Lenworth N. Johnson and Michael A. Meyer

Lumbar puncture (LP) is essential for the diagnosis of pseudotumor cerebri and suspected life-threatening central nervous system infections, such as bacterial meningitis. It also may help diagnose central nervous system inflammatory disease and neoplasia.

PROCEDURE

Initial preparation for LP involves the assessment of existing contraindications such as mass effect from an intracranial lesion, or coagulopathy, which can result in intrathecal bleeding. A computed tomographic (CT) scan or magnetic resonance imaging (MRI) scan should be used to exclude the presence of a large tumor or other mass lesion that could predispose to brain herniation. For patients who recently discontinued heparin or coumadin, a prothrombin time and partial thromboplastin time should be obtained before the LP is performed. The use of antiplatelet agents does not pose an absolute contraindication, but ideally should be discontinued before nonemergent elective LPs.

To obtain an accurate measurement of the opening and closing cerebrospinal fluid (CSF) pressure, the patient should be positioned in the horizontal lateral decubitus position with maximal flexion.

Sterile precaution should be observed. The area for needle insertion is cleansed with alcohol and then iodine. Palpation of the midline spinous processes with sterile gloves confirms the intended landmarks for LP entry at the L3–L4 or L4–L5 level in adults, to ensure that the LP is below the termination of the spinal cord. When performing an LP in a newborn, the needle insertion must be lower, at L4–L5 because the spinal cord terminates at a lower level in newborns than in adults.

If there is no history of allergy to novocaine or related anesthetics, then local anesthesia may be administered as a subcutaneous injection to raise a wheal at the desired site. A larger needle is then used to deliver anesthetic into the deeper paraspinal muscles along the anticipated track of the LP needle. Immediately thereafter, a fine-gauge LP needle containing a stylet is inserted with the needle bevel pointed laterally (upward if the patient is in lateral decubitus) so that it splits, rather than cuts, the longitudinal fibers of the dura. The first give or sudden reduction in resistance to passage of the needle occurs as the needle punctures the ligamentum flavum. The second give in resistance is encountered when the dura is punctured. The stylet of the needle is then slowly removed to see if clear CSF emerges from the needle. If there is no fluid, the needle is slowly rotated 90 degrees toward the cephalad direction. With the stylet in place, the needle may be withdrawn and repositioned, particularly if bone is encountered. Should the patient experience pain or tingling in an extremity, this indicates that the needle is too lateral to the affected side. If the physician is unsuccessful after two or three attempts, then LP under fluoroscopy should be considered.

OPENING PRESSURE