Mechanism The brain and spinal cord are enveloped by a layer of cerebrospinal fluid, 125–150 ml in total (in adults) which acts as a shock absorber and provides a medium for the transfer of nutrients and waste products. The majority is produced by the choroid plexus in the brain and circulates from there to other areas, before being reabsorbed into the circulation (predominantly by the arachnoid granulations). The cerebrospinal fluid can be accessed most safely in the lumbar cistern. Below the first or second lumbar vertebrae (L1 or L2) the spinal cord terminates (conus medullaris). Nerves continue down the spine below this, but in a loose bundle of nerve fibers called the cauda equina. There is lower risk with inserting a needle into the spine at the level of the cauda equina because these loose fibers move out of the way of the needle without being damaged. The lumbar cistern extends into the sacrum up to the S2 vertebra. Procedure Illustration depicting<unk>  (spinal tap) Spinal needles used in<unk> . Illustration depicting common positions for<unk>  procedure. The person is usually placed on their side (left more commonly than right). The patient bends the neck so the chin is close to the chest, hunches the back, and brings knees toward the chest. This approximates a fetal position as much as possible. Patients may also sit on a stool and bend their head and shoulders forward. The area around the lower back is prepared using aseptic technique. Once the appropriate location is palpated, local anaesthetic is infiltrated under the skin and then injected along the intended path of the spinal needle. A spinal needle is inserted between the lumbar vertebrae L3/L4, L4/L5 or L5/S1 and pushed in until there is a "give" as it enters the lumbar cistern wherein the ligamentum flavum is housed. The needle is again pushed until there is a second 'give' that indicates the needle is now past the dura mater. The arachnoid membrane and the dura mater exist in flush contact with one another in the living person's spine due to fluid pressure from CSF in the subarachnoid space pushing the arachnoid membrane out towards the dura. Therefore, once the needle has pierced the dura mater it has also traversed the thinner arachnoid membrane. The needle is then in the subarachnoid space. The stylet from the spinal needle is then withdrawn and drops of cerebrospinal fluid are collected. The opening pressure of the cerebrospinal fluid may be taken during this collection by using a simple column manometer. The procedure is ended by withdrawing the needle while placing pressure on the<unk>  site. The spinal level is so selected to avoid spinal injuries. In the past, the patient would lie on their back for at least six hours and be monitored for signs of neurological problems. There is no scientific evidence that this provides any benefit. The technique described is almost identical to that used in spinal anesthesia, except that spinal anesthesia is more often done with the patient in a seated position. The upright seated position is advantageous in that there is less distortion of spinal anatomy which allows for easier withdrawal of fluid. Some practitioners prefer it for<unk>  in obese patients, where lying on their side would cause a scoliosis and unreliable anatomical landmarks. However, opening pressures are notoriously unreliable when measured in the seated position. Therefore, patients will ideally lie on their side if practitioners need to measure opening pressure. Reinsertion of the stylet may decrease the rate of post<unk>  headaches. Although not available in all clinical settings, use of ultrasound is helpful for visualizing the interspinous space and assessing the depth of the spine from the skin. Use of ultrasound reduces the number of needle insertions and redirections, and results in higher rates of successful<unk> . If the procedure is difficult, such as in people with spinal deformities such as scoliosis, it can also be performed under fluoroscopy (under continuous X-ray imaging). Children In children, a sitting flexed position was as successful as lying on the side with respect to obtaining non-traumatic CSF, CSF for culture, and cell count. There was a higher success rate in obtaining CSF in the first attempt in infants younger than 12 months in the sitting flexed position. The spine of an infant at the time of birth differs from the adult spine. The conus medullaris (bottom of the spinal cord) terminates at the level of L1 in adults, but may range in term neonates (newly born babies) from L1–L3 levels. It is important to insert the spinal needle below the conus medullaris at the L3/L4 or L4/L5 interspinous levels. With growth of the spine, the conus typically reaches the adult level (L1) by 2 years of age. The ligamentum flavum and dura mater are not as thick in infants and children as they are in adults. Therefore, it is difficult to assess when the needle passes through them into the subarachnoid space because the characteristic "pop" or "give" may be subtle or nonexistent in the pediatric<unk> . To decrease the chances of inserting the spinal needle too far, some clinicians use the "Cincinnati" method. This method involves removing the stylet of the spinal needle once the needle has advanced through the dermis. After removal of the stylet, the needle is inserted until CSF starts to come out of the needle. Once all of the CSF is collected, the stylet is then reinserted before removal of the needle.
Lumbar puncture
Ultrasound-guided lumbar puncture
Brain biopsy
Hemoencephalography
Cranioscopy
CSF tap test
Meningeal biopsy
Intracranial pressure monitoring