Malignant Hyperthermia - 11
Masseter Muscle Rigidity (Spasm) (MMR, MMS)

MMR is part of a spectrum of responses to intravenous succinylcholine.
  • Some slight, subclinical increase in masseter muscle tone may be normal.
  • Jaw stiffness that interferes with mouth opening for direct laryngoscopy and orotracheal intubation occurs in about 2% of children. This may include an unknown number at risk for MH.
  • "True MMR" occurs when "jaws of steel" make mouth opening impossible. This is relatively rare. In one series of children referred for biospy, 50% proved to be MHS.
Differential diagnosis of difficult mouth opening after succinylcholine
  • not enough time
  • not enough succinylcholine (use a peripheral nerve stimulator to test for block)
  • temporomandibular joint dysfunction (Could patient open mouth before induction?)
  • amyotonic myotonia
Dealing With MMR
If succinylcholine has caused marked MMR, then
  • patient may be MHS
  • consideration should be given to canceling an elective case
  • emergent case may be continued with nontriggering agents (patient may require flexible fiberoptic nasotracheal intubation)
  • observe closely for (additional) signs of MH
  • be prepared to treat with dantrolene
  • extend postoperative observation period
  • consider referral for muscle biopsy after discussion with family
Perhaps easier, if reasonably possible:
avoid sux
No succinylcholine = no masseter muscle rigidity! Consider rocuronium, mivacurium, atracurium, cisatracurium, vecuronium, pancuronium, ...

Greg Gordon MD