neurologic disorders
How To Do Lumbar Puncture
Step by Step Description of Procedure
Place the patient in proper position, using an assistant if needed. Identify the needle-insertion site clinically: Palpate the lumbar vertebral spinous processes to identify the one closest to an imaginary line between the top of the superior posterior iliac crests; the closest spinous process is usually L4 (sometimes L3 in women). The insertion point is the depression just caudal to this spinous process (ie, in the L3-L4 interspace for the L4 process). Mark the site with a skin-marking pen. Although the L3-L4 interspace is the usual insertion site, the L4-L5 or L2-L3 interspace is acceptable. For children, apply topical skin anesthetic and allow time for it to take effect. Swab the insertion site with antiseptic solution using a series of expanding concentric circles that reach about 20 cm diameter. Allow the antiseptic solution to dry for at least 1 minute. If iodine orchlorhexidineis used, wipe it off with alcohol to prevent its introduction into the subarachnoid space by the spinal needle. Place sterile equipment on a sterile equipment tray and cover with a sterile drape. Don sterile gloves. If you have any respiratory symptoms, wear a face mask. If isolation protocols are in place, wear a gown, face mask, and cap. Assemble the manometer, stopcock, and short connecting tubing. The short connecting tubing allows some free motion, helping to prevent unexpected movement of the connected apparatus (eg, if the patient unexpectedly moves) from dislodging the needle. Ensure smooth working motion of the stopcock and of the spinal needle and stylet. Place sterile drapes around the site. Place a wheal of anesthetic at the needle-entry site using a 25-gauge needle and then anesthetize deeper in the soft tissues along the anticipated path of needle insertion. Fully seat the stylet in the spinal needle whenever the needle is advanced or withdrawn. Hold a beveled spinal needle with the bevel facing the patient's right or left side (ie, facing up for a patient in the lateral decubitus position). The needle can be held with one hand, but holding the needle with two hands gives better control in case the patient moves or jerks. Palpate the iliac crest and spinous processes to reconfirm the insertion site. Insert the needle pointing toward the umbilicus, roughly 15° cephalad, and advance it in small increments of about 2 to 3 mm. A pop may be palpable as the needle pierces the ligamentum flavum (to enter the epidural space) and sometimes when the needle pierces the dura (to enter the subarachnoid space). In children, such pops are usually less apparent. Often the needle needs to be advanced for much of its length before reaching the subarachnoid space. Remove the stylet between incremental needle advances to check for CSF flow (indicating access to the subarachnoid space); reinsert the stylet before continuing to advance the needle. If the needle meets bone, withdraw it to the subcutaneous level, point it more cephalad and then reinsert it. Make sure the needle is directed toward the spinal canal, and not off to one side. If blood returns with a deep insertion, the needle may have entered the venous plexus on the ventral side of the cord. Withdraw the needle in successive small (eg, 1-mm) steps, checking for CSF return with each step. The needle tip sometimes is blocked by nerve roots or other tissue. If CSF is not obtained despite what seems to be successful penetration of the subarachnoid space, turn the needle 90° and recheck. Once the subarachnoid space is reached, reinsert the stylet and do not advance the needle further. This lumbar puncture is done with the patient in the lateral decubitus position and the lumbar puncture needle inserted at the L3-L4 interspace. Manometry is usually done but can be omitted if patients are critically ill and should be omitted for patients in the sitting position because these measurements are unreliable. Remove the stylet from the spinal needle. Attach the assembled manometer/stopcock/extension tubing to the needle. Hold the manometer level with the spinal needle. Open the stopcock. CSF will rise in the manometer tube to a height representing the opening CSF pressure, which is read on markings on the tube. The CSF meniscus should fluctuate slightly with respirations. Normal height of the fluid column (CSF pressure) is 7 to 18 cm. If the pressure is low, instruct the patient to gradually straighten the legs a bit to improve CSF flow. If there is no variation in pressure with respiration, particularly if the pressure is low, rotate the needle 90° to relieve possible obstruction by a nerve or if the needle is not completely in the subarachnoid space. Never aspirate CSF fluid. If CSF manometry has been done, drain the CSF from the manometer into the first collection tube. When the manometer has emptied, remove the manometer. In numerical sequence, allow about 1 to 2 mL of CSF to drip into each of the 4 collection tubes. Greater volumes (up to 30 to 40 mL) may be useful for some tests, such as detection of acid-fast bacilli, fungi (eg,Cryptococcus neoformans,Coccidioides immitis), or carcinomatous meningitis. Do cell count and differential on the 1st and 3rd tubes so that counts can be compared if red blood cells are present. (A sharp decrease in red blood cell count from the 1st to the 3rd tube is consistent with a traumatic puncture.) Do other tests, including protein and glucose levels, oligoclonal bands (if needed) and microbiologic testing, from the 2nd tube and, if necessary from CSF that remains from the 1st and/or 3rd tube. Consider freezing the 4th tube in case additional, unanticipated studies later become necessary. Check with the lab to see how long it holds specimens, and ask them to hold the sample for a longer period of time if needed. Reinsert the stylet into the needle. Gently withdraw the needle. Apply an adhesive dressing. 