Lumbar Puncture

A lumbar puncture (also called a spinal tap) is a procedure to collect and look at the fluid (cerebrospinal fluid, or CSF) surrounding the brain and spinal cord. During a lumbar puncture, a needle is carefully inserted into the spinal canal low in the back (lumbar area). Samples of CSF are collected.
Evaluation of CSF is essential for the diagnosis of suspected meningeal infection, subarachnoid hemorrhage, leptomeningeal neoplastic disease, and noninfectious meningitis.

Relative contraindications to LP include 

• local skin infection in the lumbar area, 
• suspected spinal cord mass lesion, and 
• a suspected intracranial mass lesion. 

Preparation: 

• Any bleeding diathesis should also be corrected prior to performing LP to prevent the possible occurrence of an epidural hematoma. 
• A functional platelet count >50,000/μL and an international normalized ratio (INR) <1.5 are advisable to perform LP safely.
• In patients with an altered level of consciousness, focal neurologic deficits, or evidence of papilledema, an imaging study should generally be obtained prior to performing LP.

Technique: 

• Proper positioning of the patient is essential; either the lateral decubitus or the sitting position can be used. 
• Most routine LPs should be performed using the lateral decubitus position .The sitting position may be preferable in obese patients. 
• With either position, the patient should be instructed to flex the spine as much as possible. 
• In the lateral decubitus position, the patient assumes a fetal position with the knees flexed toward the abdomen; the shoulders and pelvis should be vertically aligned without forward or backward tilt. 
• In the sitting position, the patient should bend over a bedside table with the head resting on folded arms.
• The entry site for an LP is below the level of the conus medullaris, which extends to L1–L2 in most adults. Thus, either the L3–L4 or L4–L5 interspace can be utilized as the entry site. 
• The posterior superior iliac crest should be identified and the spine palpated at this level. This represents the L3–L4 interspace, with the other interspaces referenced from this landmark. 
• The midpoint of the interspace between the spinous processes represents the entry point for the spinal needle. 
• For elective LPs, topical anesthesia can be achieved by application of lidocaine 4% cream 30 min prior to the procedure. 
• The skin is then prepped and draped in a sterile fashion with the operator observing sterile technique at all times. 
• A small-gauge needle is then used to anesthetize the skin and subcutaneous tissue; this is usually performed with multiple small (0.5–1 mL), serial injections of 1% lidocaine as the needle is progressively advanced. 
• The spinal needle should be introduced perpendicular to the skin in the midline and advanced slowly. The needle stylet can be withdrawn periodically as the spinal needle is advanced to assess if the subarachnoid space has been reached. 
• As the needle enters the subarachnoid space, a “popping” sensation can sometimes be felt. 
• If bone is encountered, the needle should be withdrawn to just below the skin and then redirected more caudally. 
• Once CSF begins to flow, the opening pressure should be measured in the lateral decubitus position with the patient shifted to this position if the procedure was begun in the sitting position. 
• The CSF can then be collected in a series of specimen tubes. 
• A minimum of 10–15 mL of CSF is usually obtained. 
• The stylet is then replaced and the spinal needle removed

Specimen Collection: 

Diagnostic evaluation of CSF is based on the clinical scenario. In general, spinal fluid should always be sent for cell count with differential, protein, and glucose. 

Other studies that can be obtained on CSF include bacterial mycobacterial fungal, viral cultures, polymerase chain reaction (PCR) tests for microorganisms, smears (Gram and acid-fast stains), VDRL, cryptococcal antigen, gamma globulins, oligoclonal bands, and cytology.

Post Procedure:

• The patient is often instructed to lie in the recumbent position for 30–60 min, although this practice does not appear to reduce the risk (estimated at 10–30%) of post-LP headache, caused by a fall in CSF pressure. 
• The post-LP headache is dramatically positional; it begins when the patient sits or stands upright and is relieved upon reclining.
• Bedrest, hydration, and oral analgesics are often helpful; IV caffeine (500 mg in 500-mL saline given over 2 h) is sometimes tried. 
• For post-LP headaches lasting more than a few days, consultation with an anesthesiologist should be considered for placement of an epidural blood patch; this usually provides rapid relief.