History and exam
Key diagnostic factors
common
presence of risk factors
Key risk factors include exposure to potent inhalation anaesthetic and/or succinylcholine (suxamethonium), susceptibility to MH, a previous MH episode, and a positive family history.
exposure to potent inhalation anaesthetic and/or succinylcholine (suxamethonium)
The majority of cases occur following exposure to inhalation anaesthetics and/or succinylcholine (suxamethonium). Any volatile inhalation anaesthetic can cause MH.[1]
susceptibility to MH
A positive muscle contracture test is diagnostic of susceptibility to MH.
Patients with susceptibility to MH may develop MH if exposed to a trigger.
In patients with susceptibility to MH, 50% to 70% have mutations in RYR1.[7][21][24][25] The reported yields vary with the population selected and the details of the genetic test.[32][48]
previous MH episode
Patients with a confirmed previous episode of MH may have a further episode if exposure to a trigger is repeated. However, a history of uneventful anaesthesia with a triggering agent does not preclude the possibility of a future episode.[1]
positive family history
Susceptibility to MH is an autosomal dominant condition, so first-degree relatives have a 50% chance of inheriting the genetic predisposition. Subsequent generations are also at an increased risk compared with patients with no family history of an MH event.
increased minute ventilation
MH is almost always associated with an increase in minute ventilation in a spontaneously breathing patient. If the patient is mechanically ventilated then a substantial increase in minute ventilation is required to maintain a normal end tidal carbon dioxide.
elevated core temperature
The onset of hyperthermia varies from early to several hours after exposure to inhalation anaesthetics.
Emphasis on temperature monitoring is stressed in every patient receiving an anaesthetic, specifically when changes in body temperature are possible. An increase in deaths due to MH from 2007 to 2012 was reported to be associated with inadequate and inaccurate temperature monitoring.[53]
muscle rigidity
Muscle rigidity can result from inadequate anaesthesia: that is, the patient is reacting to surgical stimulation. If anaesthetics and non-depolarising neuromuscular blockers in large doses do not remove rigidity, then it must be attributed to a process in muscle. This process can be either MH, Brody’s disease, or myotonia.
About 40% of MH cases have muscle rigidity as one of the clinical findings during the MH event.[6] Masseter muscle rigidity may be an early sign of an episode of MH.[2][6]
Other diagnostic factors
common
tachycardia
A non-specific sign, but useful to guide treatment.
decreased urine output
Decreased urine output, accompanied by an increase in urinary myoglobin, is a sign of acute kidney injury.
uncommon
excessive sweating with exercise
People who have susceptibility to MH may feel uncomfortable during exercise in the heat.
muscle cramps
People with susceptibility to MH report severe muscle cramps more often than people who are MH normal by muscle contracture testing.[3]
spontaneous episodes of severe muscle stiffness
Rarely, episodes of MH may occur without exposure to anaesthetics.[70]
These typically present as spontaneous episodes of muscle stiffness.
Risk factors
strong
exposure to potent inhalation anaesthetic and/or succinylcholine (suxamethonium)
The majority of cases occur following exposure to inhalation anaesthetics and/or succinylcholine (suxamethonium). Any volatile inhalation anaesthetic can cause MH.[1]
susceptibility to MH
A positive muscle contracture test is diagnostic of susceptibility to MH.
Patients with susceptibility to MH may develop MH if exposed to a trigger.
In patients with susceptibility to MH, 50% to 70% have mutations in RYR1.[7][21][24][25] The reported yields vary with the population selected and the details of the genetic test.[32][48]
previous MH episode
Patients with a confirmed previous episode of MH may have a further episode if exposure to the trigger is repeated. However, a history of uneventful anaesthesia with a triggering agent does not preclude the possibility of a future episode.[1]
positive family history
Susceptibility to MH is an autosomal-dominant condition, so first-degree relatives have a 50% chance of inheriting the genetic predisposition. Subsequent generations are also at an increased risk compared with patients with no family history of an MH event.
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