Myasthenia Gravis vs Lambert-Eaton — MRCP Part 1
- Crack Medicine
- 11 minutes ago
- 4 min read
TL;DR
For MRCP Part 1, differentiating myasthenia gravis vs Lambert-Eaton is a classic neurology exam theme. Myasthenia gravis causes fatigable weakness due to postsynaptic acetylcholine receptor antibodies and commonly presents with ocular symptoms. Lambert-Eaton syndrome is a presynaptic disorder associated with small-cell lung cancer and autonomic dysfunction, where muscle strength improves after repeated activity. Recognising these patterns quickly helps secure easy exam marks.
Why this matters
Neuromuscular junction disorders are a high-yield topic in MRCP Part 1. Questions frequently test the candidate’s ability to distinguish between myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) using subtle clinical clues.
Although both conditions cause muscle weakness, they differ in pathophysiology, reflex patterns, associated diseases, and electrophysiological findings. In exam scenarios, these conditions often appear in short clinical vignettes where recognising a single key clue can lead directly to the correct answer.
For a broader overview of the syllabus and exam strategy, review the MRCP Part 1 overview.
Core differences: Myasthenia Gravis vs Lambert-Eaton
The most effective way to remember the distinction between these conditions is through a direct comparison.
Feature | Myasthenia Gravis | Lambert-Eaton Syndrome |
Site of pathology | Postsynaptic neuromuscular junction | Presynaptic neuromuscular junction |
Autoantibody target | Acetylcholine receptors (AChR) or MuSK | Voltage-gated calcium channels |
Muscle weakness | Fatigable, worsens with activity | Improves with repeated activity |
Reflexes | Normal | Reduced or absent |
Ocular involvement | Very common (ptosis, diplopia) | Rare |
Autonomic symptoms | Rare | Common (dry mouth, constipation, impotence) |
Malignancy association | Thymoma | Small-cell lung carcinoma |
EMG findings | Decremental response | Incremental response |
This comparison table covers multiple commonly tested MRCP Part 1 points.
Pathophysiology: A favourite MRCP exam theme
Understanding the neuromuscular junction physiology helps explain the clinical differences between MG and LEMS.
Myasthenia gravis
Myasthenia gravis is an autoimmune disorder affecting the postsynaptic membrane.
Key mechanisms include:
Autoantibodies against acetylcholine receptors
Reduction in functional receptors at the neuromuscular junction
Impaired neuromuscular transmission
Decreased end-plate potentials
The consequence is fatigable muscle weakness, meaning muscle strength deteriorates with repeated use.
The condition is frequently associated with thymic abnormalities, including thymoma or thymic hyperplasia.
More detailed guidance on MG can be found via the National Institute for Health and Care Excellence (NICE) clinical overview:https://cks.nice.org.uk/topics/myasthenia-gravis/
Lambert-Eaton myasthenic syndrome
LEMS is a presynaptic disorder.
Key features include:
Antibodies against P/Q-type voltage-gated calcium channels
Reduced calcium influx into nerve terminals
Decreased release of acetylcholine
Repeated nerve stimulation increases calcium accumulation, temporarily improving neurotransmitter release. This leads to the classic exam finding where muscle strength improves after exercise.
LEMS is strongly associated with small-cell lung carcinoma, making it a classic paraneoplastic syndrome.
Clinical information is summarised by NHS guidance:https://www.nhs.uk/conditions/lambert-eaton-myasthenic-syndrome/
Clinical features: Recognising the pattern
Myasthenia gravis
Typical clinical features include:
Fluctuating muscle weakness
Ptosis and diplopia
Bulbar symptoms (dysphagia, dysarthria)
Proximal muscle weakness
Normal reflexes
Symptoms worse later in the day
Normal sensory examination
Ocular symptoms are usually the earliest manifestation.
Lambert-Eaton syndrome
The clinical pattern is distinct:
Proximal limb weakness
Reduced or absent reflexes
Autonomic symptoms such as dry mouth
Temporary improvement after repeated activity
Rare ocular involvement
Association with malignancy
A common MRCP vignette involves a long-term smoker with proximal weakness and dry mouth.
Investigations: Key exam points
Myasthenia gravis
Important investigations include:
Anti-acetylcholine receptor antibodies
Anti-MuSK antibodies
CT chest to assess for thymoma
Repetitive nerve stimulation → decremental response
Single-fibre electromyography
Authoritative information is available via StatPearls (NCBI):https://www.ncbi.nlm.nih.gov/books/NBK559331/
Lambert-Eaton syndrome
Typical findings include:
Antibodies against voltage-gated calcium channels
Incremental response on repetitive nerve stimulation
Screening for small-cell lung carcinoma using CT chest
A detailed clinical review is available from StatPearls:https://www.ncbi.nlm.nih.gov/books/NBK507891/
Five most tested subtopics in MRCP
When preparing for MRCP Part 1, focus particularly on these high-yield themes:
Neuromuscular junction physiology
Antibody targets in MG vs LEMS
Paraneoplastic associations
Electrophysiology findings
Clinical differentiation using reflexes and autonomic symptoms
These areas are commonly assessed in question banks and exam papers.
You can practise these patterns with Free MRCP MCQs.
Practical examples / mini-cases
MRCP-style question
A 62-year-old man presents with progressive proximal muscle weakness. He struggles to climb stairs. Examination shows reduced deep tendon reflexes that improve after repeated testing. He also reports dry mouth.
What is the most likely diagnosis?
A. Myasthenia gravisB. Lambert-Eaton syndromeC. Motor neurone diseaseD. PolymyositisE. Multiple sclerosis
Answer: B — Lambert-Eaton syndrome
Explanation
Key clues include:
Proximal muscle weakness
Reduced reflexes
Reflex improvement after exercise
Autonomic symptoms (dry mouth)
This pattern strongly indicates Lambert-Eaton myasthenic syndrome, commonly associated with small-cell lung cancer.
Practical MRCP revision checklist
Use this checklist to rapidly revise the topic:
✔ Identify presynaptic vs postsynaptic disorders✔ Remember thymoma → myasthenia gravis✔ Remember small-cell lung cancer → Lambert-Eaton syndrome✔ Reflexes normal in MG, reduced in LEMS✔ Weakness worsens with use in MG✔ Weakness improves with activity in LEMS✔ Autonomic symptoms strongly suggest LEMS✔ Ocular symptoms strongly suggest MG
Structured teaching sessions can reinforce these concepts through our MRCP lectures.
Common pitfalls (MRCP traps)
Assuming reflexes are reduced in both conditions
Forgetting the malignancy association in LEMS
Confusing the effect of exercise on muscle strength
Ignoring autonomic symptoms such as dry mouth
Overlooking the high prevalence of ocular symptoms in MG
Avoiding these traps helps answer exam questions more confidently.
FAQs
How do MRCP questions differentiate MG from Lambert-Eaton?
Exam questions typically highlight differences in reflexes, autonomic symptoms, and response to exercise. MG shows fatigable weakness with normal reflexes, whereas LEMS features reduced reflexes and improvement with repeated activity.
Which cancer is associated with Lambert-Eaton syndrome?
Lambert-Eaton syndrome is most commonly associated with small-cell lung carcinoma, making it a key paraneoplastic neurological disorder.
Why does strength improve after exercise in Lambert-Eaton syndrome?
Repeated activity increases presynaptic calcium accumulation, improving acetylcholine release at the neuromuscular junction.
What antibody is most common in myasthenia gravis?
The most common antibody is anti-acetylcholine receptor (AChR). Some patients also have antibodies against MuSK (muscle-specific kinase).
Are reflexes normal in myasthenia gravis?
Yes. Deep tendon reflexes are usually preserved in MG, which helps distinguish it from Lambert-Eaton syndrome.
Call to action
Mastering pattern-recognition topics like MG vs Lambert-Eaton can significantly improve your MRCP Part 1 performance.
Start practising with Free MRCP MCQs and explore the full MRCP Part 1 overview to structure your revision.
Sources
MRCP(UK) Examination Blueprint – https://www.mrcpuk.org/mrcpuk-examinations/part-1
NICE Clinical Knowledge Summary: Myasthenia Gravis – https://cks.nice.org.uk/topics/myasthenia-gravis/
NHS: Lambert-Eaton Myasthenic Syndrome – https://www.nhs.uk/conditions/lambert-eaton-myasthenic-syndrome/
StatPearls: Myasthenia Gravis – https://www.ncbi.nlm.nih.gov/books/NBK559331/
StatPearls: Lambert-Eaton Syndrome – https://www.ncbi.nlm.nih.gov/books/NBK507891/
Comments