Neuromuscular Pharmacology
Neuromuscular Pharmacology
What Do You Need to Know?
- MG pharmacotherapy — pyridostigmine (symptomatic), prednisone (may initially worsen), steroid-sparing agents, complement inhibitors (eculizumab), FcRn inhibitors (efgartigimod)
- Myasthenic crisis — ICU monitoring, IVIG or PLEX, intubate if FVC <15–20 mL/kg; avoid aminoglycosides/fluoroquinolones/magnesium
- Drugs that worsen MG — aminoglycosides, fluoroquinolones, magnesium, beta-blockers, checkpoint inhibitors, telithromycin, D-penicillamine
- Lambert-Eaton — 3,4-DAP (amifampridine) first-line; pyridostigmine less effective; treat underlying malignancy
- Botulinum toxin — cleaves SNARE/SNAP-25, onset 3–7 days, duration ~3 months; used for dystonia, spasticity, chronic migraine, sialorrhea
- NMJ blockers — succinylcholine (depolarizing, hyperkalemia risk) vs rocuronium/vecuronium (nondepolarizing, reversed by sugammadex)
- Inflammatory myopathy Rx — prednisone first-line, steroid-sparing agents, IVIG (especially dermatomyositis); IBM has limited treatment response
- Neuropathy treatments — CIDP (IVIG/PLEX/steroids), GBS (IVIG or PLEX — steroids NOT effective), neuropathic pain (duloxetine, pregabalin)
Myasthenia Gravis Pharmacology
Overview of MG Treatments
| Drug | Class / Mechanism | Role | Key Considerations |
|---|---|---|---|
| Pyridostigmine | AChE inhibitor — increases ACh at NMJ | First-line symptomatic therapy | Cholinergic side effects (diarrhea, cramping, salivation); less effective in MuSK-positive MG |
| Prednisone | Corticosteroid | First-line immunotherapy for moderate–severe MG | Initial worsening in ~50% (start low, go slow); long-term side effects require steroid-sparing agents |
| Azathioprine | Purine synthesis inhibitor | Steroid-sparing; onset 6–12 months | Check TPMT before starting; risk of myelosuppression |
| Mycophenolate | IMPDH inhibitor | Steroid-sparing; onset 6–12 months | GI side effects; teratogenic; RCTs negative but widely used |
| Rituximab | Anti-CD20 — B-cell depletion | Refractory MG; especially effective in MuSK-positive MG | Screen for hepatitis B; risk of PML (rare) |
| Eculizumab / Ravulizumab | Terminal complement (C5) inhibitor | Refractory generalized AChR-positive MG | Must vaccinate against Neisseria meningitidis before starting |
| Efgartigimod | FcRn inhibitor — accelerates IgG degradation | Generalized AChR-positive MG | Reduces total IgG levels; cyclic dosing (4 weekly infusions per cycle) |
| IVIG | Pooled immunoglobulins | Acute exacerbations, crisis, pre-thymectomy | Check IgA level (anaphylaxis risk in IgA deficiency) |
| Plasmapheresis (PLEX) | Removes circulating antibodies | Myasthenic crisis, rapid stabilization | Requires central access; effects are temporary |
Thymectomy
- Indicated for: thymoma (regardless of MG severity) and non-thymomatous generalized AChR-positive MG (MGTX trial)
- Less beneficial in: MuSK-positive MG, late-onset MG (>50 years), purely ocular MG
Board Pearl
Eculizumab requires meningococcal vaccination before initiation. It blocks terminal complement (C5), which is also the defense against Neisseria. Efgartigimod works by blocking FcRn, which normally recycles IgG — blocking it accelerates IgG catabolism, lowering pathogenic antibody levels.
Myasthenic Crisis Management
Approach to Crisis
- Definition: MG exacerbation with respiratory failure requiring intubation or noninvasive ventilation
- Monitoring: serial FVC and NIF every 2–4 hours
- 20/30/40 rule: intubate if FVC <20 mL/kg, NIF <−30 cmH2O, or MEP <40 cmH2O
- Treatment: IVIG (0.4 g/kg/day × 5 days) or PLEX (5 exchanges) — equally effective
- Hold pyridostigmine initially (excess secretions complicate airway management)
- Identify triggers: infection (#1), medication changes, surgery
- Distinguish from cholinergic crisis (excess AChE inhibitor) — salivation, diarrhea, miosis, bradycardia
Board Pearl
In myasthenic crisis, follow FVC, not oxygen saturation. SpO2 drops late in neuromuscular respiratory failure. FVC <20 mL/kg is the key threshold. Infection is the #1 trigger for crisis.
Drugs That Worsen Myasthenia Gravis
| Drug / Class | Mechanism of NMJ Impairment | Clinical Note |
|---|---|---|
| Aminoglycosides | Block presynaptic Ca²⁺ channels and postsynaptic AChR | Most dangerous antibiotic class in MG; can precipitate crisis |
| Fluoroquinolones | Pre- and postsynaptic NMJ blockade | FDA black box warning for MG exacerbation |
| Telithromycin | NMJ blockade (mechanism not fully elucidated) | Contraindicated in MG; fatal cases reported |
| Magnesium (IV) | Competes with Ca²⁺ at presynaptic terminal | Avoid IV magnesium; caution with oral supplementation |
| Beta-blockers | Impair NMJ transmission | Ophthalmic beta-blockers (timolol) also worsen MG |
| Calcium channel blockers | Reduce Ca²⁺ influx at presynaptic terminal | Verapamil and diltiazem worst; dihydropyridines less problematic |
| D-Penicillamine | Induces autoimmune MG (anti-AChR antibodies) | Drug-induced MG; usually reversible after discontinuation |
| Checkpoint inhibitors | Immune dysregulation → de novo or unmasked MG | Fulminant MG with concurrent myositis and myocarditis; high mortality |
| Botulinum toxin | Blocks presynaptic ACh release (SNARE cleavage) | Relative contraindication; systemic spread risk |
| Neuromuscular blockers | Direct NMJ blockade | MG patients exquisitely sensitive to nondepolarizing agents |
Board Pearl
Telithromycin is absolutely contraindicated in MG — fatal respiratory failure reported even with a single dose. D-penicillamine causes drug-induced MG with positive AChR antibodies. Checkpoint inhibitors can cause a devastating triad of MG + myositis + myocarditis.
Lambert-Eaton Myasthenic Syndrome Treatment
Pharmacotherapy
- 3,4-Diaminopyridine (amifampridine): first-line; blocks presynaptic K⁺ channels → prolongs AP → increased Ca²⁺ influx → more ACh release
- Pyridostigmine: modest benefit; less effective than in MG (presynaptic defect = reduced ACh release)
- Immunotherapy: prednisone, azathioprine, IVIG, or PLEX for refractory cases
- Treat underlying malignancy: ~60% associated with SCLC; tumor treatment often improves LEMS
- Cancer screening: CT chest at diagnosis; repeat every 6 months for 2 years if negative
Clinical Pearl
3,4-DAP prolongs the presynaptic action potential, allowing more Ca²⁺ entry through antibody-reduced P/Q-type channels. It directly addresses LEMS pathophysiology. Seizures are the main dose-limiting side effect.
Botulinum Toxin
Formulations
| Product | Type | SNARE Target | Key Note |
|---|---|---|---|
| OnabotulinumtoxinA (Botox) | Type A | SNAP-25 | Most widely used; doses NOT interchangeable between products |
| AbobotulinumtoxinA (Dysport) | Type A | SNAP-25 | Higher unit ratio vs onabotulinum (~2.5–3:1) |
| IncobotulinumtoxinA (Xeomin) | Type A | SNAP-25 | Free of complexing proteins; lower immunogenicity |
| RimabotulinumtoxinB (Myobloc) | Type B | Synaptobrevin (VAMP) | Used for type A–resistant dystonia; more dry mouth |
Mechanism, Pharmacokinetics, and Indications
- Mechanism: internalized into presynaptic terminal → light chain cleaves SNARE proteins → blocks vesicle fusion → no ACh release
- Onset: 3–7 days (up to 2 weeks for full effect); Duration: ~3 months (axonal sprouting restores transmission)
- Resistance: neutralizing antibodies develop with frequent/high-dose injections; switch to type B or Xeomin
- Dystonia: cervical dystonia (first-line), blepharospasm, limb and laryngeal dystonia
- Spasticity: focal spasticity in stroke, MS, TBI, cerebral palsy
- Chronic migraine: onabotulinumtoxinA; ≥15 headache days/month; 31 injection sites across 7 head/neck muscles
- Sialorrhea: ALS, Parkinson disease; injected into parotid and submandibular glands
Board Pearl
Botulinum toxin type A cleaves SNAP-25; type B cleaves synaptobrevin. Doses are NOT interchangeable between formulations. IncobotulinumtoxinA (Xeomin) lacks complexing proteins and may have lower immunogenicity. Use type B (Myobloc) for patients with antibody resistance to type A.
Neuromuscular Junction Blockers
Depolarizing vs Nondepolarizing Agents
| Feature | Depolarizing (Succinylcholine) | Nondepolarizing (Rocuronium, Vecuronium) |
|---|---|---|
| Mechanism | Persistent depolarization → desensitization block | Competitive antagonist at AChR |
| Phase I block | Fasciculations → paralysis; no fade on train-of-four | N/A |
| Phase II block | Prolonged exposure → fade on train-of-four (resembles nondepolarizing) | N/A |
| Onset / Duration | Rapid (30–60 sec); ultrashort (5–10 min) | 1–3 min onset; intermediate (30–60 min) |
| Metabolism | Plasma cholinesterase (pseudocholinesterase) | Hepatic (vecuronium); hepatic + renal (rocuronium) |
| Reversal | No reversal agent (wait for metabolism) | Sugammadex (encapsulates drug); neostigmine + glycopyrrolate |
| Major risks | Hyperkalemia (burns, denervation); malignant hyperthermia (RYR1) | Prolonged block in MG; residual paralysis |
Succinylcholine Contraindications
- Burns (after 24–48 hours through ~1 year) — upregulation of extrajunctional AChR → massive K⁺ release
- Denervation injuries (stroke, SCI, GBS) — same mechanism; avoid after 48–72 hours
- Malignant hyperthermia susceptibility — uncontrolled Ca²⁺ release from SR via RYR1; treat with dantrolene
- Pseudocholinesterase deficiency — prolonged paralysis (hours instead of minutes)
- Hyperkalemia at baseline — succinylcholine raises K⁺ by 0.5–1.0 mEq/L
Clinical Pearl
MG patients are relatively resistant to succinylcholine (fewer functional AChR) but exquisitely sensitive to nondepolarizing agents — use 1/10th to 1/5th of the normal dose. Sugammadex rapidly reverses rocuronium and is the preferred reversal agent.
Inflammatory Myopathy Treatment
Treatment by Myopathy Type
| Myopathy | First-Line | Second-Line / Steroid-Sparing | Key Notes |
|---|---|---|---|
| Dermatomyositis | Prednisone (1 mg/kg/day) | Methotrexate, azathioprine, mycophenolate, IVIG, rituximab | IVIG has best evidence for refractory DM; screen for malignancy |
| Polymyositis | Prednisone | Methotrexate, azathioprine, mycophenolate, rituximab | Ensure not IBM misclassified as PM; check for anti-synthetase syndrome |
| IMNM | Prednisone + early steroid-sparing | IVIG, rituximab, mycophenolate | Anti-SRP or anti-HMGCR Abs; statin-triggered IMNM needs immunosuppression despite statin cessation |
| Inclusion body myositis | No proven effective treatment | IVIG may provide modest benefit | Does NOT respond to steroids; most common inflammatory myopathy >50 years |
Board Pearl
IBM does not respond to immunosuppressive therapy. If a patient >50 with asymmetric weakness (finger flexors, knee extensors) and rimmed vacuoles on biopsy is not improving on steroids, the diagnosis is IBM. Anti-HMGCR myopathy requires immunosuppression even after statin discontinuation.
Neuropathy Treatments
CIDP
- First-line options (all equivalent): IVIG, PLEX, or corticosteroids
- IVIG: 2 g/kg loading over 2–5 days, then 1 g/kg maintenance every 3–4 weeks
- Subcutaneous Ig (SCIg): alternative for maintenance; better tolerability
- Key point: unlike GBS, steroids ARE effective in CIDP
GBS
- IVIG (0.4 g/kg/day × 5 days) or PLEX (5 exchanges) — equally effective; start early
- Steroids are NOT effective in GBS — critical board distinction from CIDP
- Do NOT combine IVIG + PLEX (PLEX removes the IVIG)
- Monitor FVC closely; DVT prophylaxis; autonomic monitoring
Neuropathic Pain Pharmacotherapy
| Drug | Class | FDA Indication | Key Note |
|---|---|---|---|
| Duloxetine | SNRI | Diabetic neuropathy pain | First-line per AAN; also treats comorbid depression |
| Pregabalin | α2δ Ca²⁺ channel ligand | Diabetic neuropathy, PHN, fibromyalgia | First-line; dizziness and weight gain common |
| Gabapentin | α2δ Ca²⁺ channel ligand | Postherpetic neuralgia | Widely used off-label; renal dosing required |
| TCAs (amitriptyline) | Tricyclic antidepressant | Off-label | Anticholinergic side effects; avoid in elderly |
| Capsaicin 8% patch | TRPV1 agonist | PHN, HIV neuropathy | Applied in clinic 30–60 min; repeat every 3 months |
Clinical Pearl
Steroids work in CIDP but NOT in GBS — one of the most frequently tested distinctions on neurology boards. Both respond to IVIG and PLEX. Never combine IVIG and PLEX sequentially (PLEX removes IVIG).
Quick Reference
High-Yield Summary Table
| Topic | Key Fact | Board Buzzword |
|---|---|---|
| MG symptomatic Rx | Pyridostigmine; less effective in MuSK-positive | Cholinergic vs myasthenic crisis |
| MG crisis | IVIG or PLEX; intubate if FVC <20 mL/kg | Follow FVC, not SpO2 |
| Drugs worsening MG | Aminoglycosides, fluoroquinolones, Mg²⁺, telithromycin | Telithromycin = absolute contraindication |
| LEMS treatment | 3,4-DAP first-line; screen for SCLC | K⁺ channel blocker → more ACh release |
| Botulinum toxin | Type A → SNAP-25; onset 3–7 days; lasts 3 months | Xeomin = no complexing proteins |
| Succinylcholine | Depolarizing; hyperkalemia in burns/denervation; MH risk | Pseudocholinesterase deficiency → prolonged block |
| Nondepolarizing agents | Rocuronium/vecuronium; reversed by sugammadex | Use 1/10th dose in MG |
| IBM treatment | No effective immunosuppressive therapy | Does NOT respond to steroids |
| GBS treatment | IVIG or PLEX; steroids NOT effective | Steroids work in CIDP, not GBS |
| Neuropathic pain | Duloxetine + pregabalin first-line per AAN | α2δ ligands; avoid TCAs in elderly |
References
- Narayanaswami P, Sanders DB, Wolfe G, et al. International Consensus Guidance for Management of Myasthenia Gravis: 2020 Update. Neurology. 2021;96(3):114–122.
- Wolfe GI, Kaminski HJ, Aban IB, et al. Randomized Trial of Thymectomy in Myasthenia Gravis (MGTX). N Engl J Med. 2016;375(6):511–522.
- Howard JF, Utsugisawa K, Benatar M, et al. Safety and efficacy of eculizumab in refractory generalised myasthenia gravis (REGAIN). Lancet Neurol. 2017;16(12):976–986.
- Howard JF, Bril V, Vu T, et al. Safety, efficacy, and tolerability of efgartigimod in generalised myasthenia gravis (ADAPT). Lancet Neurol. 2021;20(7):526–536.
- Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 12th ed. McGraw-Hill; 2023.
- Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
- Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL. Bradley and Daroff’s Neurology in Clinical Practice. 8th ed. Elsevier; 2022.
- Dalakas MC. Inflammatory muscle diseases. N Engl J Med. 2015;372(18):1734–1747.
- Bril V, England JD, Franklin GM, et al. Evidence-based guideline: Treatment of painful diabetic neuropathy. Neurology. 2011;76(20):1758–1765.