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Malignant hyperthermia

Malignant hyperthermia (MH or MHS for "malignant hyperthermia syndrome", or "malignant hyperpyrexia due to anesthesia") is a life-threatening condition resulting from a genetic sensitivity of skeletal muscles to volatile anaesthetics and depolarizing neuromuscular blocking drugs that occurs during or after anaesthesia. It is related to, but distinct from, the neuroleptic malignant syndrome. more...

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Signs, symptoms and diagnosis

The phenomenon presents with muscular rigidity, followed by a hypermetabolic state showing increased oxygen consumption, increased carbon dioxide production and hypercarbia, and increased temperature (hyperthermia), proceeding to rhabdomyolysis with rapid rising of blood levels of myoglobin, creatine kinase (CK/CPK) and potassium.

Halothane, a once popular but now rarely used volatile anaesthetic, has been linked to a large proportion of cases, however, all volatile anesthetics are potential triggers of malignant hyperthermia. Succinylcholine, a neuromuscular blocking agent, may also trigger MH. MH does not occur with every exposure to triggering agents, and susceptible patients may undergo multiple uneventful episodes of anesthesia before developing an episode of MH. The symptoms usually develop within one hour after anesthesia.

Susceptibility testing

Testing for susceptibility to MH is by muscle biopsy done at an approved center under local anesthesia. The fresh biopsy is bathed in a solution containing caffeine and halothane (the "caffeine-halothane contracture test", CHCT) and observed for contraction; under good conditions, the sensitivity is 97% and the specificity 78% (Allen et al., 1998). Negative biopsies are not definitive, so any patient who is suspected to have MH by history is generally treated with non-triggering anesthetics even if the biopsy was negative. Some researchers advocate the use of the "calcium-induced calcium release" test in addition to the CHCT to make the test more specific.

Litman & Rosenberg (2005) give a protocol for investigating people with a family history of MH, where first-line genetic screening of RYR1 mutations is one of the options.

Pathophysiology

Disease mechanism

Malignant hyperthermia is caused in a large proportion (25-50%) of cases by a mutation of the ryanodine receptor (type 1) on sarcoplasmic reticulum (SR), the organelle within skeletal muscle cells that stores calcium (Gillard et al., 1991). In normal muscle, the receptor releases small amounts of calcium when triggered, which is then reabsorbed into the SR for the next cycle of contraction. In MH, the receptor does not close properly after having opened in response to a stimulus. The result is excessive release of calcium, which is reabsorbed into the SR in a futile cycle; this process consumes large amounts of ATP (adenosine triphosphate), the main cellular energy carrier, and generates the excessive heat (hyperthermia) that is the hallmark of the disease. The muscle cell is damaged by the depletion of ATP and possibly the high temperatures, and cellular constituents "leak" into the circulation, including potassium, myoglobin, creatine and creatine kinase.

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Divergent effects of the malignant hyperthermia-susceptible Arg(615)-->Cys mutation on the Ca(2+) and Mg(2+) dependence of the RyR1
From Biophysical Journal, 10/1/01 by Balog, Edward M

ABSTRACT The sarcoplasmic reticulum (SR) Ca^sup 2+^ release channel (RyR1) from malignant hyperthermia-susceptible (MHS) porcine skeletal muscle has a decreased sensitivity to inhibition by Mg^sup 2+^. This diminished Mg^sup 2+^ inhibition has been attributed to a lower Mg^sup 2+^ affinity of the inhibition (I) site. To determine whether alterations in the Ca^sup 2+^ and Mg^sup 2+^ affinity of the activation (A) site contribute to the altered Mg^sup 2+^ inhibition, we estimated the Ca^sup 2+^ and Mg^sup 2+^ affinities of the A- and I-sites of normal and MHS RyR1. Compared with normal SR, MHS SR required less Ca^sup 2+^ to half-maximally activate [^sup 3^H]ryanodine binding (K^sub A,Ca^: MHS = 0.17 +/- 0.01 (mu)M; normal = 0.29 +/- 0.02 AM) and more Ca^sup 2+^ to half-maximally inhibit ryanodine binding (K^sub I,Ca^: MHS = 519.3 - 48.7 AM; normal = 293.3 - 24.2 AM). The apparent Mg^sup 2+^ affinity constants of the MHS RyR1 A- and I-sites were approximately twice those of the A- and I-sites of the normal RyR1 (KA,,: MHS = 44.36 + 4.54 AM; normal = 21.59 + 1.66 (mu)M; K^sub I,Mg^: MHS = 660.8 +/- 53.0 (mu)M; normal = 299.2 +/- 24.5 (mu)M). Thus, the reduced Mg^sup 2+^ inhibition of the MHS RyR1 compared with the normal RyR1 is due to both an enhanced selectivity of the MHS RyR1 A-site for Ca^sup 2+^ over Mg^sup 2+^ and a reduced Mg^sup 2+^ affinity of the I-site.

INTRODUCTION

Supported by grants from the National Institutes of Health (GM-31382 to C.F.L.) and the American Heart Association, Northland Affiliate (9704662A to E.M.B.).

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Edward M. Balog, Bradley R. Fruen, Nirah H. Shomer, and Charles F. Louis

Department Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455 USA

Received for publication 27 October 2000 and in final form 11 July 2001. Address reprint requests to Dr. Edward M. Balog, Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, 6-155 Jackson Hall, 321 Church St., S.E., Minneapolis, MN 55455. Tel.: 612-- 625-3292; Fax: 612-625-2163; E-mail: balog004@tc.umn.edu.

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