The placement of local anesthetics at various sites along the neural axis is an important tool in the diagnosis and treatment of a variety of pain disorders, such as complex regional pain syndrome and postherpetic neuralgia. Peripheral nerve blocks also can provide muscle relaxation and pain relief to facilitate an active physical therapy program.
When the point of injection has been determined, it is best marked with the tip of a retracted ballpoint pen or a needle hub by pressing the skin to produce a temporary indentation to mark the point of entry. The skin is then prepared in a standard sterile fashion, and sterile technique is used throughout the procedure. The skin and subcutaneous tissue at the injection site may be anesthetized by injecting 1% lidocaine with no epinephrine using a 25- to 30-gauge needle. Alternatively, a vapocoolant spray or analgesic cream applied to the skin surface may be used to provide adequate anesthesia. Before injecting the medication, an attempt to aspirate should always be made to avoid accidental intravascular injection. After ensuring that the needle is in the joint space, the medication should be injected in a slow, steady fashion.
Indications for Nerve Blocks
Neural blockade may be used for the diagnosis, prognosis, and treatment of pain. Selective nerve blocks are indicated to determine the etiology of pain by isolation of specific anatomic structures. Selected nerve blockade is used to determine specific nociceptive pathways and other mechanisms involved in pain generation. Diagnostic blocks assist in narrowing the differential diagnosis of the site and cause of pain. Prognostic neural blockade is used to evaluate the possible outcome from neurolytic procedures. Therapeutic nerve blocks are indicated to decrease morbidity in acute postoperative pain, posttraumatic pain, and pain resulting from self-limiting conditions. Nerve blockade may provide rapid relief of pain and facilitate the patient’s participation in a comprehensive rehabilitation program. Therapeutic nerve blocks may interrupt the pain cycle sufficiently to provide prolonged pain relief.
Contraindications for Nerve Blocks
Absolute contraindications for regional anesthesia include patient refusal, localized infection, a skin condition that prevents adequate skin preparation, the existence of a tumor at the injection site, a history of allergy to local anesthetics, the presence of severe hypovolemia (for blocks that could result in significant sympathetic blockade), gross coagulation defects, septicemia, and increased intracranial pressure (spinal, caudal, and epidural).
Prilocaine should not be used in doses greater than 600 mg, because significant methemoglobinemia may result. The use of corticosteroids with preservatives is contraindicated in epidural and subarachnoid techniques because the preservative may result in seizures and permanent CNS damage.
Relative contraindications include general medical conditions that would put the patient at increased risk. These include aortic stenosis, severe lung disease, sickle cell anemia, and preexisting neurologic diseases such as multiple sclerosis or amyotrophic lateral sclerosis, which could be worsened during regional anesthesia.
Complications common to nerve blocks include hypotension from sympathetic blockade. This usually occurs in patients who are hypovolemic and receive a block covering a large portion of the sympathetic chain ganglia, for example, during spinal or epidural blockade. Local anesthetic overdose or intravascular injection can result in CNS toxicity and, in some cases, pulmonary and cardiac arrest. Nerve injury from contact with the needle may occur but is rare, especially when a short, beveled needle is used. Other complications are dependent on the location of the block and are discussed separately.
Before the injection, the appropriate landmarks are located and marked. The skin is scrubbed with antiseptic and allowed to dry for 2 minutes. The wearing of sterile gloves is required so that the bony landmarks in the sterile field may be palpated throughout the procedure. The standard sterile technique is required to minimize the risk for infection. It is preferable to use single-dose vials of the local anesthetic because this further reduces the risk for infection. A 25- to 27-gauge needle is used with 1% lidocaine with no epinephrine to raise a small skin wheal for skin anesthesia. Routinely a 1½-in. (4-cm), 21- to 25-gauge needle transverses the skin, joint capsule, synovial lining and then slides smoothly into the joint cavity.
Aspiration is done to ensure there is no intravascular penetration. If penetration occurs, the needle should be repositioned and aspirated to ensure that blood vessels have been avoided; then the medication is slowly injected. After the medication has been injected, the needle should be cleared with a new syringe containing a small amount of lidocaine or saline. The needle is then withdrawn with pressure applied to minimize bleeding.
Peripheral nerves may be localized with a nerve stimulator using a small adjustable amount of electrical current to depolarize neural tissue in proximity to the needle. The cathode (negative) terminal is connected to the needle, and the anode (positive) terminal is connected to a grounding patch. The stimulator initially is set to deliver 10 to 20 mA of current to detect the general area of the nerve. The current is then reduced to further localize the nerve. The needle is positioned to produce the maximal twitch at the lowest stimulus. The needle is usually adjacent to the nerve when 0.5 to 0.1 mA produces motor stimulation with an insulated needle and 1 mA with an uninsulated needle (Fig. 67-1).
FIGURE 67-1. Nerve stimulator attached to regional block nerve. The negative (black) lead is attached to the exploring needle, whereas the positive (red) lead is connected to the reference electrocardiogram pad used as the ground reference. Note the current distribution pattern for this uninsulated needle.
Nerve stimulators do not substitute for knowledge of anatomy and proper needle placement. Insulated needles increase the point of maximal current density at the needle tip and are used for precise localization of specific nerves. Uninsulated needles are often accurate enough for many nerve blocks; however, both the tip and shaft of the needle have sufficient current density to stimulate a nerve. Local muscle twitches from the shaft of the uninsulated needle should not be confused with the response from the nerve to be blocked (Fig. 67-2).