Deciphering malignant hyperthermia
Dr Michelle Casey, a specialist anaesthetist, explains the rare condition of malignant hyperthermia during anaesthesia and the precautions taken to avoid it.
What is malignant hyperthermia (MH)?
Malignant hyperthermia is a potentially life-threatening condition which can occur under general anaesthesia (state of unconsciousness induced by medication to facilitate surgery) in certain people who have a genetic disorder of their muscles.
It’s triggered by the use of specific medications that are commonly used for general anaesthesia, namely inhalational anaesthetic agents (the gases that are inhaled and keep you asleep during your operation), and a specific type of muscle relaxant called suxamethonium.
When these medications are used in a person who has the genes that make them susceptible to developing malignant hyperthermia, a chain reaction is set off. Once triggered, the body’s metabolism goes into overdrive, with a massive increase in the person’s metabolic rate. There is a large increase in the person’s heart rate and their temperature. The person’s muscles become stiff and can start to break down which leads to further metabolic disturbances, including deranged electrolyte levels. MH can result in death unless the condition is recognised and treated early. Extreme exercise and heat can rarely also trigger a malignant hyperthermia crisis in susceptible people.
Who is at risk?
Malignant hyperthermia is suspected in those people who have previously had an MH crisis under anaesthesia, or in those people who have a family member known to have MH.
Many people are otherwise unaffected by this condition, with the reaction occurring only under general anaesthesia. It’s therefore important to ask your parents and grandparents if any family members died unexpectedly under anaesthesia or had a bad reaction to anaesthesia in the past.
MH can occur in people who have no known family history of this condition, but who have undergone a new gene change or mutation. MH is also known to occur in people who have certain diseases of their muscles, e.g. central core disease. Interestingly, there appears to be a higher incidence of MH in children and in men, and MH can also occur in pigs, dogs and horses.
How is MH diagnosed?
If there is a suspicion that you are at risk of malignant hyperthermia, specific tests can be done on a small biopsy of muscle taken from your thigh. These tests are done at a specific MH testing centre and are the best way to make the diagnosis. Genetic DNA blood tests can also be done, but they aren’t as sensitive as the tests done on a muscle biopsy. Should a diagnosis of MH be made, your entire family can be screened for the condition.
Incidence of MH
MH is very rare, with one episode of MH estimated to occur in every 10 000 to 150 000 anaesthetics. On average, 20 new cases of MH are reported in the UK every year.
Increased awareness and treatment
Over the years, the mortality rate due to MH has greatly decreased from 70-80% in the 1960s to the current estimated mortality rate of 4%. This is largely due to an increased awareness of the condition among anaesthetists, as well as the use of an antidote medication called dantrolene.
If a MH crisis does occur, the anaesthetist will also use various methods to help cool the body down and correct the electrolyte abnormalities.
MH safety during anaesthetic
Patients who are known to have MH can safely undergo anaesthesia as long as they inform their anaesthetist of their MH risk, or any family history of MH, so that their anaesthetist can take the necessary precautions to avoid the known trigger agents.
The anaesthetist will then decide on either a regional anaesthetic (such as spinal anaesthesia) or a special form of general anaesthesia, known as total intravenous anaesthesia. The anaesthetic machine will also need to be specially prepared. Various other forms of muscle relaxants can be used safely if required, thereby avoiding the need to use suxamethonium.
Hopkins PM, Girard T, Dalay S, et al. Malignant Hyperthermia 2020. Guideline from the Association of Anaesthetists. Anaesthesia. 2020. https://doi.org/10.1111/anae.15317
MEET THE EXPERT – Dr Michelle Casey
Dr Michelle Casey is a specialist anaesthetist at the University of Cape Town’s Department of Anaesthesia and Perioperative Medicine. She has an interest in paediatric anaesthesia, liver transplants and adult and paediatric chronic pain. In 2017, she completed a postgraduate diploma in Interdisciplinary Pain Management.