Myasthenia gravis (MG) is one of those USMLE-friendly diagnoses you can spot in 5 seconds if you train your brain to look for the right pattern: fatigable weakness that gets worse with use and better with rest, especially in the eyes and bulbar muscles.
The 5-Second Rule (What you should think immediately)
If the stem says:
- Ptosis and/or diplopia
- Weakness worsens over the day or with repeated use
- Normal sensation
- Normal reflexes
- Improves with rest
→ Think Myasthenia gravis until proven otherwise.
One-liner: Autoantibodies against postsynaptic ACh receptors (or MuSK) → fewer functional receptors → fatigable weakness.
“MG = Muscle Gets tired” Mnemonic + Visual
The sticky-note mental image
Picture a neuromuscular junction where the muscle-side receptors are missing:
Nerve releases ACh → muscle has “empty parking spots” → each repeated signal finds fewer spots → muscle response fades.
Mini-mnemonic (shareable)
Myasthenia Gravis = Muscle Gives out
- Use it → lose it (temporarily)
- Rest it → best it
High-Yield Clinical Pattern (Step-style)
What’s weak?
- Extraocular: ptosis, diplopia (often first)
- Bulbar: dysphagia, dysarthria (nasal speech)
- Proximal limb weakness
- Respiratory involvement in severe cases (myasthenic crisis)
What’s not affected (classic clue)
- Sensation: intact
- Pupils: normal (helps separate from botulism)
- Reflexes: typically normal
Step 1 Pathophys in 20 Seconds
| Feature | Myasthenia gravis |
|---|---|
| Problem location | Postsynaptic neuromuscular junction |
| Antibody target | ACh receptor (most common) or MuSK |
| Mechanism | Receptor blockade + complement-mediated damage → ↓ endplate potentials |
| Result | Weakness that worsens with repeated activity |
| Key association | Thymic hyperplasia or thymoma |
Thymoma association is exam gold: MG can be a paraneoplastic clue—get chest imaging when suspected.
How to Diagnose (USMLE-High Yield)
Bedside / rapid testing concepts
- Ice pack test: improves ptosis (cold inhibits acetylcholinesterase → ↑ ACh in cleft)
Confirmatory testing
- AChR antibodies (binding/blocking/modulating)
- MuSK antibodies (esp if AChR Ab negative and bulbar prominent)
- Electrophysiology:
- Repetitive nerve stimulation: decremental response
- Single-fiber EMG: increased jitter (most sensitive)
Older “edrophonium test” is classic historically but less used clinically—still shows up on exams.
Treatment Framework (Quick + Testable)
Symptomatic
- Pyridostigmine (acetylcholinesterase inhibitor)
Disease-modifying / immunotherapy
- Corticosteroids
- Steroid-sparing agents: azathioprine, mycophenolate
- Thymectomy: especially with thymoma (and often beneficial even without thymoma in AChR+ generalized MG)
Myasthenic crisis (respiratory failure risk)
- IVIG or plasmapheresis (fastest immunologic improvement)
- Plus airway/ventilation support as needed
Clinical trigger clue: infection, surgery, pregnancy/postpartum, certain meds can precipitate crisis.
Medications That Worsen MG (Classic Step Trap)
Common “don’t do it” culprits:
- Aminoglycosides
- Fluoroquinolones
- Macrolides
- Magnesium
- Beta-blockers
- Neuromuscular blockers (esp peri-op)
MG vs Lambert-Eaton vs Botulism (5-second differentiator table)
| Feature | Myasthenia gravis | Lambert-Eaton (LEMS) | Botulism |
|---|---|---|---|
| Lesion | Postsynaptic AChR/MuSK | Presynaptic Ca channels | Presynaptic ACh release blocked |
| Weakness pattern | Worse with use | Improves with use (facilitation) | Descending paralysis |
| Reflexes | Normal | Decreased | Variable, often decreased |
| Pupils/autonomics | Usually normal | Autonomic symptoms common | Dilated pupils, autonomic symptoms |
| Cancer link | Thymoma | Small cell lung cancer | Foodborne/infant/wound source |
The 5-Second Takeaway (Ultra-shareable)
Myasthenia gravis = fatigable weakness (eyes/bulbar first) + normal pupils/sensation/reflexes + improves with rest → AChR (or MuSK) antibodies; treat with pyridostigmine + immunotherapy; crisis = IVIG/plasmapheresis.