Malignant hyperthermia is also called as malignant hyperpyrexia and anesthesia-related hyperthermia. It is a very severe reaction to certain general anesthetics given during surgery. Its incidence is 1 in 5000-50,000 cases receiving anesthetic gases during surgery. Some people have a genetic susceptibility to developing malignant hyperthermia. Its inheritance is autosomal dominant (that is, only one copy of mutated gene in every cell is enough for you to be at risk).
Other risk factors are muscle disorders, previous history of heat stroke, muscle pain on exercise etc. It is characterized by muscle fiber breakdown (rhabdomyolysis), muscle rigidity, high fever, acidosis (increased levels of acid in blood and tissues), and increased heart rate. It can be fatal if not recognized and treated early. It is diagnosed by muscle biopsy. The main line of treatment is with dantrolene and monitoring.
There is clear evidence for increased susceptibility to malignant hyperthermia in the presence of certain genetic defects. To date, eighty such genetic defects have been identified. So, if you have a family history of malignant hyperthermia you are at greater risk. However, not everyone with a genetic susceptibility develops it.
People with certain muscular disorders like myotonia and muscular dystrophy are at increased risk. Other people at risk are those with osteogenesis imperfecta (a bone disorder), deficiency of the enzyme carnitine palmityl transferase, and those who have had a previous attack of heat stroke.
Malignant hyperthermia is an environmentally triggered response of the body in the presence of genetic susceptibility to it. It occurs as a reaction to certain general anesthetic agents, particularly inhalation (volatile) anesthetic agents. The anesthetic agents responsible are desflurane, enflurane, isoflurane, sevoflurane, methoxyflurane, ether, halothane, and succinylcholine. These drugs cause an uncontrolled increase in metabolism in skeletal (voluntary) muscles.
The body then is not able to meet the demand for supply of oxygen and removal of carbon dioxide. Regulation of body temperature is also impaired. The other drugs implicated are phenothiazines, monoamine oxidase inhibitors, and catecholamines. Malignant hyperthermia can also occur due to stress brought on by excess heat or heavy physical exercise.
Symptoms And Signs
Typically, the symptoms develop within an hour of exposure to the triggering agents mentioned above. However, sometimes they can occur even after several hours after such exposure.
The symptoms are mainly due to increased metabolic activity, characterized by high temperature, increased breathing rate and heart rate, increased production of carbon dioxide, increased oxygen consumptions, rigid muscles, acidosis, and rhabdomyolysis. There may be bleeding. Urine will be dark brown. Muscles will be aching, without specific causes like injury or exercise.
If it is not diagnosed and treated early it can lead to several complications. Muscle tissue will break down. Hands and feet can become swollen, leading to impaired nerve function and blood flow (compartment syndrome). In some cases, amputation may be required.
There will be abnormal clotting of blood and bleeding. Functioning of lungs, heart and kidneys will get impaired. Eventually, death may occur if left untreated.
If it is recognized during surgery, the triggering agent should be stopped. Dantrolene 2 mg/kg IV is the drug of choice. The maximum dose of dantrolene is 10 mg/kg, which should not be exceeded. Azumolene is an alternative to dantrolene. Monitoring of all vital functions should be done. Body temperature should be brought down with cooling and antipyretics.
Sodium bicarbonate may be necessary to correct the acidosis. Other treatment will depend on the specific symptoms you have, such as mannitol if urine output is less and anti-arrhythmics if heart rhythm is disturbed.
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