Selective Nerve Root Block



Selective Nerve Root Block


L. Mark Dean



Introduction


Selective nerve root block (SNRB) is a fundamental technique used to perform regional anesthesia. This procedure is also used as a means for diagnosis and medical treatment. The physician attempts to prove that a particular nerve is the root source of the patient’s symptoms by placing a needle adjacent to the nerve root sleeve as it emerges from the intervertebral foramen. The evaluation attempts to reproduce the patient’s typical pain and then interrupt and alleviate the pain. This procedure is performed under the guidance of fluoroscopy, ultrasound, or computed tomography (CT) scan to confirm proper needle placement along the nerve root sleeve. This interventional procedure tests for the cause of pain that may be undiagnosed by magnetic resonance imaging (MRI), CT, discograms, and electromyograms.


The practice of recognizing and understanding dermatomal patterns has given practitioners the opportunity to better characterize pain and analyze the process of radicular pain. The understanding and application of techniques that interrupt pain from the neck, thorax, visceral organs, or lower extremities that can be adequately blocked at the spinal level continues to evolve.1 As we better characterize pain patterns and as this process is readily adopted into clinical practice, the awareness of its efficacy will be more pervasive.2


This chapter attempts to place the practitioner within a standard, safe, and effective algorithm for the clinical evaluation of patients. Working through the patient’s description of their symptoms, their behavior, and the pictures and pain diagrams they draw, the practitioner can determine the location and type of injection to be performed. This information will help practitioners who perform SNRB to determine the location or level for needle placement.




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. 163-1).



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. 163-2).



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.



Nerves


There are 31 pairs of spinal nerves that extend from the cord. These nerve rootlets are covered by all layers of the meninges initially. The anterior and posterior rootlets coalesce to form the dorsal root ganglion, and the nerve root extends peripherally beyond the neural foramen. As the nerve roots approach the neural foramen, the dura thins.


The dorsal and ventral rootlets of the cord are covered by pia and arachnoid. Beyond the neural foramen, the pia and arachnoid form the perineural epithelium covering of the peripheral nerve. The dura extends primarily along the dorsal surface of the rootlets and extends lateral to the point where the anterior and posterior rootlets fuse to form the ganglia. This termination of dural tissue forms a small sleeve. The sleeve is of variable lengths.3 The sleeve is pierced by arteries, veins, and lymphatics that communicate with the underlying subarachnoid space.


Also along this sleeve are permeations in the thinned dura where a layer of arachnoid mushrooms through and creates areas of granulation. The granulation is a permeable membrane for anesthetic to enter the subarachnoid space. These arachnoid granulations are continuous with the epineurium that surrounds the nerve beyond the neural foramen. This allows for a ready route for passage of local anesthetic onto the nerve root. The caudal nerve roots in the sacrum are more susceptible to neural blockade because they are only surrounded by epineurium.


Nerve fibers carry afferent and efferent activity. Both motor activity and sensory perception travel along the peripheral nerve trunks. The dermatomal area a nerve innervates and its designated function differentiate the type of primary afferent fibers and the type of sensation that travels in the nerve root. The target or end muscle innervated is responsible for gross motor movement or fine motor activity. Information about proprioception and muscle spindle tone are transmitted to the brain along afferent tracts.


The anatomy of the nerve fiber, its diameter, and myelination can determine its physiologic response to anesthetics.4 The myelination and diameter of the nerve are important in this process.


Nerve permeability regulates the readily available amount of anesthetic for blocking neural transmission. The amount of anesthetic, such as lidocaine, around the nerve root also affects its neural blockade activity. In addition, pH can enhance permeability of the perineural membrane. Local anesthetics are weakly acidic. These molecules are more stable at the acidic level, but the neutral anesthetic molecule passes across the membrane more readily.5,6 Therefore, raising the pH with small aliquots of sterile sodium bicarbonate prevents significant dilution and is beneficial in creating the alkalized form of the anesthetic.7



Sources of Pain


There are potentially five perspective sources of back- or spine-mediated pain: the disc, facet joint complex, spinal cord and its extension as the nerve root, the bone or vertebral body, and the myofascia. Other than bone-mediated pain, which is transmitted by segmental nerves within the vertebral column, pain is transmitted via the nerve root. A nerve root block is one of the dominant pathways for halting pain transmission. SNRB is designed to determine which level is producing provocative pain.


Pain from the zygapophyseal joint is referred to the adjacent paraspinal tissues. The pattern and potential distribution of perceptive pain in the cervical spine have been defined by Bogduk and Marsland, and in the thoracic spine by Fukui et al.8,9 The lumbar spine and paraspinal and adjacent structures produce perceptive pain from the low back, buttock, and thigh areas. However, the lower lumbar medial branch nerves may convey pain that is perceived as low as the knee and occasionally the calf.


Disc-mediated pain is described as being mechanical if the disc is degenerating. Increased pressure and inflammation within the disc can create back, neck, or thoracic pain or paresthesias. Disc-mediated pain can arise in discs that are “normal appearing” and in discs that are degenerating. The input source of the innervated disc and the annulus is the sinuvertebral nerve. The sinuvertebral nerves are branches of the ventral rami, and the gray rami communicantes of the sympathetic chain.


Myofascial pain emanates from the superficial tissues that surround the vertebral column, such as the ligaments and muscle, and is conveyed through sensory fibers and relayed via the nerve root. SNRB can interrupt the specific pathway based on the dermatome being innervated.


Pain originating from within the cord or a primary nerve root can be due to several sources, including inflammation, tumor, or defects such as a syrinx. Cord-mediated pain, however, is blocked more centrally.



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. 163-3). 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.10 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. 163-4). 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.





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.14 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.15 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.16 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. 163-5). In patients with scoliosis, CT is the preferred imaging tool over MRI, which may have limitations with visualization of the sagittal plane (Fig. 163-6).




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. 163-7). 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 163-1). 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 (COPD). In the setting of a suspected tension pneumothorax, chest trauma or COPD pulmonary clearance is recommended to avoid an intraoperative crisis.17



Drugs Used for Selective Nerve Root Block


Anesthetics


Because local anesthetics are infiltrated as a fluid medium around the nerves, the fluid must diffuse through layers of fibrous tissue before it reaches the axons.5 The rate of diffusion depends on adjacent scar tissue, and diffusion can be especially difficult in the setting of previous surgery. The rate of absorption by fat, uptake into the microvasculature, and local metabolism all determine the clinical potency of the injection. The final concentration that reaches the nerve is affected by these factors and by the length of nerve exposed to the drug solution. Injections in close proximity to the nerve reduces the need for diffusion and the volume of anesthetic required to achieve the desired block. Intimate proximity to the nerve root can be confirmed radiographically by CT scan or fluoroscopically.


Injection of water-soluble iodinated contrast or gadolinium in iodine-sensitive patients is beneficial to outline the nerve root and proximity to the ganglion and confirm the location of the needle. It also serves to exclude direct vascular uptake.


Blockade of the peripheral nerve can also be affected by drug absorption in surrounding tissues. This can be affected by use of a vasoconstrictor such as epinephrine. The rate of absorption in the vasculature is reduced, thereby leaving more anesthetic available for neural blockade and prolonging the anesthetic affect.18



Corticosteroids


Depot formulations of epidurally placed medications appear to prolong the local anesthetic effect. The slow release should also be accompanied by a decrease in the rate of systemic drug absorption and reduction of the potential for systemic toxicity.19,20


The analgesic effect of anesthetics is enhanced with corticosteroids. Steroids relieve pain by reducing nociceptive C-fiber transmission and by decreasing inflammation. Steroids inhibit the action of phospholipase A2, which releases arachidonic acid from cell membranes at the site of inflammation. Membrane injury causes accumulation of edema and release of unsaturated fatty acids. Altered membrane permeability in the setting of inflammation leads to intraneural edema and causes abnormal neural transmission.21,22 Abnormal transmission along the nerve fiber results in creation of pain. Stabilization of the neural membrane by corticosteroids can inhibit sensitization of the nerve fiber and prevent the neural discharge that results in pain creation.


Corticosteroid medications have a therapeutic benefit. The recommended dose is not more than 50 mg triamcinolone or 80 mg methylprednisolone.23 Corticosteroids have been used to prolong the therapeutic effect of the diagnostic test injection. The antiinflammatory effect decreases pressure along the nerve root as swelling subsides and autologous histamine substances such as substance P are diluted away. Symptom improvement can increase the time for the patient to confirm the injection is beneficial. An injection of 0.5 to 1.5 mL of contrast material is used to outline the nerve root. The volume of injected anesthetic should mirror the injected volume of contrast material. The goal of the contrast is to not only localize the nerve root but also demonstrate the extent of the spread of medication and perhaps avoid injection into the epidural space.



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.24 Its benefit is best seen in the cervical region where the avoidance of major blood vessels is critical (Fig. 163-8). 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 recognition of 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. 163-9). 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 prepped 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-site or down-the-barrel projection is used when the target is the foramen. The goal is perineural placement of the needle (Fig. 163-10). 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. 163-11). 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.25





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. 163-12). 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. 163-13).




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. 163-14). 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. 163-15).


Dec 23, 2015 | Posted by in INTERVENTIONAL RADIOLOGY | Comments Off on Selective Nerve Root Block
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