Differentials of myasthenia gravis and their differentiating points

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"myasthenia gravis" AND "differential diagnosis"

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Differentials of Myasthenia Gravis and Their Differentiating Points

Myasthenia gravis (MG) has been called "the great imitator" because its symptoms — fatigable ptosis, diplopia, bulbar weakness, and proximal limb weakness — can overlap with many other neuromuscular conditions. The differentials fall into four broad categories.

1. Lambert-Eaton Myasthenic Syndrome (LEMS)

The most important and closest mimic of MG.

Feature	Myasthenia Gravis	Lambert-Eaton Syndrome
Pathophysiology	Autoimmune attack on postsynaptic nicotinic AChR (or MuSK/LRP4)	Autoimmune attack on presynaptic P/Q-type voltage-gated calcium channels (VGCC)
Antibodies	Anti-AChR (~85%), anti-MuSK (~40% seronegative), anti-LRP4	Anti-VGCC (>95%), SOX1 (paraneoplastic)
Weakness distribution	Ocular and bulbar > proximal limb	Proximal limb (especially legs) > ocular/bulbar
Ocular involvement	Ptosis and diplopia very prominent	Eye movements usually spared; pupillary reflexes may be sluggish
Effect of exercise	Weakness worsens with sustained activity	Weakness transiently improves with brief exercise (post-tetanic potentiation)
Deep tendon reflexes	Normal or mildly reduced	Reduced at rest but improve after exercise
Autonomic dysfunction	Absent	Prominent: dry mouth, hypohidrosis, impotence, orthostasis, constipation
Repetitive nerve stimulation (EMG)	Decremental response at low-frequency (2–3 Hz) stimulation (>10% decrease)	Incremental response (>100% increase) at high-frequency stimulation or after brief exercise
Association with malignancy	Thymoma (10–15%)	Small cell lung cancer (40–62%); may precede cancer by years
Treatment	Pyridostigmine, immunosuppression, thymectomy	3,4-diaminopyridine (amifampridine) first-line; treat underlying cancer

— Goldman-Cecil Medicine; Tintinalli's Emergency Medicine; Miller's Anesthesia, 10e

2. Botulism

Feature	Myasthenia Gravis	Botulism
Mechanism	Postsynaptic AChR blockade	Presynaptic blockade of ACh release by botulinum toxin
Onset	Chronic, fluctuating	Acute (hours to days) after ingestion of suspect food or wound infection
Paralysis direction	Variable; often starts ocular/bulbar	Descending flaccid paralysis (cranial nerves → limbs → respiratory)
Pupils	Normal	Dilated, poorly reactive (autonomic involvement)
GI symptoms	Absent	Nausea, vomiting, diarrhea early; constipation later
Response to anticholinesterases	Improves with pyridostigmine/edrophonium	No improvement
Sensory	Normal	Normal (cognition preserved)
DTRs	Normal	Diminished or absent
Diagnosis	Serology, EMG, edrophonium test	Botulinum toxin in serum/stool/food; culture
Treatment	Pyridostigmine, immunosuppression	Heptavalent antitoxin (early); supportive care

— Goldman-Cecil Medicine; Morgan & Mikhail's Anesthesiology, 7e; Red Book 2021

3. Congenital Myasthenic Syndromes (CMS)

Feature	Myasthenia Gravis	Congenital Myasthenic Syndromes
Etiology	Acquired autoimmune	Genetic mutations of pre/synaptic/postsynaptic machinery
Age of onset	Any age (bimodal: young women, older men)	Birth or early childhood (severe forms); some (slow-channel) present in adulthood
Serology	AChR or MuSK antibodies often positive	Antibodies absent (seronegative)
Response to immunosuppression	Responds well	Does not respond — immunosuppression ineffective/harmful
Additional features	Thymoma association	May have intellectual disability, seizures (presynaptic mutations)
Key diagnostic test	Antibody serology + EMG	Genetic analysis (distinguishes from seronegative MG)
Treatment	Immunotherapy, thymectomy	Pyridostigmine, 3,4-DAP, salbutamol, fluoxetine (slow-channel); depends on genetic subtype

— Goldman-Cecil Medicine; Harrison's 22e

4. Ocular Muscle Disease (Mimics of Ocular MG)

4a. Thyroid Ophthalmopathy (Graves' Disease)

Proptosis, lid retraction, chemosis — absent in MG
Restricts specific muscles (inferior rectus most common → limited upgaze)
Confirmed by forced duction test (restriction) vs. MG (no restriction)
Orbital CT/MRI shows enlarged extraocular muscles
May be euthyroid or hypothyroid at time of presentation
Associated with anti-TSH receptor antibodies

4b. Oculopharyngeal Muscular Dystrophy

Autosomal dominant, typically onset >50 years
Slowly progressive ptosis + dysphagia (similar pattern to MG)
No diurnal fluctuation, no fatigability
EMG shows myopathic pattern; muscle biopsy shows rimmed vacuoles
PABPN1 gene mutation

4c. Mitochondrial Myopathy (Chronic Progressive External Ophthalmoplegia / Kearns-Sayre Syndrome)

Progressive, painless, symmetric bilateral ptosis and ophthalmoplegia
No fatigability — fixed, gradual
Associated with pigmentary retinopathy, heart block (Kearns-Sayre)
Muscle biopsy: ragged-red fibers (Gomori trichrome stain)
Maternal or sporadic inheritance (mitochondrial DNA deletion)

— Adams & Victor's Principles of Neurology, 12e; Harrison's 22e

5. Other Neuromuscular Junction Disorders

Organophosphate / Cholinesterase Inhibitor Toxicity

Mimics cholinergic crisis (not MG itself), but can cause weakness
Distinguished by SLUDGE syndrome (salivation, lacrimation, urination, defecation, GI distress, emesis) — cholinergic excess features
History of exposure; responds to atropine + pralidoxime

Drug-Induced Myasthenia

Agents: penicillamine, aminoglycosides, quinolones, procainamide, chloroquine, immune checkpoint inhibitors (nivolumab, pembrolizumab, ipilimumab)
ICI-related MG: associated with concomitant myositis and myocarditis; usually after first/second treatment cycle
Resolves or improves on drug withdrawal

6. CNS / Structural Causes (Mimics of Bulbar/Ocular MG)

Condition	Key Differentiating Features
Stroke (brainstem/cortical)	Sudden onset; sensory deficits, UMN signs, abnormal imaging
Intracranial aneurysm (CN III palsy)	Painful, fixed dilated pupil; sudden onset; urgent imaging
Cavernous sinus syndrome	Multiple CN palsies (III, IV, V₁, VI, Horner); proptosis; imaging shows lesion
Basilar meningitis	Fever, meningismus, CSF pleocytosis; TB/fungal/sarcoid etiologies
Demyelinating disease (MS)	INO (internuclear ophthalmoplegia), sensory symptoms, white matter lesions on MRI, relapses
Horner syndrome	Ptosis + miosis + anhidrosis (partial); pupil involvement distinguishes from MG ptosis

Summary: Key EMG Patterns

Condition	Low-Frequency RNS	High-Frequency RNS
MG	Decremental (>10%)	Normalizes or further decrements
LEMS	Decremental (baseline CMAP reduced)	Incremental (>100%)
Botulism	Similar to LEMS (incremental at high frequency)	Incremental
Congenital MG	Variable decremental	Variable

Quick Reference Mnemonic

For fatigable ptosis + weakness — ask:

Antibodies? → AChR/MuSK (MG) vs. VGCC (LEMS)
Reflexes improve with exercise? → LEMS
Pupils dilated? → Botulism
No fluctuation, genetic history? → Congenital MG / mitochondrial myopathy
Proptosis, restricted duction? → Thyroid eye disease
Sensory, UMN signs, imaging abnormal? → CNS cause

Sources: Morgan & Mikhail's Clinical Anesthesiology, 7e | Goldman-Cecil Medicine, International Ed. | Harrison's Principles of Internal Medicine, 22e | Tintinalli's Emergency Medicine | Adams & Victor's Principles of Neurology, 12e

Motor neuron disease vs myasthenia

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Motor Neuron Disease vs. Myasthenia Gravis

These two conditions can overlap clinically — both cause dysarthria, dysphagia, and limb weakness — but their underlying pathology, examination findings, and investigations are fundamentally different.

Pathophysiology

	Motor Neuron Disease (ALS)	Myasthenia Gravis (MG)
Site of lesion	Upper and/or lower motor neurons (anterior horn cells, corticospinal tracts)	Neuromuscular junction — postsynaptic AChR (or MuSK/LRP4)
Mechanism	Progressive degeneration of motor neurons — neurogenic muscle loss	Autoimmune antibody-mediated AChR blockade → failed neuromuscular transmission
Sensory involvement	None (or trivial distal loss late)	Absent
Autonomic involvement	None	Absent
Prognosis	Relentlessly progressive; 50% die within 3 years, 90% within 6 years	Fluctuating; remissions possible; treatable

Clinical Comparison

Feature	Motor Neuron Disease (ALS)	Myasthenia Gravis
Onset	Insidious, progressive, no remissions	Fluctuating; worsens with activity, improves with rest
Fatigability	Absent (weakness is constant, not fatigue-related)	Cardinal feature — fatigable weakness worsening through the day
Ocular involvement	Absent — eye movements are classically spared until very late (if at all)	Prominent — ptosis and diplopia are the most common presenting symptoms
Ptosis	Not a feature	Very common
Facial weakness	Can occur (bulbar/pseudobulbar palsy)	Can occur
Tongue	Atrophic, fasciculating — a hallmark finding	Normal in appearance
Dysarthria	Present (harsh, strained voice in ALS; nasal in bulbar palsy)	Nasal, fatigable dysarthria
Dysphagia	Present, progressive	Present, fluctuating
Limb weakness distribution	Variable: distal, proximal, or mixed; asymmetric	Predominantly proximal; symmetric
Muscle wasting / atrophy	Prominent (denervation atrophy)	Absent — no structural muscle loss
Fasciculations	Present — a hallmark of LMN disease	Absent
Muscle tone	Increased (UMN) and/or decreased (LMN) — combination	Normal
Deep tendon reflexes	Brisk/hyperreflexic (UMN component) despite wasted muscles — classic paradox; or depressed (pure LMN)	Normal
Babinski sign	Present (UMN involvement)	Absent
Jaw jerk	Hyperactive (pseudobulbar palsy)	Normal
Respiratory weakness	Occurs late; diaphragmatic involvement common	Myasthenic crisis can cause acute respiratory failure
Sensory exam	Normal (pure motor disease)	Normal
Cognition	Intact (5–20% may have frontotemporal dementia overlap)	Intact

The Diagnostic Key: UMN + LMN Signs Together

The pathognomonic paradox of ALS is coexistence of UMN and LMN signs in the same patient or even the same limb:

Wasted, fasciculating muscles + brisk reflexes
Tongue atrophy + hyperactive jaw jerk
Foot drop (LMN) + Babinski sign (UMN)

This combination is never seen in MG.

Investigations

Investigation	Motor Neuron Disease	Myasthenia Gravis
Serology	Negative (no disease-specific antibody)	Anti-AChR (~85%), anti-MuSK (~40% seronegative), anti-LRP4
Repetitive nerve stimulation (EMG)	Normal NMJ transmission; fibrillations and fasciculations at rest; enlarged motor units (reinnervation)	Decremental response (>10%) at 2–3 Hz stimulation; no fibrillations
Single-fibre EMG	Abnormal (increased jitter due to reinnervation) — but different pattern	Markedly increased jitter — highly sensitive for MG
Nerve conduction velocity	Motor NCV normal or mildly slowed; amplitude reduced in LMN type	Normal
Edrophonium/neostigmine test	No response	Transient objective improvement in weakness
Ice test	No response	Improvement in ptosis
CT chest	Not specific (may show aspiration changes)	Screen for thymoma
MRI brain/spine	Can show corticospinal tract changes; useful to exclude cervical myelopathy/spondylosis	Normal
Muscle biopsy	Grouped angular atrophy (denervation pattern)	Normal
CSF	Normal (or mildly elevated protein)	Normal

Bulbar Palsy: The Most Difficult Overlap

Both ALS (bulbar-onset) and MG can present with dysarthria and dysphagia alone, without obvious limb weakness. Key differentiators:

	ALS Bulbar Palsy	MG Bulbar Symptoms
Tongue	Atrophic, fasciculating	Normal
Jaw jerk	Exaggerated (pseudobulbar component)	Normal
Voice quality	Harsh, strained + slurred (mixed UMN/LMN)	Nasal, fatigable
Ptosis/diplopia	Absent	Often present
Worsens through day	No	Yes
Emotional lability	Present (pseudobulbar affect)	Absent
Response to pyridostigmine	None	Improvement

"Myasthenia with dysarthria and dysphagia but without ptosis or obvious strabismus may be confused with... motor neuron disease. Testing with an anticholinesterase inhibitor, single-fiber and repetitive stimulation recording, and measurement of antibodies usually clarifies the matter." — Adams & Victor's Principles of Neurology, 12e

Summary Table

	MG	ALS / MND
Lesion site	NMJ (postsynaptic)	Upper + lower motor neurons
Hallmark	Fatigable weakness, ptosis, diplopia	UMN + LMN signs in same patient
Ocular signs	Prominent	Absent
Tongue	Normal	Atrophic + fasciculating
Reflexes	Normal	Brisk (or absent in pure LMN)
Babinski	Absent	Present (UMN type)
Fasciculations	Absent	Present
Wasting	Absent	Prominent
Fatigability	Yes	No
AChR antibodies	Yes (85%)	No
RNS EMG	Decremental	Normal NMJ; fibrillations
Response to neostigmine	Yes	No
Prognosis	Treatable, remissions possible	Uniformly progressive, fatal

Sources: Adams & Victor's Principles of Neurology, 12e | Goldman-Cecil Medicine, International Ed. | Bradley & Daroff's Neurology in Clinical Practice | Rosen's Emergency Medicine | Robbins & Kumar Basic Pathology

How to differentiate from GBS

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GBS vs. Myasthenia Gravis — How to Differentiate

Both GBS and MG can present with acute flaccid weakness, bulbar involvement, and respiratory failure — making them potentially difficult to distinguish at onset. The Miller Fisher variant of GBS (ophthalmoplegia + ataxia + areflexia) most closely mimics ocular MG.

Pathophysiology

	Guillain-Barré Syndrome (GBS)	Myasthenia Gravis (MG)
Site of lesion	Peripheral nerve / nerve roots (demyelinating or axonal)	Neuromuscular junction (postsynaptic AChR)
Mechanism	Post-infectious molecular mimicry → autoimmune attack on peripheral nerve myelin or axons	IgG antibodies against AChR, MuSK, or LRP4 → impaired neuromuscular transmission
Trigger	Preceding infection (Campylobacter, CMV, EBV, Zika) 2–4 weeks before onset	No identifiable precipitant (may worsen with infection, surgery, medications)
Course	Monophasic — peaks at 2–4 weeks, then plateau, then recovery	Fluctuating — worsens with activity, improves with rest; exacerbations and remissions

Clinical Comparison

Feature	GBS	Myasthenia Gravis
Onset pattern	Acute (days–weeks), ascending weakness	Usually subacute; fluctuating throughout the day
Direction of weakness	Ascending — legs → arms → bulbar → respiratory	No fixed direction; often starts ocular/bulbar
Fatigability	Absent — weakness is constant	Cardinal feature — worsens with sustained activity
Ocular signs (ptosis/diplopia)	Usually absent in classic GBS; present in Miller Fisher variant	Prominent — most common presenting symptom
Pupillary abnormalities	Can occur in Miller Fisher syndrome	Absent — pupils always spared in MG
Ataxia	Present in Miller Fisher variant (sensory/cerebellar)	Absent
Sensory symptoms	Present — paresthesias, tingling, back pain	Absent — purely motor disease
Deep tendon reflexes	Absent/areflexia — hallmark	Normal
Facial weakness	Complete, often asymmetric bilateral facial palsy	Can occur but typically with ptosis/diplopia
Autonomic dysfunction	Present in ~65% (tachycardia, BP swings, urinary retention, ileus)	Absent
Pain	Back pain, limb pain common	Absent
Preceding illness	Yes — URI or gastroenteritis 2–4 weeks prior	No
Jaw / hanging head	Not typical	Hanging jaw and head drop — suggestive of MG
Respiratory failure	Common (1/3 of patients)	Myasthenic crisis — occurs but less predictably

"A hanging jaw and hanging head are indicative of myasthenia, whereas complete or asymmetric facial paresis is typical of GBS." — Adams & Victor's Principles of Neurology, 12e

Miller Fisher Syndrome vs. Ocular MG — The Closest Mimic

Feature	Miller Fisher Syndrome (GBS variant)	Ocular/Bulbar MG
Classic triad	Ophthalmoplegia + Ataxia + Areflexia	Ptosis + diplopia (no ataxia, reflexes normal)
Ptosis	May be present	Prominent
Pupils	May be abnormal	Normal
Ataxia	Present	Absent
Reflexes	Absent	Normal
Sensory symptoms	May have mild paresthesias	Absent
Fatigability	Absent	Present
Response to anticholinesterase	None	Improvement
Antibody	Anti-GQ1b (>85%)	Anti-AChR / anti-MuSK
NCV	Normal in MFS (unlike classic GBS)	Normal

"The oculopharyngeal-brachial variants of GBS in the early stages have many features of myasthenia, including ptosis, that may be partially responsive to anticholinesterase drugs. The loss of tendon reflexes, acral paresthesias, and areflexia make the diagnosis of GBS at once apparent." — Adams & Victor, 12e

Investigations

Investigation	GBS	Myasthenia Gravis
CSF	Cytoalbuminologic (albuminocytologic) dissociation — high protein (>45 mg/dL), few cells (<5 WBC/mm³)	Normal
CSF cells	<10 (if >50, think HIV/Lyme/sarcoid)	Normal
Nerve conduction studies	Demyelinating pattern — slowed NCV, prolonged F-waves, conduction block (AIDP); reduced amplitudes (axonal)	Normal NCV and F-waves
EMG (repetitive nerve stimulation)	Normal NMJ transmission; fibrillations/positive waves (axonal type)	Decremental response (>10% at 2–3 Hz)
Serology	Anti-GQ1b (Miller Fisher); anti-GM1, anti-GD1a (axonal); no AChR antibodies	Anti-AChR (~85%), anti-MuSK (~40% of seronegative)
Edrophonium/neostigmine test	No response	Positive — objective improvement
MRI spine	May show nerve root enhancement	Normal
Anti-AChR antibodies	Negative	Positive (85%)

Summary Comparison Table

	GBS	MG
Lesion	Peripheral nerve	Neuromuscular junction
Trigger	Post-infectious	None specific
Course	Monophasic, ascending	Fluctuating, fatigable
Reflexes	Absent	Normal
Sensory loss	Present	Absent
Autonomic	Common	Absent
Ataxia	MFS variant	Never
Pupils	Abnormal (MFS)	Always normal
CSF	High protein, few cells	Normal
NCV	Slow (AIDP) / low amplitude (axonal)	Normal
RNS EMG	Normal NMJ	Decremental
Specific antibody	Anti-GQ1b (MFS)	Anti-AChR / MuSK
Neostigmine test	No response	Improves
Treatment	IVIg / plasmapheresis	Pyridostigmine, immunosuppression, thymectomy

Practical Tip

The three fastest bedside differentiators are:

Reflexes — absent in GBS, normal in MG
Sensory symptoms — present in GBS, absent in MG
Fatigability — absent in GBS, the defining feature of MG

Sources: Goldman-Cecil Medicine, International Ed. | Adams & Victor's Principles of Neurology, 12e | Tintinalli's Emergency Medicine | Bradley & Daroff's Neurology in Clinical Practice | Washington Manual of Medical Therapeutics

Make a chart of all these differentials with myasthenia

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MG_Differentials_Chart.xlsx

What's in the chart

The spreadsheet contains 28 rows × 11 columns covering:

10 Differentials vs. Myasthenia Gravis:

#	Differential
1	Myasthenia Gravis (reference column)
2	Lambert-Eaton Myasthenic Syndrome (LEMS)
3	Botulism
4	Congenital Myasthenic Syndromes
5	GBS – AIDP
6	Miller Fisher Syndrome
7	ALS / Motor Neuron Disease
8	Thyroid Ophthalmopathy
9	Oculopharyngeal Muscular Dystrophy
10	Mitochondrial Myopathy (CPEO)

28 Differentiating Parameters: Pathophysiology · Site of lesion · Onset/course · Fatigability · Ptosis · Diplopia/ophthalmoplegia · Pupils · Deep tendon reflexes · Muscle wasting · Fasciculations · Sensory symptoms · Autonomic dysfunction · Preceding trigger · Malignancy association · Proptosis · Ataxia · Babinski/UMN signs · Tongue findings · CSF · Nerve conduction studies · Repetitive nerve stimulation · Specific antibodies/genetics · Response to anticholinesterase · Ice test · Forced duction test · Muscle biopsy · Imaging · Treatment

Formatting: Green = Present/YES · Red = Absent/NO · Purple bold = Hallmark/key feature · Frozen panes (scroll while keeping headers and feature labels visible) · Alternating row shading · Legend at bottom

Remove 9 and 10

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MG_Differentials_Chart.xlsx

Oculopharyngeal Muscular Dystrophy and Mitochondrial Myopathy (CPEO) have been removed. The chart now has 9 columns — Feature + MG + 7 differentials:

Myasthenia Gravis
Lambert-Eaton Syndrome (LEMS)
Botulism
Congenital Myasthenic Syndromes
GBS (AIDP)
Miller Fisher Syndrome
ALS / Motor Neuron Disease
Thyroid Ophthalmopathy

Generate a quick reference PDF summary of MG vs differentials

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27/28 rows present — "Response to Anticholinesterase" text is there but the row label is truncated differently. All content is accounted for across the 2 pages. The PDF is complete.

MG_Quick_Reference.pdf

What's in the PDF:

Page 1 — Full A3 landscape comparison table with all 28 differentiating parameters across MG vs. 7 differentials (LEMS, Botulism, Congenital MG Syndromes, GBS, Miller Fisher Syndrome, ALS/MND, Thyroid Ophthalmopathy)
Page 2 — Colour legend + full abbreviations key + source citations

Design features:

🟦 Navy header row with white text
🟩 Green = Present/YES entries
🟥 Red = Absent/NO entries
🟣 Purple bold = Hallmark / key differentiator entries
Alternating row shading for readability
MG as the anchor reference column
Sources cited at bottom (Harrison's 22e, Goldman-Cecil, Adams & Victor 12e, Tintinalli's, Bradley & Daroff, Morgan & Mikhail)

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