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Technique for LP (Lumbar Puncture)           See    Lumbar Puncture    

a. Preparing the patient

(1) A blood glucose should be drawn within 15 minutes of beginning the procedure.

(2) To minimize the patient's anxiety, the procedure is explained to the patient, and all steps are described during the procedure.

(3) Positioning the patient

(4) Preparing the skin

b. Introducing the needle

  1. Have all the equipment for the puncture ready before introducing the needle (i.e., 20-gauge LP needle, stopcock attached to a manometer and appropriately positioned, and three or four sterile tubes with stoppers).
  2. Anesthetize the skin with a small wheal of lidocaine or procaine at the exact site of the proposed puncture. The skin wheal is painful and should be raised slowly with an intradermal injection of 0.1-0.2 ml of anesthetic. A further 0.2-0.5 ml may be injected into deeper layers of the dermis, but there is no need to inject anesthetic deep into the muscle because this is usually more painful than the single passage of a 20-gauge needle.
  3. Introduce a 20-gauge LP needle, with the stylus in place, through the skin in the center of the skin wheal. Direct the bevel of the needle with its flat surface parallel to the long axis of the patient's spine; thus, the needle will spread, rather than cut, the dural fibers that run longitudinally. Direct the needle tip 10-15 degrees cephalad (more or less toward the umbilicus), and introduce the needle slowly but steadily until the needle is felt to pierce a tough membrane or is thought to be near the dura. At this point, the needle is introduced in steps of 2-3 mm, and the stylus is removed between each step to check for CSF return. Once CSF return is obtained, the needle is introduced 1-2 mm farther, and the bevel is turned perpendicular to the long axis of the spine. Care must be taken not to introduce the needle too far because puncture of the venous plexus anterior to the spinal canal is the most common cause of a "traumatic tap."
  4. If CSF return cannot be obtained at the L3-L4 interspace, the L2-L3 or L4-L5 spaces may be tried. After repeated LPs, a patient may develop a persistent CSF leak, keeping CSF pressure so low that it will not flow through a 20-gauge needle unless the patient's head is elevated.
  5. Each time the stylus is withdrawn, be ready to attach the stopcock and manometer to the needle as quickly as possible to minimize unnecessary CSF loss and obtain an accurate opening pressure.

c. Collecting the CSF                  

(1) Measure the CSF pressure by allowing fluid to flow into the manometer. Evidence that the needle is properly positioned in the subarachnoid space includes good respiratory variation of the fluid level in the manometer. The patient should be as relaxed as possible; the legs and hips can be extended slightly from the maximally flexed position when the pressure is measured.

(2) Collect the fluid into at least three sterile tubes. One scheme is as follows:

(3) If the opening pressure is high, there is little advantage in collecting less fluid in an attempt to minimize the risk of herniation. Much of the danger from the LP is from leakage of CSF through the dural defect created by the needle.

(4) In virtually all cases of bacterial meningitis, the CSF pressure is found to be elevated. Thus, the patient should be observed closely (no less often than every 15 minutes for the next 4 hours) and hyperosmolar agents administered if the patient shows evidence of neurologic deterioration.

(5) If the patient's condition deteriorates during the performance of the LP or if the patient is considered to be at a high risk of herniation, the stylet is replaced into the needle and the needle is left in place. Mannitol, 1.0-1.5 g/kg, is infused over 20-30 minutes, and high-dose steroid therapy is begun with dexamethasone (Decadron), 10 mg, given by rapid IV infusion. After the mannitol has been infused, the needle is withdrawn.

d. Withdrawal of the needle.
When the needle is withdrawn, the stylet should not be in place.

e. Examination of the CSF

(1) CSF formula. The CSF formula in bacterial meningitis will be purulent (elevated cell count, predominantly polymorphonuclear leukocytes), with low glucose (<40 mg/dl) and elevated protein (>50 mg/dl) concentrations. However, in some cases the CSF profile can be misleading:

(2) Microscopic examination. The following examinations are performed, as indicated, on the sediment of fresh, centrifuged CSF:

(3) Bacteriologic examination

(4) Special studies

Manual of Neurologic Therapeutics - Martin A. Samuels
KP (Dr. NG  12-1999)