Procedure

After explaining the procedure to the patient and obtaining informed consent, the patient is placed on the side on a flat stretcher with a pillow under the head. The procedure is typically started with the patient placed in the fetal position and the person performing the procedure seated facing the patient’s back. Administration of a small dose of an anxiolytic, such as midazolam, is helpful in anxious patients. Depending on the response to the medication, pulse oximetry may be appropriate.

The iliac crest is located and the thumbs used to palpate and identify the spinous processes in the midline of the back. This is roughly the L4-L5 interspace and is an appropriate interspace to use. In adults, the L3-L4 interspace can also be used because both lie below the termination of the spinal cord within the spinal canal. An absorbent towel is placed under the patient, and the patient’s back is cleansed with the skin preparation solution (povidone-iodine or chlorhexidine) by making circular motions to scrub the back in gradually expanding circles, starting at the midline of the L4-L5 interspace and out to the iliac crests and lower thoracic spine; the skin should be allowed to dry between wipes (Figs. 105-1 and 105-2). The procedure may be facilitated by having the patient sit and lean over a bedside table while being stabilized by a helper, but this prevents measurement of OP. In anatomically challenging patients, ultrasonography and fluoroscopic guidance may aid in the identification of landmarks.

Fig. 105.1 Lumbar puncture procedure showing patient preparation.

Fig. 105.2 Lumbar puncture procedure showing identification of the interspace.

Once the patient and physician are in positions of comfort, the landmarks are reassessed and lidocaine is injected subcutaneously with a 27- or 25-gauge needle at the identified interspace to anesthetize the skin (Fig. 105.3). In children, topical application of anesthetic before injection may reduce the initial discomfort. After 1 to 2 minutes the deeper tissues are anesthetized by advancing a longer, 22-gauge needle slowly and injecting lidocaine. Achieving sufficient anesthesia is critical to success of the procedure.

Fig. 105.3 Lumbar puncture procedure showing placement of local anesthetic.

Once anesthesia is achieved, the landmarks are checked yet again. A 22-gauge spinal needle is used to advance into the previously identified space (Fig. 105.4