Karla Alexander, age 14, of Erie Pa. for her question

HOW IS A SPINAL TAP DONE?

The spinal cord and the brain float in a liquid called cerebrospinal fluid, abbreviated to CSF. In order to test the CSF, a doctor performs a spinal tap by inserting a long, slender needle through an opening in the lower back area. The pressure of the CSF is measured and a sample is removed for laboratory testing and analysis.

The CSF acts as a cushion between the soft brain and spinal cord matter and the hard bone of the skull and spine, allowing the body to withstand jolts and jars without bruising the soft tissues

The CSF has a very definite composition, and variations in this composition, such as the presence of blocd or pus, can indicate to a doctor the presence of certain diseases or injuries. The pressure of this fluid, as well as its chemical makeup, is important in diagnosis.

Some patients complain of severe headaches lasting as long as three days after a spinal tap, especially when they do not lie flat. Although it seems that this could happen because some of the "cushion" has been removed, usually not enough fluid is removed to cause any problem at all.