Myasthenia Gravis - The Daily PANCE Blueprint
A 35-year-old man comes to the emergency department because of double vision and difficulty chewing for the past day. Symptoms improve with rest and worsen with extended periods of activity. Infusion with an agent produces improvement in his symptoms. Which of the following agents is most likely administered in such a scenario?
A. Botulinum toxin
B. Edrophonium
C. Pilocarpine
D. Scopolamine
E. Sumatriptan
Answer and topic summary
The answer is B, Edrophonium
- Myasthenia Gravis is characterized by muscle weakness that is worsened with continued activity. Alleviation of symptoms involves administration of an acetylcholinesterase inhibitor such as edrophonium (Tensilon)
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Pearls
Myasthenia gravis is a type II hypersensitivity disorder where the body produces antibodies that most commonly target nicotinic acetylcholine receptors on the surface of muscle cells.
- The antibodies block the receptors which means the signal to contract isn’t received
- The classical clinical presentation of a patient with myasthenia gravis is a patient with (eye symptoms) ptosis, diplopia and muscle weakness that worsens with use
Diagnosis
The diagnosis of myasthenia gravis is suggested by symptoms and signs and confirmed by measurement of:
- Myasthenic antibodies in the blood
- AChR antibodies – antibodies to the acetylcholine receptor – about 80 to 85 percent of MG patients test positive for AChR antibodies
- Muscle-specific serum kinase (MuSK) – 5 to 10 percent are found to have antibodies to muscle-specific serum kinase (MuSK)
- LRP4 antibodies – found in approximately 13 percent of patients who are seronegative for both AChR and MuSK antibodies
- Nerve conduction studies/repetitive nerve stimulation – RNS studies are the most frequently used electrodiagnostic test for myasthenia gravis due to their wide availability
- Single-fiber electromyography (SFEMG) is the most sensitive diagnostic test for myasthenia gravis and shows a decremental response to repetitive stimulation of motor nerves
- Ice Test – cooling of muscles affected by MG may transiently improve weakness. This test is simple and easily performed on a droopy eyelid, often helping to confirm a diagnosis of MG
- CT of the chest: Thymomas affect about 10-20% of patients with myasthenia gravis. Patients diagnosed with myasthenia gravis should, therefore, have thymoma ruled out via
Treatment
- Acetylcholinesterase inhibitor (pyridostigmine/neostigmine) = first line ⇒ stops breakdown of acetylcholine
- Immunosuppressive drugs (prednisone) ⇒ reduce the production of autoantibodies
- Thymectomy ⇒ reduces muscle weakness
- Myasthenic crisis: neuromuscular respiratory failure from dysphagia/aspiration (treat with plasma exchange, IVIG)
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