-L5 level. In unusual circumstances, the spinal tap can be done at higher levels. The spinal cord typically ends at L1 level in adults and slightly lower in children.
Lumbar punctures are most commonly performed to test the cerebrospinal fluid to detect or rule out suspected diseases or conditions.
- Diagnose infection such as meningitis, inflammation, cancer, or bleeding in the area around the brain or spinal cord.
- Diagnose diseases of the brain and spinal cord, such as multiple sclerosis (MS) or Guillain-Barré syndrome.
- Assess and measure the pressure of cerebrospinal fluid (CSF) in the space surrounding the spinal cord. Pressure above normal can cause problems.
- Inject anaesthetics or chemotherapeutic agents into the CSF to treat leukaemia and other types of cancer of the central nervous system.
- Insert dye in the CSF to act as a contrast for special X-ray tests enabling the spinal cord and fluid to appear clearer on the test.
The most common reason for a lumbar puncture is to assess cerebrospinal fluid for suspected meningitis, a very serious infection that is highly treatable when diagnosed. The test may also be ordered to diagnose subarachnoid haemorrhage, benign intracranial hypertension, hydrocephalus, among others.
How it is done?
The patient will be positioned with the back curved out so the spaces between the vertebrae are as wide as possible. This allows the doctor to easily find the spaces between the lower lumbar bones where the needle will be inserted. Older children may be asked to either sit on an exam table while leaning over with their head on a pillow or lie on their side. Infants and younger children are usually positioned on their sides with their knees under their chin.
The doctor marks the lower back with a pen where the puncture will occur. The area is cleaned with a special soap and draped with sterile towels. A numbing medicine (local anaesthetic) is applied to the skin.
Then a long, thin needle is inserted into the spinal canal. It has a hollow core, and inside the hollow core is a "stylet," another type of thin needle that acts like a plug. When the spinal needle is inserted into the lower lumbar area, the stylet is carefully removed, which allows the cerebrospinal fluid to drip out into the collection tubes.
After the CSF sample is collected, the needle is withdrawn and a small bandage is placed on the site. Collected samples are sent to a lab for analysis and testing.
What are the Risks?
A lumbar puncture is considered a safe procedure with minimal risks. Significant risks include the following:
- In some cases, a leak of cerebrospinal fluid (CSF) may develop after a lumbar puncture causing a headache. Drinking plenty of fluids and lying down for a few hours after the procedure can help prevent headaches.
- About 1 in 1,000 people who have a lumbar puncture have a minor nerve injury.
- There is also a small chance of infection of the CSF (meningitis), bleeding inside the spinal canal, or damage to the cartilage between the vertebrae.
A lumbar puncture may cause serious problems for people who have high pressure in the brain caused by a tumour, a pocket of infection in the brain (abscess), or major bleeding inside the brain.