A lumbar puncture, or spinal tap, is a common neurological procedure that removes cerebrospinal fluid (CSF) from the spinal canal so a doctor can study the fluid more thoroughly. While neurologists perform this test frequently, the idea of a lumbar puncture makes most people nervous. Here's a few of the questions many people ask.
Why do I need a lumbar puncture?
Lumbar punctures are usually done to run diagnostic tests on the spinal fluid. This helps the doctor better diagnose a disease, or understand how an already diagnosed disease is affecting a person. The cerebrospinal fluid can be tested for infectious diseases like meningitis, inflammatory diseases like multiple sclerosis, and some cancers. In addition, the pressure of the fluid inside the spinal canal can be measured to evaluate for disorders such as pseudotumor cerebri. To measure for any increase in pressure, neurologists may use a gauge known as a manometer during the procedure.
What happens during a lumbar puncture?
The procedure takes about 45 minutes to an hour. You will lay on your side in a fetal position with your knees tucked close to your chest. Alternatively, you may be asked to sit in a hunched position with your back to the neurologist. Your lower back will be cleaned with a sterilizing agent such as iodine, and local anesthetic such as lidocaine will be injected into the skin to numb the area. The injection of the numbing medication can cause a slight burning sensation. Once your back is numb, the neurologist will slip a hollow needle between two backbones into the thecal sac, which is the space containing the CSF. Sometimes the needle has to be repositioned to find the right spot.
Once the needle is in place, the neurologist may measure the pressure using the manometer. The fluid then drips out and is collected in test tubes to be sent to the lab. Once the fluid is collected the needle is removed. A bandage is usually placed over the site of the lumbar puncture. The site may be sore for a day or two after the procedure.
Can I be paralyzed from having a spinal tap? What complications do I need to worry about?
The spinal cord ends a few inches above the spot where the needle is inserted. Nerves branching off the spinal cord dangle loosely down through the thecal sac in what is called the cauda equina ("horse's tail"). Because the needle is inserted well below where the cord ends, there is almost no chance of cord damage or paralysis. Sometimes, the needle may brush against one of the nerves in the cauda equina, but this only causes damage in about 1 out of 1,000 people, and usually heals with time. More commonly you feel a brief electric twinge that goes down your leg.
Between 10 to 30 percent of people who get a lumbar puncture will later develop a headache. While there is some controversy on how best to avoid these headaches, most physicians will recommend good hydration and lying flat for an hour after the lumbar puncture. Caffeine may also help if a headache develops. Sometimes the headache persists for a few days after the lumbar puncture. This may mean that the needle has left a hole in the thecal sac, and fluid is still leaking out. In this case, a procedure called a blood "patch" -- in which your own blood is injected into the thecal sac -- may relieve the headache.
In patients with a mass in the brain such as an abscess or tumor, the pressure change caused by removing spinal fluid can lead to dangerous shifting of brain tissue. A CT or MRI scan of the brain prior to the procedure can rule out any such mass. Other possible complications include infection or bleeding. All of these complications are uncommon given adequate preparation.
How do I prepare for my spinal tap?
No special preparations are needed. A spinal tap may be performed in the doctor's office or at the bedside of a hospitalized patient. However, if you take a blood-thinning medication, it may be stopped prior to the procedure to reduce the risk of bleeding. Also ensure the doctor has answered all of your questions prior to the procedure. This may help relieve anxiety you have about the lumbar puncture.
Does it hurt to get a lumbar puncture?
The discomfort associated with a lumbar puncture seems to vary widely from patient to patient. I've treated an eleven-year old boy who laughed throughout the procedure, and I've seen grown men wince as I was just cleaning the skin. No one will say that a spinal tap is comfortable, but it is usually less painful than imagined. Sometimes the worst part is the pinch felt with the injection of the numbing medicine. As the longer needle is placed to draw the fluid, many people describe more of a deep pressure sensation, and sometimes an electric sensation in the back or moving down a leg. No more than local anesthetic is normally needed to control the pain.
Are there any other ways to get this fluid?
For people with problems such as severe scoliosis, arthritis, or obesity, it may be too difficult for the neurologist to retrieve the CSF at the bedside. In these situations, a lumbar puncture may be done under fluoroscopic guidance with the help of a radiologist. However, in most cases, this is unnecessarily complicated and expensive.
In short, a lumbar puncture, while uncomfortable, is not as bad as most patients fear. While complications can occur, they are very infrequent, and generally can be avoided by using good technique. The benefits of obtaining the fluid for testing usually outweigh the risks and discomfort of the procedure. For more information, ensure all your questions are addressed by your doctor.
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