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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

• (a) Whenever possible, the LP is performed with the patient lying on his or her side with knees and hips flexed. The patient may be left relatively relaxed while being prepped and draped, and only when the physician is prepared to introduce the needle are the patient's hips and back maximally flexed. Care must be taken to avoid airway obstruction when flexing the neck of an infant; risk of respiratory compromise is especially high in infants with cyanotic heart disease.
• (b) The L3-L4 interspace is palpated at the level of the superior iliac crests, and any scoliosis or abnormalities of the spine are noted.
• (c) If the puncture cannot be performed with the patient lying on his or her side, it may be done with the patient in a sitting position, leaning forward onto a table. It is essential that the patient's spine be perfectly vertical. The CSF pressure cannot be measured accurately in this position; however, once puncture of the dura is achieved, the patient can be carefully repositioned in the lateral decubitus position and the opening pressure obtained.

(4) Preparing the skin

• (a) Wash the patient's back with iodine-soaked sponges; begin at the site of the proposed puncture and wash in concentric circles. Wash with iodine 2 or 3 times, and then wash with alcohol-soaked sponges 3 times, taking care to wash off all the iodine.
• (b) To avoid introducing iodine into the subarachnoid space on the LP needle, change gloves before proceeding.
• (c) Cover the patient with a sterile drape.

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:

• Basic studies:
  Tube No. 1: Collect 2 ml for protein and glucose. This tube is centrifuged, and the sediment is used for microscopic examination, as Gram stain & India ink exam for fungus .
  Tube No. 2: Collect 1-2 ml for bacteriologic culture.
  Tube No. 3: Collect 1 ml for cell count  with differential and 1 ml for serology as VDRL.
• Other studies: when suspecting subacute or chronic meningitis, or meningitis in immunocompromised/HIV+ pts, or pts with recent antibiotic Rx:
  Bacterial antigen panel; Cryptococcal antigen, fungal culture, AFB smear & culture (suggest 5-10 mL), & cytology (requires 10 mL of CSF)
• Advanced studies: after discussion with Infectious Disease Specialist
  Herpes virus culture, enterovirus culture, Herpes simplex PCR, encephalitis panel, TB PCR.

(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:

• (a) In a "partially treated" bacterial meningitis.
• (b) Early in overwhelming infection, especially with Streptococcus pneumoniae, where there can be a poor polymorphonuclear response.
• (c) When the patient is markedly leukopenic or immunosuppressed.
• (d) When meningitis is caused by bacteria that induce a lymphocytic rather than neutrophilic CSF profile (e.g., Listeria monocytogenes, Treponema pallidum, Borrelia burgdorferi, Leptospira interrogans).

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

• (a) Gram's stain.
• (b) Acid-fast bacilli (AFB) stain. The greater the volume of CSF examined, the higher the yield of this procedure. Therefore, the CSF sediment may be concentrated by allowing four or five drops of CSF to dry sequentially on one area of a slide.
• (c) India ink preparation is not as good a screening test for Cryptococcus species as is Gram's stain, in which the organisms appear as large gram-positive cocci. When Gram's stain suggests fungus, India ink can be used to identify the cryptococcal capsule. Identification of cryptococcal antigen titers in CSF by latex agglutination is the most sensitive means of detecting cryptococcal meningitis.
• (d) Wet smear for fungi and amebae.
• (e) Examination of polymorphonuclear cells with polarized light to look for keratin fragments. These fragments indicate chemical meningitis, secondary to the spillage of the contents of a dermoid cyst or craniopharyngioma.

(3) Bacteriologic examination

• (a) Routine cultures for bacteria. CSF is smeared on a blood agar plate or a chocolate agar plate or slant or is inoculated into a nutrient broth. It is essential to inoculate all cultures as soon as possible after the LP.
• (b) If indicated, fluid may be cultured for mycobacteria, fungi, and amebae.

(4) Special studies

• (a) Meningitis due to Neisseria meningitidis, Haemophilus influenzae, and S. pneumoniae may be rapidly diagnosed, even in the absence of a positive Gram's stain, by the documentation of the presence of specific bacterial antigens in the CSF. The latex particle agglutination and coagglutination tests have replaced counterimmunoelectrophoresis for rapid detection in CSF.
• (b) Serologic test for syphilis. Peripheral blood serology should be used as a screening test for syphilis and the CSF serology examined only when the peripheral serology is positive or there is clinical suspicion of CNS syphilis.
• (c) Viral isolation studies or viral antibody titers as indicated based on clinical suspicion.
• (d) Immunoassays for fungal antigens. Latex particle agglutination for cryptococcal antigen is more sensitive than Gram's stain or India ink preparation for the diagnosis of cryptococcal meningitis. Complement fixation tests can be obtained from the State Department of Health Laboratory when coccidioidomycosis or histoplasmosis is suspected.

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

2006