Understanding Malignant Hyperthermia, Plasma Cholinesterase Deficiency, and Related Considerations

First recognised in Australia in 1960, malignant hyperthermia or malignant hyperpyrexia (MH) is characterised by an unexplained rise in body temperature and muscle rigidity during anaesthesia, stemming from a significant increase in metabolism. This metabolic surge leads to a marked increase in oxygen consumption and carbon dioxide production. A predisposition to malignant hyperthermia is an inherited condition, occurring in approximately 1 in 40,000 patients. MH is triggered by exposure to specific anaesthetic drugs – the volatile anaesthetic agents (such as isoflurane) and suxamethonium. Triggering may occur upon the first exposure to these drugs, or even after repeated and seemingly uncomplicated anaesthetics.

Treatment of an MH episode involves immediately stopping the triggering drug, halting the operation if feasible, and administering a drug called dantrolene. This is the sole specific treatment for this syndrome; without it, approximately half of all patients experiencing a malignant hyperthermia reaction will succumb. Additional supportive measures, including supplemental oxygen, cooling, and resuscitative drugs and fluids, are also crucial.

The primary diagnostic test for MH involves examination of a biopsied muscle tissue sample. However, it’s important to note that some tests may yield false positive results, indicating the condition when it is not actually present. As genetic patterns are increasingly identified in families with MH, genetic testing may be used to diagnose susceptible individuals. Testing is recommended for the patient and close relatives. Individuals with a history of MH reaction or a positive test should obtain a Medic Alert notification and carry it at all times.

For patients with known MH requiring surgery, special preparation of the Operating Room is essential. The use of volatile anaesthetic agents should be avoided for 12 hours beforehand, and ideally, the patient should be scheduled as the first case of the day. A ‘safe’ anaesthetic technique, avoiding known triggering agents, is generally achievable. Drugs considered ‘safe’ include nitrous oxide, thiopentone, propofol, midazolam, narcotics, muscle relaxants such as curare or vecuronium, and all local anaesthetic drugs. Careful monitoring of the patient’s condition, including temperature, is vital, as with any general anaesthetic, and should continue into the postoperative period. While rare, some patients have reported reactions following a ‘safe’ anaesthetic; however, these reactions have typically been mild.

Plasma Cholinesterase Deficiency (PChD)

Plasma cholinesterase deficiency or pseudocholinesterase deficiency (PChD) is an enzyme deficiency that affects the metabolism of certain anaesthetic drugs, thereby prolonging their action. These drugs include specific types of local anaesthetic agents and suxamethonium. It’s important to understand that having PChD does not signify an ‘allergy’ to these drugs, but simply indicates that the drug’s effects persist for a longer duration.

If a patient with PChD receives suxamethonium, the muscle relaxation induced by the drug may last for several hours, rather than the usual few minutes. During this extended period, the patient is unable to move or breathe spontaneously and requires artificial ventilation. Sedation is employed to make this period of profound weakness more comfortable.

PChD is typically inherited and affects less than 0.01 percent of the population. It can also occur in patients with liver failure and certain tumours, as well as in those exposed to specific drugs, such as ecothiopate, and certain insecticides. Some women near the end of pregnancy may develop a mild form of PChD that resolves after childbirth. The enzyme deficiency can be confirmed through a special blood test.

Frequently Asked Questions (FAQs)

Q: What is malignant hyperthermia and how common is it? A: It’s a rare inherited condition causing a dangerous rise in body temperature and muscle rigidity during anaesthesia, occurring in about 1 in 40,000 patients.

Q: What is plasma cholinesterase deficiency, and what does it mean for anaesthesia? A: It’s an enzyme deficiency that slows down the metabolism of certain drugs, prolonging their effects. It doesn't mean a patient is allergic, just that the drug's action lasts longer.

Q: Why is it important for patients with known MH or PChD to carry a Medic Alert notification? A: To alert medical professionals to their condition in case of an emergency.

Glossary of Terms

(Expanded from previous version)

• Anaesthesia: The use of drugs to induce a loss of sensation or consciousness.
• Anaesthetist: A medical professional trained in administering anaesthesia.
• Dantrolene: A specific medication used to treat malignant hyperthermia.
• Malignant Hyperthermia (MH): A rare, life-threatening reaction to certain anaesthetic drugs.
• Medic Alert: A system for identifying individuals with specific medical conditions.
• Nitrous Oxide: An anaesthetic gas.
• Plasma Cholinesterase Deficiency (PChD): An enzyme deficiency affecting the metabolism of certain drugs.
• Pseudocholinesterase: An enzyme that breaks down certain drugs.
• Suxamethonium: A muscle relaxant.