This patient with masseter muscle spasm, acute hypercarbia, and elevated temperature shortly after induction of anesthesia likely has malignant hyperthermia (MH), a condition characterized by excessive muscle contracture.

Normally, muscle contraction is mediated by a cascade of events:

• Membrane depolarization releases calcium from the sarcoplasmic reticulum into the intracellular space
• Calcium binds to troponin to expose myosin bridging sites
• Exposed myosin cross-links to actin and contracts
• Sarcoplasmic reticulum calcium ATPase reuptakes calcium to reverse this binding and cause relaxation

In patients with MH, exposure to certain anesthetic agents (eg, succinylcholine, inhalational gasses) causes unregulated passage of calcium from the sarcoplasmic reticulum into the intracellular space.  Excessive myoplasmic calcium accumulation results in sustained muscle contraction, which is often first noted in the masseter.

Sustained muscle contraction leads to increased cellular metabolism, which causes hypercarbia and acidosis.  Muscle hypermetabolism consumes ATP and leads to excessive heat production.  Once ATP and oxygen stores are depleted, muscle breakdown (ie, rhabdomyolysis) occurs, which releases potassium, myoglobin, and creatine kinase, leading to a further cascade (eg, arrhythmias, acute renal failure, disseminated intravascular coagulation) that may be fatal.  Dantrolene is used to treat MH because it blocks further release of calcium into the intracellular space.

(Choice A)  Thyroid storm (ie, thyrotoxic crisis) can be precipitated by surgery.  Hyperpyrexia occurs because excessive circulating levels of T3 and T4 increase cellular metabolism and adrenergic tone.  However, thyroid storm does not cause sustained muscle contractions.

(Choice B)  A true fever results from a change in the hypothalamic set point, typically due to an infection (not anesthesia).  Shivering may occur, but not sustained muscle contraction or acute hypercarbia.

(Choice D)  Heat dissipation occurs primarily by cutaneous vasodilation and sweating.  These mechanisms can be impaired by aging or medications (eg, atropine).  During an MH crisis, cutaneous heat removal is unaffected, but heat production far outstrips heat dissipation.

(Choice E)  Excessive serotonergic activity in the CNS can result in hyperpyrexia (due to serotonin syndrome).  However, it classically presents with hyperreflexia and clonus (rather than rigidity) and is seen in patients taking serotonergic medications (eg, selective serotonin reuptake inhibitors, tricyclic antidepressants), especially in combination, rather than with anesthesia.

Educational objective:
In patients with malignant hyperthermia, exposure to certain anesthetic agents (eg, succinylcholine, inhalational gasses) causes unregulated passage of calcium from the sarcoplasmic reticulum into the intracellular space.  This results in sustained muscle contraction that leads to hypercarbia, hyperthermia, and acidosis.