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LUMBAR PUNCTURE (SPINAL TAP, LP)

Click on the small picture to right to see a larger view of LP needle inserted in lumbar region on patient on their left side. Click 'back' in your browser to return.

Definition

Lumbar puncture (LP) is the insertion of a needle into the subarachnoid space (the area under the membrane that surrounds the brain and spinal cord) of the lumbar (lower back) region for diagnostic or therapeutic purposes. This allows access to the cerebrospinal fluid (CSF) in which the brain and spinal cord float.

Although the subarachnoid space can be accessed from other levels, the lumbar region is most often used as it allows the needle to be inserted below the end of the spinal cord.

What's it used for?

Most frequently LP is used to obtain a sample of CSF for testing. This is a critical test for identifying infections such as meningitis and encephalitis and in cases where they are suspected must be carried out quickly. It is also useful for identifying CSF abnormalities in other diseases of the nervous system including multiple sclerosis, peripheral neuropathies and some tumors. 

Preparation

There is no preparation required. If you will be going home after the test you should ensure that you have a ride and do not drive yourself.

The test is usually done with the patient lying on their side, with their knees drawn up and their back arched towards the doctor. This spreads apart the bones between which the needle is passed. Occasionally it is done in the sitting position or under X-ray guidance.

Are there side effects?

There is minor discomfort, but the test is usually tolerated quite well.  Local anesthetic is usually injected before the spinal needle is inserted. 

Despite the inordinate amount of anxiety that people often have towards LP, serious side effects are remarkably rare. The most common side effect is headache which occurs in about 40% of people and begins within 48 hours and resolves spontaneously in  less than 5 days. It is typically better with lying flat and worse while standing. Other side effects that together occur in about 0.3% (1 in 330) of people include the following:

-prolonged headache lasting longer than 7 days (by far the most most common of the rare occurrences on this list)

-infection 

-nerve root irritation, herniation or transecton

-injury to one of the nerves that exit directly from the head (known as cranial neuropathies)

-bleeding (in the head, spine or locally)

-low back pain

-movement of the brain downward if there is abnormally higher pressure in the head than in the spinal canal (known as uncal or tonsillar herniation, reversible tonsillar descent or spinal coning depending on which part moves; extremely rare but very serious)

-fainting

Where is it done?

LPs are most often done in hospitals or clinics.

R. O'Brien MD