NMJ Disorders — Bone Wizardry

The Patient Who Gets Weaker ▶

Before we teach you anything — read this patient and commit to a diagnosis. No peeking.

A 32-year-old woman presents with 3 weeks of drooping eyelids and double vision, worse in the evening. She says her jaw gets tired halfway through dinner, and by the end of the day she can barely keep her eyes open. This morning she felt fine. A CT chest shows an anterior mediastinal mass.

What's the most likely diagnosis?

Lambert-Eaton Myasthenic Syndrome

Myasthenia Gravis

Botulism

Multiple Sclerosis

Nice. Weakness that worsens with use + ptosis + diplopia + anterior mediastinal mass (thymomaThe thymus is where T cells mature. In MG, the thymus is often hyperplastic or contains a thymoma. The thymus may be "teaching" T cells to attack AChRs. Thymectomy improves symptoms in ~70% of MG patients, even without a thymoma.) = textbook Myasthenia Gravis. The evening worsening is the giveaway — she uses those muscles all day, they fatigue. Let's learn why.

Good instinct, but look at the pattern: weakness that worsens through the day (better in morning, worse by evening), ptosis, diplopia, and an anterior mediastinal mass. That's MG with a thymoma. LEMS would improve with use. Botulism would be acute, with descending paralysis. MS has different temporal patterns. Let's break all of these down.

The Junction — Why This All Makes Sense ▶

Every NMJ disorder attacks a different part of the same synapse. Once you know WHERE the problem is, the clinical picture writes itself.

The Normal NMJ in 30 Seconds

Action potential arrives at nerve terminal → VGCCsVoltage-Gated Calcium Channels. When the action potential depolarizes the nerve terminal, these channels open and let calcium flood in. Calcium is the signal that tells vesicles to fuse with the membrane and dump ACh into the cleft. No calcium entry = no ACh release. open → Ca²⁺ floods in → AChAcetylcholine. The neurotransmitter at the NMJ. Synthesized from choline + acetyl-CoA by choline acetyltransferase. Released in quanta (packets) from vesicles. Degraded by acetylcholinesterase in the cleft. Every step here is a potential drug target. vesicles fuse and release → ACh crosses the cleft → binds AChRsNicotinic Acetylcholine Receptors. These are ligand-gated ion channels (not muscarinic). When ACh binds, the channel opens, Na+ enters, the muscle depolarizes. In MG, antibodies block or destroy these receptors — fewer receptors = weaker signal = fatigable weakness. on muscle → muscle contracts.

Now break each step and watch what happens.

MG — Attacks the Postsynaptic Receptor

Anti-AChR antibodiesFound in ~85% of MG patients. These IgG antibodies bind to the nicotinic AChR, causing: (1) complement-mediated destruction of the postsynaptic membrane, (2) accelerated internalization of receptors, (3) direct blocking of ACh binding. Fewer functional receptors = each successive nerve impulse is less effective = fatigue. destroy or block AChRs on the muscle. The nerve releases ACh just fine, but there are fewer receptors to catch it.

First few contractions? Enough receptors still available. But with repeated use, the safety factor drops — each impulse recruits fewer and fewer receptors. The muscle gets weaker and weaker and weaker.

Worsens with repeated use. Better after rest.

LEMS — Attacks the Presynaptic Channel

Anti-VGCC antibodiesAntibodies against P/Q-type voltage-gated calcium channels on the presynaptic nerve terminal. These channels are what let calcium in to trigger vesicle fusion. Block them = less calcium = less ACh released per impulse. But with repeated stimulation, residual calcium BUILDS UP inside the terminal, eventually enough to trigger release. That's why repeated use HELPS. block calcium channels on the nerve terminal. Less Ca²⁺ gets in → less ACh gets released.

But here's the twist: with repeated nerve firing, residual calcium accumulates inside the terminal. Eventually enough builds up to trigger vesicle release despite the blocked channels. The muscle starts weak and gets stronger.

Improves with repeated use. The exact opposite of MG. 🔑 LEMS = Less at first, Eventually More Strength

Botulism — Blocks Vesicle Release Entirely

Botulinum toxinClostridium botulinum produces the most potent toxin known. It's a zinc metalloprotease that cleaves SNARE proteins (SNAP-25, synaptobrevin, syntaxin) — the machinery that fuses vesicles with the membrane. No fusion = no ACh release = no muscle contraction. Same mechanism as Botox injections (which just use tiny, localized doses). cleaves SNARE proteins — the molecular machinery that fuses ACh vesicles with the membrane. The calcium channels work fine. The receptors work fine. The vesicles just can't release their contents.

Unlike MG or LEMS, this doesn't wax and wane — it's an acute, descending paralysis starting with cranial nerves (eyes, face, swallowing) and spreading downward.

⚠ THE Classic Board Trap: "Patient has weakness that improves with repeated muscle use." If you pick MG, you lose the point. MG worsens with use. That's LEMS. Improves with use = LEMS. Every time. The board loves this inversion.

The Big Three — Side by Side ▶

One table. Three disorders. Every board-tested difference.

Feature	MG	LEMS	Botulism
Target	Postsynaptic AChR	Presynaptic VGCC	SNARE proteins (presynaptic)
Antibody	Anti-AChR (85%) Anti-MuSK (5-8%)	Anti-VGCC (P/Q-type)	None (toxin-mediated)
Repeated use	WORSENS	IMPROVES	No significant change
Weakness pattern	Ocular → bulbar → generalized Ptosis, diplopia first	Proximal legs first "Can't get out of a chair"	Descending Cranial nerves → trunk → limbs
Reflexes	Normal	Decreased (improve after exercise)	Decreased to absent
Autonomic	Not typical	Dry mouth, constipation, impotence	Dilated pupils, constipation, urinary retention
Cancer link	Thymoma (10-15%)	Small cell lung cancer (50-60%)	None
EMG pattern	Decremental response on repetitive stim	Incremental response at high-frequency stim	Decremental; incremental at high freq
Treatment	Pyridostigmine, thymectomy, immunosuppression	3,4-DAP, treat underlying cancer	Antitoxin, supportive care
Crisis	Myasthenic crisis (resp failure) → IVIG or plasmapheresis	Rarely crises	Respiratory failure, ICU

🎯 MG age demographics: Young women (20-30s) and older men (60-70s) = bimodal distribution. The young woman + ptosis + thymoma vignette is the boards' favorite.

🎯 LEMS cancer screening is mandatory: >50% of LEMS patients have an underlying small cell lung cancerSCLC expresses VGCCs on its surface. The immune system makes antibodies against the tumor's VGCCs, but those same antibodies cross-react with the VGCCs on nerve terminals. This is paraneoplastic — the cancer causes the syndrome. Finding LEMS = hunt for SCLC. CT chest + PET scan. If negative, repeat every 6 months for 2 years.. LEMS can be the FIRST sign of cancer, appearing months before the tumor is found.

Botulism — The Three Types (Boards Love #3)

Food-borne: Preformed toxin in improperly canned/preserved food. Symptoms within 12-36 hours.
Wound: C. botulinum infects a wound and produces toxin in vivo. Think IV drug users with skin-popping (black tar heroin).
Infant: Baby ingests spores (not preformed toxin) — spores germinate in the immature gut. Classic board trigger: honey given to infant <1 year old. Presents with constipation, "floppy baby," weak cry, poor feeding. 🔑 Honey = Harmful to infants. Baby's gut can't Handle the spores.

⚠ Anti-MuSK MG: ~5-8% of MG patients have anti-MuSKMuscle-Specific Kinase. A receptor tyrosine kinase on the postsynaptic membrane that helps cluster AChRs. Anti-MuSK antibodies prevent AChR clustering = fewer functional receptors. These patients are often "seronegative" for anti-AChR but still have MG. They tend to have more bulbar symptoms (speech, swallowing) and respond POORLY to pyridostigmine. antibodies instead of anti-AChR. They're "seronegative" on the standard AChR antibody test but still have MG. Board clue: MG presentation + negative AChR antibodies → check anti-MuSK.

The Diagnostic Decision Tree ▶

A patient walks in with weakness. You need to figure out if it's the junction. Let's practice the algorithm — you answer BEFORE the tree reveals.

Step 1: A patient has fluctuating weakness — some days better, some worse. It affects the eyes and face prominently. Does the weakness get BETTER or WORSE with repeated muscle use?

Worse with use (fatigable)

Better with use (facilitating)

Neither — it's constant and getting worse rapidly

The Branch:
• Worse with use → Go to Step 2 (MG workup)
• Better with use → Go to Step 3 (LEMS workup)
• Acute descending → Go to Step 4 (Botulism)

Elimination Game — 5 Patients, 3 Diagnoses ▶

Each clue eliminates one diagnosis. By the end, only the answer remains. Your working memory stays on the screen, not in your head.

Myasthenia Gravis

LEMS

Botulism

MG Treatment — The Board Loves This ▶

Step 1: Symptom Control

PyridostigmineAn acetylcholinesterase inhibitor. It blocks the enzyme that breaks down ACh in the synaptic cleft. More ACh sticks around = more chances to bind to whatever AChRs remain. It treats symptoms but doesn't change the underlying autoimmune process. Side effects are cholinergic: diarrhea, cramping, salivation, bradycardia (think "SLUDGE" — Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis). (Mestinon) = first-line.

Blocks acetylcholinesterase → more ACh hangs around in the cleft → better chance of finding a receptor. Treats symptoms, NOT the autoimmune process.

Step 2: Immunosuppression

For moderate-severe MG beyond symptom control:

Corticosteroids (prednisone) — fastest onset, but watch for initial worsening in first 2 weeks
Azathioprine, mycophenolate — steroid-sparing agents, take months to work
Rituximab — especially for anti-MuSK MG

Step 3: Thymectomy

Recommended for:

All patients with thymoma (surgical oncology indication)
Generalized MG patients age 18-50, even WITHOUT thymoma (MGTX trial showed benefit)

Myasthenic Crisis

Respiratory failure from MG = emergency. The diaphragm fatigues.

IVIG or plasmapheresis — both work fast (days)
Intubate if needed — watch for negative inspiratory forceNIF (Negative Inspiratory Force) measures diaphragm strength. Normal is about -60 cmH2O. In myasthenic crisis, if NIF weakens to less negative than -20 cmH2O, intubation is often needed. Serial NIF monitoring is how you track respiratory deterioration in MG. < -20 cmH2O
Hold pyridostigmine during crisis (excess cholinergic stimulation can worsen secretions)

⚠ Myasthenic vs Cholinergic Crisis: Both cause weakness + respiratory failure. Myasthenic crisis = too little ACh at receptors (underdose). Cholinergic crisis = too much ACh (overdose of pyridostigmine). Cholinergic crisis adds SLUDGE symptoms (salivation, lacrimation, urination, defecation, GI, emesis) + miosis. If they're drowning in secretions → cholinergic crisis. Stop the pyridostigmine.

⚠ Drugs that worsen MG: Aminoglycosides, fluoroquinolones, beta-blockers, magnesium, D-penicillamine. The board LOVES giving a stable MG patient a new medication and asking why they crashed. If they got an aminoglycoside for a UTI and now can't breathe → drug-induced MG exacerbation.

Quiz — 5 Patients, Don't Kill Them ▶

12 questions in the pool, 5 random per load, shuffled answers. Every visit is different. Let's see if you were paying attention.