Malignant hyperthermia (MH) is a rare but very serious complication of general anaesthesia (GA) that occurs in genetically predisposed subjects (subjects susceptible to MH) following exposure to triggering factors

It is therefore of paramount importance to recognise potentially susceptible subjects, precisely because it is at this level that the first intervention to prevent exposure to triggering factors can be carried out.

Who is at risk of Malignant Hyperthermia?

MH is a genetic disease with autosomal dominant transmission.

Molecular genetic studies have shown that the primary defect in MI lies in the skeletal muscle calcium channel commonly known as the ryanodine receptor (RYR1).

The gene coding for this protein, which in tetrameric form makes up the calcium channel, is located on chromosome 19q.

Malignant Hyperthermia: What is the incidence?

In its clinical expression, MI is a rare condition.

It can be estimated that the overall incidence is around 1:15,000 anaesthesia in the paediatric population and 1:50,000 in the adult population, with a slight predisposition for the male sex and the paediatric age, although no predictive factors can be recognised (a subject already exposed to triggering factors, in whom the syndrome has not appeared, cannot be considered to be without risk at a subsequent exposure).

Mortality currently stands at 7% of cases worldwide.

What triggers Malignant Hyperthermia?

• Drugs that can certainly trigger an IM crisis in susceptible individuals are halogenated volatile anaesthetics and/or succinylcholine.
• Three types of MH crisis can be distinguished, depending on the clinical picture: fulminant forms, moderate forms, and abortive forms.
• It is important to remember that in MHS patients, surgery under general anaesthesia with triggering agents does not always induce a crisis; an MHS subject may be subjected to general anaesthesia several times without consequences and react with an MH crisis the next time.
• Once triggered, the MH crisis may progress to death. In fatal cases, the progression of events may occur rapidly, even within 15 minutes, or last for over an hour.
• The fundamental pathogenetic feature of the condition consists of defective regulation of cytoplasmic free calcium in the striated muscle cell due to a genetic alteration of calcium channels.

What happens during IM?

Triggering drugs in susceptible individuals cause a prolonged opening of the calcium channels with an abnormal increase in the concentration of this ion in the cytoplasm of the muscle fibrocell.

The unregulated flow of calcium causes pathological muscle contraction and increases the metabolic activity of muscles.

Muscles activated under such conditions consume energy and thus an excessive amount of oxygen, resulting in the release of heat, water, carbon dioxide and lactates.

Energy production becomes insufficient and the integrity of the cell membrane is lost, proteins such as creatine kinase (CPK) and myoglobin are released into the blood.

Increased potassium in the blood causes tachycardia and tachyarrhythmia to the point of cardiac arrest, if not intervened in time.

Lack of oxygen can cause brain damage; increased carbon dioxide in the blood stimulates rapid and deep breathing.

Myoglobin moves from muscle cells into the kidneys where it can cause acute renal failure.

The release of large amounts of heat by overactive muscles causes the patient’s temperature to rise faster than the natural thermoregulatory system can keep under control.

Within minutes, a rise to 41°C and above can occur.

How is Malignant Hyperthermia treated?

• Discontinue volatile anaesthetics and succinylcholine.
• Hyperventilate with 100% O2.
• Administer dantrolene 2.5 mg/kg ev. Repeat as necessary adjusting dosage to signs of MH. Suggested upper limit is 10mg/kg, but may be exceeded if necessary.
• Avoid calcium antagonists. Persistent arrhythmias can be treated with all other antiarrhythmics. Most antiarrhythmics respond to correction of hyperkalemia and acidosis.
• Monitor core temperature.
• Cool the patient with nasogastric and rectal lavage and body surface cooling – avoid excessive cooling.
• Continue dantrolene for at least 36 hours after resolution of the episode.
• Watch out for recrudescences of MH by monitoring the patient in intensive care for 24-36 hours. Recrudescences occur in about 25% of MH cases.
• Avoid parenteral administration of potassium.
• Follow the coagulation profile – DIC may occur.
• Dose CK every 12 hours until normalised.

Diagnosis of Malignant Hyperthermia

The susceptibility to MI does not present a peculiar clinical picture to identify with certainty those who may develop an acute episode, and there are still no bloodless tests that can be used for this purpose.

The identification of patients at risk is based only on in vitro contracture tests (IVCT) after exposure of muscle tissue to halothane and caffeine, the only universally recognised valid test.

This test requires a muscle biopsy and is therefore performed on subjects previously selected by diagnostic centres and is not useful as mass screening.

IVCT is performed on subjects who have manifested

• definite or suspected episodes of MI and/or unexplained perianesthetic death in blood relatives;
• previous adverse reaction to anaesthesia, suspected for MI;
• previous postoperative complications (fever, CPK elevation, myoglobinuria);
• rhabdomyolysis after even modest exertion;
• intense, frequent cramps and easy fatigability;
• known and suspected neuromuscular disorders;
• previous episode of neuroleptic malignant syndrome (SNM).

How is Malignant Hyperthermia prevented?

When surgery is necessary, precautions are taken for patients recognised to be at risk: anaesthetics other than halogenated anaesthetics and succinylcholine are administered, the operating theatre is prepared by setting up an anaesthesia apparatus that is not polluted by halogenated anaesthetic vapours, and the necessary instrumentation to monitor cardiac parameters, blood pressure, temperature, and to perform laboratory tests.

The following must be available in the operating room: injectable Dantrolene, all drugs needed to treat the crisis, and glucose and electrolyte solutions cooled to 4-5°C.

Prophylactic administration of injectable Dantrolene is only envisaged in cases where the operating theatre cannot be prepared in the manner described above or when the patient is undergoing emergency or particularly demolitive surgery.

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