Condition
Symptoms and characteristics
Comments
Drug-induced myasthenia gravis (penicillamine, nondepolarizing muscle relaxants, aminoglycosides, procainamide)
Induced weakness in normal persons by triggering autoimmune MG; exacerbation of MG
Distinguished by improvement of symptoms after discontinuation of the drug
Eaton–Lambert syndrome
Weakness improves after repetitive use; commonly seen in small cell lung cancer
Caused by antibodies to calcium channels
Grave’s disease
Diplopia, exophthalmos
Thyroid-stimulating immunoglobulin present
Botulism
Generalized weakness, ophthalmoplegia, mydriasis
Incremental response on repetitive nerve stimulation
Progressive external ophthalmoplegia
Ptosis, diplopia, generalized weakness in some cases
Mitochondrial abnormalities
Intracranial mass
Ophthalmoplegia, cranial nerve weakness
Abnormalities on CT or MRI
What are the diagnostic tests for myasthenia gravis?
Tensilon test (administration of anticholinesterase, i.e., edrophonium)
Positive if strength improves with inhibition of cholinesterase. Works by increasing the amount of acetylcholine available to interact with the decreased number of postsynaptic nACRs, improving the likelihood of adequate end-plate depolarization.
Edrophonium is usually administered in small doses (2-8 mg), and improvement is seen within 5 min and lasts for about 10 min.
Electromyography
Confirmed by the decremental response in compound muscle action potential after repetitive nerve stimulation.
Radioimmunoassay
Detection of anti-acetylcholine antibodies in the serum, however, the antibodies may not be detectable or not be present in all patients.
What are the treatments for myasthenia gravis?
Treatment of MG can be categorized into medical versus surgical methods:
Medical:
Anticholinesterase drugs—first line of treatment
Mechanism: Inhibit enzyme responsible for hydrolysis of acetylcholine, therefore increasing the amount of neurotransmitter available at the NMJ.
Pyridostigmine (Mestinon)
Most widely used as it is well tolerated orally, with few muscarinic side effects and has a long duration of action.
Onset is around 30 min with peak effect in 2 h and overall duration around 3-6 h
Dosing-tailored to response (max dose 120 mg PO q3hrs), 30 mg PO = 1 mg IV/IM
Higher dosages may actually induce muscle weakness, leading to cholinergic crisis
Confirmed by onset of muscarinic side effects (salivation, miosis, bradycardia), accentuated muscle weakness after administration of edrophonium
Although anticholinesterase drugs benefit most patients, the improvements may be incomplete and may wane after weeks or months of treatment.
Immunosuppression—indicated when muscle weakness not adequately controlled by anticholinesterase drugs
Short-Term Immunotherapy
Plasmapheresis—used for short-term symptomatic improvement in patients who are experiencing myasthenic crisis, respiratory compromise, or are being prepared for thymectomy [11]
Mechanism: removes antibodies from the circulation, allowing receptors to proliferate
Transient effects, improvement occurs over days with decreased ventilatory dependence
Repeated treatments could lead to increased risk of infection, hypotension, and pulmonary embolism.
Immunoglobulins—same indications and mechanism as plasmapheresis
Does not have effect on circulating concentrations of acetylcholine receptor antibodies.
Surgical:
Thymectomy—goal is to induce remission or at least reduce the dosage of pharmacotherapy [12]
Mechanism: Speculative, hypothesized removal of antigenic stimulus by the removal of myoid cells, or alterations in immune regulation by removal of the thymus
Acetylcholine receptor antibody titer usually decreases following successful thymectomy with clinical improvement [13]
Surgical approach:
Median sternotomy—optimizes visualization and removal of all tissues
Mediastinoscopy through a cervical incision – associated with a smaller incision and less postoperative pain
Postoperative:
Decreased need for anticholinesterase medication, full benefit often delayed for months after surgery.
Preoperative
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How would you manage a MG patient based on their preoperative medications?
Anticholinesterase—the decision to continue or hold the dose on the morning of surgery is per the discretion of the surgeon or the anesthesiologist. Some choose to hold the dose to avoid interactions with neuromuscular blocking agents [14].
Corticosteroids—often will require perioperative stress dose steroid coverage
Plasmapheresis—will require preoperatively if the patient’s disease is poorly controlled [15]
Anxiolytics/opioids—try to avoid given likely preexisting respiratory muscles weakness.
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What are some of the anesthetic considerations preoperatively?
Cardiovascular—MG patients have increased risk for heart disease because the culprit antibodies have a high affinity for β1 and β2 adrenergic receptors [16]. Patients often have comorbid atrial fibrillation, heart block, or cardiomyopathy.
Airway protection—bulbar involvement can severely compromise the patient’s ability to cough and clear secretions, as well as protect or maintain a patent airway
Respiratory muscle strength:
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Pulmonary function tests (PFTs)—useful for quantifying respiratory muscle strength, specifically negative inspiratory pressure and forced vital capacity (FVC).
Optimizing strength and respiratory function—if vital capacity <2L, plasmapheresis is usually performed preoperatively to increase the likelihood for adequate pulmonary function postoperatively.
It is important to inform the patient about the possibility of postoperative ventilator support.Related posts:
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