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Malignant Hyperthermia - Essential Charactistics:
>An inherited disorder of skeletal muscle triggered in susceptibles (human or animal) in most instances by inhalation agents and/or succinylcholine, resulting in hypermetabolism, skeletal muscle damage, hyperthermia, and death if untreated.
>Underlying physiologic mechanism – abnormal handling of intracellular calcium levels.
MH is inherited as an autosomal dominant trait. This means that susceptibility is passed on to half the children from an affected adult. Both males and females may be affected.
Only the potent gas anesthetics and the paralyzing agent succinylcholine trigger MH. Other drugs are safe.
In MH, anesthetic “trigger” drugs affect the metabolism of muscle through effects on cell calcium movements. Muscle damage, increased metabolism, increased acid content of the blood, abnormalities of electrolytes result. If not treated, death is likely.
Other data show that up to 1 of every 3,000 people harbor the genetic change that predisposes to MH. This is the same order of magnitude as muscular dystrophy. Unfortunately, there are no outward signs to indicate who is at risk.
Studies are showing that when MH occurs in other than a hospital setting, the mortality is much higher. This may reflect lack of personnel, lack of coordination in moving the patient to the hospital, or other factors.
The diagnosis of MH can be straightforward if all the signs occur rapidly, but the diagnosis can be subtle depending on the time of presentation of the signs of MH and the progression of the changes. In general, the first signs are an increase in exhalation of carbon dioxide, increase in heart rate and muscle rigidity (not always present). Several other conditions can lead to similar changes and the anesthesia provider needs to sort out the cause of the changes.
One of the sentinel signs of MH is jaw muscle rigidity after administering the paralyzing drug succinylcholine. This is more common in children than adults. When it occurs, clinical signs of MH may follow soon after. Even if no overt signs of MH occur, the patient needs to be observed for signs of muscle breakdown.
Several muscle disorders have been associated with MH susceptibility. These are generally very uncommon conditions, but nevertheless patients with these conditions need to be treated as being MH susceptible.
Hyperkalemia= high serum potassium levels. This occurs as a result of muscle membrane breakdown. In addition increased release of the muscle pigment , myoglobin , may occur and lead to kidney damage.
It is important to have a well defined and rehearsed plan for treatment of MH since many things have to be done simultaneously and treatment must be initiated immediately.
Dantrolene needs to be immediately available. The drug needs to be reconstituted with bacteriostatic sterile water, 60ml/vial. The average patient needs at least 9 vials to begin treatment and may need much more depending on the response to treatment. Dantrolene sodium for injection is available as Dantrium® IV from JHP Pharmaceuticals and as dantrolene sodium for injection from US WorldMeds.
Once the crisis is controlled, it is important to continue treatment with dantrolene and have the patient recover in an ICU since the syndrome can recur.
Prompt recognition and treatment is crucial in managing MH. It should be emphasized that all patients undergoing general anesthesia for more than about 30 minutes should have their body temperature monitored.
Diagnostic tests are most useful when making treatment decisions for surgical patients where there is a high level of suspicion that the patient is susceptible to MH. Before any diagnostic testing is recommended, an evaluation of the patient’s susceptibility to MH should be completed using available medical data. Although the CHCT is the gold standard, it is an invasive, costly test.
At this time, genetic testing is recommended as a confirmatory diagnostic measure for individuals known to be at high risk for an MH event, as determined by their own or a first-degree (sibling, parent, offspring) family member’s clinical episode of MH or positive muscle contracture test (caffeine-halothane contracture test).
It is hoped that , as more information is gathered about the genetics of MH, the DNA test will become more useful for diagnosing all patients as MH susceptible or not susceptible.
The anesthesia machine needs to be purged of residual gases prior to use in a susceptible. The above recommendation will need to be modified for the newer generation of anesthesia machines. They may need a longer time for purging of the gases. The practitioner should consult the manufacturer of the machine.
MH patients may be anesthetized with local anesthesia for example with spinal or epidural techniques, or using general anesthetics that are not MH triggers. An MH susceptible should not be deprived of general anesthesia if indicated by the surgery.
Although it is clear that the animal model for MH, certain swine, regularly develop awake signs of MH when stressed, this does not seem to occur with humans. However, there is evidence that the rare patient may develop muscle breakdown or even MH under certain conditions such as extreme heat and exercise. Much more work is necessary to clarify the relation to MH.