A number of articles have reported a relationship between local anesthetics and myotoxicity—damage to muscle; for example: Zink, et.al. (2004), Zink, et.al. (2003), Irwin et.al., (2002), Nonaka, et.al. (1983). Published before the widespread use of pain pumps, a 1994 article by Hogan, et.al., begins with the following accepted generalizations:
All local anesthetics that have been tested are myotoxic. Procaine produces the least and bupivacaine the most severe injury. Injection of local anesthetics intramuscularly or into adjacent subcutaneous tissue results in myonecrosis. The extent of muscle injury from local anesthetics is dose dependent and worsens with serial administration.
Local anesthetics are injected into muscle for treatment of myofascial pain, in wound margins during surgery, and for neural blockage during surgical anesthesia.
The authors describe a female patient who underwent capsular release of the left shoulder. Because continuous passive shoulder motion and physical therapy were planned immediately following the surgery, a nerve block during and after the procedure was planned. A catheter was placed in the left interscalene groove at the level of the cricoid cartilage. Bupivacaine (0.5%) with epinephrine was injected incrementally to a total volume of 45 ml (200mg), producing sensory and motor blockade of the shoulder, arm, and hand.
When the woman began experiencing shoulder pain approximately 16 hours post-operatively, she received additional bolus doses of the 0.5% bupivacaine with epinephrine through the catheter. The doctors used additional injections of the same solution through the catheter when the woman’s shoulder pain did not resolve. After 34 hours, the catheter was removed. The total dose was 228 ml (1140 mg). The woman developed persistent pain in the left side of her neck. Imaging was suggestive of a tissue injury. Approximately 8 weeks post-op, a muscle biopsy showed injury to the muscle fibers.
Large doses of bupivacaine were used on the patient because of the authors’ desire to provide pain relief suitable for the expected post-operative manipulations of her shoulder. Because the injections failed to produce the desired effect, they suspected the catheter tip became dislodged.
According to the authors, while myotoxicity of local anesthetics has been widely produced in experimental settings, reports in human patients are uncommon. The authors’ next observations are why this article is interesting regarding pain pumps:
Local anesthetic injection for neural blocks only occasionally requires intramuscular injections of large volumes…. These are not usually repeated, and the injection site is difficult to examine. Small volumes are used with injections for intercostal, supraspinatus or musculocutaneous nerve blocks and with trigger point injections and stellate ganglion blocks.
Because experimental studies show myonecrosis after single injections of even minimal doses of local anesthetic, it is likely that myopathy occurs after most injections but is not recognized because of rapid and complete recovery.
Local pain for which trigger point injections are performed may disguise myopathic changes, and discomfort and dysfunction after injections performed for surgical anesthesia can be readily attributed to surgery or concealed by surgical pain. Splinting prevents tenderness from being identified. The pain of inflammation develops only after 3 or 4 days, and the appearance of atrophy takes longer; thus, these events frequently may be missed or not correlated to the administration of anesthetic agents. (My emphases)
Caveats: this 1994 article pre-dates the widespread use of pain pumps. The authors utilized Bupivacaine (0.5%) with epinephrine. Several years ago, pain pump manufacturers began recommending against using local anesthetics with epinephrine. Further, the large volume of Bupivacaine injected into the woman’s shoulder undoubtedly exceeded the manufacturer of Bupivacaine’s maximum recommended dose of 400 mg within a 24-hour period. In light of the numerous recent reports of chondrolysis, the fact that such a large volume of Bupivacaine with epinephrine was injected into the woman’s shoulder near cartilage is alarming (although the authors seem to believe that the catheter had become dislodged from its original location).
Nonetheless, the article raises a number of concerns regarding continuously infused local anesthetics and pain pumps. As the authors’ comments make clear, at the time this article was written, local anesthetics were typically used in relatively small amounts around the surgical site and to produce nerve blocks. Even small amounts, however, routinely cause cells to die, but they typically regenerate without incident. Post-operative pain and inflammation often mask symptoms that may actually be associated with damage to tissues.
In small amounts, given the beneficial pain relief afforded by local anesthetics, the temporary harm they cause would seem to be an acceptable side effect. What happens, though, when these drugs are continuously infused, in or near an open wound, in total volumes larger than commonly used in the past? There must be factors that cause the usually temporary harm to tissues to take the form of more visible and potentially permanent complications such as wound dehiscence, blistering, sloughing, and necrosis. These would seem to include:
- Local anesthetic volume
- Local anesthetic concentration
- Rate of infusion
- Duration of infusion
- Surgery/catheter site
- Patient-specific factors-age, weight, risk-factors
The pain pump and local anesthetic manufacturers have failed to properly considered these factors and, instead, make blanket, one-size fits all recommendations to surgeons.
A bigger question: Why is the medical community still so routinely using higher concentrations of Bupivacaine when it is known to cause damage to a variety of human cells-- cartilage, muscle, nerve, renal, and undoubtedly others?
