Lumbar Puncture Technique

Wearing nonsterile gloves, locate the L3-L4 interspace by palpating the right and left posterior superior iliac crests and moving the fingers medially toward the spine (see the image below). Palpate that interspace (L3-L4), the interspace above (L2-L3), and the interspace below (L4-L5) to find the widest space. Mark the entry site with a thumbnail or a marker. To help open the interlaminar spaces, ask the patient to practice pushing the entry site area out toward the practitioner.

Open the spinal tray, change to sterile gloves, and prepare the equipment. Open the numbered plastic tubes, and place them upright (see the image below). Assemble the stopcock on the manometer, and draw the lidocaine into the 10-mL syringe.

Use the skin swabs and antiseptic solution to clean the skin in a circular fashion, starting at the L3-L4 interspace and moving outward to include at least 1 interspace above and 1 below (see the video below). Just before applying the skin swabs, warn the patient that the solution is very cold; application of an unexpectedly cold solution can be unnerving for the patient.

Place a sterile drape below the patient and a fenestrated drape on the patient (see the video below). Most spinal trays contain fenestrated drapes with an adhesive tape that keeps the drape in place.

Use the 10-mL syringe to administer a local anesthetic (see the video below). Raise a skin wheal using the 25-gauge needle, then switch to the longer 20-gauge needle to anesthetize the deeper tissue. Insert the needle all the way to the hub, aspirate to confirm that the needle is not in a blood vessel, and then inject a small amount as the needle is withdrawn a few centimeters. Continue this process above, below, and to the sides very slightly (using the same puncture site).

This process anesthetizes the entire immediate area so that if redirection of the spinal needle is necessary, the area will still be anesthetized. For this reason, a 10-mL syringe may be more beneficial than the usual 3-mL syringe supplied with the standard lumbar puncture kit. The 20-gauge needle can also be used as a guide for the general direction of the spinal needle. In other words, the best direction in which to aim the spinal needle can be confirmed if the 20-gauge needle encounters bone in one direction but not in another.

Next, stabilize the 20- or 22-gauge needle with the index fingers, and advance it through the skin wheal using the thumbs (see the video below). Orient the bevel parallel to the longitudinal dural fibers to increase the chances that the needle will separate the fibers rather than cut them; in the lateral recumbent position, the bevel should face up, and in the sitting position, it should face to one side or the other.

Insert the needle at a slightly cephalad angle, directing it toward the umbilicus. Advance the needle slowly but smoothly. Occasionally, a characteristic “pop” is felt when the needle penetrates the dura. Otherwise, the stylet should be withdrawn after approximately 4-5 cm and observed for fluid return. If no fluid is returned, replace the stylet, advance or withdraw the needle a few millimeters, and recheck for fluid return. Continue this process until fluid is successfully returned.

For measurement of the opening pressure, the patient must be in the lateral recumbent position. After fluid is returned from the needle, attach the manometer through the stopcock, and note the height of the fluid column. The patient’s legs should be straightened during the measurement of the open pressure, or a falsely elevated pressure will be obtained (see the video below).

Collect at least 10 drops of cerebrospinal fluid (CSF) in each of the 4 plastic tubes, starting with tube 1. If possible, the CSF that is in the manometer should be used for tube 1. If the CSF flow is too slow, ask the patient to cough or bear down (as in the Valsalva maneuver), or ask an assistant to press intermittently on the patient’s abdomen to increase the flow. Alternatively, the needle can be rotated 90° so that the bevel faces cephalad.

Replace the stylet, and remove the needle (see the video below). Clean off the skin preparation solution. Apply a sterile dressing, and place the patient in the supine position.

If the CSF has been collected under sterile conditions, microbiologic studies can now be performed. Stains, cultures, and immunoglobulin titers may be obtained; the last are of special importance with diseases in which peripheral manifestations fade while central nervous system (CNS) symptoms persist (eg, syphilis and Lyme disease). ^{[1, 2, 3]}

Different institutions have different protocols for the studies performed on the various CSF tubes. The classic approach is to send the 4 CSF tubes for the following studies:

• Tube 1 - Cell count and differential

• Tube 2 - Glucose and protein levels

• Tube 3 - Gram stain, culture and sensitivity (C&S)

• Tube 4 - Cell count and differential

At some institutions, only 3 tubes are sent for analysis, and tube 4 is reserved for special studies when indicated. In this approach, the following studies are done:

• Tube 1 - Protein and glucose levels

• Tube 2 - Gram stain, C&S

• Tube 3 - Cell count and differential

When indicated, viral titers or cultures, Venereal Disease Research Laboratory (VDRL) tests, Cryptococcus antigen assays, India ink stains, angiotensin-converting enzyme (ACE) levels, or other studies are ordered. Additional tests may be warranted, depending on the clinical situation. All specimens should be taken to the laboratory promptly to prevent hemolysis and specimen misplacement.

Separate specimens should be sent for microscopic study and for centrifugation. The latter must be done promptly because red blood cells (RBCs) hemolyze within a few hours. The lymphocyte count in normal CSF may be as high as 5/µL.

Cytologic assessment

A larger-than-usual number of white blood cells (WBCs) suggests an infection or, more rarely, leukemic infiltration. Although bacterial infections are traditionally associated with a preponderance of polymorphonuclear leukocytes (PMNs), many cases of viral meningitis and encephalitis also show a high percentage of PMNs in the acute phase of the illness (when most lumbar punctures are done). In addition, inflammation from any source (eg, CNS vasculitis) can raise the WBC count.

A traumatic tap, of course, introduces WBCs and RBCs into the CSF (see Complications). An approximation of 1 WBC for every 1000 RBCs can be made, though a repeat tap may be preferable. Although no normal value for RBCs in the CSF is known, an occasional RBC may be incident to the tap itself.

Multiple lumbar puncture examinations may be required in testing for leptomeningeal malignancies. At least 3 negative cytologic evaluations (ie, 3 separate samplings) are required to rule out leptomeningeal malignancy (eg, leptomeningeal carcinomatosis).

Protein assessment

Assessment of CSF protein level, though nonspecific, can be a clue to otherwise unsuspected neurologic disease. The high protein levels in demyelinating polyneuropathies, or postinfectious states, can be informative. A traumatic tap can introduce protein into the CSF. An approximation of 1 mg of protein for every 750 RBCs may be used, but a repeat tap is preferable.

Glucose assessment

The CSF glucose level normally approximates 60% of the peripheral blood glucose level at the time of the tap. A simultaneous measurement of blood glucose (especially if the CSF glucose level is likely to be low) is recommended.

A low CSF glucose level is usually associated with bacterial infection (probably due to enzymatic inhibition rather than to actual bacterial consumption of the glucose). This finding is also seen in tumor infiltration and may be one of the hallmarks of meningeal carcinomatosis, even with negative cytologic findings. A high CSF glucose level has no specific diagnostic significance and is most often spillover from an elevated blood glucose level.

Xanthochromia

The best way of distinguishing RBCs related to intracranial bleeding is to examine the centrifuged supernatant CSF for xanthochromia (yellow color). Although xanthochromia can be confirmed visually, it is more accurately identified and quantified in the laboratory.

Although xanthochromia can be produced by spillover from a very high serum bilirubin level (> 15 mg/dL), patients with severe hyperbilirubinemia (eg, from jaundice or known liver disease). usually have been identified before lumbar puncture. With this exception, xanthochromia in a freshly spun specimen is evidence of preexistent blood in the subarachnoid space. However, it should be remembered that an extremely high CSF protein level, as seen in lumbar punctures below a complete spinal block, also renders the fluid xanthochromic, though without RBCs.

Xanthochromia can persist for as long as several weeks after a subarachnoid hemorrhage (SAH). Thus, it has greater diagnostic sensitivity than computed tomography (CT) of the head without contrast, especially if the SAH occurred more than 3-4 days before presentation. Patients with aneurysmal leaks (eg, sentinel hemorrhages) may present days after the onset of headache, and this increases the likelihood of a false-negative head CT scan.

In some cases, the CSF may be another color that strongly suggests a diagnosis. For example, pseudomonal meningitis may be associated with bright-green CSF.

Possible lumbar puncture–related complications include the following ^{[1, 2, 3, 4]} :

• Post–spinal puncture headache

• Bloody tap

• Dry tap

• Infection

• Hemorrhage

• Dysesthesia

• Post–dural puncture cerebral herniation

Post–spinal puncture headache

Headache is the most common complication of lumbar puncture, observed in 20-70% of patients. ^{[21, 22, 23, 24]} It usually begins 24-48 hours after the procedure and is more common in young adults. The probable etiology is continued leakage of CSF from the puncture site. ^[25] The headache is usually fronto-occipital and improves in the supine position.

This condition is usually self-limited (≤7 days) and responds to analgesics and caffeine (300-500 mg every 4-6 hours). Severe cases can be treated with an epidural blood patch performed by an anesthesiologist or a pain specialist. Pencil-tip (Whitacre) needles are associated with a significantly lower incidence of post–spinal puncture headaches than are standard bevel-tip (Quincke) needles. ^[26]

Bloody tap

More than 50% of lumbar punctures are falsely positive for RBCs in the CSF as a result of microtrauma caused by the spinal needle. This is an uncomplicated occurrence in healthy patients with a normal coagulation system.

Dry tap

Dry taps usually result from misplacement of the spinal needle. The most common mistake is a lateral displacement, which can easily be corrected by withdrawing the needle completely, reevaluating the patient’s anatomy, and reinserting the needle in the correct place and at the proper angle. In obese patients, the regular spinal needle might be too short, in which case a longer one should be used.

If the patient is dehydrated, a falsely negative dry tap may be obtained as a result of very low CSF volume and pressure. If this is suspected, attempt to rehydrate the patient before the procedure.

Infection

Cellulitis, skin abscesses, epidural abscesses, spinal abscesses, or diskitis can result from a contaminated spinal needle. Adherence to sterile technique, including gloves, gowns, hair covers, and face masks, as well as thorough skin cleansing and disinfecting, should minimize this risk.

Hemorrhage

Epidural, subdural, and subarachnoid hemorrhage are rare complications that might carry significant morbidity and mortality in coagulopathic patients. Lumbar puncture should be deferred in patients with low platelets counts (< 50,000/µL) or patients with other coagulopathies (eg, hemophilia or supratherapeutic international normalized ratio [INR]) until the abnormality is corrected.

Dysesthesia

Irritation of nerves or nerve roots by the spinal needle can cause different lower-extremity dysesthesias. Withdrawing the needle without replacing the stylet can cause aspiration of a nerve or arachnoid tissue into the epidural space. To prevent this complication, always replace the stylet before moving the needle.

Post–dural puncture cerebral herniation

Cerebral herniation is the most serious complication of a lumbar puncture. It is very rare, and there is considerable debate in the literature regarding whether it is caused by the lumbar puncture or by the underlying disease process. There is increasing evidence that a diagnostic lumbar puncture is safe even in patients with increased intracranial pressure (ICP), such as most patients with meningitis, but there is no firm consensus regarding the safety of lumbar puncture in patients with ICP.

Until further data are available, a reasonable approach would be to avoid lumbar puncture when the disease process has progressed to the point where neurologic findings associated with impending cerebral herniation (deteriorating level of consciousness and brainstem signs including pupillary changes, posturing, irregular respirations, and very recent seizure) are seen. ^{[11, 12]}

Different institutions have different protocols for the studies performed on the cerebrospinal fluid. Typical studies include the following:

• Cell count and differential

• Glucose and protein levels

• Gram stain, culture and sensitivity (C&S)

When indicated, viral titers or cultures, Venereal Disease Research Laboratory (VDRL) tests, Cryptococcus antigen assays, India ink stains, angiotensin-converting enzyme (ACE) levels, or other studies may also be ordered. Additional tests may be warranted, depending on the clinical situation.

Cytologic Studies

A larger-than-usual number of white blood cells (WBCs) suggests an infection or, more rarely, leukemic infiltration. In addition, inflammation from any source can raise the WBC count. A traumatic tap, of course, introduces WBCs and red blood cells (RBCs) into the CSF. An approximation of 1 WBC for every 1000 RBCs can be made, though a repeat tap may be preferable. Although no normal value for RBCs in the CSF is known, an occasional RBC may be incident to the tap itself.

Protein Assessment

Assessment of CSF protein level, though nonspecific, can be a clue to otherwise unsuspected neurologic disease. The high protein levels in demyelinating polyneuropathies, or postinfectious states, can be informative. A traumatic tap can introduce protein into the CSF. An approximation of 1 mg of protein for every 750 RBCs may be used, but a repeat tap is preferable.

Glucose Assessment

The CSF glucose level normally approximates 60% of the peripheral blood glucose level at the time of the tap. A simultaneous measurement of blood glucose (especially if the CSF glucose level is likely to be low) is recommended. A low CSF glucose level is usually associated with bacterial infection but is also seen in tumor infiltration. A high CSF glucose level has no specific diagnostic significance and is most often spillover from an elevated blood glucose level.