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Malignant hyperthermia
Malignant hyperthermia is a life-threatening elevation in body temperature usually
resulting from a hypermetabolic response to concurrent use of a depolarizing muscle
relaxant and a potent, volatile inhalational general anesthetic.
Manifestations can include muscle rigidity, hyperthermia, tachycardia, tachypnea,
rhabdomyolysis, and respiratory and metabolic acidosis.
Diagnosis is clinical; patients at risk can be tested for their susceptibility. The highest
priority treatments are rapid cooling and aggressive supportive measures.
The muscle relaxant involved is usually succinylcholine; the inhalational anesthetic is
most often halothane, but other anesthetics (eg, isoflurane, sevoflurane, desflurane)
may also be involved. This drug combination causes a similar reaction in some
patients with muscular dystrophy and myotonia.
Pathophysiology
Malignant hyperthermia affects about 1/20,000 people.
Susceptibility is inherited, with autosomal dominant inheritance and variable
penetrance. Most often, the causative mutation affects the ryanodine receptor of
skeletal muscle; however, > 22 other causative mutations have been identified.
In susceptible patients.
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Anesthetic-induced potentiation of ca exit from the sarcoplasmic reticulum of skeletal
muscle
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Ca-induced biochemical reactions are accelerated
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Causing severe muscle contractions & elevation of the metabolic rate.
Complications
1. K↑
2. Respiratory and metabolic acidosis,
3. Ca↓
4. Rhabdomyolysis with CK elevation and myoglobinemia may occur,
5. Coagulation abnormalities (particularly DIC). In older patients and patients
with comorbidities, DIC may increase the risk of death.
Symptoms and Signs
Malignant hyperthermia may develop during anesthesia or the early postoperative
period. Clinical presentation varies, depending on the drugs used and the patient's
susceptibility.
1. Muscular rigidity, especially in the jaw, is often the first sign, followed by
2. tachycardia,
3. other arrhythmias,
4. tachypnea,
5. acidosis,
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6. shock, and
7. hyperthermia. Temperature is usually ≥ 40° C and may be extremely high (ie,
> 43° C).
8. Urine may appear brown or bloody if rhabdomyolysis and myoglobinuria have
occurred.
Diagnosis
o Clinical evaluation
o Testing for complications
o Susceptibility testing for people at risk
The diagnosis is suspected by the appearance of typical symptoms and signs within 10
min to, occasionally, several hours after inhalational anesthesia is begun. Early
diagnosis can be facilitated by prompt recognition of jaw rigidity, tachypnea,
tachycardia, and increased end-tidal CO2.
There are no immediately confirmatory tests, but patients should have testing for
complications, including ECG, blood tests (CBC with platelets, electrolytes, BUN,
creatinine, CK, Ca, PT, PTT, fibrinogen, d-dimer), and urine testing for
myoglobinuria.
Other diagnoses must be excluded. Perioperative sepsis may cause hyperthermia but
rarely as soon after induction. Inadequate anesthesia can cause increased muscle tone
and tachycardia but not elevated temperature. Thyroid storm and pheochromocytoma
rarely manifest immediately after anesthetic induction.
Susceptibility testing
Recommended for people at risk based on a
o Family history of the disorder
o Personal history of a severe or incompletely characterized previous adverse
reaction to general anesthesia.
The caffeine halothane contracture test (CHCT) is the most accurate. It measures the
response of a muscle tissue sample to caffeine and halothane. This test can be done
only at certain referral centers and requires excision of about 2 g of muscle tissue.
Genetic testing has limited sensitivity (about 30%) but is quite specific; patients in
whom a mutation is identified do not require the CHCT.
Treatment
1. Rapid cooling and supportive measures
It is critical to cool patients as quickly and effectively as possible to prevent
damage to the CNS and also to give patients supportive treatment to correct
metabolic abnormalities. Outcome is best when treatment begins before
muscular rigidity becomes generalized and before development of
rhabdomyolysis, severe hyperthermia, and DIC.
2. Dantrolene; (a muscle relaxant) (2.5 mg/kg IV q 5 min as needed, up to a total
dose of 10 mg/kg) should be given in addition to the usual physical cooling
measures. In some patients, tracheal intubation, paralysis, and induced coma
are required to control symptoms and provide support.
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3. Benzodiazepines given IV, often in high doses, can be used to control
agitation.
4. Malignant hyperthermia has a high mortality and may not respond to even
early and aggressive therapy.
Prevention
1. Local or regional anesthesia is preferred to general anesthesia when possible.
2. Potent inhalational anesthetics and depolarizing muscular relaxants should be
avoided in patients who are susceptible.
3. Nondepolarizing muscular blockers are the preferred preanesthetic drugs.
Preferred anesthetics include barbiturates (eg, thiopental), etomidate, and
propofol.
4. Dantrolene should be available at the bedside.
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