Lumbar puncture: A procedure in which cerebrospinal fluid is removed from the spinal canal for diagnostic testing or treatment. Abbreviated LP. The patient usually lies sideways for the procedure, although LPs in infants are often done upright. After local anesthesia is injected into the small of the back (the lumbar area), a needle is inserted between two vertebrae and into the spinal canal. Spinal fluid pressure can then be measured, and cerebrospinal fluid can be removed for testing. LP is particularly helpful in the diagnosis of inflammatory diseases of the central nervous system (CNS), especially meningitis and other infections. It can also provide clues to the diagnosis of stroke, spinal cord tumor, and cancer in the CNS. An LP can also be done for therapeutic purposes, as a way of administering antibiotics, cancer drugs, or anesthetic agents into the spinal canal. Spinal fluid is sometimes removed via LP to decrease spinal fluid pressure in patients with conditions such as normal-pressure hydrocephalus or benign intracranial hypertension. Risks related to LP include headache, brain herniation, bleeding, and infection. These complications are uncommon, with the exception of headache, which can appear up to a day after LP. Headaches are less likely to occur if the patient remains lying flat for 1 to 3 hours after the procedure. Also known as spinal tap, spinal puncture, thecal puncture, and rachiocentesis.