A lumbar puncture (spinal tap) is carried out by inserting a needle into the lumbar subarachnoid space to withdraw CSF.
The test may be performed to obtain CSF for examination, to measure and reduce CSF pressure, to determine the presence or absence of blood in the CSF, to detect spinal subarachnoid block, and to administer antibiotics intrathecally (into the spinal canal) in certain cases of infection.
The needle is usually inserted into the subarachnoid space between the third and fourth or fourth and fifth lumbar vertebrae. Because the spinal cord divides into a sheaf of nerves at the first lumbar vertebra, insertion of the needle below the level of the third lumbar vertebra prevents puncture of the spinal cord.
A successful lumbar puncture requires that the patient be relaxed; an anxious patient is tense, and this may increase the pressure reading. CSF pressure with the patient in a lateral recumbent position is normally 70 to 200 mm H2O. Pressures of more than 200 mm H2O are considered abnormal. A lumbar puncture may be risky in the presence of an intracranial mass lesion because intracranial pressure is decreased by the removal of CSF, and the brain may herniate downward through the tentorium and the foramen magnum.
Herniation of the intracranial contents, spinal epidural abscess, spinal epidural hematoma, and meningitis are rare but serious complications of lumbar puncture. Other complications include temporary voiding problems, slight elevation of temperature, backache or spasms, and stiffness of the neck.