Local Anesthetic Blocks and Epidurals

Diagnostic Nerve Blocks

These blocks are useful in searching for the location of the cause of the pain; infiltration of a local anesthetic into a neuroma, a painful joint, or a trigger point may indicate the source of the pain.

However, it may be difficult to categorically determine whether the pain has only a peripheral source or whether the pain is also or mainly due to CNS pathology. Reducing input from the periphery may dampen hyperexcitable neurons in the dorsal horn of the spinal cord. Failure to appreciate this has caused many peripheral nerves to be cut, burned, or frozen, which produces only short-lasting effects and leaves numbness and additional abnormal pain in the denervated area.

Specific sympathetic nerve blocks may help determine any sympathetically maintained pain component in more complex chronic pain states; see Box 37-3 .

For a diagnostic block to be useful, detailed anatomical knowledge, technical skills, and experience are mandatory. The physician must have thorough knowledge of the pain syndromes being diagnosed and their overall evaluation and management. It is crucial that the block be conducted with finesse and gentleness so that nervous tissue is not damaged and the patient is not disturbed by a traumatic and painful experience.

The physician must be fully aware of potential side effects and complications of nerve blocks and must be prepared and skilled at dealing with any that do arise.

Nerves must be accurately targeted with a nerve stimulator, ultrasound, or fluoroscopy. Initially, a short-acting local anesthetic such as lidocaine or chloroprocaine should be used. A small volume (0.5–3 mL, depending on the site) must be used so that nearby nociceptors and nerve fibers do not become contaminated. If a short-acting local anesthetic block relieves the pain for about an hour, the block should be repeated with a longer-acting agent such as bupivacaine, which should give pain relief for at least 2 hours, depending on the concentration and volume injected.

If a diagnostic block is performed to localize a nerve to be treated with a neurolytic agent, cryoneurolysis, or radiofrequency denervation, repeated blocks (preferably with a placebo injection) should be performed ( , , , ).

Pain relief can follow the injection of saline alone. This can be a true placebo response but might also be caused by a counter-irritation effect, or peripheral input that dampens a central component of a chronic pain condition. Pain relief from a saline “block” certainly does not by itself mean that the pain is “psychogenic” or that the patient is malingering.

Prognostic Blocks

These blocks are meant to indicate whether destruction of the peripheral nerve will give long-lasting pain relief. However, this is unreliable, in part because the pain may be maintained by central dysfunction in the spinal cord or at higher CNS sites.

Preventive Nerve Blocks (Not “Pre-emptive” Blocks)

This concept received a lot of attention after the hypothesis was launched 2 decades ago that postoperative pain could be prevented by a nerve block established before surgery. This is not the case. However, a prophylactic block established before surgery and continued during and after surgery for as long as the patient has severe movement-triggered pain constitutes optimal postoperative pain management and may have prolonged beneficial effects, such as facilitating active rehabilitation after surgery and possibly reducing risk for the development of chronic pain after surgery ( ).

Therapeutic Local Anesthetic Blocks

For acute pain after surgery or trauma, an appropriate nerve block can relieve the pain completely for the duration of the local anesthetic effect. The duration of pain relief can be extended by administering a dilute local anesthetic by continuous infusion or by patient-controlled bolus injections into a catheter placed near a nerve or a nerve plexus.

For chronic pain the efficacy of nerve blocks is less impressive. However, experienced pain clinicians are convinced that in some patients local anesthetic blocks may give pain relief that far outlasts the specific local anesthetic block of the peripheral nerve ( ). This may be due to effects on hypersensitive nociceptors and peripheral C and Aδ fibers and/or to effects in the CNS from systemically absorbed local anesthetic drugs.

Regional blocks may facilitate a mobilization regime for patients with complex regional pain syndromes (CRPS).

Finally, there is a strong, non-specific, positive “context-sensitive therapeutic effect” when the physician demonstrates to the patient that the pain can be taken away completely, though transiently. It also helps in explaining pain and pain mechanisms to patients, reducing anxiety, and improving coping. A successful block may reinforce the effects of other measures taken to help the patient.

Neurolytic, Neurodestructive Nerve Blocks

Phenol in aqueous solution (up to 6.7%), phenol (8%) in glycerol (requiring a large-bore needle), or ethanol (up to 96%) is used to interrupt conduction of impulses in peripheral nerve fibers. Initially a burning and tender inflammatory reaction occurs, followed by a numbing pain relief that peaks after a few days. Unfortunately, the duration of nerve impulse block with these neurolytic agents is often disappointingly brief. Even worse, they induce a deafferentation–neuropathic type of pain after a few weeks to months in up to one-third of cases. Except for classic trigeminal neuralgia, they are not indicated for patients with chronic pain and a normal life expectancy. In patients with localized pain from advanced cancer disease, the duration of effect of such neurolytic blocks may be sufficient ( ).

Cryoneurolysis (“cryoanalgesia”) causes its effect by freezing nerve segments to −70°C for 2–4 minutes; it is repeated two to three times with thawing in between. This is probably the most benign of the neurolytic procedures, with a low incidence of neuritis, although the risk for development of neuropathic pain is not zero ( ).

Denervation by heating the nerve to 70–80°C for brief periods with a radiofrequency probe that has a smaller dimension than the cryoprobe may cause more profound destruction in a localized area ( ).

Agents

• The local anesthetics most commonly used are short-acting lidocaine (lignocaine) and chloroprocaine and the longer-acting bupivacaine, ropivacaine, and levobupivacaine.

• Adrenaline may be added to slow absorption and thereby increase safety and the duration of effect. Adrenaline, being an α ₂ -agonist, has analgesic effects of its own in the spinal cord ( ). Adrenaline at a concentration of 1.5–2 μg/mL is optimal ( ); higher concentrations may cause local ischemia or cardiac dysrhythmias.

• Glucocorticoids enhance the acute pain-relieving effect of local anesthetics, in part by their anti-inflammatory effect, but also perhaps because of separate analgesic effects ( ). Glucocorticoids appear to prevent and reduce the hyperexcitability of nociceptors and afferent nerve fibers and also to reduce secondary, central hypersensitivity ( , ). In experimental animal studies, chronic neuropathic pain behavior can be reduced by local glucocorticoid treatment ( , ).

Acute Pain

• Infiltration of a dilute local anesthetic solution directly in loco dolente of acute painful conditions via a single shot or infusion through a catheter placed in the wound area is used widely after surgery ( ).

• Bursitis of the shoulder region and knee region, trochanteric bursitis, tendonitis of the elbow (lateral: “tennis elbow”; medial: “golfer’s elbow”), and biceps and rotator cuff tendonitis of the shoulder are commonly relieved with local anesthetic infiltration with or without a glucocorticoid. If a glucocorticoid is added, this should not be repeated more than once or twice a year because of the risk for tendon rupture.

• Muscle spasm causes severe pain, which can be effectively relieved for a few hours by local anesthetic infiltration of the spastic muscle fibers, during which time massage, stretching, and corrective exercises may be applied. Only dilute concentrations of a local anesthetic should be used (e.g., bupivacaine, 1 mg/mL), and it is not wise to repeat the injections more than three times because of possible myotoxic effects of repeated exposure to local anesthetic drugs.

Chronic Pain Conditions

When such conditions are localized, they can be relieved temporarily with local anesthetic blocks, sometimes for prolonged periods. Spontaneous ectopic discharges in a “trigger point,” in a painful scar after surgery or trauma, or in an amputation stump can be suppressed by infiltration with a local anesthetic with a depot glucocorticoid added ( ).

“Trigger points” of myofascial syndromes are treated by infiltration of dilute local anesthetic solutions, with or without glucocorticoid. Depending on what is done after the infiltration (stretching, cold, massage, appropriate exercises), the beneficial effects may persist for longer periods. Patients with uncomplicated myofascial pain syndromes will benefit; patients with more complex chronic pain conditions will need a comprehensive approach, including appropriate cognitive therapy ( ).

Arthritic pain can be relieved effectively by intra-articular injection of dilute local anesthetic with glucocorticoid added ( ). All major limb joints, as well as facet joints of the spine, can be injected ( ). The duration of pain relief depends on the degree and duration of the arthritic changes.

Topical application of local anesthetic drugs is also a form of local infiltration. Jellies or creams are useful for mucous membrane pain from the urethra, urinary bladder, and rectum. Ointments and concentrated solutions can temporarily relieve the often excruciating pain from oral mucositis in cancer patients and bone marrow transplant patients. Eutectic mixtures, ointments, and patches are used on the allodynic and painful skin of patients with, for instance, post-herpetic neuralgia ( ). Relief is obtained locally, but the local anesthetic is absorbed and the systemic effects may dampen the CNS hyperexcitability of this complex peripheral and central neuropathic pain ( ). Systemic toxic concentrations can occur if patches are kept on the skin for more than 12 hours.