To the Editor:--The report by Vita et al. correlates the inherited disorder myotonia fluctuans and its specific chromosomal abnormality with muscle rigidity produced by succinylcholine and with malignant hyperthermia (MH)-positive muscle contracture responses to halothane. The correlation weakens with their presumed logical tie to clinical episodes of MH. As the accompanying editorial states, there is contradictory evidence for a tie to MH, and the anesthetic complications do not appear to be MH. Most of the evidence regarding myotonic disorders and MH do not support a connection, i.e., a positive biopsy is not enough evidence by itself; there must be undisputed evidence for clinical episodes of MH. .
The criteria for what constitutes a "clinical episode of MH" seem to have become less certain. Rigidity per se, even that involving the entire body, is insufficient evidence for MH, as has been recognized with succinylcholine and other choline derivatives in myotonia since Orndahl described it in the early 1950s. Vita et al. admit that the clinical tie to MH is weak: "certain forms (of masseter spasm) do not progress to a recognizable hypermetabolic state." After the anesthetic has been aborted because of trismus, "there is usually little clinical evidence to distinguish which patients would have proceeded to a hypermetabolic response."
Goats inbred for myotonia congenita do not demonstrate MH when exposed to halothane and succinylcholine. They demonstrate the characteristic rigidity and have an associated modest increase in carbon dioxide production and oxygen consumption related to the metabolic demands of the rigidity. Two human cases seem to contradict the findings in goats: Haberer et al. report a boy with myotonia congenita who experienced severe rigidity, fever, and acidosis 7 h after an orthopedic procedure, with eventual arrest and CK of 11,000. Was this rhabdomyolysis that led to hyperkalemia? Patients with myotonia can exhibit greater hyperkalemia with exercise, which may be a predisposing factor if excess muscle activity develops (related to their myotonia) and they cannot relax. In this case, the evidence for MH per se is not convincing, although the blood gas and temperature findings are consistent with MH. This boy had been given oral dantrolene preoperatively and what appear to be nontriggering agents, and the complications in the late postoperative period were severe. Saidman et al. describe a young girl with presumed myotonia congenita who experienced typical MH and died. However, data from Moulds and Denborough on normal biopsy findings in patients with myotonia and their analysis of Saidman et al.'s case weaken the argument for a direct tie between MH and myotonia. Further, a 1978 review on relaxants notes that the contracture produced by succinylcholine in normal patients (their Figure 2) is exaggerated in patients with myotonia. .
Several additional reports that include myotonia in their considerations state that the association with MH is either none, uncertain, or of uncertain extent. In other words, the contracture produced by succinylcholine in those muscle groups affected by myotonia complicates the evaluation of the patient, and a cautious approach would include MH. This is an appropriate course of action early in the consideration of a case.
Myotonia should be considered "myotonias," an array of muscle disorders caused by a variety of genetic aberrations in chloride and sodium channels and perhaps yet to be discovered mutations in other ion channels. Consequently, variation in clinical behavior to depolarizing stimulation by succinylcholine may be anticipated, in some cases making differentiation from "true MH" (also a multigenetic disorder) impossible in its early stages.
The findings in myotonia fluctuans appear to confirm this opinion, and the conclusion from the report of Vita et al. is that they have demonstrated a mutation associated with myotonia fluctuans and that this is one explanation for masseter spasm occurring with the use of succinylcholine.
Gerald A. Gronert, M.D., Department of Anesthesiology, University of California, TB 170, Davis, California 95616.
