Diagnostic and Therapeutic Nerve Blocks



Diagnostic and Therapeutic Nerve Blocks


Michele Curatolo

Nikolai Bogduk



Local anesthetic blocks are procedures in which a local anesthetic agent is deliberately injected onto a peripheral nerve in order to temporarily stop conduction of action potentials. Local anesthetic agents block conduction of action potentials by acting on the sodium channels of the nerve cell membranes. Their effect is temporary because their pharmacologic action is reversible.

Nerve blocks are an attractive tool in pain medicine because they are the only means by which to stop pain completely. Analgesics may reduce pain, but only local anesthetics stop it.

Virtually any nerve in the body can be blocked in order to relieve pain. All that is required is an access to the nerve and the agent with which to anesthetize it. Many textbooks and manuals have been published that describe the techniques used to do so, and it is not the intention or purpose of this chapter to duplicate these publications. Many of the available procedures have an application in anesthesia for surgical procedures, but they are not germane to the practice of pain medicine. Some procedures have a role in securing relief of pain postoperatively of after a trauma; for those, the distinction between the discipline of pain medicine and the practice of anesthesia is uncertain. These procedures will not be the focus of this chapter.

The short duration of action of local anesthetic agents limits the utility of local anesthetic blocks in pain medicine. For acute pain, their premier utility lies in providing protection from incident pain. For chronic pain, their most straightforward use is as a diagnostic test. Otherwise, they have putative roles as prognostic tests and as therapeutic interventions.

This chapter focuses on the principles of local anesthetic blocks used in pain medicine. It illustrates the more commonly used and useful procedures and reviews the evidence concerning their validity and utility.


Common Principles

Irrespective of the purpose for which local anesthetic blocks are used, certain principles apply. These pertain to the physician, the patient, the preparation for the procedure, its contraindication, the procedure itself, and its complications.


PHYSICIAN

Any physician who performs nerve blocks should be suitably trained and experienced. He or she needs to understand the pain problem being addressed and how the proposed procedure relates to the patient’s management. He or she needs to be familiar with the patient, the patient’s complaints, and how the patient expresses him- or herself. The physician should be able to explain to the patient the nature and significance of the procedure to be undertaken and, together with the patient, be able to assess and interpret the response. These requirements require training in more than the execution of the procedure.

The physician needs a detailed knowledge of the anatomic basis of the procedure not only to execute it accurately but also to avoid complications. Similarly, he or she needs to be able to deal with the possible side effects and complications of local anesthetic agents. For procedures performed under image guidance, the physician needs to be able to obtain the correct views and to interpret them accurately.

More difficult to define and to achieve is technical excellence to perform the procedure expeditiously yet accurately with the minimum of stress to the patient. This includes avoiding painful structures during the insertion of the needle and keeping the number of adjustments during its course to a minimum. Acquisition of these skills requires good mentoring and forethought on the part of the practitioner and takes years of training.


PATIENT

The patient should understand the purpose of the procedure to be undertaken. This requires distinguishing between diagnostic and therapeutic procedures. Unless they are properly informed, patients may mistake a diagnostic block as a treatment and be disappointed when it appears not to have worked when the effect wears off. They should understand the limitations of prognostic and therapeutic blocks as outlined in the corresponding sections of this chapter. In this regard, there is a difference between a patient consenting to a procedure and being fully informed about it.

The patient should be aware of the potential side effects and complications of the procedures, what actions are to be taken to avoid them, and what will be done in the event that they occur.

Both for diagnostic blocks and therapeutic blocks, the patient will be required to assess and report their response. Therefore, before the procedure, the patient should be instructed in how to use instruments such as pain-rating scales and measurements of function. Patients who have pain in multiple sites but who undergo blocks for only one site need to understand which pain is being tested and must be able to report the effects of the block on that particular site of pain. Relief of pain will need to be corroborated by testing activities that usually are limited by pain. For this purpose, the patient must be able to specify a set of suitable activities before the block and be able to test for their restoration after the block.


PREPARATION

The preparation of the patient differs according to the nature of procedure to be undertaken and the region in which the target nerve lies. Orders for “nil by mouth” are indicated only for procedures in which loss of consciousness and aspiration of gastric contents are possible complications. Examples include cervical transforaminal blocks and procedures that involve high doses of local anesthetic. In the opinion and experience of the authors, the majority of nerve blocks can be performed
safely with the patient having had a light meal even before the procedure. Indeed, fasting may compromise the procedure if the patient is uncomfortable or distressed for not having eaten.

For most local anesthetic blocks, establishing an intravenous line is not indicated, nor is continuous monitoring of respiratory and cardiovascular function required. The doses of local anesthetic typically administered do not pose hazards that require these preparations. However, appropriate precautions are indicated for procedures that are performed close to vital structures, such as the spinal cord or the vertebral artery, or when large doses of a local anesthetic agent are to be administered. In such cases, an intravenous line should be placed before the procedure in order to ensure access for the rapid administration of fluids and resuscitation drugs, should these be necessary. Vital functions should be monitored (i.e., electrocardiogram, pulse oximetry, and blood pressure). A means of ventilating the patient with 100% oxygen with a bag and mask or via endotracheal tube and suction should be available, along with drugs and equipment to manage a cardiac arrest.

Local anesthetic blocks do not require routine sedation. Proper explanation, continuous communication, and good technique will provide patient comfort without the need for systemic medication. In the case of diagnostic and prognostic blocks, using sedation may confound the validity of response by increasing the rate of false-positive blocks,1 for which reason it is best avoided.

Only occasional patients will require sedation: those who are markedly apprehensive or who have a manifest anxiety over needles or invasive procedures. In such cases, sedation can be secured by oral agents (benzodiazepines) or intravenous administration of modest doses of sedatives (midazolam, propofol). If needed, analgesia can be provided by short-acting opioids (fentanyl, alfentanil, remifentanil), provided that the procedure is not diagnostic or prognostic. The dose should be titrated to provide sufficient sedation only for the duration of the procedure; the patient should be fully awake or be readily awakened during the procedure in order to be able to report its effects. If intravenous sedation or analgesia is used, it should be complemented with monitoring of respiratory and cardiovascular function. Supplemental oxygen may be required.





Test Blocks

Test blocks are ones that are not therapeutic injections, prognostic tests, or diagnostic tests in the strict and correct sense of those terms. They are performed simply to test if a particular nerve is involved in the patient’s symptoms. A nerve is anesthetized and the response is either that the patient’s pain is relieved or not. The block is not prognostic because it is not used to predict the outcomes of any subsequent treatment. Nor is it diagnostic because the block is not used to distinguish one source of pain from another. It is used only to see if the nerve anesthetized is involved in the patient’s symptoms.

Virtually any nerve in the body can be blocked in this way. Examples include supraorbital blocks for headache; blocks of the median, ulnar, or radial nerves for pain in the hand; ilioinguinal and iliohypogastric nerve blocks for pain after herniorrhaphy; and tibial nerve blocks for pain in the foot.

The nature of the information derived by test blocks, and its utility, have been poorly elaborated. In some cases, the blocks are used to “confirm” the diagnosis, but the diagnosis will already be evident from other clinical information. The pivotal question—both clinically and philosophically—is what then.

If the patient’s response is used to predicate a subsequent treatment, the test must convert to a prognostic block (see following section). In that event, however, the block and the response to it need to be valid, and the treatment needs to be effective. If those requirements are not satisfied, the utility of the blocks remain unclear.


Prognostic Blocks

A prognostic block is one undertaken to test if a definitive treatment will be successful. The rationale is that if a nerve block with local anesthetic relieves the pain, then a treatment capable of interrupting conduction along the nerve should relieve the pain for a prolonged period, if not permanently.

For prognostic blocks to be valid, evidence is required to complete a table like that depicted in Table 98.1. There needs to be a strong association between a positive response to the block and a successful outcome from treatment and between a negative response to the block and failure of treatment. Such data are hard to produce and, to the authors’ knowledge, are not available. They would require a study in which patients undergo treatment irrespective of their response to blocks. They also require a treatment that is dependably effective. Unless that is the case, failures may be due to the failure of treatment rather than an error in the response to blocks. Few such treatments exist.

In the absence of proper evidence, practitioners who use prognostic blocks assume that a positive response to a block should be predictive of a favorable outcome to treatment. This has not always proven to be the case. For instance, prognostic blocks of peripheral somatic nerves may be used to test the outcome of surgical neurectomy. However, there is no evidence for validity of these prognostic blocks to predict the efficacy of neurectomy. Furthermore, neuroma formation and deafferentation pain complicate the treatment, and worsening due to the treatment can occur also in the presence of positive prognostic blocks. Positive responses to sympathetic blocks are unclearly associated with favorable or sustained outcomes from sympathectomy—either surgical or chemical.


SPINAL NERVE BLOCKS

In some patients with radicular pain, medical imaging is not diagnostic. CT or magnetic resonance imaging (MRI) may show more than one spinal nerve affected by a disk herniation or by foraminal stenosis, and the clinical features do not help to determine which nerve is the source of symptoms. Spinal nerve blocks were developed to assist surgeons to determine the symptomatic level in such cases so that surgery could be directed at the correct segmental level.

Spinal nerve blocks involve placing a needle, under fluoroscopic guidance, into the intervertebral foramen that lodges the target nerve. If anesthetizing that nerve relieves the patient’s pain, then that nerve is implicated as the source of pain. If anesthetizing the nerve does not relieve the pain, then that nerve is excluded as the source of pain, and investigations can be redirected to another nerve. Steps in the execution of a cervical spinal nerve blocks are shown in Figure 98.1.

Because of the proximity of the medullary arteries and the spinal cord, and of the vertebral artery in the cervical spine, spinal nerve blocks require consummate skill in obtaining the correct view, delivering the needle, and checking its location. Guidelines for the safe conduct of the procedure have been published.19,20

Data on the validity of spinal nerve blocks are limited because of two factors. First, nerve blocks should be performed under
controlled conditions in order to guard against false-positive responses, but only one study of spinal nerve blocks has used control blocks.21 Second, the predictive validity of blocks requires a study in which patients with positive responses to blocks and patients with negative responses both be submitted to treatment. Only such a study would provide proper data on the sensitivity and specificity of the blocks. Such studies are difficult to justify ethically and, to the authors’ knowledge, have not been conducted.








TABLE 98.1 Contingency Table Illustrating the Nature of Evidence Required to Establish the Validity of a Prognostic Block























Response to Treatment




Relief


No relief


Response to Prognostic Block


Relief


a


b


No relief


c


d


Partial data are available from studies that report the proportion of patients with positive responses to blocks in whom pathology is found at surgery. This proportion is not the sensitivity of the test but its positive predictive value in the sample tested. Because that value is dependent on the nature of the sample studied, it is not generalizable to other samples or to practice at large.

For lumbar spinal nerve blocks, various studies have reported positive predictive values that ranged between 80% and 100%.22,23 These high values imply that the false-positive rate of lumbar spinal nerve blocks is low and, therefore, that their specificity is likely to be high. In one study, a small number of patients with negative responses to blocks nevertheless underwent surgery.23 Pathology was found in all cases but not at the level tested; in no case did surgery find pathology at the level tested when the block had been negative. These patients proved to have multilevel disease or anomalous nerve roots. This implies that the false-negative rate of lumbar spinal nerve blocks is low and, therefore, that their sensitivity is high.

One study measured sensitivity by performing lumbar spinal nerve blocks in 46 patients with clinical and radiologic evidence of nerve root compression, subsequently confirmed at surgery.24 The sensitivity was 100%, with 95% confidence intervals of 88% to 100%. That same study measured specificity by performing blocks in 23 patients at levels known by radiology not to be symptomatic. No false-positive responses were encountered, and the specificity of lumbar spinal nerve blocks was estimated at “around 90%.24

The one study that used controlled blocks enlisted 18 patients with cervical radiculopathy and 83 with lumbar radiculopathy but did not stratify the results according to region investigated.21 Using a criterion standard of good outcome from surgery, it found a sensitivity of 93% but a specificity of only 33%. Spinal nerve blocks were no better than MRI in predicting good outcome, but the particular advantage of spinal nerve blocks demonstrated in this study was the ability of negative responses to blocks to predict poor outcome, particularly when MRI was negative or ambiguous.


SYMPATHETIC BLOCKS

Blocks of various elements of the sympathetic nervous system have a status somewhere intermediate between diagnostic and prognostic. They are not diagnostic in the strict sense of that adjective, for the cause of pain is usually evident from clinical assessment or other investigations. Fundamentally, they are used to test if the patient’s clinical features are mediated by sympathetic nerves. Knowing this does not affect management unless the response to blocks is used to prognosticate the response to neurolytic treatment of the sympathetic nerves.

Cervical sympathetic blocks have been used largely in the evaluation of patients with complex regional pain syndromes of the upper limb. Other applications have been for patients with pain in the face or head. The procedure requires delivering an aliquot of local anesthetic onto the stellate ganglion. The traditional approach has been a blind technique based on surface markings and palpation. The blind technique classically involved injecting a large volume of local anesthetic in order to ensure saturation of the ganglion. Large volumes compromise the validity of cervical sympathetic blocks because structures other than the target nerve are anesthetized. In a study of eight volunteers, MRI showed that injectate was not delivered to the stellate ganglion but passed anterior to it.25 Several complications following stellate ganglion blocks have been described, mostly due to inadvertent puncture of sensitive structures.26,27,28,29,30 For these reasons, image-guided techniques are preferred.

Ultrasound guidance is an alternative to fluoroscopic guidance and offers certain advantages in this region. Ultrasound demonstrates the common carotid artery, internal jugular vein, thyroid gland, and esophagus—all of which need to be avoided during passage of the needle (Fig. 98.2). In a study that compared ultrasound guidance with the blind technique, the volume of local anesthetic required was reduced to 5 mL, the onset of block was sooner, and hematoma was avoided (but occurred in 3 out of 12 cases with the blind technique).31 A study in healthy volunteers showed that the esophagus, the vertebral artery, and other arteries are located in the needle path of the traditional blind technique and that manual dislocation did not displace these structures from the needle path in the majority of cases.32 Although comparative studies are lacking, it is reasonable to avoid the blind approach. Ultrasound seems preferable to fluoroscopy due to its ability to visualize structures that must be avoided during the needle passage.

Lumbar sympathetic blocks involve anesthetizing the lumbar sympathetic trunk, typically at the L3 level. They have traditionally been used to test if they relieve pain or other features of complex regional pain syndrome. The standard technique is fluoroscopically guided (Fig. 98.4).

The use of fluoroscopy or ultrasound establishes the face validity of sympathetic blocks (i.e., the sympathetic trunk is accurately and selectively infiltrated). Face validity can be confirmed physiologically by observing a rise in skin temperature in the limb of the target side, which suggests that the sympathetic trunk has indeed been anesthetized.

What has not been shown is that sympathetic blocks have construct validity (i.e., that the physiologic response is due to the effects of the local anesthetic and not to nonspecific factors such as placebo). This requires the execution of controlled blocks. Failing to use controlled blocks in the past may have led to overestimates of so-called sympathetic-mediated pain. Unless and until studies are conducted using controlled blocks, its true prevalence will not be known. To the authors’ knowledge, no studies have tested the validity of lumbar sympathetic blocks. Nor has the prognostic validity of sympathetic blocks—cervical or lumbar—in predicting the outcomes of sympathectomy been demonstrated by rigorous research.

Other blocks of the sympathetic nervous system are practiced, ostensibly widely. These include blocks of the celiac plexus, splanchnic nerves, hypogastric plexuses, and blocks of the ganglion impar. However, there is no evidence on their validity as either diagnostic or prognostic procedures.


Diagnostic Blocks

Many conditions associated with chronic pain have no detectable anatomical correlate. The cause of pain is not evident on conventional medical imaging. For some such conditions, although the cause might not be evident, the source of pain can be established using diagnostic blocks.

The rationale for diagnostic, local anesthetic blocks is that if a structure is the source of pain, then anesthetizing it or its nerve supply should relieve the pain. If the suspected structure is not the source of pain, then anesthetizing it should not relieve the pain.

Two types of blocks have been used to pinpoint sources of pain: intra-articular blocks and nerve blocks. Intra-articular blocks involve injecting local anesthetic into a joint suspected of being the source of pain. Diagnostic nerve blocks are restricted to small nerves that have a limited and specific distribution.
Blocks of larger nerves or of nerves that supply multiple tissues are not diagnostic of a particular source because of their widespread distribution; they fall under the category of test blocks.






FIGURE 98.4 Stages in the conduct of a lumbar sympathetic block. A: An oblique view showing a needle passing tangential to the lower anterolateral surface of the L3 vertebral body. B: Anteroposterior view, showing the needle in place over the anterolateral aspect of the L3 vertebral body. C: Lateral view showing the needle in place over the anterolateral aspect of the L3 vertebral body. D: Lateral view showing contrast medium spreading longitudinal over the anterior surface of the psoas major muscle. (Illustrations kindly provided by Dr. Milton Landers.)


PRINCIPLES


Controls

There are no objective tests for the presence of pain or for its relief. Investigators rely only on what the patient reports, but patients can report relief of pain for reasons other than the effect of a local anesthetic injected during a diagnostic block. They may have an expectation that the block will relieve their pain, particularly if they have been coached or instructed to expect a positive response. Unprompted, they might want the response to be positive so that they qualify for the resultant treatment that promises to relieve their pain in the long term. Patients with medicolegal claims might choose to report relief in order to vindicate their claim.

Because of these extraneous, confounding factors, a positive response to a block cannot be summarily attributed to the effect of the local anesthetic injected. Some form of control is necessary if the response is to be valid and for the interpretation of the response to be correct. The most rigorous form of control is a placebo control in which an inactive agent is injected to test for spurious responses. However, for two reasons, placebos cannot be administered in an arbitrary manner. In the first instance, administering a placebo on a single-blind basis is considered unethical in some jurisdictions. Patients would need to be informed that they might receive an inactive agent. Furthermore, a positive response to placebo does not necessarily rule out that the tested structure is symptomatic.

Comparative local anesthetic blocks can be used as alternatives to placebo-controlled blocks. These involve administering a particular agent on the occasion of the first block but using a different agent on a second occasion. The agents advocated are lidocaine and bupivacaine. The paradigm maintains that a genuine patient will report short-lasting relief when lidocaine is used and long-lasting relief when bupivacaine is used. However, it is unclear whether a discordant response truly rules out a positive diagnosis. Patients may display prolonged pain relief after lidocaine for different reasons: The block may reduce central sensitization processes that contribute to pain intensity; the pain relief may have positive psychological effects, leading to long-lasting pain relief; the patient may experience prolonged pain relief because he or she will rest after the block, giving the false impression that the block is responsible for the absence of pain; etc.

Despite the limitations associated with placebo-controlled and comparative blocks, data on diagnostic blocks for zygapophysial joint pain presented in the following sections have shown that single blocks are associated with high false-positive rates. Therefore, a form of control is currently an appropriate standard of care.


Criteria for Positive Response

For a response to a diagnostic block to be valid, certain criteria should be satisfied. On each occasion that the nerve is blocked, the patient should obtain substantial relief of pain. That should be validated by recording the intensity of pain before and after the block. Moreover, for the block to be physiologically and pharmacologically meaningful, the pain should return as the
effect of the local anesthetic wears off. Recording the return of pain, therefore, becomes part of the assessment. A painful structure should resume being painful once the local anesthetic has ceased to operate. Prolonged relief of pain, lasting days, weeks, or months, occasionally occurs but is of difficult interpretation. Prolonged responses suggest either a placebo effect or some sort of modulatory effect on pain perception, the nature of which has not been determined.

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Sep 21, 2020 | Posted by in PAIN MEDICINE | Comments Off on Diagnostic and Therapeutic Nerve Blocks
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