Confusable diagnoses · PANCE / PANRE

Guillain-Barré Syndrome vs Myasthenia Gravis

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Guillain-Barré Syndrome and Myasthenia Gravis are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.

Guillain-Barré Syndrome vs Myasthenia Gravis at a glance

  • Guillain-Barré Syndrome: Acute immune-mediated ascending demyelinating polyneuropathy; can cause respiratory failure.
  • Myasthenia Gravis: Autoimmune neuromuscular junction disease with fluctuating fatigable weakness.

Try two board-style questions on Guillain-Barré Syndrome vs Myasthenia Gravis

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Question 1NeurologyMedium
A 32-year-old male develops ascending weakness and areflexia over 5 days, two weeks after gastroenteritis. He now has bilateral facial weakness, dysphagia, and orthostatic hypotension. Lumbar puncture (LP) shows protein 180 mg/dL with 4 WBCs (cytoalbuminous dissociation). Nerve conduction studies (NCS) show prolonged F-wave latencies and reduced compound muscle action potential (CMAP) amplitudes. Anti-GQ1b antibodies are positive. Which of the following is the most likely diagnosis?
  • AMyasthenia gravis
  • BWest Nile virus encephalitis
  • CTransverse myelitis
  • DGuillain-Barre syndrome
Reveal answer & full explanation
Correct answer: D — Guillain-Barre syndrome
  • AMyasthenia gravis
  • BWest Nile virus encephalitis
  • CTransverse myelitis
  • DGuillain-Barre syndrome✓

Why Guillain-Barre syndrome is correct

  • Guillain-Barre syndrome (GBS) is an acute immune-mediated polyradiculoneuropathy; the post-infectious onset (2 weeks after gastroenteritis), ascending symmetric weakness with areflexia, bilateral facial weakness (occurs in 50%), and autonomic dysfunction (orthostatic hypotension) are classic.
  • CSF shows cytoalbuminous dissociation: protein above 45 mg/dL with fewer than 10 WBCs (here protein 180 mg/dL, 4 WBCs).
  • Nerve conduction studies (NCS) show prolonged F-wave latencies and reduced compound muscle action potential (CMAP) amplitudes.
  • Anti-GQ1b antibodies are positive and are associated with the Miller Fisher variant.

Why the others are wrong

  • Myasthenia gravis — causes fatigable weakness, not areflexia, cytoalbuminous dissociation, or post-infectious ascending paralysis (confused-with neuromuscular junction disease).
  • West Nile virus encephalitis — causes encephalitis or acute flaccid paralysis with fever and altered consciousness, and would not produce the characteristic CSF cytoalbuminous dissociation seen here (anchoring on viral cause).
  • Transverse myelitis — causes a sensory level, bowel/bladder dysfunction, and bilateral leg weakness, not ascending polyradiculopathy with cranial nerve involvement and cytoalbuminous dissociation (confused-with cord lesion).

Additional high-yield points

  • Common triggering infections: Campylobacter jejuni (most common, especially GQ1b-positive variants), cytomegalovirus (CMV), Epstein-Barr virus (EBV), COVID-19; rarely vaccines.
  • Respiratory failure occurs in 20-30% of patients; monitor forced vital capacity (FVC) and intubate if FVC falls below 15-20 mL/kg.
  • Treatment: IV immunoglobulin (IVIG) 2 g/kg over 5 days OR plasmapheresis — the two are equivalent; steroids are ineffective.
  • Miller Fisher variant: ophthalmoplegia, ataxia, areflexia (associated with anti-GQ1b antibodies).
  • Prognosis: most recover; 5-10% have residual weakness; 3-5% mortality.
Question 2NeurologyMedium
A 35-year-old female has a 2-year history of intermittent diplopia, worse with sustained upgaze, and bilateral ptosis that worsens throughout the day. She notices her voice becomes nasal after prolonged speaking. Ice pack test improves ptosis. Anti-acetylcholine receptor (anti-AChR) antibodies are positive. CT chest shows an anterior mediastinal mass. Which of the following is the most likely diagnosis?
  • AOcular myasthenia gravis
  • BMyasthenia gravis with thymoma
  • CLambert-Eaton myasthenic syndrome
  • DThyroid eye disease
Reveal answer & full explanation
Correct answer: B — Myasthenia gravis with thymoma
  • AOcular myasthenia gravis
  • BMyasthenia gravis with thymoma✓
  • CLambert-Eaton myasthenic syndrome
  • DThyroid eye disease

Why Myasthenia gravis with thymoma is correct

  • Anti-acetylcholine receptor (anti-AChR) antibodies are positive in 85% of generalized myasthenia gravis (MG).
  • Thymoma is identified in 10-15% of MG patients; an anterior mediastinal mass combined with AChR-positive MG indicates thymoma-associated MG.
  • Thymectomy is mandatory for thymoma (oncologic indication) and is also beneficial for non-thymomatous generalized MG in patients under 65 (MGTX trial).

Why the others are wrong

  • Ocular myasthenia gravis — ocular MG is limited to ocular symptoms; this patient also has bulbar involvement (nasal voice) and a thymoma, making generalized thymoma-associated MG the correct classification (premature closure).
  • Lambert-Eaton myasthenic syndrome — Lambert-Eaton causes proximal limb weakness that briefly improves with repeated activity (facilitation), not fatigable ocular/bulbar weakness with positive AChR antibody and thymoma (confused-with NMJ disorder).
  • Thyroid eye disease — thyroid orbitopathy produces lid retraction, proptosis, and restrictive diplopia from enlarged extraocular muscles rather than fatigable ptosis that improves with an ice pack, and it explains neither the AChR antibodies nor the anterior mediastinal mass (confused-with ocular diagnosis).

Additional high-yield points

  • Symptomatic therapy: pyridostigmine.
  • Immunosuppression: prednisone plus azathioprine or mycophenolate.
  • Crisis management: intravenous immunoglobulin (IVIG) or plasmapheresis.
  • Refractory disease: eculizumab (REGAIN trial) or rozanolixizumab (anti-neonatal Fc receptor (anti-FcRn)).
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Side-by-side comparison

FeatureGuillain-Barré SyndromeMyasthenia Gravis
At a glanceAcute immune-mediated ascending demyelinating polyneuropathy; can cause respiratory failure.Autoimmune neuromuscular junction disease with fluctuating fatigable weakness.
Classic presentationAcute ascending symmetric weakness + areflexia + antecedent infection + albuminocytologic dissociation in CSF (elevated protein with normal cell count).; Symmetric ascending weakness beginning in the legs, progressing over hours to days (peak by 2-4 weeks); Distal paresthesias and pain (often missed by clinicians but very common — back…Fatigable ptosis and diplopia worse at end of day, improving with rest.; Fluctuating, fatigable weakness — worse with use, better with rest; Symptoms typically worse at the end of the day; Ocular (initial in ~50%, eventually in ~85%): ptosis (often asymmetric), diplopia; Bulbar: dysphagia, dysarthria, nasal speech, jaw fatigue (chewing…
Workup / key labsClinical: progressive symmetric weakness + areflexia, peak by 4 weeks, exclusion of alternative causes. Supportive: CSF albuminocytologic dissociation, electrodiagnostic features of demyelination.; CBC, BMP, LFTs, ESR, CRP, HIV, HBV, HCV; Antiganglioside antibodies (anti-GM1, anti-GD1a, anti-GQ1b — Miller Fisher); Stool for…Clinical features + positive antibodies (AChR or MuSK) and/or supportive electrodiagnostics (decremental RNS, abnormal single-fiber EMG).; Acetylcholine receptor (AChR) antibodies — binding, blocking, modulating (binding most useful); If AChR negative: anti-MuSK antibodies; If both negative: anti-LRP4 antibodies (low yield); TSH, free…
ImagingMRI spine with gadolinium — exclude compressive lesion; nerve root enhancement (especially cauda equina) supports GBS; Nerve conduction studies / EMG: slowed conduction velocities, prolonged distal latencies, conduction block, F-wave abnormalities (early), low amplitudes (axonal variants); Lumbar puncture: albuminocytologic dissociation…CT chest with contrast (or MRI) — evaluate for thymoma in ALL patients with confirmed MG; Repetitive nerve stimulation: decremental response (>10% drop in CMAP amplitude) at low frequency; Single-fiber EMG: most sensitive — increased 'jitter' and blocking; Ice pack test, edrophonium (Tensilon) test (rarely used now due to side effects)
First-line treatmentICU admission for any patient with autonomic instability, bulbar weakness, or rapidly progressive disease; Frequent pulmonary monitoring (FVC, NIF q4-8h); intubate at 20/30/40 thresholds — do not wait for hypoxia; IVIG 0.4 g/kg/day × 5 days (total 2 g/kg) — first-line, equivalent efficacy to PLEX, easier to administer; Plasma exchange…Symptomatic: pyridostigmine (acetylcholinesterase inhibitor) 30-60 mg PO every 4-6 hours, titrate to effect — improves transmission but does not modify disease; Immunosuppression: prednisone (initiate gradually — high doses can transiently worsen weakness; start low and titrate up, or hospitalize for monitoring); Steroid-sparing agents:…

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Full outlines

Guillain-Barré Syndrome Myasthenia Gravis All 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.