Lumbar puncture

A lumbar puncture is a medical procedure undertaken to remove and test fluid from the spinal cord. The fluid (cerebrospinal fluid), which is crystal clear and looks like water, surrounds the brain and spinal cord to act as a protective agent as well as to supply nutrition and fight infection. A lumbar puncture is usually performed in a hospital, but it only requires a brief stay of less than a day. The technique is very similar to the procedure of an epidural anesthesia given to women during childbirth. The needle is similar, however in an epidural, anesthetic agent is introduced near the spinal cord, whereas in a lumbar puncture the hollow needle is inserted into the fluid sac surrounding the spinal cord and a sample, usually a teaspoon or two, is taken.

The first known lumbar puncture was performed almost 120 years ago. The reasons why a lumbar puncture is undertaken today are listed below, but because of much more sophisticated ways of veiwing the brain, with MRI scans especially, it is less often indicated in the diagnosis and management of neurologic illness than it was 100 plus years ago.

The patient is often required to lie on his or her side with knees up into the stomach, in the"fetal position." The pressure of the fluid in the spinal cord is often measured. Sometimes, to make it easier, the patient may be required to sit upright and lie forwards. The fluid is sent to a laboratory for analysis to look for any evidence of infection or inflammation or possibly other cells. Normally there should be no cells in the fluid, unless perhaps a small amount of blood was introduced during the procedure.

The most common reasons for a lumbar puncture would be in aid in diagnosis of the following:

• meningitis
• a suspicion of a ruptured blood vessel in the brain (subarachnoid hemorrhage)
• inflammation of the nerve roots as they leave the spinal cord, the most common cause of which is Guillain-Barré syndrome
• multiple sclerosis
• if there is the suspicion of increased pressure in the brain for what ever reason
• evidence that perhaps cancer from somewhere else in the body may have spread to the coverings of the brain (metastatic infiltration)

A lumbar puncture is usually only undertaken after a CT or MRI of the brain is performed to rule out any tumour or swelling of the brain.

Sometimes a lumbar puncture will be undertaken not for diagnosis, but for treatment to introduce chemotherapy or antibiotics into the fluid around the brain. This is not common, but may be neccessary because some drugs do not easily pass from the blood to the cerebrospinal fluid.

Complications
A lumbar puncture is extremely safe, and provided that unexpected raised pressure has been excluded by a CT or MRI scan, and that standard infection control measures and a good technique are adopted, the risk of complications is minimal. Such complications may include, in approximate descending order of occurrence:

(1) post lumbar puncture headache
(2) mild bleeding and hematoma formation (bruising)
(3) minor back pain and occasional tingling and numbness in the buttock and legs
(4) infection possibly around the lumbar puncture site or the spinal canal
(5) herniation of the brain contents into the beginning of the spinal canal, especially in situations in which there is undiagnosed raised pressure within the brain
(6) a very rare late manifestation, often years afterwards, of benign swelling and tumors of the sack around the spinal cord

• Post lumbar puncture headache
The most common complication of a lumbar puncture is a mild headache, usually the next day, although this is by no means universal. Most studies of these risks indicate that about 20% to 25% of patients will experience a headache following the procedure and about the same number will have a mild persistent backache for some days. Neurologists are not entirely confident that they understand the mechanism of this headache; it used to be thought that the cerebrospinal fluid continued to drip out, causing reduced pressure inside the skull. Such a headache is often not present when lying down but is felt as soon as the patient stands upright. The treatment is to spend as much time lying down after the lumbar puncture as possible, and to drink copious amounts of water with simple pain relief.

The American Academy of Neurology recently issued practice guidelines and recommendations based on scientific evidence to help minimize this common and inconvenient side effect. There are 2 major points of recommendation:

• That the procedure be undertaken by someone who has experience in the procedure or by someone who is assisted by an experienced person
• That the thinnest needle possible be used and that it have a smooth pointed end rather than a cutting-edge

These newer needles are called atraumatic lumbar puncture needles. The opening through which the fluid drips is situated on the side of the needle rather than at the very tip; this allows the manufacture of a very thin, tapered, and sharp point that "pushes its way" through the skin and tissues rather than cutting.

Finally, adequate and complete explanation and reassurance that lumbar puncture headache is not serious and will resolve with time is a third important aspect of the management of post lumbar puncture headache.

• Bleeding
The single most important factor in terms of predicting the risk of prolonged bleeding or significant bruising from lumbar puncture is whether or not the patient is taking treatment to thin the blood, especially anticoagulants such as Warfarin. Less commonly there may be an unrecognized problem with the patient's clotting mechanism, for example they may have an abnormally low platelet count or some other congenital disorder that increases the risk of a bleeding tendency. In this situation the neurologist will be faced with discussing the relative risks with the patient and weighing the benefits and indication for the lumbar puncture versus the potential risk. For example, the benefit of the accurate diagnosis of a potentially fatal bacterial meningitis far outweighs the risk of bruising and bleeding if someone had been taking aspirin for headache before the test.

This information was written by Graham Norton FRACP, and is herewith used with permission. It was last reviewed February 1, 2013.

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