Myasthenia Gravis for USMLE Step 2 & COMLEX-USA Level 2
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Clinical Presentation
Diagnosis
Complications
- Ocular Symptoms: Ptosis and diplopia are often the first symptoms, with variability throughout the day.
- Generalized Weakness: Affects bulbar, limb, and respiratory muscles; muscle weakness typically worsens with activity and improves with rest.
- Myasthenic Crisis: Acute exacerbation leading to respiratory failure, often precipitated by infection, surgery, or medication noncompliance.
Diagnosis
- Serological Tests: Anti-acetylcholine receptor (AChR) antibodies; consider anti-MuSK antibodies if AChR-negative.
- Electrophysiology: Repetitive nerve stimulation and single-fiber EMG for diagnostic confirmation.
- Imaging: Chest CT or MRI to evaluate for thymoma.
- Bedside Tests: Ice pack test can temporarily improve ptosis.
- Acetylcholinesterase Inhibitors: Pyridostigmine is the first-line treatment to enhance neuromuscular transmission.
- Immunosuppressive Therapy: Corticosteroids, azathioprine, or mycophenolate mofetil for long-term management.
- Acute Management:
- Plasmapheresis or IVIG: Used in myasthenic crisis or preoperatively.
- Thymectomy: Recommended in patients with thymoma or generalized disease, especially in younger patients.
Complications
- Myasthenic Crisis: Requires immediate intervention with respiratory support and plasmapheresis or IVIG.
- Cholinergic Crisis: Caused by excessive acetylcholinesterase inhibitors, leading to increased salivation, diarrhea, and muscle twitching; must differentiate from myasthenic crisis.
- Generally good with appropriate treatment, but requires ongoing monitoring for disease exacerbation and complications.
Commonly Tested Items on USMLE Step 2 & COMLEX-USA Level 2
- Diagnosis: Recognizing the clinical presentation and confirming with antibody testing and EMG.
- Management: Initiation of acetylcholinesterase inhibitors and when to use immunosuppressants or thymectomy.
- Myasthenic Crisis: Identification, acute management, and differentiation from cholinergic crisis.
- Thymoma: Association with myasthenia gravis and the role of imaging and surgery.