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What is Malignant Hyperthermia?
Malignant Hyperthermia (MH) is an acute pharmacogenetic
(autosomal dominant) disorder, which develops during or
immediately after the application of general anaesthesia
involving volatile agents and/or depolarising muscle relaxants.
The disorder is likely to be a result of a defect in calcium
channel regulation in the muscle cell. Volatile anaesthetic
agents and depolarising muscle relaxants interact with the
calcium channel resulting in the clinical crisis.
The classic MH crisis consists of a hyper-metabolic state caused
primarily by continued contraction of the skeletal muscles,
which leads to massive CO2 production, skeletal muscle rigidity,
tachyarrhythmias, unstable haemodynamics, respiratory acidosis,
cyanosis, hyperkalaemia, lactic acidosis, fever, and eventually
(if untreated) death. MH can present with a few or all of these
features.
How do we test for it?
In Vitro Contracture Test
Survivors of an MH crisis and their family members can be tested
for susceptibility to MH with an in vitro contracture test (IVCT),
which exposes excised live muscle to increasing concentrations
of caffeine and halothane. The test involves an open muscle
biopsy, anaesthesia and specialised laboratory equipment and
expertise. This test is only performed in 4 sites in Australia
and New Zealand. These are the Royal Melbourne Hospital
(Victoria), The Royal Perth Hospital (Western Australia),
Westmead Children’s Hospital (New South Wales) and Palmerston
North Hospital (New Zealand).
Genetic Testing
Some genetic testing is available in Australia and New Zealand
for MH diagnosis. Simple
screening of the population is not practical or useful.
Genetic testing is directed at tracking a specific mutation through
a family after having determined that mutation in a known IVCT
positive family member. Up to 50% of persons with a positive
muscle biopsy will have a mutation identified. Genetic testing
cannot yet be used to prove that a person does not have
MH.
Patient Information
What is Malignant Hyperthermia?
Malignant Hyperthermia (malignant hyperpyrexia, MH) is a
syndrome that can occur in susceptible people when they are
given certain anaesthetic drugs. It occurs very rarely and is
genetically inherited (i.e. runs in a family). It can result in
a very high temperature and muscle rigidity developing during an
anaesthetic that can be deadly if not recognised and treated.
How do I know if I have it?
You may have had a reaction under general anaesthetic which had
some or all of the features of MH. Your anaesthetist may have
referred you to the MH testing centre. You may have a family
member who has been found to be susceptible to MH.
The only way we currently know to diagnose MH is by muscle
biopsy. If someone in your family has it we will usually test
all members’, as half of them may be susceptible. For some
people genetic tests (blood tests) may be available.
How will it affect me if I have a positive diagnosis for MH?
If the biopsy results are positive this will alter the types of
anaesthetics that you can safely be given. The anaesthetist must
not use potent volatile anaesthetic agents or a drug called
suxamethonium. These are the only drugs that have been shown to
trigger MH and they would not be given in any other situation
than an anaesthetic or emergency situation (i.e. your GP would
not prescribe them). There are other available alternatives to
these drugs. Therefore you can have adequate anaesthesia for
future operations.
Being susceptible to MH only affects the type of anaesthetic
drugs you can receive safely. In general it does not effect any
other aspect of your health.
Will my family have to be tested?
If you have a positive muscle biopsy it would be prudent to test
all of your relatives. If they do not have the diagnosis they
can rest assured of no problems with triggering of malignant
hyperthermia during anaesthesia, but if they test positive much
can be done to avoid life-threatening problems
intra-operatively. Such testing may involve the IVCT or genetic
testing.
Can MH be treated?
Since the 1970’s a drug has been available that can help in the
treatment of MH. It is called dantrolene and it helps to relax
muscle. It has been very effective in decreasing the death rate
from MH if used early in the developing syndrome. It is not a
cure but rather a treatment and should therefore not be relied
upon if the diagnosis of MH has been made prior to surgery.
Does previous uneventful anaesthesia mean I’m safe?
Unfortunately, on average, patients found to be susceptible to
MH have had 3 previous uneventful anaesthetics so this is no
guarantee that you are not susceptible.
How many people have this syndrome?
MH occurs in approximately 1 in 15,000 to 1 in 50,000
anaesthetics. Recent European studies have suggested a genetic
predisposition of about 1 in 5,000 persons.
What Anaesthetics are safe?
Local anaesthetics, spinal, epidural and other regional
(numbing) anaesthetics are safe. General anaesthesia with
intravenous maintenance is also safe as is the gas, nitrous
oxide.
What is a muscle biopsy?
This is a relatively small, day case surgical procedure that
involves a cut - under regional or non-triggering general
anaesthetic - to the outer thigh. A sample of muscle is removed
which is then delivered to the anaesthetic laboratory where it
is tested. There may be some residual soreness in the area of
the biopsy for a few days but the procedure is low risk. You
will usually need 2 to 3 days off work.
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