Myasthenia Gravis vs Lambert-Eaton Syndrome on COMLEX

Myasthenia gravis is a postsynaptic disease: antibodies block the acetylcholine receptor, so weakness gets worse with use, starts in the eyes and throat, and leaves reflexes and the autonomic system alone. Lambert-Eaton syndrome is a presynaptic disease: antibodies block the voltage-gated calcium channel, so less acetylcholine is released, weakness starts in the hips and briefly improves with use, reflexes are reduced, and the mouth is dry. Myasthenia gravis goes with thymoma, and Lambert-Eaton goes with small cell lung cancer. On COMLEX, the direction of fatigue, the reflexes, and the autonomic symptoms separate them.

Both diseases live at the same synapse and both are antibody-mediated. That's why boards love pairing them. The question writers give you a weak patient and ask you to decide which side of the neuromuscular junction is broken. Once you can picture the synapse, the stem tells you the answer in the first three sentences.

Why do boards pair myasthenia gravis with Lambert-Eaton syndrome?

They are the two classic autoimmune disorders of the neuromuscular junction. Both are type II hypersensitivity reactions, where IgG antibodies bind a specific protein and disable it. Both cause weakness without sensory loss. Both can show a decrement on slow repetitive nerve stimulation.

That overlap makes them a perfect trap. A student who reads "weakness, normal sensation, antibody-mediated" and stops will pick the more familiar answer, which is usually myasthenia gravis. The writer is counting on that.

The diseases also lead to different workups. A Lambert-Eaton diagnosis means you go hunting for a lung cancer, sometimes for years. A myasthenia diagnosis means you image the anterior mediastinum for a thymoma. So the "next best step" questions only work if you got the diagnosis right first.

What goes wrong at the synapse in each disease?

Picture the normal sequence. An action potential reaches the motor nerve terminal and opens P/Q-type voltage-gated calcium channels. Calcium rushes in, vesicles fuse, and acetylcholine is released into the cleft. Acetylcholine binds nicotinic receptors on the muscle endplate, the muscle depolarizes, and the fiber contracts. Acetylcholinesterase then clears the cleft.

Each disease breaks a different step in that chain.

Myasthenia gravis: the receptor is the target

In myasthenia gravis, antibodies bind the postsynaptic nicotinic acetylcholine receptor. They block it, cross-link it so it gets internalized, and activate complement that damages the endplate. Fewer working receptors means each nerve impulse produces a smaller endplate potential.

Normal people have a safety margin. The first few impulses in a train release the most acetylcholine, and later impulses release a bit less. A healthy endplate still reaches threshold every time. A myasthenic endplate has no margin left, so as release tapers with repeated firing, more fibers fail. That is the mechanism behind fatigable weakness.

Anti-acetylcholine receptor antibodies are found in about 85% of patients with generalized myasthenia gravis and about half of patients with purely ocular disease. A smaller group of seronegative patients has antibodies against muscle-specific kinase (MuSK) instead. MuSK-positive patients tend to have prominent bulbar and neck weakness and respond less to pyridostigmine.

Lambert-Eaton syndrome: the calcium channel is the target

In Lambert-Eaton myasthenic syndrome (LEMS), antibodies bind the presynaptic P/Q-type voltage-gated calcium channel. Less calcium enters the nerve terminal with each impulse, so fewer vesicles release acetylcholine. The receptors on the muscle side are normal. There just isn't enough transmitter reaching them.

Here's the twist that every board question is built on. With sustained or rapid firing, calcium accumulates inside the nerve terminal faster than it can be cleared. That buildup boosts release with each successive impulse. So for a few seconds of effort, strength and reflexes actually get better. Prolonged effort still fatigues the patient, but the initial improvement is the classic sign.

Antibodies against the P/Q-type calcium channel are present in about 85% to 90% of patients with LEMS. Roughly half of all cases are paraneoplastic, and nearly all of those are small cell lung cancer. The tumor's neuroendocrine cells express the same calcium channels, and the immune response against the tumor cross-reacts with the nerve terminal.

Myasthenia gravis vs Lambert-Eaton syndrome side by side

How does weakness behave with use?

This is the single highest-yield discriminator. Myasthenia gravis gets worse with use. Lambert-Eaton gets better with use, at least at first.

A myasthenia stem describes a patient who is fine in the morning and worse by evening. Ptosis appears after reading or driving. Double vision shows up late in the day. Chewing a steak gets harder halfway through the meal. Speech gets nasal or slurred after talking for a few minutes. The exam maneuver is sustained upgaze for about a minute, which makes the eyelid droop further.

A Lambert-Eaton stem describes a patient who has trouble rising from a chair or climbing stairs. Grip strength is weak on the first squeeze and stronger on the third or fourth. The examiner taps the patellar tendon and gets nothing, then asks the patient to contract the quadriceps hard for 10 seconds, taps again, and gets a reflex. That return of a reflex after brief exercise is called post-exercise facilitation, and it is essentially pathognomonic.

The improvement in LEMS is short-lived. Ask a LEMS patient to walk a long distance and they still tire. Boards test the initial facilitation, not the endurance.

Where does the weakness start?

Myasthenia gravis tends to start at the top. About half of patients present with ptosis or diplopia, and most develop eye findings at some point. The ptosis is often asymmetric and can switch sides. Pupils are always spared, which separates it from a third nerve palsy. Bulbar weakness follows: dysphagia, dysarthria, nasal speech, and a fatigable jaw. Proximal limb and neck weakness can follow later.

Lambert-Eaton tends to start at the bottom. The hips and thighs go first, so the complaint is trouble with stairs, chairs, and getting out of a car. The weakness spreads upward to the shoulders. Mild ptosis can occur, but prominent eye findings or isolated ocular weakness should push you toward myasthenia.

So the shorthand is that myasthenia descends from the eyes and Lambert-Eaton ascends from the legs. Neither has sensory loss, and neither has upper motor neuron signs. If you see a Babinski sign or a sensory level, you're in a different chapter. The UMN vs LMN guide covers how to localize that first.

What do the reflexes and autonomic symptoms tell you?

Reflexes are normal in myasthenia gravis. The muscle can still generate a brisk twitch from a single stretch, because one impulse is usually enough to fire the endplate before fatigue sets in. A myasthenia patient with absent reflexes should make you question the diagnosis.

Reflexes are reduced or absent in Lambert-Eaton. A single tendon tap produces one impulse, and one impulse releases too little acetylcholine. After brief voluntary contraction, calcium builds up in the terminal and the reflex reappears.

Autonomic symptoms belong to Lambert-Eaton. The same calcium channels help release acetylcholine at cholinergic autonomic nerve terminals. Patients report dry mouth, which is the most common autonomic complaint, plus constipation, erectile dysfunction, reduced sweating, and orthostatic lightheadedness. Myasthenia gravis targets the nicotinic receptor at the skeletal muscle endplate, which is a different receptor subtype from the autonomic ganglia, so autonomic function is spared.

When a stem lists "dry mouth" in a weak patient, read it as a deliberate clue. It's rarely there by accident.

Which tumors go with each disease?

Myasthenia gravis and the thymus

About 10% to 15% of patients with myasthenia gravis have a thymoma, and many younger patients have thymic hyperplasia without a tumor. The board picture is an anterior mediastinal mass on chest imaging in a patient with fatigable ptosis. Every new diagnosis of myasthenia gets a CT or MRI of the chest to look for it.

Thymoma is the "T" in the classic anterior mediastinal mass list (thymoma, teratoma, thyroid tissue, and lymphoma). If the stem shows a mass in front of the heart, connect it to myasthenia before anything else. The chest X-ray guide walks through mediastinal compartments.

Lambert-Eaton and small cell lung cancer

The paraneoplastic form of LEMS is almost always small cell lung cancer. The classic patient is an older man with a long smoking history and new proximal leg weakness. LEMS symptoms can appear months to years before the cancer is visible on imaging.

That timeline drives the workup. A patient with LEMS gets a CT of the chest, and if that's negative, often a PET scan. Screening is then repeated for about two years, because the risk of finding a tumor is highest in that window. A question that asks for the "next best step" after confirming LEMS is usually chest imaging.

Small cell lung cancer is a paraneoplastic factory. The same tumor causes SIADH, ectopic ACTH with Cushing syndrome, and anti-Hu encephalomyelitis. If a stem gives you hyponatremia and proximal weakness in a smoker, you're looking at one cancer, not two problems.

How do you confirm the diagnosis?

Antibody testing

Order anti-acetylcholine receptor antibodies first when you suspect myasthenia. If they're negative in a patient with generalized weakness, test for anti-MuSK antibodies. For Lambert-Eaton, order anti-P/Q-type voltage-gated calcium channel antibodies.

Repetitive nerve stimulation

Slow repetitive stimulation at 2 to 3 Hz produces a decrement in both diseases. The difference shows up in the baseline and after high-frequency stimulation.

In myasthenia gravis, the first compound muscle action potential is usually normal in size, then it drops with repeated stimuli. In Lambert-Eaton, the first response is already small. After 10 seconds of maximal voluntary contraction or stimulation at 20 to 50 Hz, the response in LEMS jumps by 100% or more. That increment is the electrodiagnostic version of post-exercise facilitation.

Single-fiber EMG shows increased jitter in both. It's the most sensitive test for myasthenia but doesn't distinguish the two.

Bedside tests

The edrophonium (Tensilon) test is mostly historical, but boards still ask about it. Edrophonium is a short-acting acetylcholinesterase inhibitor. It produces a quick, visible improvement in myasthenic ptosis and little or no change in Lambert-Eaton.

The ice pack test is its modern replacement. Cold slows acetylcholinesterase, so placing ice over a closed eyelid for about two minutes improves myasthenic ptosis. A stem that describes ptosis improving after an ice pack is describing myasthenia.

How does treatment differ?

Myasthenia gravis

Pyridostigmine is first-line symptomatic therapy. It inhibits acetylcholinesterase, so acetylcholine stays in the cleft longer and has more chances to hit the remaining receptors. It does not treat the autoimmune process.

Disease-modifying therapy is immunosuppression: corticosteroids, then steroid-sparing agents such as azathioprine or mycophenolate. Thymectomy is indicated for every patient with a thymoma. The MGTX trial also showed that thymectomy improved outcomes in adults with generalized, acetylcholine receptor antibody-positive myasthenia without a thymoma.

Myasthenic crisis is respiratory failure from weakness of the diaphragm and bulbar muscles. Follow forced vital capacity and negative inspiratory force, not oxygen saturation, because saturation drops late. Intubate early. Acute treatment is IVIG or plasmapheresis, which remove or neutralize the circulating antibody quickly.

Lambert-Eaton syndrome

In the paraneoplastic form, treating the small cell lung cancer is the most effective therapy for the weakness. Removing the antigen source removes the drive for the antibody.

The symptomatic drug is 3,4-diaminopyridine (amifampridine). It blocks presynaptic potassium channels, which prolongs depolarization of the nerve terminal. A longer depolarization keeps the remaining calcium channels open longer, so more calcium enters and more acetylcholine is released. Pyridostigmine helps a little and is sometimes added. Immunosuppression and IVIG are used for refractory disease.

Drugs that make both worse

Several medications impair neuromuscular transmission and can unmask or worsen either disease. Boards favor aminoglycosides, fluoroquinolones, magnesium, beta blockers, and neuromuscular blocking agents. A classic stem is a patient with undiagnosed myasthenia who goes into respiratory failure after gentamicin, or a preeclamptic patient who gets weak on magnesium sulfate.

Penicillamine can induce a myasthenia-like syndrome, and immune checkpoint inhibitors can trigger myasthenia in patients being treated for cancer.

Myasthenic crisis vs cholinergic crisis

Too much pyridostigmine causes cholinergic crisis. The patient is weak because the endplate is persistently depolarized, and they also show muscarinic signs: salivation, lacrimation, diarrhea, sweating, bradycardia, miosis, and bronchorrhea. Fasciculations may appear. Myasthenic crisis has weakness without those muscarinic signs. Management of both starts with protecting the airway, and the cholinesterase inhibitor is held.

What else can mimic these two?

Botulism is the presynaptic mimic. The toxin cleaves SNARE proteins, so acetylcholine can't be released. Weakness is descending, starts with the cranial nerves, and comes with dilated, poorly reactive pupils, dry mouth, and constipation. The stem usually gives you home-canned food, a wound in an injection drug user, or an infant fed honey. Pupils are the giveaway, since neither myasthenia nor LEMS involves them.

Guillain-Barré syndrome causes ascending weakness with areflexia, which can look like Lambert-Eaton at first glance. The differences are sensory symptoms, a preceding infection, a subacute course over days, and albuminocytologic dissociation in the CSF. The ALS vs MS vs Guillain-Barré guide covers that pattern in detail.

Brainstem stroke can cause diplopia, dysarthria, and dysphagia, but it's sudden and comes with crossed sensory or motor findings. Fatigability and fluctuation don't fit a vascular event. The brainstem stroke buzzwords guide lists the syndromes.

Stem clues that crack these questions fast

The clues I look for in every question are age, acuity, unusual findings, matching timelines, laterality, labs and imaging, and the question part. Here's how they apply to the neuromuscular junction.

1. Age. Myasthenia gravis has two peaks: women in their 20s and 30s, and men in their 60s and 70s. A 65-year-old smoker with leg weakness is LEMS until proven otherwise.
2. Acuity. Both diseases develop over weeks to months. A crisis that develops over hours after a new antibiotic or magnesium points to a drug unmasking myasthenia.
3. Unusual findings. Dry mouth, constipation, and impotence in a weak patient point to Lambert-Eaton. An anterior mediastinal mass points to myasthenia.
4. Matching timelines. Weakness that worsens through the day is myasthenia. Strength that improves over the first few seconds of effort is Lambert-Eaton.
5. Laterality. Myasthenic ptosis is often asymmetric and can shift sides between exams. Symmetric proximal leg weakness fits Lambert-Eaton.
6. Labs and imaging. Acetylcholine receptor antibodies and a decrement with a normal baseline point to myasthenia. Calcium channel antibodies, a small baseline response, and a large increment after exercise point to LEMS.
7. The question part. "Mechanism," "most likely associated tumor," "best next step," and "best symptomatic treatment" each want a different answer for the same patient. Read the last sentence slowly before you commit.

When two answers both look reasonable on a neuro question, the guide to choosing between two answers on COMLEX walks through how to break the tie.

Common board pitfalls and how to avoid them

Calling every weak patient with ptosis myasthenia. Check the reflexes and ask about dry mouth. Absent reflexes and autonomic symptoms move you to Lambert-Eaton.

Forgetting the pupils. Neither neuromuscular junction disease touches the pupils. Ptosis with a dilated pupil is a third nerve palsy or botulism.

Ordering a thymus scan for LEMS. The tumor to find in Lambert-Eaton is in the lung. The thymus belongs to myasthenia.

Choosing pyridostigmine as the best treatment for LEMS. It helps only modestly. The symptomatic drug of choice is 3,4-diaminopyridine, and the best overall treatment for paraneoplastic disease is treating the cancer.

Following oxygen saturation in myasthenic crisis. Pulse oximetry stays normal until late. Serial vital capacity and negative inspiratory force tell you when to intubate.

Giving an aminoglycoside to a known myasthenic. Gentamicin, fluoroquinolones, and magnesium can precipitate crisis. Pick a different antibiotic when the stem offers one.

Frequently asked questions about myasthenia gravis vs Lambert-Eaton syndrome

What is the main difference between myasthenia gravis and Lambert-Eaton syndrome?

Myasthenia gravis is postsynaptic. Antibodies against the nicotinic acetylcholine receptor cause weakness that worsens with use, starts in the eye and bulbar muscles, and spares the reflexes and autonomic system. Lambert-Eaton syndrome is presynaptic. Antibodies against the P/Q-type voltage-gated calcium channel reduce acetylcholine release, causing proximal leg weakness that briefly improves with use, reduced reflexes, and autonomic symptoms like dry mouth. Myasthenia is linked to thymoma, and Lambert-Eaton is linked to small cell lung cancer.

Why does weakness improve with exercise in Lambert-Eaton syndrome?

With repeated or sustained nerve firing, calcium builds up inside the presynaptic terminal faster than it can be cleared. The extra calcium partly overcomes the blocked channels and increases acetylcholine release. Strength and reflexes improve for a few seconds, which is called post-exercise facilitation. On repetitive nerve stimulation at 20 to 50 Hz, or after 10 seconds of maximal contraction, the muscle response increases by 100% or more.

Which cancer is associated with Lambert-Eaton syndrome?

Small cell lung cancer. Roughly half of Lambert-Eaton cases are paraneoplastic, and nearly all of those are small cell lung cancer in a current or former smoker. The tumor cells express the same voltage-gated calcium channels as the nerve terminal. Weakness can come months to years before the cancer is found, so patients get chest CT, often PET, and repeated screening for about two years.

Do reflexes help distinguish myasthenia gravis from Lambert-Eaton?

Yes. Reflexes are normal in myasthenia gravis, because a single stretch still produces enough acetylcholine release to fire the endplate. Reflexes are reduced or absent in Lambert-Eaton, because each impulse releases too little transmitter. After the patient contracts the muscle hard for about 10 seconds, the reflex reappears in Lambert-Eaton. Absent reflexes in a weak patient with dry mouth should move you away from myasthenia.

What is the treatment for myasthenia gravis vs Lambert-Eaton syndrome?

Myasthenia gravis is treated symptomatically with pyridostigmine, an acetylcholinesterase inhibitor, and long term with corticosteroids, steroid-sparing immunosuppressants, and thymectomy when indicated. Crisis gets IVIG or plasmapheresis plus airway support. Lambert-Eaton is treated by finding and treating small cell lung cancer when present. The symptomatic drug is 3,4-diaminopyridine (amifampridine), which blocks presynaptic potassium channels to prolong depolarization and increase acetylcholine release.

Does Lambert-Eaton syndrome cause ptosis?

It can, but it is usually mild and rarely the presenting complaint. Lambert-Eaton typically starts with proximal leg weakness and spreads upward. Prominent ptosis, diplopia, or purely ocular weakness strongly favors myasthenia gravis. If a stem leads with fluctuating ptosis that worsens with sustained upgaze and improves with an ice pack, the answer is myasthenia.

Related guides

• UMN vs LMN lesions: the board guide
• ALS vs MS vs Guillain-Barré: board-relevant differences
• High-yield chest X-ray for board exams
• Brainstem stroke buzzwords for COMLEX and USMLE
• How do I choose between two answers on COMLEX?
• Doctor Lucas DO on YouTube: neurology question breakdowns and high-yield clinical comparison videos