Musculoskeletal Anesthesia Considerations

  • Nerve to Muscle Interaction (sequence of actions)

  1. Spinal Cord

  2. Motor Nerve Axon

  3. Synaptic Cleft

  4. NMJ

  5. Muscle Unit

  6. Muscle Fibers

  7. Muscle Cell

  • Mechanism of muscle contraction:

    • As the action potential approaches the motor nerve ending, calcium ions (Ca2+) influx into the nerve terminal, resulting in Ach being released into the synaptic cleft.

    • The release of Ach prompts the release of Ca2+ in the muscle fibers, leading to muscle fiber shortening and thus contraction.

    • Following this, Ach undergoes hydrolysis, Ca2+ is pumped back into the muscle fiber, allowing the muscle to relax.

  • Muscle Cell Physiology

    • Typical ion distribution:

      • Potassium ions (K+) predominantly inside the cell, Sodium ions (Na+) predominantly outside.

    • During contraction (CTX):

      • Outflow of K+ and inflow of Na+.

Myasthenia Gravis

  • Myasthenia Gravis (MG) is a heterogenous autoimmune disorder characterized by chronic dysfunction at the neuromuscular junction (NMJ) and altered function of the thymus gland.

  • Risk Factors

    • Predominantly affects individuals under 50 years old, with a higher incidence in women

    • Older adults affected equally

    • Abrupt onset

    • Characterized by periods of exacerbation and remission

  • Symptoms (Sx)

    • Muscle weakness that worsens with activity and does not fully resolve with rest

    • Fatigue

    • Ptosis (drooping of eyelids) and Diplopia (double vision)

    • Dysphagia (difficulty swallowing)

  • Treatment (Tx)

    • Cholinesterase Inhibitors (e.g., Pyridostigmine)

      • Mechanism: Inhibits hydrolysis of Ach, increasing its availability at the NMJ.

    • Corticosteroids to reduce inflammation and immune response.

    • IVIg for immune modulation.

    • Thymectomy (surgical removal of the thymus gland)

  • Emergent Situations

    • Myasthenic Crisis: A condition resulting from inadequate Ach leading to exacerbation of MG and resultant profound muscle weakness.

    • Cholinergic Crisis: A condition caused by overdose of medications leading to excess Ach which causes a depolarizing block and profound muscle weakness.

Anesthesia Considerations for MG

  • Mild Disease

    • Elective procedures should be performed when patients are stable and on minimal medications.

    • Cholinesterase Inhibitors should be withheld as they can cause a vagal response during intubation.

  • Severe Disease

    • Only proceed with urgent or emergent cases

    • Cholinesterase Inhibitors taken up to the morning of surgery

  • General Guidelines:

    • Increased risk of aspiration

    • Preferential use of Regional or Local Anesthesia due to:

      • Improvement of post-operative analgesia and respiratory function

      • Elimination of the need for peri-operative neuromuscular blockade

      • Amide local anesthetics preferred over esters

      • Earlier neuraxial anesthesia is recommended for laboring patients to mitigate risks associated with fatigue/weakness/stress

    • Avoid pre-operative sedation and opioid usage to minimize risks.

    • Use of Laryngeal Mask Airway (LMA) is advisable to reduce the requirement for muscle relaxants.

    • Reversal of neuromuscular blockade may precipitate a cholinergic crisis.

    • Extubation Criteria:

      • Complete and sustained return of muscle strength

      • Intact upper airway reflexes

      • Resumption of spontaneous ventilation

      • Tracheal intubation with ventilatory support may be necessary in the post-procedural phase.

    • Predictor Factors for Difficulties:

      • Body Mass Index (BMI) > 28

      • Duration of surgery > 2-6 hours (or years of age?)

      • Pyridostigmine dose > 750 mg/day

Malignant Hyperthermia

  • Definition

    • A hypermetabolic disorder of skeletal muscle, primarily inherited in an autosomal dominant pattern.

    • Commonly triggered by inhaled volatile anesthetics and succinylcholine used during anesthesia. Other rarer triggers include:

      • Stress

      • Vigorous exercise

      • Heat

  • Risk Factors

    • Individuals <15 years old

    • Male population

  • Symptoms (Sx)

    • Sustained muscle contractions (e.g., jaw clenching)

    • fever

    • Tachycardia

    • Tachypnea

    • Skin mottling

    • Hyperkalemia

    • Dysrhythmias

  • Treatment (Tx)

    • Discontinue inhaled anesthesia and succinylcholine immediately

    • Administer Dantrolene Sodium intravenously for a minimum of 24 hours

    • Use cooling blankets to manage body temperature

    • Provide hyperventilation with 100% O2 at a rate of 10L/min

    • Emergent Situations: The condition can rapidly progress to muscle cell destruction and lead to excessive hyperkalemia, resulting in life-threatening cardiac arrest.

  • Prophylaxis

    • Inquire about family history of malignant hyperthermia.

    • Avoid the use of volatile inhaled anesthetics and succinylcholine.

    • Utilize cooling blankets to prevent hyperthermia.

    • Monitor core body temperature vigilantly.

    • Ensure Dantrolene Sodium is readily available in the clinical setting.