Patients who undergo minor surgery don't expect to have the debilitating side effect of myalgia from their anesthesia. An agent commonly found on anesthesia carts could be to blame — and it isn't the propofol or the lidocaine.

Succinylcholine is the drug of choice to facilitate tracheal intubation in a rapid sequence induction. It's widely used in outpatient settings for short surgical procedures. It's known to be associated with post-op myalgia, and the pain's intensity is more frequently severe for those treated as an outpatient (67 percent) than those treated inpatient (20 percent).[1]

The pathogenosis of this myalgia is still unclear. It could be caused by inflammation but there's no convincing evidence of this. Recent studies suggest that myalgia may have a multifactorial origin, so it may not be realistic to expect one agent to be able to prevent myalgia.[2]

A meta-analysis of 52 randomized trials of more than 5,300 patients detailing the prevention of succinylcholine-induced fasciculation and myalgia[2] and a review of the literature[3] showed that:

• The incidence of succinylcholine-induced myalgia is high and the symptoms may last for more than 72 hours (or up to a week, say some reports).
• Small doses of nondepolarizing muscle relaxants (about 10 to 30 percent of the effective dose in 95 percent of patients) prevent fasciculation and myalgia to some extent. But these agents have a significant risk of potentially serious side effects, such as muscle weakness, difficulty in swallowing and in controlling airways, as well as minor events such as blurred or double vision and voice disorders.
• The use of anticholinesterases to reverse the nondepolarizing muscle relaxants may increase the incidence of nausea and vomiting as well as post-op curarization.
• Higher doses of succinylcholine (1.5mg/kg) actually decrease the risk of myalgia compared with lower doses (1.0mg/kg).
• Giving opioids at induction doesn't seem to affect the outcome, but thiopentone seems to lead to less myalgia than propofol.
• There is no clear relation between succinylcholine-related fasciculation and myalgia.
• Pretreatment with sodium channel blockers such as lidocaine, NSAIDs diclofenac and aspirin, may prevent myalgia. It may be possible to prevent succinylcholine-related fasciculation with lidocaine, muscle relaxants or magnesium.

Relaxants of the future
Although there are many unanswered questions about what causes post-operative myalgia, we already know what we want to use in the outpatient setting. The perfect muscle relaxant for our short-duration surgeries would:

• have a rapid onset;
• be short-acting;
• have a fast reversibility with no metabolites;
• be associated with no adverse effects such as histamine release, and
• for safety's sake, not be administered to patients with a cardiac implant.

There's an agent called sugammadex in the pipeline that may get us closer to reducing the use of succinylcholine and effectively reversing nondepolirizing muscle relaxant without complications.[4] Sugammadex is a neuromuscular blocker binding agent designed to encapsulate aminosteroid nondepolarizing muscle relaxants. It works by binding selectively at the neuromuscular junction and preventing the nondepolarizing muscle relaxants from exerting their effects, which would make cholinesterase inhibitors unnecessary. A reversal of neuromuscular blockade enables spontaneous breathing to recommence earlier, helping patients leave the OR quicker.

Sugammadex has a rapid onset time of about 1.1 minutes in a dose of 4mg/kg in the rocuronium group and 1.4 minutes at a dose of 8mg/kg in the vecuronium group. Thanks to its mechanism of action, it can reverse the effects of nondepolarizers in short cases where the patient needs tracheal intubation. It is also a valuable agent for difficult intubation cases. Because of its rapid reversal of blockade, it would be possible to keep patients paralyzed to the last few minutes of the case and return them to spontaneous ventilation without the adverse effects we see from anticholinesterases and succinylcholine-induced myalgia.

As of this writing, sugammadex is in phase III trials; so far it seems to be well-tolerated and effective. It could be FDA-approved in 2008, giving us a new agent in our armamentarium that could reduce the number of calls we get about patients being in pain from post-op myalgia as well as allow us the ability to keep patients paralyzed right up to the end of their surgical procedure, if need be. Until then, we have to be aware of the risks of our old, trusted, reliable, polarizing muscle relaxants.

References
1. Fleisher L. Evidence-Based Practice of Anesthesiology. 1994;415-418.
2. Schreiber JU, Lysakowski C, Fuchs-Buder T, Tramer MR . Prevention of succinylcholine-induced fasciculation and myalgia: a meta-analysis of randomized trials. Anesthesiology. 2005; 103:877-884.
3. Bettelli G. Which muscle relaxants should be used in day surgery and when. Curr Opin Anesthesiology. 2006;19:600-605.
4. Fields AM, Vadivelu N. Sugammadex: a novel neuromuscular blocker binding agent. Curr Opin Anesthesiology. 2007;20:307-310.