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Myasthenia Gravis & NMJ Disorders

What Do You Need to Know?

MG: autoimmune postsynaptic NMJ disorder; AChR Ab (~85%), MuSK Ab (5–8%), LRP4 Ab (1–2%), seronegative (5–10%); fatigable weakness — worse with activity, better with rest
MuSK MG: bulbar/facial/respiratory predominance; NO thymic pathology; POOR response to pyridostigmine; rituximab is preferred immunotherapy; complement inhibitors do NOT work (IgG4)
Diagnosis: SFEMG is most sensitive test (~95–99%); RNS shows ≥10% decrement at 2–3 Hz; AChR Ab is most specific serologic test; CT chest mandatory for thymoma screening
Crisis: respiratory failure requiring intubation; monitor FVC — intubate if FVC <20 mL/kg or NIF <−20 cmH₂O (“20/20/20 rule”); treat with IVIg or PLEX (equally effective)
Medications to AVOID: aminoglycosides, fluoroquinolones, beta-blockers, magnesium, D-penicillamine, immune checkpoint inhibitors, botulinum toxin
LEMS: presynaptic NMJ disorder; P/Q-type VGCC antibodies; triad = proximal weakness + areflexia + autonomic dysfunction; ~60% have SCLC; incremental response ≥100% on high-frequency RNS; treat with 3,4-DAP
Botulism: presynaptic blockade (SNARE cleavage); descending paralysis + autonomic dysfunction + large unreactive pupils; treat with antitoxin
New therapies: eculizumab/ravulizumab (anti-C5 complement); efgartigimod/rozanolixizumab (FcRn inhibitors) — all approved for AChR Ab+ generalized MG

NMJ Physiology

Normal Neuromuscular Transmission

Presynaptic terminal: ACh synthesized from choline + acetyl-CoA by choline acetyltransferase (ChAT); packaged into vesicles by vesicular ACh transporter (VAChT)
Action potential arrival → Ca²⁺ influx through P/Q-type VGCCs → vesicle fusion via SNARE complex (synaptobrevin, SNAP-25, syntaxin) → ACh release into synaptic cleft
Synaptic cleft: acetylcholinesterase (AChE) rapidly hydrolyzes ACh; choline recycled back into nerve terminal
Postsynaptic: ACh binds nicotinic AChRs on muscle endplate → Na⁺ influx → endplate potential (EPP) → if EPP exceeds threshold, muscle fiber action potential fires
Safety factor: normal EPP is ~3–4× greater than threshold needed to fire — ensures reliable transmission even with physiologic variability

Feature	Presynaptic NMJ Disorders	Postsynaptic NMJ Disorders
Mechanism	Impaired ACh release	Impaired ACh receptor function
Safety factor reduced by	Fewer quanta released per impulse	Reduced postsynaptic response per quantum
Prototypes	LEMS, botulism	Myasthenia gravis
RNS at low frequency	Decrement (small baseline CMAP)	Decrement (normal baseline CMAP)
RNS at high frequency	Incremental response (≥100%)	No increment
Reflexes	Diminished/absent (improve post-exercise)	Normal
Autonomic features	Common (LEMS, botulism)	Absent

💎 Board Pearl

The safety factor explains why NMJ disorders cause fatigable weakness — with repeated stimulation, fewer quanta are released (normal presynaptic depletion), but if the safety factor is already reduced, EPP falls below threshold
P/Q-type VGCCs are the primary calcium channels at the NMJ presynaptic terminal — targeted in LEMS
SNARE complex is the target of botulinum toxin (cleaves SNAP-25/synaptobrevin) and tetanus toxin (cleaves synaptobrevin in inhibitory interneurons)

Myasthenia Gravis — Classification & Antibodies

MGFA Clinical Classification

Class	Description
I	Ocular only — ptosis, diplopia; no other weakness
II	Mild generalized — may have ocular symptoms
IIa	Predominantly limb/axial
IIb	Predominantly bulbar/respiratory
III	Moderate generalized
IIIa	Predominantly limb/axial
IIIb	Predominantly bulbar/respiratory
IV	Severe generalized
IVa	Predominantly limb/axial
IVb	Predominantly bulbar/respiratory
V	Intubation required (with or without mechanical ventilation) = myasthenic crisis

Antibody Subtypes

Feature	AChR Ab	MuSK Ab	LRP4 Ab	Seronegative
Frequency	~85% of generalized MG	5–8%	1–2%	5–10%
Antibody class	IgG1, IgG3 (complement-fixing)	IgG4 (non-complement-fixing)	IgG1, IgG2	May have low-affinity AChR Ab
Demographics	Bimodal: F <40, M >60	Young women (F ≫ M)	Variable	Variable
Clinical pattern	Ocular → generalized; limb + ocular predominant	Bulbar/facial/respiratory; neck/shoulder weakness; tongue/facial atrophy	Mild; often ocular or mild generalized	Similar to AChR Ab+ MG
Thymus	Hyperplasia (~65%); thymoma (~10–15%)	Normal — NO thymic pathology	Normal	May have hyperplasia
Pyridostigmine	Good response	Poor response (may worsen)	Variable	Variable
Thymectomy	Indicated (non-thymoma + thymoma)	NOT indicated	Not established	May benefit
Preferred immunotherapy	Standard immunotherapy	Rituximab (first-line after steroids)	Standard immunotherapy	Standard immunotherapy
Complement inhibitors	Approved (eculizumab, ravulizumab)	NOT effective (IgG4 → no complement)	Not studied	Not studied
Crisis risk	Moderate	High (respiratory predominance)	Low	Low–moderate

Early-Onset vs Late-Onset AChR+ MG

Feature	Early-Onset MG (<40 yr)	Late-Onset MG (>50 yr)
Sex	Female predominance (F:M 3:1)	Male predominance (M:F 2:1)
Thymus	Thymic hyperplasia (65–75%)	Thymoma (~15%); thymic involution
HLA association	HLA-B8, DR3	HLA-B7, DR2
Anti-striated muscle Ab	Rare	85% if thymoma present

🎯 Clinical Pearl

MuSK MG is the most board-tested subtype — remember: bulbar predominance, NO thymoma, POOR pyridostigmine response, rituximab preferred, complement inhibitors do NOT work (IgG4 does not fix complement)
~50% of ocular MG patients are AChR Ab negative — but ~50% of “seronegative” patients convert to seropositive on repeat testing or with cell-based assay
Thymoma-associated MG: more severe, older onset, higher AChR Ab titers, anti-striated muscle Ab in ~85%; thymectomy mandatory regardless of MG severity

💎 Board Pearl

AChR binding Ab = most commonly ordered (~85% sensitive in generalized MG, only ~50% in ocular MG); AChR modulating Ab increases sensitivity when combined
MuSK Ab is IgG4 — this is why complement inhibitors (eculizumab) don’t work and why the disease is “antibody-mediated” without complement deposition at the NMJ
~10–15% of MG patients have a thymoma; conversely, 30–50% of thymoma patients develop MG — CT chest is mandatory in ALL newly diagnosed MG
Anti-striated muscle Ab (titin, ryanodine receptor) in a patient with MG = search for thymoma (~85% specificity)
Purely ocular MG that remains ocular >2 years has <5% chance of generalizing

Diagnosis

Bedside & Serologic Tests

Test	Method	Key Points
Ice pack test	Apply ice to closed eyelid ×2 min; assess ptosis improvement	Cooling inhibits AChE → more ACh available; ~80% sensitivity for ocular MG; no risk (unlike edrophonium)
Rest test	Eyes closed ×5 min; assess improvement	Supports fatigable weakness; nonspecific
Edrophonium (Tensilon) test	IV AChE inhibitor; rapid onset (~30 s), short duration (~5 min)	Rarely used now — risk of bradycardia/bronchospasm; atropine must be at bedside; largely replaced by ice pack test
AChR binding Ab	Radioimmunoassay	~85% sensitive in generalized MG; ~50% in ocular MG; most specific serologic test (>99%)
MuSK Ab	Cell-based assay or RIA	Order if AChR Ab negative; 5–8% of MG
LRP4 Ab	Cell-based assay	1–2% of MG; order after AChR and MuSK negative
Striational Ab (anti-titin, anti-RyR)	Immunofluorescence	Associated with thymoma — if positive, CT chest urgently; ~85% specificity for thymoma
CT chest	Imaging	Mandatory in ALL MG patients — screening for thymoma

Electrophysiology: RNS vs SFEMG

Feature	Repetitive Nerve Stimulation (RNS)	Single-Fiber EMG (SFEMG)
What it measures	Decrement in CMAP amplitude with repeated stimulation	Variability in time interval between 2 muscle fiber action potentials from same motor unit (jitter)
Technique	Stimulate motor nerve at 2–3 Hz; compare 1st and 4th/5th CMAP	Voluntary activation or stimulated; measure jitter in ≥20 fiber pairs
Positive result	≥10% decrement between 1st and 4th/5th response	Increased jitter (>55 µs in most muscles) and/or blocking
Sensitivity — generalized MG	~75–80%	~95–99%
Sensitivity — ocular MG	~30–50%	~85–95% (frontalis/orbicularis oculi)
Specificity	High (when ≥10% decrement)	Low — increased jitter seen in any NMJ or myopathic process
Best muscles to test	Proximal: trapezius, deltoid, nasalis; test clinically weak muscles	Frontalis, orbicularis oculi, extensor digitorum communis
Post-exercise facilitation	Brief improvement in decrement after 10 s of exercise (repair of decrement)	Not applicable
Post-exercise exhaustion	Worsened decrement 2–5 min after exercise	Not applicable

💎 Board Pearl

SFEMG is the single most sensitive test for MG (~95–99%) but is NOT specific — increased jitter can occur in motor neuron disease, myopathy, and other NMJ disorders
AChR Ab is the most specific test (>99%) — a positive result essentially confirms MG
RNS decrement at 2–3 Hz: the 4th or 5th response is compared to the 1st — decrement is maximal at 3rd–4th stimulus (before mobilization store replenishes); must be ≥10% to be significant
Post-exercise facilitation in MG: brief repair of decrement immediately after exercise; post-exercise exhaustion: worsened decrement 2–5 min later — test BOTH to increase sensitivity
Ice pack test works because cold inhibits AChE → more ACh at NMJ; it does NOT work in non-NMJ causes of ptosis
Edrophonium test is largely obsolete — replaced by ice pack test (safer) and serology

Treatment

Symptomatic Therapy

Agent	Mechanism	Key Points
Pyridostigmine	Reversible AChE inhibitor → increases ACh at NMJ	First-line symptomatic therapy; dose 60 mg TID (typical starting); max ~120 mg q3h; onset 30 min, peak 2 hr, duration 4–6 hr; side effects = SLUDGE (Salivation, Lacrimation, Urination, Diarrhea, GI cramping, Emesis); overdose → cholinergic crisis
Neostigmine	AChE inhibitor (parenteral)	Used intraoperatively or when PO not feasible; shorter duration than pyridostigmine

Immunotherapy

Agent	Mechanism	Onset	Key Points
Prednisone	Broad immunosuppression	2–4 weeks	Most effective conventional immunotherapy; start low and titrate (risk of initial worsening at high doses — consider inpatient initiation for severe disease); long-term side effects limit use
Azathioprine	Purine synthesis inhibitor	6–12 months	Steroid-sparing; check TPMT before starting (deficiency → life-threatening myelosuppression); monitor CBC, LFTs
Mycophenolate mofetil	Purine synthesis inhibitor	6–12 months	Steroid-sparing; teratogenic; monitor CBC; RCTs did not show benefit at 3 months (but slow onset explains negative trials)
Rituximab	Anti-CD20 → B-cell depletion	3–6 months	Preferred for MuSK MG (often first-line IS); increasingly used early in AChR+ MG; risk of PML (rare), hepatitis B reactivation
Cyclosporine	Calcineurin inhibitor	1–3 months	Fastest-onset steroid-sparing agent; nephrotoxicity, hypertension; alternative in refractory disease
Tacrolimus	Calcineurin inhibitor	1–3 months	Similar to cyclosporine; may be better tolerated; less commonly used in Western countries

Rapid Immunotherapy (Rescue / Bridge)

Agent	Mechanism	Onset	Duration	Key Points
IVIg	Immunomodulation; Fc receptor blockade; accelerated IgG catabolism	Days to 1–2 weeks	3–6 weeks	0.4 g/kg/day ×5 days (total 2 g/kg); risk of aseptic meningitis, thrombosis, renal failure; check IgA level (anaphylaxis in IgA deficiency)
Plasma exchange (PLEX)	Removes circulating antibodies	Days	2–4 weeks	5 exchanges over 10–14 days; requires large-bore IV or central venous access; risk of hypotension, coagulopathy, line infection; equally effective as IVIg for crisis

Novel Targeted Therapies

Agent	Mechanism	Target	Key Points
Eculizumab	Anti-C5 monoclonal antibody	Terminal complement (C5)	Blocks membrane attack complex (MAC) formation; approved for AChR Ab+ generalized MG; IV q2 weeks; risk of Neisseria meningitis → meningococcal vaccination required; very expensive
Ravulizumab	Anti-C5 (long-acting)	Terminal complement (C5)	Same mechanism as eculizumab but longer half-life → IV q8 weeks; same meningococcal risk
Zilucoplan	C5 inhibitor (peptide)	Terminal complement (C5)	SC daily; approved for AChR Ab+ generalized MG; smaller molecule; same meningococcal risk
Efgartigimod	FcRn inhibitor	Neonatal Fc receptor (FcRn)	Blocks IgG recycling → accelerates IgG degradation → lowers pathogenic antibodies; IV q1 week ×4 (cyclic); approved for AChR Ab+ generalized MG; also available SC (+ hyaluronidase)
Rozanolixizumab	FcRn inhibitor	Neonatal Fc receptor (FcRn)	SC injection; approved for AChR Ab+ generalized MG; similar mechanism to efgartigimod

Thymectomy

Feature	Details
MGTX Trial (NEJM 2016)	RCT: thymectomy + prednisone vs prednisone alone in AChR Ab+ non-thymomatous generalized MG; thymectomy group had lower prednisone requirements, better QMG scores, fewer immunosuppressants at 3 years
Indications	ALL thymoma-associated MG (mandatory); AChR Ab+ generalized MG age 18–65 (MGTX criteria); consider in AChR Ab+ MG not controlled with immunotherapy
NOT indicated	MuSK MG (no thymic pathology); isolated ocular MG (debated); LRP4 MG; seronegative MG (limited evidence)
Timing	Optimize MG control first; avoid surgery during crisis; benefit may take 1–3 years to manifest
Approach	Extended transsternal or robotic/VATS thymectomy; must remove ALL thymic tissue

🎯 Clinical Pearl

Initial worsening on prednisone: start low (10–20 mg) and titrate up over weeks — high-dose initiation can trigger myasthenic exacerbation; consider inpatient initiation for severe disease
FcRn inhibitors (efgartigimod, rozanolixizumab) lower ALL IgG subclasses, not just pathogenic antibodies — monitor for infection risk
Complement inhibitors only work in AChR Ab+ MG (complement-mediated) — they do NOT work in MuSK MG (IgG4, non-complement-fixing)

💎 Board Pearl

MGTX Trial: thymectomy is now Level 1 evidence for AChR Ab+ generalized MG — know this trial by name for boards
Check TPMT before azathioprine — deficiency causes life-threatening myelosuppression
Meningococcal vaccination is MANDATORY before starting any complement inhibitor (eculizumab, ravulizumab, zilucoplan) due to Neisseria meningitidis risk
MuSK MG treatment ladder: steroids → rituximab (preferred over azathioprine/mycophenolate); pyridostigmine may worsen symptoms; NO thymectomy; complement inhibitors ineffective

Myasthenic Crisis

Overview & Triggers

Feature	Details
Definition	MG exacerbation requiring intubation/mechanical ventilation (MGFA Class V)
Incidence	~15–20% of MG patients experience ≥1 crisis
Mortality	~5% with modern ICU care (down from ~40% in pre-ICU era)
Common triggers	Infection (#1 trigger), surgery, medication changes, tapering immunotherapy, pregnancy/postpartum, emotional stress
Highest risk period	First 2–3 years after diagnosis

Medications to AVOID in MG

Drug Class	Examples	Mechanism of Worsening
Aminoglycosides	Gentamicin, tobramycin, amikacin	Presynaptic: decrease ACh release; postsynaptic: block AChR ion channel
Fluoroquinolones	Ciprofloxacin, levofloxacin, moxifloxacin	Pre- and postsynaptic NMJ blockade; FDA black box warning for MG
Macrolides	Azithromycin, erythromycin, telithromycin	NMJ blockade; telithromycin has FDA black box warning specifically for MG — may cause fatal exacerbation
Beta-blockers	Propranolol, atenolol, timolol (eye drops)	Curare-like postsynaptic NMJ blockade; even topical ophthalmic beta-blockers can worsen MG
Magnesium	IV magnesium sulfate	Presynaptic: competes with Ca²⁺ → reduces ACh release; dangerous in eclampsia with coexisting MG
D-Penicillamine	Penicillamine	Induces autoimmune MG (AChR Ab production); resolves after discontinuation (weeks–months)
Immune checkpoint inhibitors	Nivolumab, pembrolizumab, ipilimumab	Immune dysregulation → de novo MG or unmasking; can be fulminant with concurrent myocarditis; high mortality
Botulinum toxin	OnabotulinumtoxinA, abobotulinumtoxinA	Presynaptic blockade at NMJ — absolute contraindication
Neuromuscular blockers	Succinylcholine, vecuronium, rocuronium	MG patients are resistant to depolarizing agents (fewer AChRs) and hypersensitive to non-depolarizing agents
Others	Quinine, quinidine, procainamide, lithium, phenytoin	Various NMJ-blocking mechanisms

Respiratory Monitoring & the 20/20/20 Rule

Parameter	Threshold for Intubation	Notes
FVC	<20 mL/kg (or <1 L)	Normal FVC ~60–70 mL/kg; serial monitoring q2–4h; trend more important than single value
NIF (MIP)	Less negative than −20 cmH₂O	Measures inspiratory muscle strength; −15 is worse than −25 (less negative = weaker)
MEP	<40 cmH₂O	Measures expiratory force / cough strength

20/20/20 rule: intubate if FVC <20 mL/kg, NIF <−20 cmH₂O, or MEP <40 cmH₂O — do NOT wait for ABG changes (late finding)
Management: ICU admission → serial FVC/NIF q2–4h → IVIg or PLEX (equally effective) → identify and treat trigger → optimize long-term immunotherapy after stabilization
Hold pyridostigmine during intubation — reduces secretions and eliminates variable of cholinergic crisis; restart when extubated
Do NOT initiate high-dose steroids acutely in crisis — can worsen weakness in first 1–2 weeks; stabilize with PLEX/IVIg first

Myasthenic Crisis vs Cholinergic Crisis

Feature	Myasthenic Crisis	Cholinergic Crisis
Cause	Disease exacerbation (undertreated)	AChE inhibitor overdose (pyridostigmine excess)
Weakness	Worsening MG weakness	Weakness from depolarization block
Pupils	Normal	Miotic (small)
Secretions	Normal	Excessive (SLUDGE) — salivation, lacrimation, diarrhea
Fasciculations	Absent	Present (nicotinic excess)
Response to edrophonium	Improves (more ACh helps)	Worsens (ACh excess)
Treatment	IVIg or PLEX; intubation PRN	Stop AChE inhibitor; atropine for muscarinic symptoms; intubation PRN

💎 Board Pearl

Infection is the #1 trigger for myasthenic crisis — but treat carefully because aminoglycosides and fluoroquinolones are contraindicated in MG
Do NOT rely on pulse oximetry or ABG to decide on intubation — by the time SpO₂ drops or CO₂ rises, the patient is in extremis; use serial FVC and NIF
Cholinergic crisis is now rare with modern dosing but a classic board question — key differentiator is SLUDGE symptoms + fasciculations
D-penicillamine causes a true autoimmune MG with AChR antibodies — resolves weeks to months after discontinuation
Telithromycin has an FDA black box warning specifically for MG — may cause fatal exacerbation
Checkpoint inhibitor-induced MG: can present with fulminant crisis + myocarditis + elevated CK (overlap syndrome) — hold immunotherapy, give steroids + IVIg/PLEX

Lambert-Eaton Myasthenic Syndrome (LEMS)

Pathophysiology & Clinical Features

Feature	Details
Mechanism	Antibodies against P/Q-type VGCCs on presynaptic motor nerve terminal → decreased Ca²⁺ influx → reduced ACh vesicle release
Antibody	P/Q-type VGCC Ab (~85–90%); SOX1 Ab (paraneoplastic marker, ~65% of SCLC-LEMS)
Cancer association	~60% paraneoplastic (SCLC); ~40% autoimmune; VGCCs expressed on SCLC tumor → immune cross-reactivity
Clinical triad	Proximal weakness + hyporeflexia/areflexia + autonomic dysfunction
Weakness pattern	Proximal > distal; legs > arms; improves transiently with initial exercise (post-exercise facilitation)
Reflexes	Absent at rest; reappear after brief exercise (post-exercise facilitation = pathognomonic)
Autonomic	Dry mouth (#1 symptom, often earliest), constipation, erectile dysfunction, orthostatic hypotension, anhidrosis
Ocular/bulbar	Mild compared to MG — mild ptosis may occur but diplopia and severe dysphagia are less prominent

MG vs LEMS Comparison

Feature	Myasthenia Gravis	LEMS
NMJ defect site	Postsynaptic (AChR)	Presynaptic (VGCC)
Antibody	AChR, MuSK, or LRP4	P/Q-type VGCC
Weakness pattern	Ocular → bulbar → generalized; fatigable	Proximal legs > arms; improves briefly with activity
Reflexes	Normal	Absent (improve post-exercise)
Autonomic	Absent	Present (dry mouth, constipation, ED)
Pupils	Normal	May be sluggish
Ocular involvement	Very common (~85%); often presenting feature	Mild or absent; rarely presenting feature
Cancer	Thymoma (10–15%)	SCLC (~60%)
Baseline CMAP	Normal amplitude	Low amplitude
RNS (2–3 Hz)	Decrement ≥10%	Decrement; low baseline CMAP
RNS (20–50 Hz) or post-exercise	No significant increment	Incremental response ≥100% (often ≥200%)
SFEMG	Increased jitter + blocking	Increased jitter + blocking (improves at higher firing rates)
Treatment	Pyridostigmine, immunotherapy, thymectomy	3,4-DAP, treat underlying cancer, immunotherapy

Screening & Treatment

Feature	Details
DELTA-P score	Predicts SCLC in LEMS: Dysarthria, Erectile dysfunction, Loss of weight, Tobacco use, Age >50, Karnofsky Performance; score ≥3 → high probability of SCLC
Cancer screening	CT chest at diagnosis; if negative, repeat q6 months ×2 years; PET/CT if high clinical suspicion; SOX1 Ab supports paraneoplastic etiology
3,4-DAP (amifampridine)	Blocks presynaptic K⁺ channels → prolongs action potential → more Ca²⁺ influx → increased ACh release; first-line symptomatic therapy; FDA-approved for LEMS
Pyridostigmine	May add as adjunct (modest benefit — limited ACh release to work with)
Immunotherapy	IVIg, PLEX, prednisone, azathioprine for autoimmune LEMS; treat tumor for paraneoplastic LEMS

🎯 Clinical Pearl

Post-exercise facilitation of reflexes is pathognomonic for LEMS — test by having patient contract quadriceps for 10 seconds, then recheck patellar reflex
Dry mouth is often the first autonomic symptom in LEMS — may precede weakness by months
If you diagnose LEMS, you MUST screen for SCLC — cancer treatment often improves neurologic symptoms

💎 Board Pearl

Low baseline CMAP that increases ≥100% at 50 Hz RNS (or post-exercise) = LEMS; in MG the baseline CMAP is normal
3,4-DAP works by blocking K⁺ channels (prolongs presynaptic depolarization) — NOT by blocking AChE — this is a common board trap
P/Q-type VGCC Ab is diagnostic but NOT specific for cancer — use SOX1 Ab to distinguish paraneoplastic from autoimmune LEMS
LEMS can coexist with cerebellar degeneration (shared P/Q VGCC target on Purkinje cells) — both are paraneoplastic in SCLC

Other NMJ Disorders

Botulism

Feature	Details
Mechanism	Clostridium botulinum toxin cleaves SNARE proteins (SNAP-25, synaptobrevin) → blocks presynaptic ACh vesicle fusion/release
Types	Foodborne (preformed toxin in canned foods); wound (IV drug users, black tar heroin); infant (spore ingestion, honey — age <1 yr); iatrogenic (cosmetic Botox)
Classic presentation	Descending paralysis + autonomic dysfunction + bulbar weakness
Pupils	Fixed, dilated (mydriasis) — key distinguishing feature from MG and GBS
Key features	Descending (cranial → arms → legs → diaphragm); symmetric; NO sensory loss; dry mouth, constipation, urinary retention; alert mental status (toxin does not cross BBB)
EMG	Low CMAP amplitude; incremental response at high-frequency RNS (similar to LEMS but often <100%); SFEMG shows increased jitter
Diagnosis	Toxin assay in serum, stool, or wound; mouse bioassay is gold standard
Treatment	Antitoxin (equine heptavalent for adults; BabyBIG/botulism immune globulin for infants); wound botulism: antibiotics (penicillin G or metronidazole) + debridement; infant botulism: do NOT give antibiotics (lysis releases toxin)

Organophosphate & Nerve Agent Poisoning

Feature	Details
Mechanism	Irreversible AChE inhibition → ACh excess at muscarinic AND nicotinic receptors
Muscarinic effects (SLUDGE + killer Bs)	Salivation, Lacrimation, Urination, Diarrhea, GI cramping, Emesis; Bradycardia, Bronchospasm, Bronchorrhea
Nicotinic effects	Fasciculations, muscle weakness, paralysis, tachycardia (initial sympathetic surge)
CNS effects	Seizures, altered mental status, coma
Pupils	Miotic (pinpoint) — opposite of botulism
Treatment	Atropine (blocks muscarinic effects; titrate to dry secretions) + pralidoxime (2-PAM) (reactivates AChE if given before “aging”; must give within 24–48h); benzodiazepines for seizures
Intermediate syndrome	Proximal weakness 24–96h after exposure; respiratory failure risk; due to persistent NMJ blockade after cholinergic symptoms resolve

Congenital Myasthenic Syndromes (CMS)

Type / Gene	Location	Key Features	Treatment
ChAT deficiency	Presynaptic	Episodic apnea in infancy; normal between episodes; triggered by infection/stress	Pyridostigmine + 3,4-DAP
AChE deficiency (COLQ)	Synaptic	Slow pupillary light response; repetitive CMAP after single stimulus; AChE inhibitors CONTRAINDICATED (worsen)	Ephedrine or salbutamol; AVOID AChEi
AChR deficiency (CHRNE)	Postsynaptic	Most common CMS; reduced AChR number; generalized weakness from birth	Pyridostigmine + 3,4-DAP
Slow-channel CMS	Postsynaptic	Gain-of-function AChR mutation → prolonged channel opening → endplate myopathy; selective hand/forearm/neck extensor weakness; repetitive CMAP; AD inheritance	Quinidine or fluoxetine; AVOID AChEi (worsens)
Fast-channel CMS	Postsynaptic	Loss-of-function AChR mutation → shortened channel opening; severe weakness	Pyridostigmine + 3,4-DAP
DOK7 CMS	Postsynaptic (signaling)	Limb-girdle pattern; proximal weakness; waddling gait; childhood onset; AChEi worsens	Ephedrine or salbutamol; AVOID AChEi
Rapsyn deficiency	Postsynaptic	AChR clustering defect; neonatal ptosis, feeding difficulty, arthrogryposis; episodic crises	Pyridostigmine + 3,4-DAP

Drug-Induced Myasthenia

Drug	Mechanism	Outcome
D-Penicillamine	Induces AChR antibody production	True autoimmune MG; resolves after drug discontinuation (weeks–months)
Immune checkpoint inhibitors	Anti-PD-1/PD-L1/CTLA-4 immune dysregulation	De novo MG or unmasking subclinical MG; can be fulminant with concurrent myocarditis + myositis (overlap); AChR Ab+ in ~65%; high mortality
Interferon-α	Immune modulation	MG with AChR antibodies; resolves after discontinuation
Chloroquine/hydroxychloroquine	Postsynaptic NMJ blockade	Usually mild; resolves after discontinuation

NMJ Toxins & Poisons

Agent	NMJ Target	Mechanism	Key Features
Botulinum toxin	Presynaptic	Cleaves SNARE proteins (SNAP-25)	Descending paralysis, mydriasis, autonomic dysfunction
Tetanus toxin	Presynaptic (inhibitory interneurons)	Cleaves synaptobrevin in Renshaw cells/inhibitory neurons	Spastic paralysis, trismus, opisthotonus; ascending
Organophosphates	Synaptic (AChE)	Irreversible AChE inhibition → ACh excess	SLUDGE + nicotinic + CNS symptoms; miotic pupils
Black widow spider (α-latrotoxin)	Presynaptic	Massive ACh release then depletion	Pain/cramping → weakness; abdominal rigidity
Tick paralysis	Presynaptic	Neurotoxin in tick saliva reduces ACh release	Ascending paralysis (mimics GBS); resolves with tick removal
Curare (tubocurarine)	Postsynaptic	Competitive AChR antagonist	Flaccid paralysis; reversed by AChE inhibitors (neostigmine)
Succinylcholine	Postsynaptic	Depolarizing AChR agonist → sustained depolarization	Phase I: fasciculations then block; Phase II: non-depolarizing block
Snake venoms (α-bungarotoxin)	Postsynaptic	Irreversible AChR binding	Elapid envenomation; respiratory paralysis

🎯 Clinical Pearl

Tick paralysis mimics Guillain-Barré syndrome (ascending paralysis, areflexia) — always check for a tick in the scalp; complete recovery after removal
In CMS, AChE inhibitors can be harmful in slow-channel syndrome, COLQ deficiency, and DOK7 — genetic diagnosis is essential before treatment
Checkpoint inhibitor MG can be fatal — occurs in ~1% of patients on anti-PD-1 therapy; may present with concurrent myositis and myocarditis (overlap syndrome)

💎 Board Pearl

Botulism pupils = dilated (mydriasis); Organophosphate pupils = miotic (pinpoint) — classic board differentiator
Pralidoxime must be given before “aging” — once the organophosphate-AChE bond ages (24–48h), the enzyme cannot be reactivated
Infant botulism: do NOT give antibiotics (lysis of C. botulinum releases more toxin); give BabyBIG (botulism immune globulin); honey exposure is the classic association
Tetanus toxin targets inhibitory interneurons (blocks glycine/GABA release) → spastic paralysis; botulinum toxin targets motor neurons → flaccid paralysis
AChEi WORSENS three CMS subtypes: slow-channel, AChE/COLQ deficiency, and DOK7 — giving pyridostigmine to these patients is harmful
Repetitive CMAP after single nerve stimulus = think AChE deficiency or slow-channel CMS (prolonged endplate current re-excites muscle fiber)

High-Yield Comparison Table

MG vs LEMS vs Botulism vs Organophosphate Poisoning

Feature	Myasthenia Gravis	LEMS	Botulism	Organophosphate
NMJ defect site	Postsynaptic (AChR)	Presynaptic (VGCC)	Presynaptic (SNARE)	Synaptic (AChE)
Antibody / Toxin	AChR Ab, MuSK Ab, LRP4 Ab	P/Q-type VGCC Ab	Botulinum toxin	Organophosphate compound
Weakness pattern	Ocular → bulbar → generalized; fatigable	Proximal legs > arms; improves briefly with activity	Descending (cranial → limbs → respiratory)	Generalized (fasciculations → paralysis)
Reflexes	Normal	Absent (improve post-exercise)	Absent	May be absent (depolarization block)
Pupils	Normal	Sluggish (may be normal)	Dilated, fixed	Pinpoint (miotic)
Autonomic	Absent	Present (dry mouth, constipation)	Present (dry mouth, constipation, ileus)	Present (SLUDGE — excessive secretions)
Secretions	Normal	Dry	Dry	Wet / excessive
Sensory	Normal	Normal	Normal	Normal (but pain/cramping)
RNS (2–3 Hz)	Decrement ≥10%	Decrement (low CMAP)	Decrement (low CMAP)	Decrement (repetitive CMAPs)
RNS (50 Hz) or post-exercise	No increment	Increment ≥100%	Increment (variable, often <100%)	Not typically tested
Cancer link	Thymoma (10–15%)	SCLC (~60%)	None	None
Treatment	Pyridostigmine, immunotherapy, thymectomy	3,4-DAP, treat cancer, immunotherapy	Antitoxin, supportive ICU care	Atropine + pralidoxime

AChR Ab+ MG vs MuSK Ab+ MG

Feature	AChR Ab+ MG	MuSK Ab+ MG
Antibody class	IgG1/IgG3 (complement-fixing)	IgG4 (non-complement-fixing)
Predominant weakness	Ocular → generalized (limb + ocular)	Bulbar, facial, respiratory, neck
Facial/tongue atrophy	Rare	Common
Thymus	Hyperplasia or thymoma	Normal — no thymic pathology
Pyridostigmine	Effective	Poorly tolerated / may worsen
Thymectomy	Indicated	NOT indicated
Complement inhibitors	Effective (eculizumab, ravulizumab)	NOT effective (IgG4 → no complement activation)
FcRn inhibitors	Effective (efgartigimod, rozanolixizumab)	Limited data (may reduce IgG4 levels)
Rituximab	Second/third line	First-line IS (after steroids)
Crisis risk	Moderate	High (respiratory predominance)

💎 Board Pearl

Board exam pattern recognition: fatigable ptosis + diplopia = MG; proximal weakness + absent reflexes + dry mouth = LEMS; descending paralysis + dilated pupils = botulism; SLUDGE + fasciculations + miotic pupils = organophosphate
Dry vs Wet: LEMS and botulism cause dry autonomic features (dry mouth, constipation); organophosphate causes wet features (SLUDGE)
High-frequency RNS increment = presynaptic disorder (LEMS or botulism); no increment = postsynaptic (MG)
The only NMJ disorder with a cancer screening protocol is LEMS — CT chest q6 months ×2 years for SCLC