A lumbar puncture (LP), also known as a spinal tap, is a diagnostic and/or therapeutic procedure performed by a doctor. The procedure is performed by inserting a hollow needle into the subarachnoid space in the lumbar area (lower back) of the spinal column. The subarachnoid space is the canal in the spinal column that carries cerebrospinal fluid (CSF) between the brain and the spinal cord.
CSF is a clear fluid that bathes the brain and spinal cord while protecting it, like a cushion, from exterior injury. The fluid is produced and reabsorbed in the brain on a continuous basis. CSF is composed of cells, water, proteins, sugars, and other vital substances that are essential to maintaining equilibrium in the nervous system.
A lumbar puncture may be performed for various reasons. The most common reason is to remove a small amount of CSF for examination and diagnosis of various disorders. CSF is tested for red and white blood cells, protein, glucose (sugar), clarity, color, and the presence of bacteria, viruses, or abnormal cells. Excess CSF may also be removed in patients who have an overproduction or decreased absorption of the fluid.
In addition, a lumbar puncture may be used to measure the pressure of the CSF, which flows freely between the spinal column and the brain. The doctor measures the pressure during a lumbar puncture using a special tube (called a manometer) that is attached to the lumbar puncture needle.
Finally, a lumbar puncture may be performed therapeutically to inject medications directly into the spinal cord. Some medications that may be given via lumbar puncture (intrathecally) include spinal anesthetics before a surgical procedure, contrast dye for X-ray studies (for example, myelography), or chemotherapeutic agents to treat cancer.