In the average adult, the skull encloses a total volume of 1475 mL, which includes brain parenchyma (~80%), blood (~10%), and CSF (~10%).26 CSF is produced at a rate of 20 mL/h, for a total of 500 mL/d, by the choroid plexus. CSF is reabsorbed across the arachnoid villi of the superior sagittal sinuses, which act as one-way valves into the venous circulation. Based on the Monro-Kellie hypothesis, the sum of the volumes of brain, CSF, and intracranial blood remains constant. This means that an increase in one will result in a decrease in one or both of the remaining two.27
CSF pressure depends on age, body posture, and clinical conditions. The normal CSF pressure in healthy adults in the horizontal position is normally 7 to 15 mm Hg.28 The spinal cord normally ends at the inferior border of L1 or the superior border of L2. The needle should be inserted into L3/L4 or L4/L5 interspinous spaces. A direct line connecting the two superior iliac crests intersects the midline at the fourth lumbar vertebral body (Figure 102–1) and this allows the clinician to identify L3/L4 and L4/L5 interspinous spaces.
Prior to performing an LP, explain the risks and benefits of the procedure to the patient and obtain an informed consent. The operator should wash their hands thoroughly and conduct a time-out at the bedside before starting the procedure. The standard prepackaged LP tray (Figure 102–2) typically includes antiseptic swab sticks, a sterile drape, 1% lidocaine solution, a syringe, needles for anesthetic (27 and 22 gauge), a spinal needle with stylet, a manometer, extension tubing, a 3-way stopcock, four collection tubes, gauze, and bandage. The operator will also need sterile gloves of the appropriate size and a face mask. Before starting the procedure, place the tray where the operator can access it without difficulty. A standard-point Quincke cutting spinal needle is most commonly supplied with the kit, but some physicians prefer to use atraumatic (noncutting) needles such as a Sprotte needle or a Whitacre needle to minimize the risk of post-LP headache (PLPH), also known as postdural puncture headache (PDPH).
Prepackaged lumbar puncture tray.
An LP can be performed with the patient in either lateral decubitus or sitting in the upright position. If opening pressure needs to be measured, it is better to position the patient in the lateral decubitus position because it allows for a more accurate measurement. The patient should lie on one side, pull the knees up to the chest, and flex the head toward the knees as much as possible. Placing a pillow under the head helps keep the head in line with the vertebral axis. Ensure that the top shoulder and hip are positioned directly above their bottom counterparts. If LP will be performed while the patient is sitting in the upright position, the patient needs to sit on the side of the bed. The patient should hunch over with the head faced down on a pillow atop a steady bedside table. The arching back will widen the intervertebral spaces. A stool can be used to support the patient’s feet as hip flexion in the sitting position optimizes interspinous space width.29 After positioning the patient, palpate the superior iliac crests again and identify the L3/L4 or L4/L5 interspace. A visual target for the needle insertion site can be made by a skin marker or by making an indentation with gentle pressure using the hub end of a needle sheath or cap of a pen.
Clean the patient’s back with povidone-iodine. It should be applied in a circular motion while starting at the L3/L4 interspace and moving outward with each motion. Place a sterile drape with an opening over the puncture site on the patient and frame the workspace. Place another sterile drape between the patient’s hip and the bed. In adults, LP is normally performed under local anesthesia using 1% lidocaine. Sedation may be necessary to facilitate the procedure for anxious or combative patients. The local anesthesia is injected subcutaneously using a 27-gauge needle, making a wheal. A longer 22-gauge needle is then used to anesthetize the deeper subcutaneous tissues. Aspirate after each advancement of the needle to make sure that the needle is not in a blood vessel. As anesthesia is taking effect, assemble the manometer with a 3-way stopcock and prepare the CSF collection tubes.
A 22-gauge spinal needle is most commonly used in adults. Hold the needle between both the thumbs and index fingers, insert a spinal needle with a stylet in the midline and within the median plane. Staying in the median plane will help avoid damage to the nerve roots. Orient the needle rostrally at a 15° angle as if aiming toward the umbilicus. The bevel of the needle should be parallel to the long axis of the spine, as this will minimize trauma to the dural fibers, which run parallel to the spinal axis. The needle is advanced through the skin, fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura, arachnoid, until it reaches the subarachnoid space (Figure 102–3). Continue advancing the needle with the stylet in place until resistance is felt. A “pop” or reduction in resistance is often felt as the needle passes through ligamentum flavum. Remove the stylet to allow for release of CSF. If no CSF is seen, reinsert the stylet, advance the needle slightly further, and reassess. If bone is encountered, reassess the patient’s position and bony landmarks, and ensure the needle is midline. Withdraw the needle into subcutaneous tissue, and reinsert at a slightly modified angle, assessing for the intravertebral space.
Midsagittal section of the spinal column with a lumbar puncture needle in place.
Upon confirming the return of CSF, quickly attach a 3-way stopcock to the needle hub using the extension tubing. Ensure to keep the “zero” mark of the manometer at the level of the spinal needle. If the patient is in the lateral decubitus position, ask the patient to relax by slowly extending the neck and legs. Turn the stopcock to allow CSF to flow up the manometer. Opening pressure is measured once the CSF column has leveled out in the manometer. Record the opening pressure and drain the CSF in the manometer into CSF collection tube 1. After removing the manometer, CSF is then serially collected in sequential tubes. Typically a total of 8 to 15 mL of CSF is removed during a routine LP. More than 15 mL may be removed when special studies are required, such as mycobacteria cultures or cytology. If the opening pressure is elevated, keep the manometer on so that a closing pressure can be measured. When completed, replace the stylet into the needle hub and withdraw the needle. Once the needle is removed, place gauze over the LP site and cover with a bandage. Bed rest after LP is frequently recommended, however, it does not prevent the onset of PDPH regardless of the duration of rest, or the body or head positions of the patient.30 Similarly, additional fluid intake does not seem to have a preventative effect on the onset of headaches.30
Bony landmarks may be difficult to palpate in obese patients as well as patients with generalized edema or scoliosis. LP with imaging guidance may be performed using fluoroscopy or ultrasound. Fluoroscopy-guided LP is performed under real-time continuous x-ray imaging. Fluoroscopy improves success rate, however, it may not be ideal in some situations because it requires a radiologist to perform the procedure, use of radiation, and transportation of potentially critically ill patients.
Imaging guidance may also be provided with ultrasound, which is noninvasive and readily available at the bedside. It is commonly used by critical care physicians for diagnostic evaluation and procedural assistance. In patients with poorly palpable spinal landmarks, ultrasound successfully identified relevant structures in 76% of cases.31 Ultrasound-guided LPs also reduce the risk of a failed or traumatic procedure, the number of needle insertions, and redirections compared to those performed without imaging.32
As the interspaces are small, direct ultrasound guidance can be technically challenging. We will describe a “mark-and-go” technique. An ultrasound-guided LP can be performed with the patient in either lateral decubitus or sitting in the upright position. A linear (high-frequency) probe works well for most patients, allowing for visualization of relatively superficial structures. A curvilinear (low-frequency) probe may be required for patients where deeper visualization is required. In an ultrasound-guided lumbar spinal imaging, the transverse and the longitudinal views are used. The transverse view is obtained by placing the probe perpendicular to the spinal column at the level of the iliac crests. This view is used to determine a midline by identifying the spinous processes. The spinous process appears as a small crescent-shaped hyperechoic (bright) structure with associated posterior hypoechoic (dark) acoustic shadow (Figure 102–4A). Once the spinous process is identified, slide the probe to center the spinous process on the screen. Mark the midline at the midpoint of the probe, both above and below the transducer. Connect these two marks, and this line represents the anatomic midline of the spine. The longitudinal view is acquired next and is used to determine the spinal interspace. This view is obtained by placing the probe parallel to the spinal column. Starting at the superior border of natal cleft, slide the probe in a cephalad direction, while keeping it in the midline, to identify the sacrum and then the spinous processes. The sacrum appears as a continuous hyperechoic band while the spinous process appears as individual hyperechoic crescent-shaped structures (Figure 102–4B). The area between the sacrum and the lowest lumbar spinous process is the L5/S1 intervertebral space. Slide the probe in the cephalad direction, along the spine, to identify L4/L5 and L3/L4 interspinous space. The interspinous space appears as a hypoechoic gray interspace between the two hyperechoic convexities. The ligamentum flavum appears at the base between the 2 vertebrae. Use the depth indicator to measure the distance between the skin and the ligamentum flavum. This approximates the needle length required to enter the subarachnoid space. Once the L4/L5 interspinous space is identified, center this interspinous space on the screen and mark the midpoint of the probe on both sides of the transducer. Move the probe and connect the two lines. The intersection of this line and the previously identified midline represents the optimal needle insertion site. It is important that patients maintain their body position throughout the ultrasound imaging and the LP so that the relationship between the labeled surface marks and the underlying structures is not altered. Cleanse the skin and perform the remainder of the procedure in the usual fashion as described previously.
A. Transverse ultrasound view of lumbar spine. Crescent-shaped white line (arrow) represents the spinous process. Shadow is cast by the spinous process and this identifies the midline of the spine. B. Longitudinal ultrasound view of lumbar spine. Crescent-shaped white lines (arrows) represent the spinous processes. The gap between the white lines represents the interspinous space.