Myasthenia gravis is an autoimmune disease that attacks post-synaptic nicotinic acetylcholine receptors at the NMJ. This leads to varying degrees of muscle weakness and fatigue and can affect ocular, bulbar (muscles involved in speech, chewing and swallowing), respiratory, and proximal skeletal muscles. Symptoms seem to be worse at the end of the day or after exertion. Classification of myasthenia depends on whether the patient has only ocular or ocular and non-ocular weakness.
These patients can pose a challenge during anesthesia not only because of their disease process but also from the medications used to treat them (acetylcholinesterase inhibitors, steroids, etc.). Preoperative evaluation should consider the recent course of the disease, muscle groups affected, drug therapy, and coexisting diseases. Perioperative management may also require a neurology consult.
- Patients should take their morning dose of acetylcholinesterase inhibitor though doing so may result in:
- Altered patient drug requirements following surgery
- Increased vagal reflexes
- Possibility of disrupting bowel anastomoses secondary to hyperperistalsis
- Significant side effects including salivation, miosis, bradycardia and even cholinergic crisis if given NMB reversal
Patients with involvement of respiratory or bulbar muscles tend to have a higher risk for aspiration so pretreatment with metoclopramide or H2 blockers may be helpful. Some MG patients are sensitive to respiratory depressants and premedication with opioids, BZDs, or barbiturates should be considered carefully or not at all.
Patients should be made aware that they might remain intubated after procedure is finished and will need to be awake prior to extubation. Predictive risk factors for post-operative ventilation support following thymectomy:
- Disease duration > 6yrs
- Concomitant pulmonary disease
- Peak inspiratory pressure < -25 cmH2O
- Vital capacity < 40 mL/kg
- Pyridostigmine dose > 750 mg/d
A steroid listed on a patient’s medication profile indicates a higher risk and elective procedures should be postponed if possible.
Of note, IV pyridostigmine is 1/30 of the PO dose.
IVIG or plasma exchange can be used in emergent cases.
Diseases associated with MG include DM, thyroid disorders, SLE and RA.
Patients with MG are resistant to succinylcholine and are exquisitely sensitive to non-depolarizing NMBs.