Lumbar Puncture a self-Directed Learning Module Technical Skills Program



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Lumbar Puncture

A Self-Directed Learning Module

Technical Skills Program

Queen’s University

Department of Emergency Medicine

Introduction

Lumbar puncture (LP) and analysis of cerebrospinal fluid (CSF) has been part of the medical profession for over one hundred years. This relatively simple procedure is widely employed, yet a surprising number of physicians are poorly educated in regards to when and how this procedure is performed. The goal of the this educational module is to familiarize the student physician on the theory behind the lumbar puncture as a diagnostic tool and to guide the student through the steps involved in performing a safe and successful lumbar puncture for the adult patient. Students should complete this module and complete the embedded multiple-choice questions prior to their scheduled lumbar puncture seminar. There will be a brief multiple-choice exam based on this material at the beginning of the seminar.



Objectives

By the completion of this teaching unit, the student physician will be able to:



  1. List the indications for lumbar puncture

  2. List the contraindications for lumbar puncture

  3. Describe when a CT scan is and is not required prior to lumbar puncture

  4. List and discuss the complications of lumbar puncture

  5. Describe the equipment employed for lumbar puncture

  6. Demonstrate a safe method for performing a lumbar puncture on a model

  7. Discuss the appropriate CSF studies to be ordered in cases of suspected meningitis or subarachnoid hemorrhage

Indications

The indications for lumbar puncture include (common uses in bold):



  • Diagnosis of central nervous system (CNS) infection

  • Diagnosis of subarachnoid hemorrhage (SAH)

  • Infusion of anesthetic, chemotherapy, or contrast agents into the spinal canal

  • Treatment of idiopathic intracranial hypertension

  • Evaluation and diagnosis of demyelinating or inflammatory CNS processes

Perhaps the most common indication for LP is to evaluate the patient for CNS infection. CSF samples are needed for the diagnosis of bacterial and viral meningitis as well as for numerous other infective processes such as CNS syphilis and other encephalopathies. The CSF may be examined for signs of infection by studying it for microbes, cells and culturing the CSF for organisms.

The diagnosis of subarachnoid hemorrhage is the other emergent indication for LP. In SAH, blood seeps into the CSF and can be detected by lumbar puncture. In this day and age, CT scan is often used to diagnose SAH, but LP is used when a CT scan may have missed the diagnosis of SAH. CT scan may miss small bleeds or bleeds which occurred greater than twelve hours prior to CT scan. In these instances, LP proves to be instrumental in the diagnosis or exclusion of SAH.

Other common indications for LP include spinal anesthesia for procedures and operations as well as injection for contrast agents into the spinal column for imaging such as in CT – myelography. CNS chemotherapy is often delivered after lumbar puncture. Other uses of LP include evaluation of inflammatory CNS processes such as multiple sclerosis.

Contraindications

As with any procedure, there are times when it is not safe to proceed with lumbar puncture. These contraindications are:



  • Skin infection near the site of the lumbar puncture

  • Suspicion of increased intracranial pressure due to a cerebral mass

  • Uncorrected coagulopathy

  • Acute spinal cord trauma

The presence of skin infection near the site of the LP increases the risk of carrying the infection into the CSF with the LP needle. Thus, infection at the potential LP site is a contraindication to performing a lumbar puncture at that area.

Perhaps the more worrisome contraindication to lumbar puncture is the suspicion of increased intracranial pressure (ICP) due to a cerebral mass lesion. In the presence of a potential brain tumor, cerebral hemorrhage, cavernous sinus thrombosis, brain abscesses, epidural or subdural hematomas , patients are at increased risk of deteriorating neurologically with LP. As the space-occupying lesion grows, ICP rises. When lumbar puncture is performed in these patients, a low-pressure shunt is formed at the site of LP where CSF can escape. As the CSF pressure drops in the spinal column, CSF and brain mass may then shift towards the low-pressure outlet (the LP site). This may lead to either trans-tentorial or uncal herniation and acute neurological deterioration. Patients with increased ICP from mass lesions often display decreased levels of consciousness, focal neurological signs or papilledema on physical exam. Any of these findings make lumbar puncture contraindicated until further evaluation can be undertaken. The next section "To CT or Not to CT", addresses the issue of when imaging is required prior to LP.

As with most invasive procedures, uncorrected coagulopathy is a contraindication to LP. This includes those on heparin, coumadin, or with clotting defects such as disseminated intravascular coagulation, hemophilia or thrombocytopenia. Having said that, when these clotting abnormalities are corrected, lumbar puncture is no longer contraindicated. Reversal of coumadin with Vitamin K or fresh frozen plasma, replacement of a hemophiliac's clotting factors or transfusion of platelets to the thrombocytopenic patient would all allow for safe lumbar puncture.

In the presence of acute spinal trauma, LP is understandably contraindicated as both the bony anatomy and spinal structure may be altered and not allow for safe placement of the LP needle.



To CT or not to CT? That is the question.

As discussed, the suspicion of increased ICP due to a mass lesion is a contraindication to lumbar puncture. In these cases, a CT scan is recommended prior to LP. Unfortunately, it has been wrongly interpreted by many that all patients require CT scan prior to their LP.

The clinical findings of decreased level of consciousness, focal neurological deficits, or papilledema make CT scan necessary prior to LP. Any of these findings place a patient into a high-risk group for having increased ICP. Having said this, LP's performed on these patients do not always lead to disaster. Studies reviewing LP in patients with known brain neoplasms, hematomas or abscesses found that neurological deterioration with LP occurs in only 0 - 5% of patients. This low rate occurs in patients known to have signs, symptoms and diagnoses of increased ICP with mass lesions!

Given this low complication rate, it has been accepted that patients with a normal level of consciousness, lack of focal neurological findings and absence of papilledema are safe to undergo LP. Many clinicians ask, "What if I cannot reliably see the fundi of my patient?". Papilledema is a late finding of increased ICP and is present in less than half of those with raised ICP. If your patient has a normal level of consciousness, no focal neurological defects, and you cannot visualize the fundi, it is still safe to proceed with the LP.

The safety of this practice has been studied in suspected meningitis. In 1993, the Canadian Medical Association Journal reviewed the need for CT prior to LP in meningitis and found that there were no clinical studies nor anecdotal reports of patients with suspected meningitis and normal neurological exam deteriorating with LP. In 1993, Durando et al reviewed almost 450 LP's in adult patients in suspected meningitis. They found that 5 cases had neurological deterioration after LP, and all these patients had either signs of increased ICP or focal neurological deficits prior to LP. In suspected meningitis, patients with a normal level of consciousness, no focal deficits and an absence of papilledema (or inability to visualize the fundi) should undergo lumbar puncture without prior CT scan. If the clinician finds reason to image the patient prior to LP, and meningitis is suspected, the patient must receive antibiotics prior to CT scan.

Delays in starting appropriate antibiotic therapy, to arrange a scan can result in increased morbidity in the setting of bacterial meningitis. Arranging, performing and interpreting a CT scan will take a minimum of an hour in most cases even if the CT is up and running. If the technician and the radiologist need to come in from home, or the patient requires transfer to another center, it will involve a considerably longer period of time. It is worth repeating that these patients must get antibiotics prior to CT scan!

The safety of LP in suspected SAH is somewhat less clear. As mentioned earlier, large intracranial bleeds have the potential to increase ICP. Patients with large bleeds either die, have decreased levels of consciousness or neurological deficits. Patients with small SAH's often have more subtle presentations. These patients may not have neurological defects. Using the same logic as in meningitis, it is inferred that lumbar puncture is safe in the suspected SAH patient who has a normal level of consciousness, no focal deficits, nor papilledema. Unfortunately, this has neither been proven nor disproven by rigorous studies.

Complications

Associated with the contraindications for LP, are the complications of LP. The potential complications of this procedure are:



  • Post LP headache

  • Post LP back pain

  • Seeding of infection to the CSF

  • Epidermoid tumor implantation

  • Uncal or transtentorial herniation and neurological deterioration

  • Spinal hematoma

Post lumbar puncture headache is the most common complication of LP. The post LP headache develops in 5 – 40% of patients undergoing lumbar puncture. It is a headache that begins within 72 hours of LP and usually lasts less than 5 days. The headache is a bilateral pressure or throbbing that is intensified in the upright position and with coughing. The headache resolves when the patient is supine. The longer the patient is upright, the longer before it resolves when the patient is supine.

From a pathophysiological viewpoint, the post LP headache occurs because of the tear in the dura mater caused by the LP needle. This opening allows for the continued leakage of CSF out of the dura and this lower pressure allows the brain to shift downward. Traction on the pain sensitive bridging vessels, dura and nerves causes the headache. When supine, the pressure column of the CSF is equal and thus there is no pull on pressure sensitive structures of the brain and the headache resolves.

There are a number of ways to minimize post LP headaches in your patient. The first is to use the smallest size spinal needle possible. The smaller the needle, the smaller the tear in the dural fibres and the lower the incidence of post lumbar puncture headache. The incidence of post LP headache is about 70% with 16 – 19 gauge needles, 20 – 40% with 20 –22 gauge needles and 5 – 12% with 24 – 27 gauge needles. The LP kit carried in our hospital contains a 20 gauge needle. This module will teach you not only how to use the provided needle, but also how to utilize smaller spinal needles for LP.

Another factor that can be utilized to minimize the chance of post LP headache is the use of a stylet in the needle. This internal stylet prevents the needle from "coring" through the tissues while placing the LP needle. It is intuitive to use the stylet to prevent the needle from being blocked up with soft tissue while doing the LP. What is of interest, though, is that the replacement of the stylet into the needle prior to the needle's removal (at the end of the procedure) has been shown to reduce the incidence of post LP headache by 50%. It is theorized that during CSF collection, a strand of arachnoid fiber may enter the needle and when the needle is withdrawn the arachnoid strand is pulled out through the dural defect and produces and prolonged CSF leak. Replacement of the stylet prior to the LP needle's removal would prevent this.



Furthermore, the type of needle and the orientation of the needle's cutting edge also influence the incidence of post lumbar puncture headache. There are three main types of needles, pictured below. The Quincke needle was the first invented and has a beveled cutting tip. The Whitacre and Sprotte are "atraumatic" or pencil point needles and have blunt tips with lateral ports for CSF collection.



Figure 1. Spinal needle types

When using the Quincke needle, the orientation of the bevel influences the incidence of post lumbar puncture headache. Post LP headache is reduced by 50% or greater when the bevel is parallel to the dural fibres' long axis. To you and me this means that if the patient is lying in the lateral decubitus position, the flat portion of the bevel should point up towards the ceiling. An easy way to remember this is that when performing the LP in the lateral position, the bead on the plastic end of the stylet and therefore the notch that the stylet fits into should be pointing up at the ceiling.

Although the usual LP kit contains a Quincke needle, there is very good evidence that the atraumatic needles significantly reduce the incidence of post lumbar puncture headache. It is theorized that these needles spread the dural fibres and cut fewer fibres. This reduces the size of the hole in the dura and reduces the tendency to develop CSF leak. Numerous studies demonstrate post lumbar puncture headache rates of only 2-6% using atraumatic needles compared to rates of 18-40% using the same sized Quincke needles. Numerous direct comparisons have borne out the superiority of using atraumatic needles for reducing post LP headache.

There is considerable conflicting literature about a variety of other methods of reducing post lumbar puncture headache. Some recommend lying flat on one's back for 4 hours, some recommend lying prone and a few recommend activity right after lumbar puncture. To date, there are no post procedural interventions that seem to influence the incidence of post LP headache.

Therefore, in order to reduce post lumbar puncture headache, we recommend using small, atraumatic LP needles when doing lumbar puncture. In this module we will demonstrate the use of both types of needles, as some practitioners may not have access to small atraumatic spinal needles.

Also common after lumbar puncture is local back pain at the site of puncture. Approximately one third of patients will experience some local back discomfort after the procedure, which lasts for a couple of days. This is due to local soft tissue trauma. In rare cases, if the needle is inserted beyond the subarachnoid space, the annulus fibrosis may be damaged and the intervertebral disk can herniate. This is very rare, though.

Thankfully, other complications from lumbar puncture are much less common. The risk of introducing organisms into the CSF from a properly performed lumbar puncture is exceedingly small. This can occur with breaks in sterile technique, use of contaminated equipment and placement of the needle through infected skin. It is estimated that the incidence of such infection after lumbar puncture is about 0.2%.

Epidermoid tumor implantation is a theoretical concern that is very rare to see. The true incidence is unknown, but thought to occur when a plug of skin is carried into the spinal canal where it presents months to years later as an expanding epidermoid tumor. The use of a stylet with lumbar puncture has made this complication mostly one of historical significance.

Spinal subdural hematoma is also a rare complication reported in lumbar puncture. It is most common in those who undergo lumbar puncture while having coagulation abnormalities, including thrombocytopenia, anticoagulation and bleeding disorders. In spinal subdural hematomas, patients present with severe low back, radicular pain, sensory loss or paraparesis hours to days post LP. These symptoms in a post LP patient warrant aggressive investigation with CT/MRI and associated decompressive laminectomy if a hematoma is present. Epidural spinal hematomas may also present with the same clinical picture post LP and have the same risk factors and investigation as the spinal subdural hemorrhage.

As discussed previously, neurological deterioration due to herniation syndromes is the most dreaded complication of lumbar puncture. As previously noted, the risk of herniation is 0 –5 % in those patients who are known to have intracranial masses. The risk of herniation in the conscious, neurologically normal patient is exceedingly small. Its occurrence obviously warrants aggressive investigation and treatment.



The anatomy of the LP

Let us review a little anatomy so that you can understand where to do your lumbar puncture. At birth, the inferior end of the spinal cord is opposite the body of the third lumbar vertebrae (L3). Distal to this point is the cauda equina and its nerve roots. As the child grows, the vertebral column grows much faster than the spinal cord itself, and by adulthood, the spinal cord only reaches the inferior border of the L1 vertebra, or the superior aspect of L2. Distal to this point is the cauda equina. In order to avoid transfixing the spinal cord during LP, the needle is placed distal to L2. This means the needle enters the subarachnoid space at the level of the mobile cauda equina.

Landmarking the interspace is quite easy, as an imaginary line that crosses the lumbar region of the back joining the posterior superior iliac crests will cross the L3-L4 interspace. Thus, one can easily identify the L2-L3(above the line), L3-L4(at the line), or L4-L5(below the line) interspaces, all of which are suitable for LP.

The CSF itself resides in the subarachnoid space between the pia mater and the arachnoid mater. In order to place the needle into the subarachnoid space, the needle passes between two vertebral processes and continues through the interspinal tissues and into the subarachnoid space. The tissues pierced are (in order): skin, subcutaneous tissue, supraspinal ligament, interspinal ligament, ligamentum flavum, dura mater, the arachnoid mater and into the subarachnoid space.



Figure 2. Local spinal anatomy


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