In real life, a spinal tap is a test that takes a small sample of spinal fluid for examination by sticking a needle into one of the body’s most sensitive areas. In a spinal tap procedure, a needle is inserted between the bones of the lower spine to “tap” into the liquid, much in the way sap is tapped from a tree.
While unpleasant, the test is usually safe and completed within a short period of time.
Spinal taps can be used to determine the presence of infections like spinal meningitis, bleeding in the brain, nervous system conditions like Guillain-Barre syndrome or multiple sclerosis. They can also be used to insert chemotherapy medications into the spinal fluid.
Cerebrospinal fluid (known by its initials, CSF) is the material the test withdraws. It is a colorless liquid that is used to transmit nutrients from the brain and spinal cord while acting as a protective buffer for both.
PERFORMING THE TEST
A spinal tap takes about 30 minutes to perform. While there may be a small pinching sensation when the needle is inserted, a spinal tap is usually not painful, although it requires that the patient lie still while fluid is withdrawn. The actual tapping procedure takes about five minutes.
Doctors use a thin needle for removing the fluid sample. Its actual length depends on the patient’s build and how many layers must be penetrated. The needle used is hollow and contains what is known as a stylet, an even thinner needle that acts like a plug. Once the needle is inserted, the stylet is removed so the CFS can enter the collection tubes.
After changing into a hospital gown, the patient will be asked to lie on his or her side, knees pulled toward the chest and chin tucked down. This will get the spinal column close to the surface. Sometimes the test is done with the patient sitting up but bent forward.
After the back area is sterilized, a local anesthetic will be injected into the lower spine. Once that takes hold, a spinal needle will be inserted. X-rays are sometimes used for guidance in a process called a fluoroscopy, which uses dye for guidance. About one to 10 millimeters of CFS are usually collected.
Once that is accomplished, the needle will be taken out, the back area re-cleaned, and a bandage put over the needle insertion spot. The patient will usually rest for a short period after the procedure.
OTHER PROCEDURES
Another spinal fluid collection procedure is called a cisternal puncture. In this procedure, a needle is positioned below the occipital bone (the back of the skull). This procedure is always done with a fluoroscopy because of its proximity to the brain stem.
In patients with a brain herniation, a procedure called a ventricular puncture may be used, but it is done rarely and requires an operating room procedure. In this process, a hole is drilled in the skull and the collection needle is inserted directly into a brain ventricle. The CSF may be collected from a tube there.
Although it is performed in an extremely delicate area of the body, the spinal tap procedure is considered to have minimal risks of complications during and after the procedure. Side effects may include a headache, fatigue and a mild backache at the point of entry. All side effects are easily relieved with rest and by drinking fluids after the procedure. There is a minor risk of infection and bleeding, but these occurrences are rare.
INTERPRETING RESULTS
Test results from a spinal tap are provided relatively quickly. But if a doctor is looking for some specific bacteria, the culture is often transported to a lab and examined. Results can take up to 48 hours. If there is an infection suspected, many doctors will start antibiotic treatments while awaiting the final results.
So what do results look like? Normal CSF values include a pressure of 70 to 180 mm H20; a clear, colorless fluid appearance; total protein of 15 to 60 mg/100 ml; gamma globulin of 3-12 percent of the total protein present; CSF glucose levels of 50 to 8- mg/100 ml (or greater than two-thirds of blood sugar level); and CSF cell count of 0 to 5 white blood cells (all mononuclear) and no red blood cells present. Chloride levels should read 110-125 mEq/t.
Note that some laboratories may have varying normal value ranges. Talk to a doctor to interpret specific test results.
If there are abnormal results -- if the spinal fluid is cloudy -- it could be a sign of infection, additional protein, or some other condition. A spinal cord obstruction may produce CSF that looks red or bloody, a sign of internal bleeding. A brown, yellow or orange fluid may signal added protein or previous bleeding issues. In some cases, blood is present from the spinal tap procedure itself.
Higher than normal CSF pressure may be attributed to increased pressure in the skull while decreased CSF pressure may be a sign of a spinal cord tumor, fainting, shock or a diabetic coma.
Higher CSF protein can be also be attributed to blood in the fluid, polyneuritis, diabetes, a tumor, previous injury to the area or an infection. A decreased protein count is a sign that the CSF is being rapidly produced.
Increased glucose levels above normal is a sign of high blood sugar while lower-than-normal readings are indicative of low blood sugar, a bacterial or fungal infection, tuberculosis, or certain types of meningitis.
If there are increased white blood cells in the CSF analysis, it can mean an acute infection, meningitis, the early sign of a chronic illness, a tumor, a potential stroke, or impending multiple sclerosis. Red blood cells in the CSF may indicate bleeding in the spinal fluid or a problem that occurred when the spinal tap was done.
An increased gamma globulin level may be a sign of diseases like multiple sclerosis, neurosyphilis, or Guillain-Barre syndrome. There is also the potential for discovering chronic inflammatory polyneuropathy; dementia, encephalitis, epilepsy, generalized tonic-clonic seizure, hydrocephalus, a pituitary tumor or Reye's syndrome.
While a spinal tap is not an easy or fun procedure to undergo, the odds of surviving it are far greater than the drummer position in the band Spinal Tap (they all met mysterious demises). Talk to a doctor if there are any fears or concerns about what’s on tap.