Nerve Root and Facet Blocks and Pain Intervention

History

The creation and development of anesthetics has been an essential part of regional anesthetics. Since the proposition and theorization by René Descartes in the 17th century that physiologically there was a neural connection between the peripheral aspects of the body and the brain, doctrine has developed to confirm the transmission of nerve impulses.

Anatomy

Tissues and Fascia

To perform SNRB, understanding the approach, location, and the layers of the tissue and fascia the needle must pass through gives the practitioner a higher level of safety and accuracy. The surrounding fascia, be it lipomatous or vascular, can affect selection of medication and determine whether adjuncts such as corticosteroids or vascular constrictors should be used. The location of the needle tip and selection of medications can greatly impact the efficacy of the injection and its perineural infiltration.

The brain continues as the spinal cord to T12 or L1 and is protected by the meninges and the bony vertebral column. The spinal meninges covering the cord is composed of the pia mater, arachnoid, and dura mater. Protection of the spinal cord is formed from 33 specialized vertebrae. The size of the central canal, the size of the alae, and curvature of the bones give the vertebrae functional ability and provide protection function to the cord and nerve roots ( Fig. 112.1 ).

The neural foramen is the passageway for the spinal nerve as it exits the central canal. The anterior border of the foramen is framed by the uncovertebral joint and vertebral body. The pedicle of the vertebral body above supplies the roof, and the pedicle and lamina of the vertebrae below supplies the floor. The posterior aspect of the neural foramen is framed from the inferior and superior articular facets of the adjoining vertebrae ( Fig. 112.2 ).

The spinal epidural space surrounds the cord and nerve roots and extends from the sacral hiatus inferiorly to the foramen magnum at the skull base. Between the epidural space and the spinal cord are the three layers of the meninges.

Indications

SNRB serves to identify a specific a level where the pain generator arises. Pain relief after SNRB implicates a specific nerve as the source of pain. A selective block is therefore not only therapeutic in the alleviation of pain but diagnostic in determining the level at which therapy may be beneficial. Radicular pain creates sensory, motor, and reflex abnormalities in a clinical pattern manifested as pain, numbness, and weakness. Other dysesthesias can accompany the pain, such as tingling, itching, or hyperesthesia. Neck pain, headaches, and nuchal stiffness are not unusual from cervical nerves. Stiffness can be due to muscle spasms secondary to nerve irritation. Radicular pain is often described as shooting, stabbing, burning, or a dull ache. The patient’s history, clinical findings, and physical examination are used to exclude causes such as tumor, infection, aneurysm, fracture, or cauda equina syndrome.

The location and distribution of pain can be diagrammed on a dermatomal map ( Fig. 112.3 ). This valuable information can assist in determining whether single or multilevel disease is present. Recording factors such as time of day and response to aggravating or relieving factors helps characterize pain. An organic pain map has considerable diagnostic potential. It has a high degree of validity in the assessment of neurogenic pain and dysfunction and is reliable in showing concordance with the physical examination. How pain is influenced by functional ability, mechanical loading, bending, sitting, standing, driving, or rest can further the evaluation.

Nerve roots cover a defined dermatome in most individuals, but the specific distribution or patterns are individualized. There is, however, considerable overlap in nerve root distribution. In the cervical region, the map is better defined ( Fig. 112.4 ). In the thoracic region, the pattern of the thoracic nerve roots is segmental and covers a band or horizontal distribution. In the lumbar and sacral region, there is a more extensive distribution with overlap along the back and involvement of the buttocks and leg to the foot.

Mapping

Needle electromyography (EMG) can detect waves and fibrillation potentials along muscle fibers. The motor nerve fibers innervating the muscles extend across two nerve root levels. EMG does not test sensory input. Based on the muscles involved, EMG helps predict the level of the radiculopathy. EMG detects signs of irritability as the process of denervation and reinnervation progresses from acute to chronic injury. ^, By sampling multiple muscles, it seeks to determine the specific root or roots involved. It is estimated that the specificity of the EMG is near 85%.

Diagnostic Imaging

Depending on the clinical condition, diagnostic imaging may provide the best clues to the diagnosis and level to be injected. Plain film evaluation of the region of concern provides a good diagnostic tool. Plain film radiography is readily available and cost-effective when diagnosing acute back pain. Plain films of the cervical, thoracic, and lumbar spine may reveal loss of disc height, end-plate sclerosis, neural foramen narrowing, or bone hypertrophy. Plain films assist in ruling out other benign and malignant conditions.

Plain film images of the cervical, thoracic, and lumbar spine can be obtained without concern for an implant device, sedation, or claustrophobia. Spinal instruments such as screws and plates may limit evaluation of plain films but do not produce the multiple artifacts that would be seen with CT and MRI. Plain film images are obtained in at least two planes, frontal and lateral. Special projections such as oblique views in the lumbar and thoracic region and pillars or odontoid views in the cervical spine are helpful in evaluating pathology and postoperative changes, including fusions. In addition, congenital anomalies, fractures, osteoarthritic impingement, or bone erosion may be differentiated from tumor involvement with plain film radiography.

CT is the imaging tool that best visualizes the bones of the spine and is capable of determining the level of disc bulging or herniation in the spine. Gas within the disc, intraosseous herniation, and loss of disc height on sagittal reconstructions can all be seen with CT and can often predict a potential source of pain.

The discogram is a diagnostic tool for identifying discogenic back pain. Intradiscal injections create pressure, provide the opportunity to characterize spread of contrast material throughout the disc, and identify spillage into the epidural space. Concordant pain can be reproduced with injection of the offending disc. Correlation with a CT scan demonstrates the spread of contrast material through the nucleus pulposis and into the fissures of partially or completely torn annular fibers.

Myelograms are performed with the assistance of CT. This procedure allows access to the cerebrospinal fluid for diagnostic purposes. Intrathecal administration of iodinated contrast material allows detection of abnormalities encroaching on the spinal cord or nerve roots, including herniated disc, meningeal carcinomatosis, or spinal vascular malformation ( Fig. 112.5 ). In patients with scoliosis, CT is the preferred imaging tool over MRI, which may have limitations with visualization of the sagittal plane ( Fig. 112.6 ).

MRI uses protons and high-field-strength magnets for imaging. It is noninvasive and does not use x-rays. MRI has revolutionized imaging of the spine and surrounding soft tissues and can determine the level of a disc herniation or nerve root impingement. However, an abnormal disc may not indeed be manifesting the pain.

MRI shows loss of disc height, annular tears, and intradisc hypointensity on T1- and T2-weighted images. In the presence of annular tears, sagittal T2-weighted images with fat saturation show focal high signal, and subchondral marrow shows T2 hyperintensity in the vertebral bodies abutting the disc. Central canal, lateral recesses, and neural foraminal narrowing can be seen on MRI ( Fig. 112.7 ). Associated changes such as facet hypertrophy, degenerative spondylolisthesis, and hypertrophy of the ligamentum flavum can also be seen by MRI. These findings are correlated with the selective block to determine which level or levels should be tested or should undergo neural blockade.

Contraindications

Contraindications to SNRB are procedural or related to medications. If sensitivity to contrast media is present, patients can be treated prophylactically, or noniodinated contrast media or gadolinium can be used.

In patients with an uncorrectable coagulopathy, spinal injections are not recommended ( Table 112.1 ). Spinal and paraspinal infection creates a risk of spread along the spinal canal.

A spinal injection or neural blockade of the nerves to the muscles used in respiration can create a thoracic block in a patient with a contralateral pneumothorax or severe chronic obstructive pulmonary disease. In the setting of a suspected tension pneumothorax, chest trauma or chronic obstructive pulmonary disease pulmonary clearance is recommended to avoid an intraoperative crisis.

Drugs Used for Selective Nerve Root Block

Technique

SNRB can be performed in the cervical, thoracic, lumbar, and sacral spine. The procedure was initially performed without guidance and involved the use of a series of landmarks. The block could be performed in the sitting, lying, or oblique position. Most thoracic and lumbar injections are performed with the patient prone, whereas a cervical block can be performed from the decubitus or supine position. The chosen position should allow maximal exposure of the spine and neural foramen. A short roll can be place beneath the hips or chest in the prone position and under the neck in the supine position.

Most injections are performed using fluoroscopic guidance, but CT can also be used for needle guidance. Its benefit is best seen in the cervical region, where the avoidance of major blood vessels is critical ( Fig. 112.8 ). The approach taken with the use of CT will be a direct axial one unless the CT scan gantry is angled. This approach allows the practitioner to avoid the primary vasculature of the vertebral and carotid arteries as well as the large branches of the subclavian artery, especially the superior thyroidal artery near the base of the neck. There are multiple small muscular branches of the vertebral artery that may not be avoidable, but recognizing their presence if bleeding develops as the vertebral artery is approached can be helpful. However, vigilance for the spinal artery during a cervical injection is important to avoid major complications. The precise location of a tortuous vertebral artery can be localized, especially near the skull base and at the base of the neck, before entering the foramen transversarium ( Fig. 112.9 ). In the setting of a neck or shoulder mass, CT scan presents an excellent option to avoid direct transgression of the mass to gain access to the cervical nerve root.

The skin is prepared and draped using sterile technique. Local anesthetic is placed in the subcutaneous tissues. A 22- or 23-gauge spinal needle with a beveled tip is often used.

The basic technique is to stabilize the needle with the thumb and index finger or use a hemostat for measured advancement of the needle tip. The fluoroscope is positioned to optimize the position and alignment of the neural foramen. A gun-sight or down-the-barrel projection is used when the target is the foramen. The goal is perineural placement of the needle ( Fig. 112.10 ). To avoid puncture of the nerve root, the advancing needle tip is also visualized in the lateral and frontal projections. Ideal needle placement depends on the level of the spine being treated. In the cervical spine, the needle tip is placed posterior for the foramen ( Fig. 112.11 ). As the needle tip nears the neural foramen, care should be taken to avoid piercing the nerve root and vertebral artery. Success of the injection is not influenced by the length of the spinal needle bevel. In comparison with long-bevel needles, short-bevel needles do not reduce the rate of vascular injection.

Cervical

For injection of the cervical spine, the needle end target is placed near the upper outer aspect of the foramen at the 10 o’clock position on the right and at the 2 o’clock position on the left of the patient. The patient can be placed in an oblique position, or the image intensifier is maneuvered to open the neural foramen and create the widest circle ( Fig. 112.12 ). The nerve exits the foramen anterolaterally and inferiorly en route to form segments of the brachial plexus. The needle should be directed to engage the lip of the foramen posteriorly. The vertebral artery commonly lies anterior to the nerves and passes through a channel or foramen transversarium. Posterior to the needle are the articular facets ( Fig. 112.13 ).

For the injection of the C2 nerve, a direct lateral or posterior approach is taken. The pathway is centered in a rectangle created between the spinal laminar line posteriorly and the odontoid anteriorly. The superior border is framed by the lamina of C1, and the floor is created by the lamina of C2. Using a posteroanterior projection, the needle tip is advanced until it passes the lateral third of the facet joint ( Fig. 112.14 ). Oftentimes the patient’s mouth has to be opened for adequate positioning and determination of landmarks.

Thoracic

For injection of the thoracic spine, the needle is angled along an inferolateral approach. The image intensifier is angled so it parallels the end plates of the vertebral bodies at the level that is being approached. The foramen is approached medially from the inferior aspect of the transverse process. A curved needle is used to touch the transverse process and then walked off its edge. Just deep to the anterior aspect of transverse process, the needle will give way beyond the bone contact with the bone. The needle is advanced in a lateral to medial direction ( Fig. 112.15 ).

From an anteroposterior projection, the position of the needle is adequate when the lateral third of the pedicle is approached. From a lateral projection, the needle should not advance more anterior than the posterior wall of the vertebral body or anterior margin of the foramen. However, the anterior margin of the foramen can be difficult to see if the view is not a true lateral projection. Contrast material is injected to locate the nerve root and view it in the frontal and lateral projections ( Fig. 112.16 ).

Lumbar

For injection of the lumbar spine, the vertebral bodies are squared and their end plates are paralleled by the image intensifier. The target is beneath the pedicle of the vertebrae or “eye of the Scotty dog” and is in the region of the safe triangle as described by Bogduk et al. ( Figs. 112.17 and 112.18 ). This region is above the exiting nerve root. A down-the-barrel approach is best; the needle is inserted into the tissues in a paramedian direction through the muscle. This approach immediately below the pedicle and along the superior border of the foramen is chosen to avoid the exiting nerve root. From a posteroanterior approach, the needle is not advanced more medial than the lateral one-fourth of the pedicle. The needle is aspirated to avoid blood or cerebral spinal fluid ( Fig. 112.19 ).

Sacral

When injecting the sacral spine, the foramina are directly posterior and are aligned so that the anterior and posterior foramina overlap. This approach creates a steep cephalocaudal view from the image intensifier. The image intensifier can also be made slightly laterally oblique to create a larger circle of the foramen. This places the image intensifier in a position perpendicular to the foramen, and the location of the individual sacral nerve roots can be predicted ( Fig. 112.20 ).

The spinal needle is inserted into the foramen perpendicular to the dorsal surface of the sacrum ( Fig. 112.21 ). The needle is advanced through the epidural space and is viewed from the lateral projection ( Fig. 112.22 ). If the needle is seen approaching the anterior foramen and presacral space, the position of the needle is adequate. Contrast material can be injected to view the outline of the sacral nerve.

At each level, a small quantity of contrast (1–2 mL) is injected to identify the nerve root. This also confirms the absence of epidural and vascular spread of contrast. Anesthetic solution and corticosteroid medications are injected around the nerve root sleeve. The diagnostic value of the injection is maintained if epidural spread of contrast is avoided ( Fig. 112.23 ). Using a 1-mL syringe, 1 mL of anesthetic solution is injected. The injected anesthetic may be lidocaine 1% in the cervical region or as strong as 4% in the lumbar region, or bupivacaine 0.5% can be used.

The injected contrast agent is viewed in two projections to confirm that vascular uptake is not occurring. The injection should be smooth, without great pressure. Small volumes are injected to exclude untoward respiratory or cardiac activity. The patient should be observed for changes in perception of pain.

Outcomes

SNRB is used primarily as a diagnostic tool, but it can have extended beneficial therapeutic effects as well. A paravertebral approach can create a zone of anesthesia to cover the ventral ramus, dorsal ramus, and the sinuvertebral nerve beyond the neural foramen. Larger volumes of injectate can extend to the medial branch nerve posteriorly or anteriorly, affect the sympathetic nerve, and cloud the diagnostic information. Awareness of where the contrast material is spreading is therefore essential.

Patients should be asked to draw and describe the location and type of pain they experience. This information is fed into the characterization of the pain and is useful to distinguish neuropathic from radicular from articular-based pain. Organic pain drawings are found to have a positive predictive value in outcome parameters and have been associated with positive radiologic and surgical findings of nerve root entrapment. ^,

Although patients often indicate that they hurt and describe a wide general area, they should be encouraged to define as specifically as possible where they perceive discomfort and where it travels to and draw it. This can be used to determine where the initial injection is to be performed, but the diagram can also serve as a follow-up to assess regression of the symptoms.

In the lower part of the back, pain described as a focal dull ache or shooting pain from the low back down the lateral thigh to the calf can have varied consequences. The patient’s description of the pain can guide the clinician in differentiating nerve-mediated pain from spondyloarthropathy ( Table 112.2 ). It is probable that some patients with nonradiating focal lumbosacral midline pain could have spondylolisthesis or facet-mediated arthrosis, and that others describing pain radiating down the leg may have a paracentral disc at the L4-L5 level. A good dermatomal map can be created by descriptions of the pain.

Concordant pain can be created along a nerve root by approaching it with the needle tip or by injecting contrast material nearby. Resolution of the primary pain by injection of the anesthetic confirms the appropriate level of injection. The efficacy of the selective block has been studied by Pang et al. and was used to predict which level was implicated in creating the radicular pain. Nerve roots in the lumbar spine were infiltrated with anesthetic for diagnostic evaluation. This, however, can be challenging in the setting of multilevel disease or complex pain secondary to disc-mediated and facet spondylotic disease.

The difficulty associated with interpretation of pain in patients was studied by Wolfe et al. They evaluated the perception of pain induced by the injection and mapped the creation of an anesthetic effect by SNRB to determine which levels matched which dermatomes. They also sought to determine what neighboring dermatomes created overlap on the pain map. The investigators found overlap by two to three dermatomal levels, both from the subjacent level below and also the suprajacent level above. The overlap created in the neural network is built-in redundancy that is a common theme seen at the level of the facet and medial branch nerves. Each rootlet provided sensory input from the level below, at the level of origin, and from the level above. Not uncommonly, however, some patients perceive pain not only from two levels above or below the primary root origin but also from the contralateral side. So the map is used as a template to predict where the initial injection should be placed.

The dermatomal map for the lower and upper extremities is well established. The thoracic region and the map for the visceral organs are less specific and less often used in clinical practice. The use of regional anesthesia can offer surgical colleagues a more specific and diagnostic clue to where and what is hurting the patient. Patients with chronic symptoms of right upper quadrant pain can respond to a positive block and help the practitioner distinguish acalculous biliary colic from renal or duodenal disease ( Table 112.3 ).

Similarly, nonischemic angina or chronic anterior thoracic pain can be found to have a positive response to SNRB at the T5, T6, or T7 region. This can offer the cardiac patient an opportunity for long-term response to an injection for noncoronary pain. This information has been beneficial to spinal surgeons seeking a positive prediction for a good surgical outcome from decompression.

In patients with breast or head and neck cancer, the cancer can presumably affect the nerve, but chemotherapy or radiotherapy or concurrent therapies can create a disabling plexopathy. There is no specific study or clinical finding that can be used to distinguish which nerve roots are responsible for the pain. Patients with breast cancer can have an incidence of radiation-induced plexopathy as high as 9%. ^, A selective block of the lower trunks of the plexus at the C5, C6, or C7 roots tends to relieve this pain.

Not only has the map been shown to have diagnostic value, but Derby et al. also used the map and SNRB to predict surgical outcomes. If perineural injection of steroid and anesthetic relieved the radicular pain by inhibiting release of arachidonic acid along the affected nerve root, it should predict a positive outcome from surgical decompression at that level.

North et al. used the diagnostic SNRB indirectly in the lumbosacral spine. Their study of a series of 33 patients led them to conclude that the selective block was not specific for a positive outcome, but that failure of the temporary block to relieve the pain would predict failure of a surgical procedure.

Slipman et al. studied 20 patients retrospectively and reported a significant reduction in pain at an average of 5.8 months in 60% of them. This clinically effective intervention was used for cervical radiculopathy in treating atraumatic cervical spondylotic pain.

Radicular pain creates a potentially dull, burning ache often associated with numbness, be it in the arm radiating from cervical nerve roots or in the thoracic or lumbar region. With radicular pain, pulsating or shooting discomfort can be perceived. This ache can be described as shocking or stabbing, but it is nonetheless annoying and not readily relieved. Patterns seen in the dermatomal map are typical for a radiculopathy across a single nerve root pathway (see Fig. 112.3 ).

SNRB can be difficult for the practitioner to master technically, but after a level of proficiency develops, diagnostic accuracy can be gained. Learning which level or levels produce the offending pain based on location and character can propel novice practitioners to an elevated status. The ability to determine the need for multiple injections or directly assess the neural pathway at the nascent levels is satisfying.

Postthoracotomy pain syndrome creates an interesting dilemma. It is probably due to intercostal nerve injury and can be predictive of which nerve roots are involved, based on the surgical site. The pain can be intense in the acute phase and distressing if it becomes chronic. However, for the practitioner, radiation of pain along nerves two or three levels above or below the surgical site can make controlling the pain difficult. Treatment of acute postoperative pain and its underrecognized neuropathic pain by regional anesthetic nerve blocks can have a favorable impact on pain management.

The level of inflammation, patients’ perception of how the pain is affecting their lives, and pain tolerance can determine how well they respond to an injection. It can be challenging to predict how patients will report their response to pain reduction, and this can be related to the presence of anxiety and depression.

The objective of evaluating the diagnostic process in assessment of the patient’s level of pain is to create a hypothesis about what is creating the pain and determine the level at which the pain is arising. The clinical information, imaging studies, physiologic analysis, and available assessments are evaluated to anticipate where the involved nerve roots are located. The experience of a seasoned practitioner, along with the use of a pain map for assessment, can create a high degree of reliability that the result of SNRB will be beneficial.

Facet Joint Injection