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). The bevel of the needle is kept parallel to the dural fibers, which run cephalad to caudad, to minimize the incidence of post–dural puncture headache (PDPH). The needle is advanced slowly while controlling it at the skin with the nondominant hand. Angling the needle slightly cephalad may be required because of the orientation of the spinous processes. The widely discussed “pop” of the needle penetrating the dura is not always felt; after traversing the ligamentum flavum, removal of the introducer every few millimeters of advancement may identify CSF. It is important to keep the orientation of the needle in the midline to avoid nerve roots as they exit the spinal canal.

Fig. 105.4 Lumbar puncture procedure showing insertion of the needle.

Once CSF is observed, the manometer is attached to measure OP (Fig. 105.5). The patient’s legs are straightened while the patient is lying on the side, and the EP waits for CSF to equilibrate in the manometer column (slight respiratory variation will be noted) (Fig. 105.6). The fluid meniscus is the OP, which in the absence of obstructive hydrocephalus is equivalent to ICP.

Fig. 105.5 Lumbar puncture procedure showing measurement of intracranial pressure.

Fig. 105.6 Lumbar puncture procedure showing measurement of intracranial pressure.

One to 2 mL of CSF is placed into each of four tubes, the introducer stylet is replaced in the needle, the needle is removed from the patient’s back, and pressure is briefly applied over the puncture site with a piece of gauze. Residual skin preparation solution is washed off and a bandage applied over the puncture site. At this point the patient may be returned to any position of comfort.

Complications

PDPH is the most common complication associated with LP, with a rough incidence of 5% to 40%. Factors associated with PDPH include the technique and the type and gauge of needle used.² PDPH develops about 2 days after the performance of a dural puncture, is worse with standing, and is partially relieved by lying down. The exact cause of this complication is uncertain, but it possibly due to continued leakage of CSF through the dural rent created by the needle. The headache is typically self-limited but can last up to a week and be fairly debilitating.

Several strategies can be used to reduce the incidence of PDPH, including orientation of the bevel of sharp-tipped needles parallel to the dural fibers to avoid cross-cutting of the fibers, the use of atraumatic blunt-tipped spinal needles (Pajunk, Sprotte) with a noncutting tip and side port, and reinsertion of the introducer into the noncutting needle before removal of the needle.^3–5 The patient’s position following dural puncture or the periprocedural use of intravenous (IV) fluids is not associated with the development or prevention of PDPH. PDPH can be treated by reassurance and administration of IV fluids, analgesics, and IV caffeine. Severe headaches may require the placement of a blood patch, which is curative in 80% to 90% of cases.

Other complications of LP are less common, but potentially more serious. Development of a spinal epidural hematoma is more common in the setting of anticoagulation, platelet disorders, thrombocytopenia, and hypocoagulable conditions and may represent a surgical emergency. As with all invasive procedures, infections such as vertebral osteomyelitis, meningitis, or abscess can be minimized by observing strict sterile technique. In procedures performed at the proper level (i.e., L3-L4 or lower in an adult and L4-L5 or lower in a child), cauda equina or nerve root injury is not a common complication.

Procedure

Shunt aspiration is a relatively simple procedure that involves palpation of the reservoir under the scalp as a landmark. Once the landmark is identified, the skin is prepared in sterile fashion. Absolute sterility is imperative because the procedure violates the sterile space of the CNS through a retained foreign body (i.e., the shunt) and infection can result.

The location of the reservoir is subcutaneous, so its depth below the skin and therefore the depth of needle penetration should be only the thickness of the scalp. Once sterility is ensured, a 23- or 25-gauge butterfly needle is introduced into the reservoir, with the bevel of the needle contained within the reservoir. If desired, a manometer can be connected to the butterfly needle so that pressure within the CNS can be assessed. If so, a larger-gauge needle such as a 23 gauge may be needed to produce more accurate results. CSF is then aspirated for standard CSF analysis as discussed previously.

Complications

The most common complication of VP shunt aspiration is infection, which can potentially result in meningitis or encephalitis. Other complications include subdural hemorrhage from overly aggressive CSF decompression, bleeding or persistent CSF leakage from the site of aspiration, and disruption of the valve by a misplaced attempt at aspiration.