Case Study: Malignant Hyperthermia
Contributed by Susan Torchen, CRNA
A 15-month-old, 22-kg male is scheduled for strabismus surgery. His medical history is unremarkable except for idiopathic childhood exotropia. He has no surgical or anesthesia history. Family anesthetic history includes maternal uncle that died during surgery with general anesthesia as a child; mother has only vague knowledge of exact cause of death. Mother has received only sedation and regional for small procedures. Father has no known surgical or anesthesia related complications.
Are there any additional questions you would want to ask the family prior to the start of this case?
With a questionable familial anesthetic complication all efforts should be made to find out as much information surrounding the questionable event. Malignant hyperthermia (MH) is an autosomal dominant heterogeneous disease that varies from family to family. Twenty-one percent of patients who carry the MH gene will have at least one uneventful exposure to general anesthesia prior to having a MH reaction. Since this patient has a questionable maternal link to MH it is important to use MH precautions until the child is proven not to be susceptible to MH (this can only be done by a negative contracture test, and cannot be performed in this child until later in childhood). Certain common pediatric disorders should heighten suspicion to a possible MH reaction; these are the myopathies caused by mutations involving the ryanodine receptor and include central core myopathy, multiminicore myopathies, and several other extremely rare variants. The anesthetic plan for this patient should include MH precautions and the use of total intravenous anesthesia.
How can MH susceptibility be determined?
The caffeine-halothane contracture test (CHCT) remains the gold standard to diagnose MH; however, there are only five remaining testing centers in North America, and the patient has to have the procedure at the testing site. Genetic analysis of known ryanodine (RYR1) mutations is possible by sending blood or tissue samples to a certified lab, but the sensitivity of the test is not more than 50%. Therefore, the vast majority of patients suspected to have susceptibility to MH are treated as such throughout their lives.
What is the pathophysiology of MH?
A mutation in the ryanodine receptor gene (RYR1) causes susceptibility to MH in persons with the mutation that are exposed to triggering general anesthetics. However, the genotype-phenotype correlations are unknown; in other words, the specific mutation is not predictive of the clinical presentation. The mutated ryanodine receptor in the sarcoplasmic reticulum allows calcium to flow unregulated into the muscle cell, which leads to unabated muscle contracture, and the classic clinical signs of MH. The exact mechanism of action that causes this interaction in the presence of a triggering anesthetic is unknown.
How would you prepare your operating room for this patient?
Patients that are known to be susceptible to MH should not receive the triggering anesthetic agents, which include all the inhaled volatile gases and succinylcholine. The procedure for preparing the anesthesia machine can be found on the MHAUS website.
Following emergence and extubation of your patient, you note that he has limited chest rise and poor gas exchange, despite the adequate placement of an oral airway. You are unable to effectively ventilate the patient, there is no etCO2, and his oxygen saturation declines rapidly.
How do you manage severe laryngospasm in an MH susceptible patient?
Since succinylcholine needs to be avoided in this patient, we would immediately administer IV rocuronium (1.5 mg/kg) and propofol (2 mg/kg). If this patient did not have IV access, rocuronium (2 mg/kg) should be administered intramuscularly, but if the airway obstruction is so severe that the patient is developing life-threatening bradycardia, then we recommend administration of IM succinylcholine (4 mg/kg) and atropine (0.02 mg/kg).