GC056 Generalized Muscle Weakness
Generalized muscle weakness is a diffuse reduction in muscle strength affecting multiple muscle groups, resulting from neurological, muscular, metabolic, or systemic disorders that impair the ability to generate normal voluntary force.
Generalized Muscle Weakness: Myasthenia Gravis, Myopathy & Myositis, Neuropathy, Neurophysiology II
This lecture (GC 056) is a cornerstone neuromuscular session that asks you to systematically evaluate a patient presenting with generalized weakness. The "big idea" is anatomical localization: once you know where the lesion is (peripheral nerve vs. neuromuscular junction vs. muscle), the differential diagnosis, investigations, and management flow logically. The lecture also covers the floppy infant syndrome and the principles of clinical neurophysiology (NCS/EMG).
Learning objectives [1]:
- Evaluate a floppy baby clinically.
- Differentiate floppiness with or without weakness to narrow the aetiology.
- Understand the simple classification of neuromuscular diseases (common types).
- Recall the neuroanatomy of peripheral nerves, NMJ, and muscle.
- Understand different entities of neuropathy and principles of neurophysiology tests.
- Learn the classification, pathophysiology, pathogenesis, clinical features, investigations, and differential diagnosis of myasthenia gravis.
- Learn entities of muscle disorders and principles of neurophysiology tests in myopathy.
The critical first step is to determine if the lesion is at the level of the CNS, peripheral nerve, NMJ, or muscle. [1][2]
This is the single most important concept in the lecture. Everything else is a branching tree from this localization.
Why Localization Matters From First Principles
The motor pathway runs: Motor cortex → Corticospinal tract → Anterior horn cell → Peripheral nerve → NMJ → Muscle. A lesion at each level produces a distinct pattern of signs because:
- UMN lesions (cortex/cord) lose inhibitory modulation → hypertonia, hyperreflexia, upgoing plantars
- LMN lesions (anterior horn/peripheral nerve) lose trophic support → wasting, fasciculations, hyporeflexia
- NMJ lesions have intact nerve and muscle architecture but impaired transmission → fatigability without wasting or sensory loss
- Muscle lesions damage the effector → proximal weakness, elevated CK, no sensory involvement
High Yield – The Localization Table
This table is directly from the lecture and senior notes and is the framework examiners use to write question stems. Memorize the discriminating features.
| Feature | CNS Lesion | Peripheral Nerve | NMJ Disease | Muscle Disease |
|---|---|---|---|---|
| Pattern of weakness | Whole limbs; UL extensors, LL flexors | Symmetrical distal | Symmetrical proximal; fatigable | Symmetrical proximal |
| Wasting | Minimal | Early wasting | NO wasting | Variable; pseudohypertrophy possible |
| Tone | ↑ (spastic) | ↓ (flaccid) | Normal | Normal or slightly ↓ |
| Reflexes | Hyperreflexia | Hypo-/areflexia | Preserved (↓ in LEMS) | Variable |
| Sensory loss | Dissociated possible | Non-dissociated, glove-and-stocking | ABSENT | ABSENT |
| Fasciculations | No | Yes | No | Possible (rarely) |
| Gait | Spastic/scissoring | Steppage gait | — | Waddling gait |
| Sphincter | May be involved | Rare | Spared | Spared |
| Key Hx clue | Higher function deficits, bulbar | Pain, autonomic dysfunction | Fluctuating, diurnal variation, ocular/bulbar | Proximal difficulty (stairs, standing), ± myalgia |
| Key Ix | MRI brain/spine ± CSF as indicated | NCS/EMG plus cause-directed bloods ± CSF | AChR/MuSK antibodies, RNS or SFEMG; chest imaging for thymoma in MG | CK, EMG, MRI, genetics ± biopsy |
Exam Trap
Do NOT confuse "no sensory loss" in NMJ/muscle disease with "no sensory symptoms." GBS patients often report sensory symptoms (tingling, pain) but the hallmark is motor > sensory deficit. In pure MG or myopathy, there is truly no sensory involvement at all.
1. Polyneuropathies
Polyneuropathies present with motor problems (weakness, wasting, fasciculations, hyporeflexia, bulbar/respiratory involvement) AND sensory problems (negative = numbness; positive = pain/paraesthesia). [1]
The autonomic-symptom figure adds a third domain that is easy to miss:
| System | Autonomic neuropathy manifestations shown |
|---|---|
| Cardiovascular/thermoregulatory | Tachycardia, exercise intolerance, cardiac denervation with painless myocardial infarction, orthostatic hypotension, heat intolerance, altered skin blood flow |
| Gastrointestinal | Oesophageal dysfunction, diabetic gastroparesis, diarrhoea, constipation, faecal incontinence |
| Genitourinary | Erectile dysfunction, retrograde ejaculation, cystopathy and neurogenic bladder |
Classify every polyneuropathy by four axes [1]:
| Axis | Options | Why it matters |
|---|---|---|
| Time course | Acute / Subacute / Chronic / Relapsing | Acute → think GBS, toxin; Chronic → think DM, hereditary, CIDP |
| Distribution | Generalized / Focal | Generalized = polyneuropathy; Focal = mononeuropathy/mononeuritis multiplex (vasculitis) |
| Fibre type | Motor / Sensory / Autonomic / Mixed | The clinical combination narrows the differential; MND is a motor-neuron disorder rather than a polyneuropathy, and pure small-fibre neuropathy can have normal routine NCS |
| Pathology | Primarily axonal / Demyelinating / Mixed | NCS distinguishes loss of axons (low amplitudes) from myelin dysfunction (slowing, prolonged latencies or block); prognosis depends on cause, severity and treatment, not this label alone |
The lecture lists diverse aetiologies [1]:
| Category | Examples |
|---|---|
| Inflammatory demyelinating | GBS, CIDP |
| Infectious/Granulomatous | Leprosy, sarcoidosis, HIV-related, Lyme disease |
| Systemic disease | Diabetes, CRF, vitamin deficiency, paraproteinaemias, CTD, critical illness polyneuropathy |
| Ischaemic | PVD, vasculitis |
| Metabolic | Various inherited metabolic |
| Hereditary | CMT disease, SMA |
| Toxins | Pharmaceuticals (anti-neoplastic, anti-TB, anti-HIV, colchicine, thalidomide, pyridoxine abuse), environmental toxins, alcohol, solvents, heavy metals |
Electrophysiological studies, look for systemic diseases, CSF, nerve biopsy (seldom indicated), genetic tests. [1]
- NCS differentiates axonal (↓ amplitude) from demyelinating (↓ conduction velocity, prolonged distal latency, conduction block).
- Blood tests: Glucose/HbA1c, CBC, L/RFT, TFT, Vit B12/folate, ESR/CRP, serum protein electrophoresis (SPE) — screen for the treatable systemic causes. [2]
- CSF: albuminocytological dissociation supports GBS, but protein can be normal early and a substantial pleocytosis should prompt an alternative diagnosis.
- Nerve biopsy: reserved for vasculitis, amyloidosis, granulomatous disease where diagnosis cannot be made otherwise.
1.4 Clinically Important Polyneuropathies (Lecture Deep Dives)
High Yield – GBS Definition
"Rapidly progressive flaccid paralysis, areflexia and raised CSF protein without increase in cells (albuminocytological dissociation)." [1]
Epidemiology & Pathogenesis [1]:
- Incidence: 1–2 per 100,000 per year
- Immune-mediated, inflammatory polyneuropathy
- Antecedent viral/bacterial infection (URTI, GE) in 2/3 of cases — molecular mimicry: antibodies against microbial antigens (e.g., Campylobacter jejuni gangliosides) cross-react with peripheral nerve components
- Onset: subacute over a few days
- Nadir: within 4 weeks (this distinguishes GBS from CIDP, which continues beyond 8 weeks)
Clinical Features [1]:
- Ascending weakness with facial involvement
- Respiratory failure may occur — this is why serial respiratory and bulbar assessment is critical
- Hypo- or areflexia — because demyelination blocks the reflex arc
- Sensory symptoms (tingling, pain) but motor predominates
- Muscle ache
- Autonomic dysfunction in > 2/3 (labile BP, arrhythmias, urinary retention)
Investigations [1]:
- Raised CSF protein with a normal or only mildly increased cell count (albuminocytological dissociation); this may be absent in the first week
- Electrophysiology supports and subtypes the neuropathy: demyelinating features include prolonged distal/F-wave latencies, slowing and conduction block, but axonal GBS variants also occur [12]
Management [1]:
- General supportive measures:
- Monitor work of breathing, cough, bulbar function, serial FVC and inspiratory strength, BP and ECG. Escalate early for a falling trajectory or clinical fatigue; do not wait for hypoxaemia or an abnormal ABG, and do not use one FVC cut-off in isolation.
- Mechanical ventilation for respiratory failure
- General care for immobility (DVT prophylaxis, pressure care, PT)
- Nutritional support (NG tube if dysphagia)
- Treat complications (autonomic instability, infections)
- Specific immunotherapy: Plasma exchange (PLEX) or IV immunoglobulin (IVIg)
- The 2023 EAN/PNS guideline recommends IVIg 0.4 g/kg/day for 5 days when a patient unable to walk unaided presents within 2 weeks (also considered at 2–4 weeks), or 4–5 plasma exchanges over 1–2 weeks when within 4 weeks [12]
- Do not routinely follow PLEX immediately with IVIg or give a second IVIg course solely because prognosis appears poor [12]
- Steroids are NOT effective in GBS (a classic exam trap!)
Prognosis: The percentages on the slide are historical cohort estimates. Recovery and residual disability vary widely by age, subtype, severity, complications and access to specialist respiratory/autonomic care; use validated tools such as mEGRIS (ventilation risk) and mEGOS (functional outcome) rather than quoting a fixed outcome to an individual [12].
Exam Trap – GBS vs CIDP
GBS nadir is within 4 weeks and then plateaus/improves. CIDP (Chronic Inflammatory Demyelinating Polyneuropathy) progresses beyond 8 weeks. CIDP responds to steroids; GBS does not.
Charcot-Marie-Tooth (CMT) disease [1]:
- Hereditary condition(s) — the most common inherited neuropathy
- Distal wasting, foot drop, pes cavus
- Slowly progressive
Historical phenotype classification shown in the lecture [1]:
The HMSN I–VII labels below are useful for recognizing older exam material, but current practice classifies inherited neuropathies by phenotype, electrophysiology, inheritance and molecular diagnosis rather than forcing every case into this table.
| Type | Pathology | Inheritance | Notes |
|---|---|---|---|
| HMSN-I | Demyelinating PN | AD, AR, X-linked | Most common; PMP22 duplication |
| HMSN-II | Axonal PN | AD, AR, X-linked | Later onset, less severe |
| HMSN-III | DSD (Dejerine-Sottas) | Severe infantile | |
| HMSN-IV | Refsum disease | Phytanic acid accumulation | |
| HMSN-V | Axonal + pyramidal signs | ||
| HMSN-VI | With optic atrophy | ||
| HMSN-VII | With retinitis pigmentosa |
Genes: PMP22, MPZ, MFN2, GJB1 etc. [1]
Hereditary motor neuropathies include Spinal Muscular Atrophy (SMA) [1]:
- Cause: most 5q SMA is due to biallelic pathogenic variants in SMN1; a higher SMN2 copy number often modifies severity
- Treatment correction: nusinersen (Spinraza) does not replace or target SMN1. It is an antisense oligonucleotide that modifies SMN2 splicing to increase full-length SMN protein; other disease-modifying options include risdiplam and onasemnogene abeparvovec, subject to local eligibility [18]
- Other genes: UBA1, DYNC1H1, VAPB
The image-heavy SMA slide classifies severity by age at onset, highest motor milestone and SMN2 copy number:
| SMA type | Onset | Highest milestone / historical supportive-care natural history | Typical SMN2 copies |
|---|---|---|---|
| 0 / 1A | Prenatal | No milestones; contractures/cardiopathy; death within weeks | 1 |
| 1B | < 3 months | Poor/absent head control; feeding and respiratory problems; death by the second or third year | 2 |
| 1C | > 3 months | Head control achieved; feeding/respiratory problems, often plateauing in the first 2 years | 3 |
| 2 | > 6 months | Sits unaided; scoliosis; survival into adolescence/adulthood | 3 |
| 3a | 18–36 months | Walks unaided, but may lose ambulation early; normal lifespan | 3 |
| 3b | > 3 years | Walks unaided; later loss of ambulation; normal lifespan | 3–4 |
| 4 | Second/third decade | Remains ambulant until late life; normal lifespan | 4 |
The percentages on the slide refer to the distribution of SMA phenotypes, not SMA's share of all hereditary neuropathies: type 1 ≈ 60%, type 2 ≈ 20–30%, and type 3 ≈ 10–20%. The milestone and survival entries describe the untreated historical natural history: newborn screening and early disease-modifying therapy now blur type boundaries and can substantially alter outcomes [18].
"Progressive degeneration of α motor neuron" affecting anterior horn cells AND corticospinal tract. [1]
Key Features [1]:
- ALS / PLS / PMA / PBP / Pseudobulbar palsy — different patterns based on UMN vs LMN predominance
- Sensory involvement: ABSENT — this is the hallmark distinguishing MND from neuropathy
- Incidence: 1 per 100,000/year
- Peak 6th decade, M:F = 3:2
- Familial ~10% (SOD1, C9orf72, TARDBP, FUS)
The name "Amyotrophic Lateral Sclerosis" encodes the pathology [1]:
- "Amyotrophic" = muscle wasting (LMN loss)
- "Lateral Sclerosis" = hardening of lateral columns of spinal cord (UMN corticospinal tract degeneration)
Combined UMN + LMN signs WITHOUT sensory loss = MND until proven otherwise.
Management — current correction to the historical slide [15][16]:
- ALS remains progressive and life-limiting, but “no specific treatment” and “die within 3 years” are not safe counselling statements: survival is highly variable, and phenotype, respiratory involvement and supportive interventions matter.
- Refer early to a multidisciplinary MND team. Reassess communication, swallowing/nutrition, mobility, secretion control, cough and respiratory symptoms; discuss gastrostomy before advanced nutritional or respiratory compromise and offer non-invasive ventilation when indicated [15].
- Riluzole is a disease-modifying option for ALS. Edaravone availability and eligibility vary. Tofersen has accelerated FDA approval for adults with a confirmed pathogenic SOD1 variant; this is genotype-specific and jurisdiction-dependent [16].
- Advance-care planning should be individualized, revisited over time, and based on the patient's informed preferences, capacity and local law. The slide's binary “euthanasia vs long-term ventilation” framing is obsolete.
Alcohol, solvents, heavy metals, pharmaceuticals (anti-neoplastic agents, anti-microbials for TB and HIV, colchicine, thalidomide, pyridoxine abuse). [1]
Why pyridoxine (vitamin B6)? In excess, it causes a sensory neuropathy through direct neurotoxicity to dorsal root ganglia neurons. This is paradoxical because B6 deficiency also causes neuropathy.
2. Myopathies
Proximal weakness, waddling gait, bulbar/respiratory involvement, tenderness. Sensory and sphincter are SPARED. [1]
Many generalized myopathies preferentially weaken the pelvic and shoulder girdles, but distribution is diagnostically important: IBM affects quadriceps and finger flexors, distal myopathies may begin distally, and ocular weakness suggests a restricted set of muscle/NMJ disorders. Do not infer localization from “proximal” alone.
Waddling gait: Hip abductor weakness → Trendelenburg sign → pelvis drops on the unsupported side during gait → patient compensates by shifting trunk toward the weight-bearing side [4].
Myopathies are classified as inherited or acquired. [1]
| Inherited | Acquired |
|---|---|
| Muscular dystrophies (Dystrophinopathies, FSHD, LGMD) | Inflammatory myopathies (PM, DM, IBM, Necrotising autoimmune myopathy) |
| Metabolic (Mitochondropathies, Glycogenoses, Lipid myopathies) | Complicating systemic diseases (CTD, Endocrinopathies, Sarcoidosis, Critical illness myopathy) |
| Congenital myopathies | Toxic myopathies (statins, steroids) |
| Channelopathies (Myotonic disorders, Periodic paralysis, Malignant hyperthermia) | Rhabdomyolysis |
| Infection-related |
Electrophysiological studies, muscle enzymes (e.g., CK), look for systemic diseases, muscle biopsy, genetic tests. [1]
- CK: a useful screening test, interpreted against the laboratory range, recent exercise/trauma, drugs and phenotype. A normal CK does not exclude myopathy (notably IBM, some congenital/metabolic myopathies or steroid myopathy), while very high values occur in dystrophinopathy, immune-mediated necrotising myopathy and rhabdomyolysis.
- EMG: Shows polyphasic, low-amplitude, short-duration motor unit potentials (MUAPs) — because fewer muscle fibres per motor unit are functional
- Also: increased insertional activity, fibrillation potentials, complex repetitive discharges (CRD) [1]
- Muscle MRI (fat suppression/STIR): Signal changes from inflammation, edema, or muscle replacement by connective tissue. Guides biopsy site and monitors treatment response. [1]
- Muscle biopsy: useful when histology will distinguish inflammatory, necrotising, metabolic or structural disease; MRI can select an involved but not end-stage muscle. Genetic testing is often first-line when an inherited disorder is suspected, so biopsy is not universally “definitive.”
3. Muscular Dystrophies (In Detail)
Old definition: (1) Inherited, (2) All symptoms due to muscle weakness, (3) Progressive, (4) No histopathological abnormalities other than degeneration and regeneration. [1]
DMD, BMD, FSHD, LGMD, Myotonic dystrophy. [1]
Phenotypic → Immunohistochemical → Genetic [1]
This sequence reflects the lecture's historical diagnostic progression. Contemporary assessment starts with phenotype and molecular testing; immunohistochemistry or biopsy then answers a focused unresolved question. The older “LGMD1/LGMD2” labels on the slide have also been superseded by gene/protein-based nomenclature.
| Level | Examples |
|---|---|
| Immunohistochemical | Dystrophinopathy, Sarcoglycanopathy, Laminin, Caveolin, Calpain, Dysferlin |
| Genetic inheritance | X-linked (DMD, BMD); AD (LGMD1, FSHD); AR (LGMD2) |
High Yield – DMD vs BMD
Both are X-linked dystrophinopathies caused by mutations in the dystrophin gene (Xp21.2). The critical difference is the reading frame rule:
- DMD: frameshift/nonsense mutation → no or very little dystrophin → severe
- BMD: in-frame deletion → partially functional dystrophin → milder
| Feature | DMD | BMD |
|---|---|---|
| Dystrophin | No or few dystrophin-positive fibres | Partial deficiency |
| Motor milestones | Mild delay | Near-normal |
| Onset of weakness | 3–5 years | Later |
| Loss of ambulation on the historical slide | By 9–12 years | Ambulation beyond 15 years |
| Respiratory/cardiac disease | Earlier | Usually later, but cardiomyopathy may be disproportionate to skeletal weakness |
| Associated features | Cardiomyopathy, mild mental impairment | Cardiomyopathy |
| Key signs | Gower's sign, pseudohypertrophy of calves, tip-toe gait | Similar but milder |
Historical natural-history warning
The ages for loss of ambulation, respiratory failure and death in this 2022 slide set describe older cohorts and must not be used as a current prognosis. Corticosteroids, mutation-specific therapies where eligible, proactive cardiac/respiratory care, rehabilitation and orthopaedic management have altered trajectories; care is multidisciplinary across the lifespan [17].
4. Inflammatory Myopathies (Autoimmune Myopathies)
Inflammatory infiltration (or necrosis) of skeletal muscle, limb-girdle pattern of weakness, occasionally myalgia, can be associated with extra-muscular manifestations. Annual incidence: 5/100,000. [1]
1. Polymyositis, 2. Dermatomyositis, 3. Overlap myositis, 4. Inclusion body myositis, 5. Necrotising autoimmune myopathy. [1]
Slides 28–29 trace the historical classification systems (Bohan & Peter 1975; Dalakas 1991; ENMC 2003; Troyanov 2005) to the 2017 EULAR/ACR criteria. These criteria are for classification, not a substitute for clinical diagnosis. Modern clinicoserological practice separates dermatomyositis, antisynthetase/overlap myositis, immune-mediated necrotising myopathy (IMNM) and IBM; “true” polymyositis is uncommon and remains a diagnosis of exclusion.
The visual antibody map links phenotype to mechanism:
| Antibody group | Main phenotype highlighted |
|---|---|
| Anti-tRNA synthetase (especially anti-Jo-1; also PL-7, PL-12, EJ, OJ, KS, Zo, Ha) | Antisynthetase syndrome: myositis, ILD, arthritis, fever, Raynaud phenomenon and mechanic's hands |
| Anti-SRP | Often severe, rapidly evolving immune-mediated necrotising myopathy; myocardial involvement may occur |
| Anti-HMGCR | Immune-mediated necrotising myopathy, often after statin exposure |
| Anti-Mi-2 / MDA5 / TIF1γ / NXP2 / SAE | Dermatomyositis phenotypes detailed below |
| Anti-cN1A | Associated with sporadic inclusion-body myositis, but sensitivity and specificity are insufficient for it to diagnose IBM alone |
The adjacent spectrum diagram places sIBM at an inflammation-dominant end, dermatomyositis/antisynthetase disease in a mixed zone, and anti-SRP/anti-HMGCR IMNM at a necrosis-dominant end. Its second visual gradient also cautions that antibody pathogenicity is not equal: anti-cN1A is mainly an association marker, whereas anti-SRP and anti-HMGCR have stronger mechanistic links. This is a teaching continuum, not a claim that the diagnoses are interchangeable.
High Yield – Dermatomyositis
F > M. Subacute or insidiously progressive proximal weakness, sometimes with myalgia. Skin manifestations: heliotrope rash, malar rash, V & shawl rash, Gottron papules, mechanic's hands, periungual erythema, gingival and oral involvement. ILD, polyarthritis. Calcinosis and GI involvement in juvenile cases. [1]
The slide quotes a wide historical malignancy range (6–45%) and lists breast, lung, pancreatic, colorectal, cervical and nasopharyngeal cancers. The clinically useful point is a temporal cluster around myositis onset, not a universal percentage.
Pathology: Complement-mediated microangiopathy → perimysial and perivascular inflammation (macrophages, B-cells, CD4+ plasmacytoid dendritic cells), perifascicular atrophy, capillary depletion. [1]
Myositis-Specific Antibodies in DM [1]:
| Antibody | Association |
|---|---|
| Anti-TIF-1γ | Neoplasia, dysphagia |
| Anti-NXP-2 | Neoplasia in adults, calcinosis in children |
| Anti-Mi2 | Rapid onset, but LOW risk neoplasia/ILD, good response to immunotherapy |
| Anti-MDA-5 | Amyopathic DM, polyarthritis, SEVERE ILD |
| Anti-SAE | Amyopathic DM, in Asians associated with ILD |
These are risk associations, not deterministic diagnoses. Current IMACS guidance stratifies adult-onset IIM cancer risk by subtype, antibody and clinical features, then selects a basic or enhanced screening panel. Continue ordinary age/sex-appropriate population screening; consider endoscopy, nasoendoscopy or FDG-PET/CT only in selected risk groups rather than ordering PET for every patient [14].
Exam Trap – Anti-MDA-5
Anti-MDA-5 is classically associated with amyopathic DM (minimal/no muscle involvement) but rapidly progressive ILD which can be fatal. Don't assume "no weakness = benign."
"Symmetric proximal myositis and the absence of histopathological signs of other myopathies or typical rash of DM." Endomysial mononuclear inflammatory infiltrates. Actually a heterogeneous group; many PM cases are in fact sIBM or OM. True idiopathic PM is rare (or even a controversial entity) — "diagnosis of exclusion." [1]
Commonest autoimmune myopathy in ≥ age 50, M > F. Affects quadriceps, finger flexors, dorsiflexors → axial muscles. Slowly progressive, life expectancy not significantly affected. CK normal or up to 10-fold above ULN. Histology: inflammatory infiltrates (CD8+ T cells, macrophages) invading non-necrotic muscle fibres, rimmed vacuoles, congophilic inclusions (amyloidogenic proteins). Anti-cN1A may be present but is not diagnostic. Routine immunotherapy is generally not beneficial. [1]
The distribution in the slide explains the bedside clues: knee buckling/falls from quadriceps weakness, weak deep finger flexion/grip, and frequent dysphagia. Management is supportive and multidisciplinary (falls prevention, physiotherapy/occupational therapy, swallowing and nutritional assessment) rather than repeated empiric immunosuppression.
High Yield – IBM vs PM
IBM is the most common inflammatory myopathy in the elderly. Unlike PM, it has asymmetric and distal involvement (quadriceps, finger flexors), does NOT respond to steroids, and has rimmed vacuoles on biopsy. If a patient labeled as "PM" doesn't respond to treatment, reconsider IBM.
Myositis with overlap features (other than rash) and/or "overlap autoantibodies." With other CTDs: Anti-U1RNP (SLE/MCTD-OM), Anti-PM/Scl, Anti-Ku, Anti-U3RNP (SSc-OM). [1]
Anti-synthetase syndrome: Acute myopathy, fever, ILD, polyarthritis, Raynaud's, mechanic's hand. Anti-Jo1 (anti-tRNA-synthetase). Also Anti-PL-7, Anti-PL-12 (amyopathic variant), Anti-OJ, Anti-KS, Anti-EJ, Anti-Zo, Anti-Ha-YRS. [1]
- Myositis-specific/associated antibodies: support phenotype and risk assessment, but assay performance varies and results must fit the clinical picture.
- CK and other muscle enzymes: quantify muscle injury but may be normal in amyopathic DM or only modestly elevated in IBM.
- EMG: increased insertional activity, fibrillation potentials, complex repetitive discharges and short-duration, low-amplitude polyphasic MUAPs may support an active myopathy, but are not subtype-specific [1].
- Muscle MRI (fat-suppressed T2/STIR): maps oedema/inflammation versus chronic fatty replacement, selects a biopsy target and can follow distribution over time [1].
- Biopsy: integrate the pattern—perifascicular/perivascular change in DM, fibre necrosis with sparse inflammation in IMNM, or endomysial invasion plus rimmed vacuoles in IBM—with serology and phenotype.
- Cancer screening: risk-stratified basic/enhanced screening per IMACS; PET/CT is not a routine stand-alone test for every IIM patient [14].
5. Neuromuscular Junction (NMJ) Disorders
NMJ disorders present with: Fluctuating weakness, fatigability, extra-ocular muscle manifestation, bulbar/respiratory involvement. Sensory and sphincter are SPARED. [1]
The NMJ is either pre-synaptic or post-synaptic:
| Location | Disease | Mechanism |
|---|---|---|
| Post-synaptic | Myasthenia Gravis | Antibodies to AChR, MuSK or less commonly LRP4; some patients are seronegative on standard assays |
| Pre-synaptic | LEMS | Antibody to P/Q-type voltage-gated Ca²⁺ channels → impaired ACh release |
| Pre-synaptic | Botulism | Clostridium botulinum neurotoxin cleaves SNARE proteins → blocks presynaptic ACh vesicle release; it is not simply “cytotoxic” |
High Yield – MG
The slide quotes a prevalence of 5 per 100,000. Autoimmune MG causes fluctuating fatigable weakness with ocular ± generalized involvement and is associated with thymic pathology and other autoimmune disease. [1]
Why "fatigability"? Anti-AChR antibodies reduce the number of functional AChR at the post-synaptic membrane. With each successive nerve impulse, fewer ACh molecules find available receptors (normal NMJ has a "safety factor" with excess receptors; this is lost in MG). So strength is initially near-normal but declines with repeated use.
Pathophysiology [4]:
- Anti-AChR Ab (90% of generalized, 50% of ocular MG): T-cell dependent; causes receptor blockade + complement-mediated destruction + accelerated receptor internalization
- Anti-MuSK Ab: more bulbar/respiratory involvement, higher risk of crisis, poorer response to anticholinesterase
- Anti-LRP4 Ab: more ocular involvement
- Thymic pathology: thymic hyperplasia or thymoma
- Autoimmune diseases (1/3): Graves', Hashimoto's, T1DM, RA, SLE
Types of MG [4]:
- Ocular MG (15%): weakness limited to eyelids and EOMs; 50% seropositive; 50% risk of generalizing within 2 years
- Generalized MG (85%): ocular + bulbar + limb + respiratory; 90% seropositive
- Early: diurnal variation (worse evening), symptom-free periods
- Late: persistent weakness
- Ocular (> 50%): ptosis (uni/bilateral), diplopia, pupil ALWAYS spared
- Bulbar (15%): dysarthria (nasal voice), dysphagia, jaw weakness
- Facial: expressionless, transverse smile ("myasthenic sneer")
- Neck: dropped head syndrome
- Limb: proximal, UL > LL
- Respiratory: SOBOE, orthopnea → respiratory failure (MG crisis)
Historical Osserman Classification [4]:
This table may appear in older notes, but contemporary clinical communication generally uses MGFA clinical classes and treatment status rather than Osserman labels.
| Type | Description |
|---|---|
| 1 | Ocular MG only |
| 2A | Mild generalized (limb) |
| 2B | Moderate generalized (bulbar) |
| 3 | Acute fulminating (respiratory, < 6 months) |
| 4 | Late severe ( > 2 years, high mortality) |
- Anti-AChR Ab → confirmatory; if negative, check anti-MuSK/anti-LRP4
- Electrophysiology: Repetitive Nerve Stimulation (RNS) → ≥ 10% decremental response in CMAP amplitude at 3 Hz
- Single-fibre EMG is highly sensitive but not specific; test a clinically involved muscle and interpret with the phenotype
- Edrophonium (Tensilon) is a historical test, not routine first-line diagnosis because of availability, false positives and potentially serious cholinergic/cardiac adverse effects. The slide should not be read as an instruction to perform it.
- Ice-pack test may support ocular MG when objectively measured ptosis improves, but does not replace serology/electrophysiology.
- CT or MRI chest: assess for thymoma
- TFT ± other autoantibodies
- Impending crisis: rapidly worsening bulbar or respiratory weakness is an emergency. Obtain specialist/critical-care input early, protect the airway as clinically indicated, assess swallowing before oral intake when bulbar weakness is present, and trend respiratory function rather than relying on a single FVC threshold.
- Anti-cholinesterase: Pyridostigmine — inhibits acetylcholinesterase → more ACh available at NMJ
- Thymectomy — required for thymoma where feasible; also discuss early in appropriate AChR-antibody-positive generalized non-thymomatous MG [19]
- Immunotherapy — corticosteroids and steroid-sparing therapy individualized by antibody, severity, comorbidity and reproductive considerations; IVIg or PLEX for crisis/rapid deterioration. Rituximab and targeted complement/FcRn therapies have specialist roles in selected generalized or refractory disease [13][19].
Medication safety in MG: Do not turn a long slide-derived list into blanket contraindications. Check an up-to-date interaction resource and balance indication against risk. Drugs with important reported exacerbations include magnesium, aminoglycosides, fluoroquinolones, macrolides, neuromuscular blockers, β-blockers, procainamide, quinine and penicillamine; monitor closely and seek specialist advice when alternatives are limited. Any new dysphagia, dyspnoea or rapidly worsening weakness after a medicine is an urgent review trigger.
Lambert-Eaton Myasthenic Syndrome (LEMS): Ab to voltage-gated Ca²⁺ channel, associated with SCLC. [1]
| Feature | MG | LEMS | Botulism |
|---|---|---|---|
| Fatigability | Worsens with exercise | Improves with brief exercise | Descending cranial/bulbar weakness; clinical facilitation is not a defining feature |
| Muscles | Generalized/ocular | LL >> UL; ocular involvement is less prominent | Eyes/bulbar > limbs |
| Autonomic | Absent | Prominent: dry mouth, constipation, erectile dysfunction | Prominent: dry mouth, ileus/constipation, fixed or poorly reactive pupils |
| Reflexes | Usually preserved | Reduced at rest, may augment after brief exercise | Reduced/variable |
| Association | Thymoma/autoimmunity | SCLC (also autoimmune non-tumour LEMS) | Food-borne, wound, infant intestinal colonisation or iatrogenic exposure |
| Antibody | Anti-AChR | Anti-VGCC | — (toxin) |
Why does LEMS improve with exercise? The antibodies impair Ca²⁺ influx needed for ACh vesicle release. With repeated stimulation, residual Ca²⁺ accumulates in the presynaptic terminal, eventually reaching threshold for vesicle release — hence post-exercise facilitation (and incremental response on RNS at high frequency).
Botulism produces an acute, often descending cranial/bulbar and autonomic syndrome. The toxin blocks ACh release by proteolysing SNARE proteins; it does not destroy the nerve terminal by nonspecific cytotoxicity. Suspected botulism requires urgent public-health/toxin-expert contact and antitoxin consideration—do not wait for confirmatory testing if the syndrome is convincing [20].
6. Floppy Infant Syndrome
High Yield – Central vs Peripheral Hypotonia
The key clinical distinction in a floppy infant is whether the hypotonia is predominantly CENTRAL (alertness, development, seizures, dysmorphism and preserved/brisk reflexes may point centrally) or PERIPHERAL (objective weakness, poor antigravity movement, weak cry/suck, muscle atrophy and reduced/absent reflexes). [1][6]
- Chromosomal: Trisomy, Prader-Willi, partial deletion/duplication
- Cerebral malformation
- Perinatal distress (HIE)
- Postnatal cerebral injury
- Single-gene/metabolic disorders on the slide: Zellweger spectrum disorder, neonatal adrenoleukodystrophy and oculocerebrorenal syndrome. Correction: acid maltase deficiency (Pompe disease) is a metabolic myopathy and belongs in the peripheral/neuromuscular branch, although cardiomyopathy or multisystem findings may coexist.
- Spinal cord: Hypoxic-ischaemic myelopathy, trauma, congenital malformation
- Motor neuronopathies: SMA, congenital cervical SMA, infantile neuronal degeneration and neurogenic arthrogryposis. “Vaccine-associated poliomyelitis” is historical and applies to rare oral live-poliovirus vaccine complications, not inactivated polio vaccine.
- Polyneuropathies: Congenital hypomyelinating polyneuropathy; “Dejerine-Sottas syndrome” is an older phenotypic label for severe early-onset demyelinating hereditary neuropathy.
- NMJ disorders: Congenital defect of NMJ, neonatal MG (transplacental maternal Ab), infantile botulism
- Myopathies: Central core disease, congenital fibre-type disproportion, X-linked myotubular myopathy and metabolic myopathy such as infantile-onset Pompe disease
| Central Hypotonia | Peripheral Hypotonia |
|---|---|
| MRI brain when central features are present | NCS/EMG when peripheral features are present |
| TFT | CK level |
| Metabolic screen | EMG |
| Early genomic testing guided by phenotype/dysmorphism | Early targeted/rapid genomic testing (for example SMN1 when SMA is suspected) |
| Infection/metabolic tests only when history/examination indicate | Muscle biopsy only when genetics, biochemistry and imaging do not answer the focused question |
In either branch, first stabilize airway/breathing and feeding, document antigravity movement and reflexes, and look for time-critical treatable disease (sepsis/metabolic derangement, infantile botulism, SMA, Pompe disease). A floppy infant often has mixed clues, so this is a localization framework rather than a binary rule.
7. Clinical Neurophysiology
Three main modalities: (1) Nerve Conduction Studies (NCS), (2) Needle Electromyography (EMG), (3) Special techniques for NMJ conduction. [1]
Electrodiagnostic (EDx) studies provide objective, quantitative and reproducible assessment of PNS function. NCS uses surface stimulation/recording and is non-invasive apart from discomfort; needle EMG is minimally invasive. [1]
- Exclude/confirm peripheral neuropathy, myopathy
- Distinguish neuropathy from myopathy
- Define aetiology of neuropathy (demyelinating vs axonal)
- Define aetiology of myopathy
- Suspected cervical myelopathy: EDx does not visualize or confirm spinal-cord compression; it helps identify coexisting radiculopathy, entrapment neuropathy, motor-neuron disease or other mimics. MRI is the key structural test when cord disease is suspected.
- Diagnose focal/multifocal neuropathies
- Assess severity and monitor progress
- Monitor treatment response
- Prognosis after nerve trauma + guide intervention
- EMG-guided botulinum toxin therapy
| Test | What it measures | Key findings in neuropathy | Key findings in myopathy |
|---|---|---|---|
| NCS (Motor + Sensory) | Large fibre conduction: amplitude, velocity, latency | Axonal: ↓ amplitude, normal velocity; Demyelinating: ↓ velocity, prolonged latency, conduction block | Usually normal (the nerve is fine) |
| EMG | Bioelectric activity of muscles at rest and during contraction | Denervation potentials (fibrillation, positive sharp waves); large, polyphasic MUAPs (reinnervation) | Increased insertional activity, fibrillation, CRD, small spiky polyphasic MUAPs |
| RNS | NMJ transmission | Normal | Normal in primary myopathy; MG: significant low-frequency decrement; LEMS: marked post-exercise or high-frequency increment |
8. Integration with Related Conditions
The lecture lists channelopathies under inherited myopathies. Key exam-relevant points from supporting notes [7][8]:
- Primary hypoK PP: AD inheritance (CACNA1S), triggered by CHO load, exercise (after delay), β2-agonists
- Thyrotoxic PP: Disproportionately affects men and people of East or Southeast Asian ancestry despite hyperthyroidism being more common in women. It causes hypokalaemic motor weakness, usually without sensory loss; replace potassium cautiously because total-body potassium is not depleted and rebound hyperkalaemia can occur, then treat the thyrotoxicosis.
- HyperK PP: AD (SCN4A), triggered by exercise, fasting, cold
- MG
- Botulism
- GBS
- Transverse myelitis
- Secondary hypokalemic PP (thyrotoxic, hyperaldosteronism, RTA type I/II)
High Yield Summary
- Localize first: CNS vs anterior horn cell/peripheral nerve vs NMJ vs muscle.
- Polyneuropathy: classify time course, distribution, fibre modalities and axonal/demyelinating physiology.
- GBS: acute progressive weakness with reduced reflexes; CSF and NCS support but may be nondiagnostic early. Monitor bulbar, autonomic and respiratory trajectory; use IVIg or PLEX when indicated, not corticosteroids.
- MG: fluctuating fatigable ocular/bulbar/limb weakness without sensory loss. Diagnose with AChR/MuSK antibodies and RNS/SFEMG; image the chest for thymoma. Edrophonium is historical, not a routine first-line test.
- LEMS/botulism: both are presynaptic. LEMS has autonomic symptoms, reduced reflexes and facilitation and may signal SCLC; botulism causes cranial/autonomic then descending weakness and is an antitoxin/public-health emergency.
- Dystrophinopathy/SMA: the slide's milestone and survival tables describe historical untreated cohorts; genetics and modern multidisciplinary/disease-modifying care have changed prognosis.
- Inflammatory myopathy: use phenotype, antibodies, MRI/EMG and selective biopsy together. DM cancer screening is risk-stratified; IBM causes quadriceps/finger-flexor weakness, falls and dysphagia and usually does not respond to immunotherapy.
- Floppy infant: central and peripheral clues guide the work-up, but stabilize breathing/feeding first and pursue time-critical treatable causes.
- NCS/EMG: NCS characterizes large-fibre nerve physiology; needle EMG samples muscle and motor-unit physiology; RNS/SFEMG assess NMJ transmission.
- MND: combined UMN and LMN signs with preserved sensation are characteristic. Prognosis is variable; management includes disease-modifying treatment where eligible plus proactive respiratory, nutritional, communication and palliative care.
Active Recall - Generalized Muscle Weakness
[1] Lecture slides: GC 056. Generalized muscle weakness.pdf (all pages) [2] Senior notes: Ryan Ho Neurology.pdf (p. 178 – Approach to Generalized Weakness) [3] Senior notes: Ryan Ho Neurology.pdf (p. 67 – Motor Weakness, Types of motor deficit table) [4] Senior notes: Maksim Medicine Notes.pdf (p. 272–276 – NMJ disorders, Myopathy) [5] Senior notes: Ryan Ho Neurology.pdf (p. 192–194 – Muscular Dystrophies, Channelopathies) [6] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 132–134 – Approach to Floppy Infant, Generalized Weakness) [7] Senior notes: Ryan Ho Neurology.pdf (p. 193–194 – Ion Channelopathies, Periodic Paralysis) [8] Senior notes: Block A - I am losing weight and sweating all the time.pdf (p. 34 – Thyrotoxic periodic paralysis) [9] Senior notes: Ryan Ho Urogenital.pdf (p. 25 – Hypokalemia clinical features) [10] Senior notes: Ryan Ho Urogenital.pdf (p. 28 – Hyperkalemia clinical features) [11] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p. 1655 – DDx of generalized weakness) [12] Van Doorn PA, et al. European Academy of Neurology/Peripheral Nerve Society guideline on diagnosis and treatment of Guillain–Barré syndrome. Eur J Neurol. 2023. https://pubmed.ncbi.nlm.nih.gov/37814552/ [13] Narayanaswami P, et al. International Consensus Guidance for Management of Myasthenia Gravis: 2020 Update. Neurology. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC7884987/ [14] Oldroyd AGS, et al. International Guideline for Idiopathic Inflammatory Myopathy-Associated Cancer Screening (IMACS). Nat Rev Rheumatol. 2023. https://doi.org/10.1038/s41584-023-01045-w [15] NICE. Motor neurone disease: assessment and management (NG42; last reviewed 2024). https://www.nice.org.uk/guidance/ng42/chapter/Recommendations [16] US FDA. FDA approves tofersen for SOD1-associated ALS under the accelerated approval pathway. 2023. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-treatment-amyotrophic-lateral-sclerosis-associated-mutation-sod1-gene [17] Birnkrant DJ, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis and neuromuscular, rehabilitation, endocrine, gastrointestinal and nutritional management. Lancet Neurol. 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC5869704/ [18] Prior TW, Leach ME, Finanger E. Spinal Muscular Atrophy. GeneReviews (revised 2026). https://www.ncbi.nlm.nih.gov/books/NBK1352/ [19] Jacob S, et al. Association of British Neurologists autoimmune myasthenia gravis management guidelines (2025 update). Pract Neurol. 2025. https://pubmed.ncbi.nlm.nih.gov/40940171/ [20] US Centers for Disease Control and Prevention. Clinical Overview of Botulism. 2024. https://www.cdc.gov/botulism/hcp/clinical-overview/
GC055 Generalized Lymphadenopathy
Generalized lymphadenopathy is the enlargement of lymph nodes in two or more non-contiguous anatomical regions, often indicating systemic infections, autoimmune disorders, or malignancies.
GC057 Glomerular And Tubulo-interstitial Diseases And Acute Kidney Injury
Glomerular and tubulo-interstitial diseases encompass inflammatory, immune-mediated, or toxic disorders affecting the glomeruli or renal interstitium and tubules, which, along with acute kidney injury, result in a rapid or progressive decline in kidney filtration, concentration, and excretory functions.