MALIGNANT hyperthermia (MH) is probably the only medical syndrome that can only be caused by the administration of anesthetic agents. Because it is almost uniformly fatal if not rapidly diagnosed and treated, it would be a significant improvement for anesthesia risk management if we could reliably predict or confirm MH susceptibility.
The mechanism of MH appears to be a defect in skeletal muscle excitation–contraction coupling. In 50% of patients, MH has been associated with one of 20 or more mutations in the type 1 ryanodine receptor (RyR1), 1–3the skeletal muscle sarcoplasmic reticulum Ca2+release channel, and in one family it was associated with a mutation in the α1s-dihydropyridine receptor, the slow voltage gated calcium channel in the sarcolemma. 4However, in the remaining 50% of patients it has not yet been linked to any genetic locus. 5This seriously limits our ability to use genetic testing methods. The diagnosis of MH is still a result of a clinical event, and these events are associated with variable symptoms. There have been efforts to grade these events and to make the diagnosis based on a clinical score, 6but it would be very desirable to have a simple laboratory test that could predict or confirm MH susceptibility with a high level of confidence. Over the years, there have been many diagnostic tests that have been proposed to predict MH susceptibility. However, only the caffeine, caffeine–halothane, and 4 chloro-m -cresol (4-C m C) contracture tests (Caffeine Halothane Contracture Test [CHCT] in North America 7and In Vitro Contracture Test [IVCT] in Europe 8) have withstood the test of time. The CHCT is not perfect, but it has had a high degree of correlation with genetic testing in the small, clinically selected population of individuals susceptible to MH. Nevertheless, contracture testing requires a large surgical muscle biopsy and, although readily available in Europe, 9is performed at a decreasing number of centers in the United States. A less invasive test that could be easily performed at any center and would be at least as reliable as the CHCT would obviously be very desirable. In this issue of Anesthesiology, there are two new tests proposed by Sei et al . 10and Klingler et al . 11that are “noninvasive” and appear to correlate well with both the CHCT and genetic testing. If the results of these preliminary studies are confirmed, the new tests may well prove to be a useful adjunct to the diagnosis of MH susceptibility. If not, they still suggest new paths to be explored in the direction of this “holy grail.”
Sei et al . 12had previously demonstrated the existence of RyR1 in B lymphocytes. Girard et al . 13demonstrated that B lymphocytes from patients susceptible to MH produced more interleukin 1β in response to caffeine and 4-C m C than those from patients who were not susceptible to MH. Based on these findings and the recent findings of others, Sei et al . 12hypothesized that Ca2+homeostasis in B lymphocytes is altered in individuals who are susceptible to MH and proposed that this could be the basis for a noninvasive diagnostic blood test. The current study by Sei et al . 10shows that Ca2+release induced by caffeine and 4-C m C in cells from individuals susceptible to MH is greater than that in patients with normal CHCT test results (MH susceptibility negative) and that in normal control subjects who did not undergo CHCT testing. This suggests that Ca2+regulation may be altered in B lymphocytes, and that this malfunction is related to the mutation of RyR1. In addition, on average, the MH-susceptible cells show a lower threshold for 4-C m C than controls. The finding that the caffeine response can be abolished by treatment with EGTA or Ca2+-free media was unexpected, since caffeine induces its pharmacologic action in the vast majority of cells not by facilitating the entry of Ca2+from the extracellular space, but by releasing Ca2+from intracellular stores. This will require further study. However, the lack of a clear-cut point between the responses from cells that are susceptible to MH and those that are not, which is probably related to the heterogeneity of the MH phenotype, limits our ability to discriminate between patients who are and are not susceptible to MH. It will also be necessary for Sei et al . to determine whether this test will have any value for use in patients who do not have a mutation of the RyR1, since B lymphocytes have not been shown to express the other proteins thought to be important for skeletal excitation–contraction coupling or MH. While these factors limit the current utility of this test for diagnosing MH susceptibility, we look forward to further work in this area, with the hope that a more usable clinical test might be developed.
In the report by Klingler et al . 11in this issue, the investigators have proposed a clever extension of the IVCT. They propose (but have not shown) that this can be done from the tissue collected from a small needle biopsy of muscle, eliminating the need for a surgical procedure and making the test almost “noninvasive.” They use the premise that proton excretion from muscle, in this case human myotubes, is the result of increased metabolic rate, and the larger the rate of proton excretion for a given stimulus, the greater the metabolic activity. Despite the fact that this is an indirect measurement, 4-C m C is highly specific for stimulating Ca2+release from RyR1, 14which, in turn, will increase the metabolic rate. Thus, any increase in proton release caused by 4-C m C is likely to be the result of Ca2+metabolism. The myotubes that Klingler et al . 11cultured were not from needle biopsies but from large biopsy samples taken for IVCT testing. This did have the advantage of allowing direct correlation of the two techniques but does not validate the proposed needle biopsy technique. From their data, they show a reasonable correlation between the threshold for proton excretion and force production for 4-C m C and a complete separation of MH susceptibility and lack of MH susceptibility using these criteria. If this test can actually be performed on needle biopsy samples, it is a big step forward in decreasing the invasiveness of MH testing, and, since it is performed on muscle, it is likely to be as useful as the IVCT (CHCT) in diagnosing individuals susceptible to MH.
Despite this step forward, it is our opinion that there is one major problem facing all diagnostic tests for MH that use skeletal muscle responses. Despite the fact that we all want to know the results of such a test, we have found, in a show-of-hands poll of more than 800 anesthesiologists after MH lectures, that simply performing the test is sufficient for 99% of the respondents in the United States to consider the patient tested to be susceptible to MH—no matter what the result! Furthermore, the CHCT (IVCT) is not a specific test for MH in and of itself. Increasing myoplasmic free Ca2+concentration in muscle that is not susceptible to MH will reversibly convert its phenotype to MH susceptible, 15and we have recently shown that lowering the myoplasmic free Ca2+concentration in MH-susceptible muscle can reversibly convert its phenotype to MH susceptibility negative (J. R. López, M.D, Ph.D., and P. D. Allen, M.D, Ph.D., unpublished data, June 2002). Several other muscle diseases besides MH have increased caffeine sensitivity due to a high resting Ca2+in the myoplasm. However, until it is possible to correlate the syndrome with a specific genetic mutation in more than 90% of patients—thus allowing a simple blood test to screen for susceptibility—any less invasive test that is at least as reliable as the IVCT (CHCT) is worthy of continued evaluation.