Intraoperative management

Modern monitoring has greatly aided the early diagnosis of malignant hyperthermia. With the introduction of capnography it soon became apparent that the earliest sign of an impending malignant hyperthermia crisis was not an increase in temperature but an unexplained increase in end-tidal CO 2 concomitant with an unexplained tachycardia. Once other diagnoses have been excluded (e.g. problems with the anesthetic circuitry etc.), these two signs should alert the anesthetist to the possibility of malignant hyperthermia. At this stage core temperature recording should be established and monitored continuously.

The onset of a malignant hyperthermia reaction varies considerably; it may be sudden and rapidly life threatening as in fulminant crisis, or it may develop insidiously. The latter often occurs with low concentrations of the anesthetic vapors currently used to prevent 'awareness' and malignant hyperthermia may take 1 to 2 h to develop.

Once the diagnosis of malignant hyperthermia has been suspected, the following treatment plan should be instituted and modified based on signs and severity.

1. Withdraw all trigger agents (this includes all vapors), flush the anesthetic machine, and substitute a clean circuit. Ventilation may need to be increased to two to three times the minute volume to aid the removal of CO2.

2. If feasible, abandon the procedure as soon as possible or continue with a 'malignant-hyperthermia-safe' technique (see later).

3. Give specific therapy: intravenous dantrolene 1 mg/kg. This can be repeated up to 10 mg/kg depending on the regression of signs. Frequently the response to dantrolene is rapid and obvious unless it is given too late. Dantrolene is expensive and rarely used, and so it is recommended that each operating theater should keep enough dantrolene to provide initial therapy while further amounts are obtained from a central store.

4. Start to cool the patient but avoid causing vasoconstriction, which impedes heat loss. Simple measures such as tepid sponging, fanning, and cool intravenous fluids are usually sufficient, although more aggressive therapy (e.g. cold gastric, peritoneal, or bladder lavage) may be needed.

5. Treat arrhythmias in the usual way. With a severe reaction a cardiac arrest may quickly ensue; thus metabolic acidosis and hyperkalemia should be corrected early.

6. Blood samples should be taken for serum K+, arterial blood gases, and an initial creatine kinase. Hyperkalemia and acidosis can be corrected in the usual way.

7. The urine output should be observed for signs of impending renal failure, and catheterization may be required. A urine sample should be sent for myoglobin evaluation. If the urine is stained red-brown, suggesting the presence of myoglobin, diuresis should be promoted with fluids and perhaps mannitol, remembering that one 20-mg vial of dantrolene contains 3 mg of mannitol. If renal failure occurs, the patient will need to be dialyzed temporarily until the kidneys recover.

8. Although disseminated intravascular coagulation rarely develops, the clotting status should be checked.

9. Core temperature should be monitored for 24 to 48 h.

10. A repeat creatine kinase measurement should be performed after 24 h as it is slow to rise compared with myoglobinuria, which occurs soon after the initial event.

11. Consider the differential diagnosis of, for example, thyrotoxic storm, pheochromocytoma, septicemia, or respiratory or other infection. The appropriate investigations need to be performed, and include white blood count, chest radiography, thyroid function studies, and urine vanilmendalic acid.

12. Consider the possibility of a myopathic disorder, particularly the myotonic diseases if muscular dysfunction is prominent. An electromyogram (EMG) and/or a neurological opinion may be required.

13. After recovery the patient and/or the family should be counselled as to the potential implications of malignant hyperthermia and the necessity of confirming or refuting the clinical diagnosis by an in vitro contracture test.

14. Refer the patient to a malignant hyperthermia screening center if one is available.

Masseter muscle spasm

Masseter muscle spasm occurring after the administration of succinylcholine (suxamethonium) is generally regarded as a warning sign of malignant hyperthermia. Much of the controversy surrounding the significance of masseter muscle spasm stems from the difficulty in defining the spasm itself and that succinylcholine has been shown to increase jaw tension in normal patients. Unexpected difficulty with jaw opening after succinylcholine which hinders intubation by an experienced anesthetist should be considered as masseter muscle spasm, and the patient should be presumed to be potentially at risk from malignant hyperthermia. If feasible, the procedure should be abandoned, blood should be taken for initial and 24-h creatine kinase, and the first voided specimen of urine sent for myoglobin estimation. It is unlikely that the patient will exhibit any other signs of malignant hyperthermia, but these should be sought and treated appropriately. If the procedure is urgent, anesthesia can be continued with a 'malignant-hyperthermia-safe' technique once the patient is stable. The incidence of malignant hyperthermia susceptibility (from in vitro contracture tests) in patients presenting with masseter muscle spasm as the sole sign is 25 to 30 per cent, but this includes many patients who were not fully investigated so that masseter muscle spasm appeared to be the only apparent sign.

Patients with additional metabolic signs (e.g. tachycardia, pyrexia) have a 50 per cent incidence of being shown to be susceptible to malignant hyperthermia after in vitro contracture tests, and those with muscle signs (e.g. very high creatine kinase and myoglobinuria) have the highest incidence of 75 to 80 per cent ( Ellis et al. 1990).

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