Myasthenia Gravis for USMLE Step 3 & COMLEX-USA Level 3

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Clinical Presentation and Diagnosis

Initial Symptoms: Fluctuating muscle weakness, especially in ocular muscles (ptosis, diplopia), with progression to generalized weakness.
Differential Diagnosis: Differentiate from other causes of muscle weakness, such as Lambert-Eaton myasthenic syndrome, botulism, and stroke.
Key Diagnostic Tests:
Serology: Anti-acetylcholine receptor (AChR) antibodies; anti-MuSK antibodies for AChR-negative patients.
Electrophysiology: Repetitive nerve stimulation and single-fiber EMG.
Imaging: Chest CT or MRI to rule out thymoma.
Bedside Tests: Ice pack test can help confirm diagnosis in ptosis.

Management and Treatment

First-line Treatment:
Acetylcholinesterase Inhibitors: Pyridostigmine for symptom management.
Long-term Management:
Immunosuppressants: Corticosteroids, azathioprine, mycophenolate mofetil, or cyclosporine.
Thymectomy: Recommended for thymoma and often in generalized disease.
Crisis Management:
Myasthenic Crisis: Requires intensive care, mechanical ventilation, and plasmapheresis or IVIG.
Cholinergic Crisis: Differentiate from myasthenic crisis; management includes discontinuing acetylcholinesterase inhibitors and supportive care.

Complications

Respiratory Failure: Particularly during myasthenic crisis, necessitating prompt recognition and treatment.
Thymoma: Associated with more severe disease; thymectomy can be curative in some cases.

Prognosis and Follow-up

Requires long-term management and monitoring for disease progression and treatment-related side effects.

Commonly Tested Items on Step 3

Crisis Management: Differentiating and managing myasthenic vs. cholinergic crisis.
Thymectomy: Indications and outcomes, especially in relation to thymoma.
Immunosuppressive Therapy: Long-term management and monitoring for side effects.
Complex Cases: Considerations in pregnancy, surgery, and infections that might exacerbate symptoms.