Malignant hyperthermia: what it is, causes, treatment and testing

St George's University Hospitals says early treatment is usually successful. Treatment involves discontinuing the anaesthetic, cooling the patient and giving dantrolene, an antidote that is available in all hospitals where anaesthetics are used.

The Scottish Muscle Network's letter templates explain why teams act quickly: MH can be a rapidly progressive reaction, so when anaesthetists become concerned it may be happening they treat for MH rather than wait for certainty, as waiting until the diagnosis is sure may be too late.

After a suspected reaction, the team will discuss your case with colleagues or the UK National MH Unit in Leeds, and consider referral to clinical genetics for testing, according to the same template letters.

Malignant hyperthermia is a reaction to certain anaesthetic drugs and gases in people who carry a genetic susceptibility. The UK MH Registry describes it as a life-threatening reaction to anaesthetic drugs, and says the word "malignant" refers to its progressive nature, ending in death associated with a very high body temperature if it is not recognised and treated promptly.

St George's University Hospitals says that, very rarely, MH can be triggered by stresses on the body such as vigorous exercise or heat. Some people who are susceptible find heat or exercise can trigger symptoms and should avoid the activity that triggers them; people who have not had adverse effects from heat and exercise do not need to avoid them.

St George's University Hospitals says susceptibility to MH most often occurs in people with variants in the RYR1 gene, but variants in other genes have also been linked. Many gene variants are found in RYR1, and it is not always certain which cause MH, so genetics teams may look at the genes and history of several family members.

Susceptibility is hereditary: people with an RYR1 variant have a 1 in 2 chance of passing it on to any children. The Scottish Muscle Network adds that it is a genetic condition passed on within families from generation to generation.

MH is recognised in the operating theatre, not by symptoms you would notice yourself, because it happens while you are anaesthetised. St George's University Hospitals says early signs are a rapid heart rate and increased carbon dioxide levels. As the reaction progresses, body temperature rises, muscles become rigid and heart rhythms can become irregular.

The UK MH Registry stresses that a reaction is not predictable. People susceptible to MH may not develop it every time they have a general anaesthetic, even when it includes the triggering drugs, so a history of uneventful anaesthetics does not exclude it. That is why every patient at pre-assessment should be asked about a family history of anaesthetic problems.

There are two routes. Genetic testing uses a blood sample. The Scottish Muscle Network says that if a gene variant is found it can only prove MH susceptibility; people with a suspected reaction in whom no variant is found will also need referral to Leeds for a muscle biopsy.

St George's University Hospitals describes the muscle biopsy test, the in vitro contracture test (IVCT): it involves an overnight stay at a hospital in Leeds, where a small sample of muscle is removed and exposed to anaesthetic drugs to observe its reaction. It is invasive, carries risks such as wound infection, bleeding and sensory loss, and is not 100% accurate, but can guide care and inform family members.

To get tests, ask your GP or hospital specialist to refer you to your regional genetics service, particularly if a relative has an RYR1 variant or has had an MH episode.

Because MH is inherited, relatives may be at risk. The Scottish Muscle Network's template says that until it is proven that a relative does not carry the genetic risk, family members should be treated as at risk if they need a general anaesthetic. St George's University Hospitals says relatives who have not been tested should be treated as though they have MH and avoid certain anaesthetic drugs and gases.

Relatives are advised to tell their GP and any doctor, surgeon or dentist planning a general anaesthetic. Depending on urgency, the hospital may advise postponing a planned procedure until the risk has been clarified.

St George's University Hospitals recommends telling your anaesthetist that you are at risk if you need a general anaesthetic, keeping an MH warning card in your wallet and wearing a warning disc, such as MedicAlert, in case you arrive in hospital unconscious. It also recommends making sure all your healthcare providers know and telling your genetics team if anyone in the family has MH symptoms.

The UK MH Registry says those at risk can receive anaesthetics safely as long as the inhalational anaesthetics and suxamethonium are avoided. The Scottish Muscle Network says that with the correct knowledge and awareness MH is a well-managed condition.

On the NHS the route is through your anaesthetist, GP and regional clinical genetics service, with the UK National MH Unit in Leeds providing specialist investigation and advice. Testing referrals are made by clinicians, not self-referral.

Private anaesthetists can also take a history of MH and plan a trigger-free anaesthetic, but confirmation of susceptibility goes through the NHS genetics pathway and the national unit, so tell any private provider about any known or suspected MH in the family.