Use of Local Anesthetics to Block a Nerve
Injection of a local anesthetic to achieve a temporary block in transmission of chronic noncancer pain and pain generated by the sympathetic nervous system sometimes allows prediction of what will happen if an attempt is made to destroy a nerve. In addition to performing such diagnostic nerve blocks, a variety of painful conditions arising from cancer are treated by blocking nerves with repeated injections or a continuous infusion of local anesthetics.
The procedure is simple: To block a nerve, the area around the nerve should be infused with a local anesthetic (often mixed with a steroid). Myoneural injections are performed in the location of trigger points, which are distinct localized areas of tenderness within a taut band of muscle that runs parallel to the direction of the muscle fibers. These tender areas are hard and are linear or nodular in shape. Palpation can cause pain, produce altered sensations, or elicit a local twitch response. Trigger points can be treated by inserting a dry needle into the site and then manipulating the needle to break up the congested area. Sometimes, small amounts of a local anesthetic or of botulinum toxin are also injected into the target muscle.
Any intervention carries a risk inherent to the procedure. The risks and benefits of all procedures, therefore, must be weighed. With nerve blocks, the patient's coagulation status and bleeding time should always be con-
sidered. Certain malignancies and anticoagulation therapy can alter blood chemistry and increase the possibility of the procedure causing bleeding into the site or neuraxis, which can cause permanent nerve damage, paralysis, and even death. Other factors that may contraindicate nerve block injections are hypovolemia, dehydration, and cachexia.
As noted, nerve blocks are performed for two purposes, diagnostic and therapeutic. Diagnostic nerve blocks differentiate among the mechanisms that may be responsible for the patient's persistent pain. Because many mechanisms can cause pain, there are also many types of nerve blocks; some are described next.
Therapeutic nerve blocks alleviate pain complaints after diagnosis. Injection of a local anesthetic with or without a steroid can provide symptom relief. There is also evidence that a repeat series of injections may provide long-term pain relief.
Evidence about the efficacy of nerve blocks in general, however, is contradictory. A review of the published reports to 1997 was inconclusive about the efficacy of epidural injections in low back pain and sciatica (4). Koes et al. conducted a similar review in 1995 and drew a similar conclusion (5). Other investigators, however, found epidural injections an effective treatment for sciatica (6). Part of the problem may rest with variations in the method used to administer the injections or in degrees of proficiency in appropriate patient selection. Or, as suggested by Hopayian and Mugford, the origin of the discrepancy may simply lie in the difference in choice of methods used to conduct the systematic reviews (7).
Blocking Specific Sites
The small, diarthrodial facet joints provide posterior support to the spine. As people age, arthritis associated with severe pain can develop in these joints. There are two types of blocks: intraarticular (steroids) and posterior rami blocks (local anesthetics followed by Demerol). Facet blocks are performed in the lumbar, thoracic, or cervical spine. A patient with facet syndrome may experience tenderness in the area of the facet joints, pain on movement or twisting of the spine, or local muscle spasms or hyperalgesia. Pain is generally exacerbated with ipsilateral extension of the spine. Discomfort on the contralateral side, however, is more consistent with myofascial pain
Patients with lumbar facet syndrome experience a deep, dull ache unilaterally or bilaterally in the lower back. This pain may radiate to the buttock, groin, or hip or from the posterior thigh to the knee.
The stellate ganglion innervates the face, upper extremities, and heart. The stellate ganglion (see Fig. 2A,B) is blocked to treat sympathetically maintained pain (regional sympathetic dystrophy or complex regional pain syndrome) of the upper extremity and face, frostbite, prolonged QT interval, hyperhidrosis, acute herpetic neuralgia, and angina. A stellate ganglion block involves injecting 10 cc local anesthetic over the C6 transverse process (casserians tubercle) above the longus coli muscle. If patients experience significant pain relief following an injection (with no blockade of the motor and sensory fibers), it can be determined that the pain is sympathetically maintained. In these cases, repeated blocks frequently reduce pain. Celiac Plexus Block
The celiac plexus innervates the upper abdominal structure; thus, celiac plexus blocks (CPBs) can relieve pain originating from viscera in
this area. Patients with pain from upper abdominal malignancy, especially pancreatic cancer, as well as those with other forms of visceral
disease, may benefit from a CPB (see Fig. 3A,B). A successful CPB allows patients to reduce consumption of analgesic drugs and thus reduce drug-related adverse effects. The diarrhea that is a common side effect of CPB usually resolves with conservative treatment. Many practitioners avoid CPB for nonmalignant pain because of the rare, but catastrophic, risk of causing a lumbar plexus injury or anterior spinal artery syndrome, which can result in paraplegia. Lumbar Sympathetic Block
Lumbar sympathetic block (LSB) may be useful for treating sympathetically mediated pain of the lower extremities (see Fig. 4A,B). In patients with advanced peripheral vascular disease, for example, neuro-lytic or radiofrequency lesioning can lead to a 50% long-term improvement in blood flow and relief of pain and ulceration. A phenol LSB can relieve pain and permit healing of gangrenous ulcers in diabetic patients who experience pain on resting and in nondiabetic patients with digital gangrene or digital ulcers. When used diagnostically, LSB can predict the result of surgical sympathectomy for causalgia following lumbar laminectomy.
Patients with short-term low back pain thought to be consistent with lumbar facet pain can be considered for intraarticular steroids (8). If pain persists, a denervation is indicated (see "Nerve Destruction").
The superior hypogastric plexus innervates the pelvic area (Fig. 5A,B). Superior hypogastric plexus block (SHPB) can help patients suffering from pelvic pain of colon or cervical cancer (9). if patients have extensive retroperitoneal disease overlying the plexus, however, the neurolytic agent may be hindered from spreading appropriately to achieve the desired results (10).
Computed tomographic guidance is useful for administering SHPB to manage chronic pelvic pain in the presence of endometriosis. SHPB has also been used to treat severe, chronic, nonmalignant penile pain.
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