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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.
Related Terms
Myasthenia Gravis