Last Updated: 1/27/2010
WHAT IS MH?
The sudden unexpected death of a healthy individual undergoing minor surgery is a tragedy almost beyond comprehension in this day of modern medical miracles. Yet this still happens to patients susceptible to malignant hyperthermia (MH). Even when treated properly, the syndrome known as the MH crisis can cause death. In rare cases, survivors might be left with brain damage, failed kidneys, muscle damage or impaired function of other major organs.
Another cause of unexpected death during or shortly after anesthesia is a sudden cardiac arrest in a young male patient with muscular dystrophy. In some cases, the patient may not be old enough to show the characteristic signs of muscle weakness. The anesthesia care team may, therefore, not realize that the patient may develop a marked increase in potassium in the blood when anesthetized which is sufficient to stop the heart. This phenomenon occurs with the use of drugs that "trigger MH," but the syndrome is distinct and different from MH. Another sign of this reaction is severe muscle breakdown manifested by brown urine and kidney failure. (See below: Are There Links Between MH and Other Diseases?)
The MH crisis is a biochemical chain reaction response, “triggered” by commonly used general anesthetics and the paralyzing agent succinylcholine, within the skeletal muscles of susceptible individuals. The general signs of the MH crisis include tachycardia, a greatly increased body metabolism, muscle rigidity and/or fever that may exceed 110 degrees F. Severe complications include: cardiac arrest, brain damage, internal bleeding or failure of other body systems. Thus, death, primarily due to a secondary cardiovascular collapse, can result.
WHO IS SUSCEPTIBLE TO MH?
There has been dramatic improvement in our understanding of what causes MH and who is at risk. Over 80 genetic defects have been associated with MH. MH susceptibility is inherited with an autosomal dominant inheritance pattern. This means that children and siblings of a patient with MH susceptibility usually have a 50% chance of inheriting a gene defect for MH, and hence would also be MH susceptible. They, therefore, may develop an MH reaction upon exposure to triggers.
Nevertheless, those who are carriers for susceptibility may be completely unaware of this risk unless they or a family member developed a life-threatening crisis during anesthesia. It is important to know that not everyone who has a gene defect linked to MH develops the MH
crisis upon each exposure to the triggering anesthetics. (See the section below on Testing for MH Susceptibility.)
WHAT DRUGS TRIGGER MH?
The volatile gaseous inhalation anesthetics are MH triggers:
sevoflurane desflurane isoflurane halothane enflurane methoxyflurane
Also, succinylcholine (Anectine), the depolarizing muscle relaxant can trigger an MH crisis.
ARE OTHER ANESTHETICS SAFE?
Yes, all other anesthetic drugs are safe. Some examples of safe anesthetics are:
local anesthetics narcotics ketamine barbiturates
nitrous oxide propofol etomidate benzodiazepines
The non-depolarizing muscle relaxants (used to temporarily produce muscle paralysis) are
also safe:
pancuronium cisatracurium atracuriummivacurium
vecuronium
rocuronium
WHAT IS THE INCIDENCE OF MH?
The exact incidence of MH is unknown. The rate of occurrence has been estimated to be as frequent as one in 5,000 or as rare as one in 65,000 administrations of general anesthesia with triggering agents. The incidence varies depending on the concentration of MH families in a given geographic area. High incidence areas in the United States include Wisconsin, Nebraska, West Virginia and Michigan.
WHAT CAUSES AN MH EPISODE?
MH-susceptible persons have a mutation that results in the presence of abnormal proteins in the muscle cells of their body. Although normal in everyday life, when these patients are exposed to certain anesthetic agents, or in rare cases when exposed to high environmental heat or strenuous exercise, it causes an abnormal release of calcium from the sarcoplasmic reticulum in the muscle cell, which results in a sustained muscle contraction and thus an abnormal increase in metabolism and heat production. The muscle cells eventually are depleted of adenosine triphosphate (ATP) the source of cellular energy, and die, releasing large amounts of potassium into the bloodstream, causing hyperkalemia, followed by ventricular (cardiac) arrhythmias. The muscle pigment myoglobin is also released from the muscle cells and may be toxic to the kidney. Left untreated, these changes can cause cardiac arrest, kidney failure, blood coagulation problems, internal hemorrhage, brain injury, liver failure, and may be fatal. A more detailed explanation of the biochemical changes in MH may be found on the MHAUS Web-site.
HOW IS MH TREATED?
Treatment is predicated upon preparation for a rare event. Every anesthetic must be associated with a plan for treatment of unanticipated MH. With the plan in place, treatment can be prompt and lifesaving. Prompt recognition of the signs of MH is essential to an optimal outcome. Preparedness is essential to prevent death from MH.
In addition to an anesthesia machine (if used), ECG monitor, pulse oximeter and capnometer, all locations where general anesthesia is administered should contain:
· A plan to treat MH, such as the poster and MH Procedure Manual available from MHAUS. For immediate emergency consultation with a volunteer anesthesiologist MH Hotline consultant, the MH Hotline can be contacted at: 1-800-644-9737.
· A means to continuously monitor end-tidal carbon dioxide levels, blood oxygen saturation and core body temperature.
· A means to actively cool a patient; e.g., a hypothermia blanket(s) (over and under the patient) and a refrigerator containing cold isotonic saline for IV infusion and for gastric, peritoneal or rectal irrigation, as appropriate. Ice is much more effective at cooling, though core cooling using iced saline intravenously may be effective (0.5°C. /liter in a 70 kg adult). The volume of IV saline that should be rapidly infused limits maximum effect.
Beware of unintentional hypothermia! Stop cooling measures when temperature falls to 38°C
An MH cart or kit containing the required drugs, equipment, supplies and forms should be immediately accessible to operating rooms.
HOW CAN MH-SUSCEPTIBLE PATIENTS BE IDENTIFIED?
Because MH is a dominantly inherited disorder, all closely related members of a family in which MH has occurred must also be considered MH susceptible and managed accordingly, unless proven otherwise. It should be noted that those who have had previous anesthetics without problem cannot be certain they are not at risk; MH related deaths have occurred even though patients have undergone multiple prior uneventful surgeries. Certainly any family with a history of anesthetic deaths or complications should make this known to the anesthesiolgist before undergoing surgerg. Additionally, they should register their MH susceptibility with the North American Registry lf MHAUS, Pittsburgh, PA by calling 412-692-6390, or toll-free 888-274-7899.
CAN MH-SUSCEPTIBLE PATIENTS HAVE SURGERY?
Yes! Surgery can be safely performed in the known MH-susceptible patients. However, nontriggering anesthetics must be used as well as special precautions and techniques, including close monitoring of appropriate vital functions. Close monitoring occurs in all anesthetics, but this is modified for MHS patient or more focused analysis of vital signs and some blood sampling
In surgery for a known MH-susceptible patient, the anesthesiologist should:
· Avoid the use of MH-triggering anesthetics.
· Be familiar with the signs and treatment of MH, e.g., re- review the routine information.
· Continuously monitor the patient's exhaled carbon dioxide concentration.
· Continuously monitor the patient's temperature (also during recovery). Skin temperature is not optimal in this situation.
· Have an MH kit or cart within the operating room suite stocked with an adequate supply of dantrolene.
CAN MH OCCUR OUTSIDE OF THE OPERATING ROOM?
Yes. While most cases of MH occur during general anesthesia, the one-hour period immediately following surgery (including the recovery room) is also a critical time. In addition, MH can occur if trigger anesthetics and/or succinylcholine are used in any location, such as emergency rooms, dental surgeries, surgeon’s offices or intensive care units.
CAN ANYTHING OTHER THAN ANESTHETIC DRUGS TRIGGER MH?
Only the anesthetic drugs already mentioned usually trigger MH. Studies have shown that a small percent of people who develop muscle breakdown following exercise only, or after heat stroke, harbor the genetic changes associated with MH susceptibility. It is still unclear if the muscle breakdown and other changes result from these non-anesthetic incidences. Generally, we advise MHS patients to lead normal lives other than when anesthesia is required
ARE THERE LINKS BETWEEN MH AND OTHER DISORDERS?
The following disorders have been shown to predispose a patient to MH:
· Central Core Disease and Multiminicore Disease These patients should not receive MH triggers.
· Duchenne Muscular Dystrophy and Becker's Muscular Dystrophy patients may develop life-threatening hyperkalemia with succinylcholine and/or potent volatile anesthetic agents, all though this is not MH per se.
· The vast majority of muscle and neuromuscular disorders do not predispose to MH.
· However, succinylcholine may precipitate muscle breakdown and/or rigidity (especially the myotonias). In general, succinylcholine should be avoided in all patients with clinical muscle or neuromuscular disorders. Mitochondrial myopathies do not predispose to MH.
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