Injection procedures use three primary classes of medication: local anesthetic agents, neurolytic agents, and corticosteroids. All of these medications have multiple clinical applications and are documented to be safe and effective when used appropriately. It is incumbent on the practitioner to understand the efficacy, complications, and common side effects of these agents.
The mechanism of local anesthetic action is a reversible block of ion flux through the axon’s sodium channels. By blocking transmission from the peripheral nerves, no input is detected at the central nervous system (CNS). The degree of neural blockade depends on the properties, absorption, amount, location, and other characteristics of the drug injected (28,29) (Table 67-2). For purposes of attaining longer duration analgesia, agents such as bupivacaine or ropivacaine are acceptable and safe when used within the dosing guidelines. Anesthetic agents with fast onset, such as lidocaine, are useful particularly for skin infiltration prior to block insertion or for rapid diagnosis of the pain generator site. Local anesthetics should be chosen appropriately for both the purpose and the site of injection. For example, commercial preparations may come with preservatives, making them potentially neurotoxic if given intrathecally. Therefore, it is imperative to verify that the particular medication preparation is approved for the desired use. This information is available in many locations, including the package insert for the medication.
aProcaine = 1.
CNS, central nervous system; CVS, cerebrovascular system.
When higher doses of local anesthetics are used (i.e., near the upper limit of recommended dose), are injected near highly vascular structures (face, intercostals), or injected near the neuraxis (paravertebral, epidural, or intrathecal), monitoring of the patient is highly recommended. Verbal communication, constant screening for symptoms, and pulse oximetry can identify any CNS toxicity, while the electrocardiogram and blood pressure can identify any cardiovascular toxicity. If signs of toxicity or high spinal block are present, injection should be suspended and the patient closely evaluated and/or supported.
Neurolytic nerve blocks were popular in the early 1900s. However, improved use of analgesics, as well as the use of radiofrequency and cryoablative techniques, has replaced their use in most instances. Alcohol and phenol are the most widely used neurolytic agents in the United States. These agents indiscriminately affect motor and sensory nerves.
Phenol can be used intrathecally and epidurally, as well as for peripheral nerve and motor point blocks. It is poorly soluble in water and is often added to glycerin to achieve concentrations higher than 7%. It also can be added to radiographic contrast to allow fluoroscopic visualization of the spread during injection. Phenol has a local anesthetic effect, resulting in less pain after the injection. Because of this, long-term effects of the block cannot be evaluated for 24 to 48 hours after the effects of the local anesthetic dissipate. Doses greater than 100 mg can result in serious toxicity (30).
Alcohol is used primarily intrathecally for nerve roots and locally for sympathetic blockade. Patient positioning with intrathecal use is extremely important, because alcohol is hypobaric in cerebrospinal fluid (CSF). It is readily soluble in body tissues and produces intense burning on injection. It requires 12 to 24 hours before block effects can be determined.
Glucocorticosteroids are used in the injection treatment of inflammatory processes (31) (Table 67-3). Some of the commonly used forms of corticosteroid available for intraarticular injections are listed as follows:
- • Betamethasone sodium phosphate and acetate (Celestone Soluspan), 6 mg/mL
- • Methylprednisolone acetate (Depo-Medrol), 40 and 80 mg/mL
- • Prednisolone sodium phosphate (Hydeltrasol), 20 mg/mL
- • Prednisolone terbutate (Hydeltra-TBA), 20 mg/mL
- • Triamcinolone acetonide (Kenalog), 40 mg/mL
- • Triamcinolone hexacetonide (Aristospan) 20 mg/mL
aRelative to hydrocortisone.
These corticosteroids vary in strength, concentration, duration, and side effects, and all are effective. Triamcinolone hexacetonide has the longest duration of suppression of inflammatory activity. Fluorinated corticosteroids (e.g., triamcinolone) are rarely chosen for soft-tissue injection, because they are more likely to cause tissue atrophy. Prednisolone tebutate and methylprednisolone acetate are often used for soft-tissue injections because of their efficacy and cost.