Temporary Neural Blockade

CHAPTER 38 TEMPORARY NEURAL BLOCKADE



Michael M. Hanania, MD, Martin R. Boorin, DMD, Charles E. Argoff, MD




1. Describe the role of temporary nerve blocks in pain management


Nerve blocks can help in the diagnosis and treatment of pain. Peripheral and central nerve blocks help to localize the origin of the specific pain problem. When pain seems to cover multiple dermatomes, a selective peripheral nerve block helps to diagnose which nerve or dermatome is primarily responsible for the pain. Temporary nerve blocks are also necessary before more permanent neurolytic procedures can be attempted. When a permanent neurolytic procedure is contemplated, a temporary nerve block should produce significant pain relief before the longer-lasting block is instituted.


For unclear reasons, a temporary nerve block sometimes results in prolonged pain relief, outlasting the duration of the local anesthetic. In cases of somatic pain, such as mechanical low back pain, temporary nerve blocks may break the pain cycle and allow increased function. This helps to facilitate physical therapy and rehabilitation, thereby preventing disuse muscle atrophy and joint dysfunction.


In some syndromes (such as nerve injury) it is unclear whether the primary pain generator is peripheral (at the site of nerve injury) or central (within the spinal cord). If peripheral nerve block produces complete pain relief, it is inferred that the generator is peripheral.



2. What is the mechanism of action of local anesthesia? What are the implications for differential blockade?


Local anesthetics are weak bases that reversibly block sodium channels and impair the propagation of an action potential along a nerve fiber. In general, thinner fibers are more sensitive to anesthetic blockade than thicker fibers. Unmyelinated fibers are more sensitive than myelinated fibers. Sympathetic fibers are very thin. Nociceptors (which are responsible for the perception of noxious stimuli) are A-delta fibers and C fibers (also quite thin, although somewhat thicker than the sympathetic nerves). Fibers subserving proprioception and light touch are relatively thick; motor fibers are the thickest. Thus, when progressively increasing concentrations of an anesthetic are used, sympathetic fibers are blocked first, followed by nociceptors, proprioceptors, and motor fibers. Function returns in the opposite order as anesthetics wear off.



3. Which are the ester and amide local anesthetics? Discuss their fundamental differences


The ester local anesthetics have an ester linkage between the aromatic moiety and amine group and are metabolized by plasma cholinesterases. Their half-lives in the circulation are short. A product of their metabolism is p-aminobenzoic acid, which is sometimes associated with hypersensitivity reactions. Procaine, cocaine, chloroprocaine, and tetracaine are ester anesthetics.


The amide local anesthetics have an amide linkage and undergo primarily hepatic metabolism. They include lidocaine, mepivacaine, bupivacaine, levobupivacaine, ropivacaine, and etidocaine. Levobupivacaine and ropivacaine are relatively new local anesthetics that produce more sensory than motor blockade and have a lower cardiotoxicity profile bit otherwise have features that are identical to bupivacaine.



4. What are the differences in duration of action among the commonly used local anesthetics?


Duration of action of the local anesthetics depends on a number of factors including the agent in question, the vascularity of the tissue into which it is injected, and, with some of the local anesthetics, the coadministration of epinephrine. Therefore, lidocaine has a shorter duration of action than bupivacaine, levobupivacaine, and ropivacaine. Topical applications to mucous membrane (which has a high vascularity) are associated with shorter anesthesia than injection into, for example, muscle. Coadministration of epinephrine (a vasoconstrictor) prolongs the duration of action of lidocaine (a vasodilator), but has no effect on the duration of action of bupivacaine, levobupivacaine or ropivacaine (which have no significant effect on vascular tone). In addition to prolonging the action of lidocaine, epinephrine increases the maximum dose of lidocaine that can be used before toxic symptoms arise.


The concentration of local anesthetic used does not significantly alter the duration of action.



5. List the toxic dose limits for peripheral nerve blockade for lidocaine and bupivacaine


Overdoses of lidocaine and bupivacaine usually produce symptoms indicative of central nervous system dysfunction (dizziness, tinnitus, metallic taste, slurred speech, and seizures). These are often followed by cardiovascular symptoms, such as hypotension, tachyarrhythmias or bradyarrhythmias, ventricular fibrillation, and, in extreme cases, electrical standstill. Toxic doses are dependent on the site of injection, with increasing risks of a toxic, as opposed to anaphylactic, reaction with injection into highly vascular tissue. Approximate maximum dose limits are as follows:
















Lidocaine 2-3 mg Kg−1
Lidocaine with epinephrine 6-7 mg Kg−1
Bupivicaine 2-3 mg Kg−1
Bupivicaine with epinephrine 2-3 mg Kg−1

Remember that epinephrine should not be added to a local anesthetic where it is to be injected in the territory of an end artery (e.g., digital or penile nerve block).



6. What is the role of neuraxial block in pain management?


Epidural or spinal blocks provide temporary block of neural function at a spinal cord level. They help to differentiate a peripheral from a central origin of pain. If a peripheral nerve or root block has failed but a spinal block relieves pain, it may be inferred that the origin of the pain is more central (within the spinal cord), provided that the operator is confident that the nerve block has actually been achieved. Epidural blockade also offers the option of leaving the catheter in place for prolonged treatment. Thus, epidural analgesia or anesthesia can be provided on an ongoing basis or through intermittent boluses. This procedure may facilitate rehabilitation. The use of neuraxial block, while well established in acute pain management and anesthetic practice, is not supported by conclusive evidence in the field of chronic pain management.



7. List the possible complications of epidural or intrathecal anesthetic blockade




image Epidural blockade involves the risk of inadvertent dural puncture. When this occurs, cerebrospinal fluid (CSF) may leak, and a postlumbar puncture (low-pressure) headache may develop.


image Rarely, the needle used for puncture may injure a nerve root.


image Also rare are epidural abscesses, epidural hematoma, or injury to the cauda equina. With spinal injection, cord trauma is possible if spinal injection is made above the level of the conus.


image Local anesthetics may induce temporary neuritis.


image Cephalad migration of the anesthetic agent may produce an anesthetic level much higher than the level anticipated. In cervical blocks, cephalad migration may lead to respiratory arrest.


image Transient hypotension is fairly common as a result of vasodilatation (sympathetic nerve block).


image Intravascular injection could occur.



8. What are the indications and contraindications for epidural steroid injections?


Indications for epidural steroid injections (ESIs) are still controversial. Efficacy for chronic low back pain is uncertain. In acute exacerbations of radicular nerve pain, injections may be of some benefit. Radicular pain is more likely to respond to steroid injections than is mechanical low back pain. Recovery from an acutely herniated lumbar or cervical disc may be hastened by ESIs. With acute low back pain, most clinicians use 2 to 3 weeks of conservative management before attempting ESIs. An interlaminar approach to ESI seems to be least helpful in spinal stenosis, mechanical low back pain, and long-standing, chronic low back pain.


A recent study by Manchikanti and colleagues suggests that a caudal approach to epidural steroid injection is more effective for chronic low back pain than lumbar epidural. A transforaminal approach under fluoroscopic guidance is felt to sometimes help more with a far lateral disc herniation with nerve root inflammation than midline epidural.


Contraindications to any epidural injection include coagulopathy, local infection, or progressive weakness that may require surgical decompression.



9. What is the appropriate timing of epidural steroid injections?


A steroid formulation such as methylprednisolone or triamcinolone is usually injected in doses of 40 to 80 mg every 2 to 4 weeks to a maximum of three injections in 6 months. Evidence suggests that additional injections offer no added benefit and may increase the risk of side effects. Adrenal suppression with decreased cortisol levels has been shown to occur for 3 to 5 weeks after a typical ESI. Multiple injections may produce ligamentous laxity.



10. What are the various brachial plexus blocks?


The brachial plexus can be blocked proximally at the level of the nerve roots and trunks by the interscalene approach; at the level of the divisions and cords by the supraclavicular or infraclavicular approach; and distally at the level of the terminal branches or nerves by the axillary approach. A brachial plexus block can be performed by single injection or continuous catheter technique for repeated injections or infusion.



11. For what pain conditions are brachial plexus blocks useful?


Evidence for efficacy of brachial plexus blocks is anecdotal. However, some find them useful in conditions such as complex regional pain syndrome (CRPS), acute or postherpetic neuralgia involving dermatomes within vertebrae C5-T1, vascular insufficiency of Raynaud’s disease, and phantom upper limb pain. Brachial plexus block also facilitates range-of-motion (ROM) exercises in cases of frozen shoulder, elbow, or wrist. Because the thoracocervical sympathetic nerves travel in close relation to the somatic nerves of the brachial plexus, brachial plexus block results in sympathetic blockade as well. It also may be used for postoperative pain management and prevention of phantom limb pain after amputation.



12. What are the possible side effects and complications of the various brachial plexus blocks?


Hematoma, neuropathy, and unintentional intravascular injection are possible with any of the brachial plexus blocks. Interscalene block may be associated with unintentional injection of local anesthetic into the epidural or subarachnoid space; recurrent laryngeal nerve block, which causes hoarseness; phrenic nerve block; and ipsilateral Horner’s syndrome. Pneumothorax is most often associated with the supraclavicular block.



13. What is the sensory innervation of the lower extremities?

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Jun 14, 2016 | Posted by in PAIN MEDICINE | Comments Off on Temporary Neural Blockade

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