Chapter 102: Lumbar Puncture

Percutaneous needle LP was first introduced by Quincke in 1891.¹ Since then, LP has become a fundamental method to access cerebrospinal fluid (CSF) in a variety of clinical settings. In the field of critical care, LP is often used to obtain CSF for analysis and to measure the opening pressure of the subarachnoid space. With the advancement of other diagnostic modalities, especially neuroimaging procedures such as CT scans and magnetic resonance imaging (MRI), the numbers of definite indications for LP have been reduced in recent years. However, analysis of CSF remains essential to the diagnosis of two potentially fatal but treatable conditions, which are CNS infections and SAH in patients with a negative CT scan. CSF analysis should always be correlated with history, physical examination findings, and other diagnostic tests. LP allows clinicians to access CSF in a relatively safe manner in the absence of significant contraindications, although on rare occasions harmful or even serious complications may result. This chapter will review the indications, contraindications, technique, and complications of performing LP in adults.

The primary indication for LP is to diagnose or exclude bacterial, viral, fungal, and parasitic infections of the CNS. LP is also an indispensable step in the exclusion of SAH when there is a strong clinical suspicion of SAH and brain imaging is nondiagnostic. In addition, CSF analysis and CSF pressure provide clinically valuable information in the diagnoses of many other noninfectious neurologic conditions such as CNS malignancies, pseudotumor cerebri, and demyelinating diseases including multiple sclerosis and Guillain-Barré syndrome.

CNS Infection

Meningitis refers to inflammation of the meninges and could be either infectious or noninfectious. Noninfectious causes of meningitis will not be discussed in this chapter. From 2003 to 2007, the United States had an estimated 4100 cases, including 500 deaths, of bacterial meningitis per year.² The annual incidence has declined significantly since the introduction of Haemophilus influenzae type b (Hib) and pneumococcal conjugate vaccines. Despite these vaccines, bacterial meningitis continues to be a serious health threat, as both the morbidity and mortality remain high. The two most common pathogens of bacterial meningitis in adults are Streptococcus pneumoniae and Neisseria meningitidis. Patients with acute bacterial meningitis commonly present with one or more of the following: fever, changes in level of consciousness, nausea or vomiting, headache, and meningeal signs. Thorough physical examinations are certainly important, but the clinical signs and symptoms may be subtle and nonspecific, and should not be used in isolation to rule out potentially life-threatening disease.

Given the high mortality and morbidity of this condition, CSF analysis is often required to diagnose or rule out CNS infection. CSF analysis is the only definitive way to establish the diagnosis and determine the causative pathogen. Therefore, when bacterial meningitis is suspected, LP should be obtained as soon as possible. Early diagnosis with prompt administration of appropriate antibiotics is crucial when managing meningitis. Ideally LP should occur before administration of antibiotics, but it should not delay the antibiotic therapy. While there are contraindications to LP, the decision to forgoing LP should not be undertaken lightly given the seriousness of this condition.

The term encephalitis refers to inflammation of brain parenchyma, most commonly caused by a virus. Viral encephalitis is often characterized by an altered level of consciousness, focal neurologic deficits, seizures, as well as neuropsychiatric symptoms such as psychosis, personality changes, and hallucinations. Clinical manifestations vary depending on the pathogen because different pathogens may affect different areas of the brain. Herpes simplex virus (HSV) 1 is the most common diagnosed pathogen of sporadic viral encephalitis in the western world. In the absence of acyclovir, the mortality for this encephalitis was greater than 70%, and even with acyclovir the 6-month mortality remains high at 14% to 28%.^3,4,5,6 Survivors often suffer from serious neurologic sequelae. Other viruses that cause acute encephalitis include other herpes viruses, rabies virus, arboviruses, enteroviruses, and human immunodeficiency virus (HIV). As with bacterial meningitis, when viral encephalitis is suspected, LP should be performed as soon as possible. Polymerase chain reaction (PCR) can detect HSV DNA in CSF rapidly and reliably with sensitivity similar to that of a brain biopsy.⁷ Brain imaging such as MRI or CT is often obtained before or after LP to support the diagnosis. When there is high suspicion for HSV encephalitis, initiation of acyclovir treatment should not be delayed. Delays, particularly beyond 48 hours, are associated with poor outcomes.^6,8

A patient’s travel history, geographic location, recreational habits, and arthropod or animal exposure may suggest more unusual pathogens. If a CNS infection is suspected in such patients, CSF analysis should be obtained. In addition, immunocompromised patients may not present with classical signs and symptoms of CNS infections. Therefore, CSF analysis should be obtained when CNS infection is suspected in these patients.

Subarachnoid Hemorrhage

SAH refers to extravasation of blood into the subarachnoid space between the arachnoid membrane and the pia. Although SAH is most commonly caused by trauma, approximately 85% of nontraumatic SAH cases are due to a ruptured cerebral aneurysm.⁹ The incidence of spontaneous aneurysmal SAH is approximated to be 27,000 to 30,000 annually in the United States.^10,11 The clinical hallmark of aneurysmal SAH is a sudden, unusually severe headache classically described as the “worst headache of my life.” Headache onset is instantaneous, usually within seconds. Additional associated features include a period of unresponsiveness, changes in level of consciousness, nausea or vomiting, preretinal subhyaloid hemorrhages, and localized neurologic signs. Nuchal rigidity and low back pain are also commonly seen in patients with SAH, however, these usually take hours to develop after the hemorrhage. If SAH is suspected, noncontrast head CT should be obtained first. Extravasated blood appears hyperdense on CT imaging. The sensitivity of head CT for detecting SAH is highest in the first 6 hours after SAH and declines over time. CT performed in neurologically intact patients has a sensitivity of 93% if the CT is performed within 24 hours of headache onset. The sensitivity declines over time to 58% on day 5.^12,13 Even if the CT is performed within 12 hours after the bleed, about 2% of patients with SAH could have false negatives.¹⁴ Delayed diagnosis of SAH leads to delays in treatment and consequently results in worse overall patient outcomes.^15,16 Therefore, LP should be performed in cases when SAH is suspected despite nondiagnostic CT. LP should not be performed if the CT is diagnostic for SAH or if possible contraindications to LP, such as obstructive hydrocephalus or an intracranial lesion causing mass effect are present. The classic CSF findings of SAH are elevated CSF pressure and xanthochromia, which represents hemoglobin degradation products. Xanthochromia is present when an LP is conducted 12 hours after the onset of bleeding and usually persists for 2 weeks.¹⁷ Although xanthochromia may be confirmed visually, spectrophotometry is a more sensitive method to differentiate xanthochromia and traumatic blood. Xanthochromia may not be apparent if LP is performed early in the disease process, because it takes approximately 4 hours to develop. Angiography should be performed and prompt neurosurgical consult should be requested once the diagnosis of SAH is made.

LP is both a diagnostic and a therapeutic procedure. LP may be used to access the intrathecal space for drainage of CSF as a treatment for elevated intracranial pressure (ICP), or for injection of contrast dye during neuroimaging studies such as myelogram. LP facilitates the intrathecal administration of medications such as chemotherapy or spinal anesthesia.

Contraindications to LP include skin or soft tissue infection at the puncture site, acute spinal cord or head trauma, uncorrected severe coagulopathy, and brain shift secondary to a SOL or diffuse cerebral edema. The term “raised ICP” is sometimes used as a contraindication to LP; however, elevated ICP itself does not necessarily incur the risk of herniation after LP, and therefore the term “brain shift” is preferred. Brain shift may end in herniation, and CT should be obtained prior to performing LP in those who are at a higher risk of developing herniation. Determining which adult patients should undergo CT before LP remains somewhat controversial. Physicians should make an attempt to select patients for CT on the basis of clinical findings rather than obtaining routine CT before LP in all patients.^18,19

Patients with a high likelihood of having intracranial pathology should be evaluated with a CT prior to LP. These include, but are not limited to, an immunocompromised state, history of CNS disease, new-onset seizures, papilledema, abnormal level of consciousness, or focal neurologic deficit.^20,21,22 CT features that are suggestive of brain shift and unequal pressures between intracranial compartments include obliteration of the ventricles, effacement of sulci, suprachiasmatic, or basilar cisterns, lateral shift of midline structures, and brain herniation.^18,23,24,25 In addition to these signs, evidence of epidural abscess, ncommunicating hydrocephalus, and large posterior fossa masses preclude an LP.

Other contraindications include coagulopathy. There are no absolute cutoff values that determine when to forgo LP. The decision for emergent LP in such patients should be decided on a case-by-case basis. If time permits and it is clinically feasible, coagulopathy should be corrected before performing LP. In general, an international normalized ratio (INR) of less than 1.4, partial thromboplastin time (PTT) below 50, and a platelet count above 50,000/mm³ are considered safe parameters. Aberrations can often be corrected with fresh frozen plasma (FFP) and/or platelet transfusions.

Anatomy and Physiology

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.

Preparation

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).

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.

Technique

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.

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

Imaging Guidance

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.

PDPH is one of the most common complications following LP, and the incidence of PDPH varies from 1% to 70%.³³ PDPH is thought to be caused by CSF leakage into the paraspinous spaces, which decreases the CSF pressure, and results in traction of the meninges and stretching of the pain-sensitive intracerebral veins. Headaches usually start within 24 to 48 hours after the procedure and resolve spontaneously within a few days. PDPH could be mild or disabling, usually exacerbated by an upright position, and relieved in the supine position. The use of smaller-gauge needles and atraumatic needles, stylet reinsertion before withdrawing, and parallel orientation of the bevel to the dural fibers have all been reported to reduce the incidence of PDPH.^{34,35,36,37,38,39} Despite common belief, bed rest and hydration post-LP do not reduce the incidence of PDPH.³⁰ Most PDPH can be managed by supportive care alone. Epidural blood patch is considered an effective treatment for persistent PDPH associated with a continued CSF leak.⁴⁰ Another relatively common complication of LP is back pain, which occurs in about 35% of patients and also resolves spontaneously after several days.⁴¹ Some patients may feel transient electric shock-like pain or dysesthesias during the procedure. The pain is usually transient, but permanent motor and sensory loss can result in very rare cases.⁴¹

The most serious complication of LP is brain herniation. Removal of CSF via LP usually results in a mild, transient reduction of lumbar CSF pressure that is rapidly communicated throughout the subarachnoid space. In the presence of brain shift, there is a relative pressure gradient with downward displacement of the brain stem and cerebrum. LP will increase this pressure gradient and potentially precipitate brain herniation. Determination of LP as the cause of brain herniation is difficult, and the exact incidence is unknown. Patients with a history of CNS disease, new-onset seizures within 1 week of presentation, papilledema, abnormal level of consciousness, focal neurologic deficit, as well as immunocompromised patients should undergo CT prior to LP.^20,21,22

Subarachnoid space infection is a rare complication of LP. Most cases of postdural puncture meningitis are thought to be caused by contamination of the puncture site from the patient’s skin flora and aerosolized bacteria from the operator’s mouth.⁴² The risk of infection can be minimized by proper sterile techniques including hand washing and the use of a face mask.

A small amount of bleeding is relatively common but serious bleeding that results in spinal cord compromise is very rare in the absence of a bleeding risk. If patients experience prolonged back pain or neurologic symptoms such as numbness, weakness, and incontinence after LP, they should be evaluated with a MRI for possible spinal hematoma. Other unlikely complications of LP include extraspinal hematoma, cervical spinal cord infarction, cortical blindness, intraspinal epidermoid tumor, and intervertebral disc herniation.^23,43

A lumbar drain is used for external drainage of CSF, monitoring of CSF flow, and evaluation of ICP. Temporary externalized lumbar catheters are usually used as a treatment of CSF leaks, pressure monitoring, and drainage trials of patients with suspected normal pressure hydrocephalus. More permanent internalized lumboperitoneal shunts are used as the treatment of communicating hydrocephalus or idiopathic intracranial hypertension. Lumbar CSF drainage is also used as a spinal cord protective strategy in thoracic aortic aneurysm repair for patients at high risk of spinal cord ischemic injury.⁴⁴ Catheter placement is performed in a similar manner as a standard LP described previously. Instead of a 22-gauge spinal needle, a 14-gauge Tuohy needle is used for the lumbar drain placement. First, approximately 5 mL of sterile saline is injected into a plastic case surrounding the catheter guidewire to lubricate the guidewire. This facilitates threading the guidewire through the catheter. The guidewire is inserted into the open end of the catheter and set it aside. Prepare the area using sterile techniques and local anesthetics as previously described. Appropriate insertion of a Tuohy needle at the L3/4 interspace is confirmed by CSF return. Once the needle is in the subarachnoid space, rotate the needle 90° so that the bevel of the needle points in the cephalad direction. While holding the hub of the needle, the lumbar drain catheter is threaded over the guidewire through the needle. The catheter should advance smoothly. Advance the catheter to approximately the 15-cm mark (3-5 cm in the spinal space is sufficient), and remove the needle over the catheter. Once the needle is removed, carefully remove the guidewire. Thread the catheter tip onto the connector, attach the strain relief device, and snap the connector closed to prevent contamination. Attach a sterile syringe to the connector, aspirate CSF, and confirm correct catheter placement. Secure the catheter to the patient’s skin using a sterile dressing. The catheter is then attached to a sterile external lumbar drainage system. CSF is allowed to drain when the lumbar pressure exceeds a previously set threshold. Complications are similar to routine LP and include bleeding, infection, CSF leaks, nerve root irritation, and supratentorial subdural hematoma secondary to CSF overdrainage.