Malignant hyperthermia (MH) is a rare genetic condition in which exposure to volatile anaesthetic agents or suxamethonium triggers a hypermetabolic state. Abnormal calcium movement results in generalised muscle rigidity, increased oxygen consumption and increased lactate and carbon dioxide production with hyperkalaemia and myoglobinaemia due to cell breakdown. Even with dantrolene, which prevents the release of calcium from the sarcoplasmic reticulum, mortality remains as high as 5%. The incidence of a fulminant syndrome was 1 in 62,000 general anaesthetics involving suxamethonium and a volatile agent in a previous study. MH is particularly rare in obstetric practice, given the preference for regional anaesthesia.
Problems and special considerations
Women with known susceptibility to MH may present in pregnancy. This is identified through muscle biopsy and the caffeine-halothane contracture test. Women with a positive family history of MH who have not been tested are best treated as susceptible to MH, owing to its autosomal dominant nature.
Alternatively, an undiagnosed case may become apparent on exposure to triggers during the peripartum period. This should be treated as per national and local guidelines. Previous uneventful general anaesthesia does not rule out susceptibility to MH.
Management options
Planning and preparation are key in the approach to the parturient with known susceptibility to MH. Antenatal review by a consultant anaesthetist in the high-risk obstetric clinic should occur. Regional techniques are recommended, and a plan should be in place in case elective or emergency surgical intervention is needed. Regional anaesthesia, opioids and nitrous oxide are not associated with MH.
On admission of the patient to the labour ward, the anaesthetist should be informed; this allows further discussion and planning of anaesthetic input. At this point, dantrolene should be readily available.
Regional analgesia is strongly indicated. In the event that general anaesthesia is required for operative intervention, careful consideration of the risk-benefit profile should occur, and local guidelines should be followed. Most departments no longer keep a vapour-free anaesthetic machine for use with MH patients. Instead, the vaporisers and carbon dioxide absorber can be removed from the anaesthetic machine, a new circuit attached and the circuit flushed with high-flow oxygen; duration of flushing is machine-dependent, and studies have indicated this may be between 5 and 104 minutes. If available, an activated charcoal filter may be placed in both the inspiratory and expiratory limbs of the ventilation circuit; this has been shown to render the anaesthetic machine vapour-free in under 3 minutes. In an emergency, hand ventilation using a self-inflating bag or Mapleson C breathing system will always be available. Anaesthesia can be induced with propofol or thiopental depending on personal preference and/or unit policy, but suxamethonium must not be used. A non-depolarising agent such as rocuronium should be used instead; an appropriate sugammadex dose should be calculated before induction and should be available in the event of airway difficulties. Total intravenous anaesthesia can be used to maintain anaesthesia. End-tidal carbon dioxide and temperature should be monitored carefully.
If MH is triggered, local and national treatment guidelines should be followed; this includes dantrolene administration, active cooling and correction of metabolic derangement. Treatment of hyperkalaemia, hypoxaemia, acidosis, hyperthermia and arrhythmias will require additional anaesthetic support, which should be requested immediately. Masseter spasm may prevent laryngoscopy; hand ventilation is usually possible until suxamethonium has worn off. Uterine atony has been described after dantrolene administration; laboratory investigation has suggested this is related to the mannitol it is provided with. Dantrolene may also be associated with neonatal hypotonia.
If the father is susceptible to MH, this may have been passed to the fetus. Triggers that cross the placenta such as volatile agents should be avoided. Although the placenta acts as a relative barrier to suxamethonium, it is probably best avoided.
Key points
• Malignant hyperthermia (MH) is a rare but potentially fatal condition that may be triggered in susceptible women in the peripartum period.
• Regional analgesia and anaesthesia should be encouraged in women susceptible to MH.
• Appropriate preparation and early administration of dantrolene if necessary are key to a successful outcome.
• When anaesthetising the mother of a potentially MH-susceptible fetus, an MH-safe anaesthetic should be administered.