EMG in myopathic disorders identifies muscle fiber pathology distinct from nerve disease. Myopathic EMG shows small, brief, polyphasic motor unit action potentials with early recruitment — opposite features from neurogenic disease. Combined with spontaneous activity (fibrillations in inflammatory myopathies, myotonic discharges in channelopathies), distribution (proximal in muscular dystrophy and inflammatory myopathies, distal in IBM), and histologic correlation, EMG narrows the muscle disease differential. This page covers the EMG findings in inflammatory myopathies, muscular dystrophies, metabolic myopathies, and channelopathies.

The Myopathic EMG Pattern

1. MUAP changes: small amplitude (often <500 μV), short duration (often <5 ms), polyphasic (often >4 phases).
2. Early recruitment: many motor units recruited with minimal effort (compensating for weak individual units).
3. Spontaneous activity: present in active inflammatory myopathies (fibs, PSWs, CRDs); minimal in dystrophies; specific patterns in channelopathies.
4. Distribution: typically proximal predominance (inflammatory myopathies, dystrophies); distal in IBM; specific to syndrome.

Why Myopathic MUAPs Are Small

• Each muscle fiber within a motor unit contributes to the MUAP amplitude.
• In myopathy, fibers within each motor unit are damaged or lost.
• Fewer functional fibers per motor unit → smaller summed MUAP.
• Polyphasic: variability of fiber loss and reinnervation within the motor unit.
• Brief duration: fewer fibers, shorter total activation time.

Inflammatory Myopathies

Polymyositis

• Acute or subacute proximal weakness.
• Often associated with autoimmune disease.
• EMG findings:
  • Increased insertional activity.
  • Fibrillations and PSWs (active inflammation).
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Distribution: proximal — deltoid, biceps, triceps, quadriceps.
• Lab: elevated CK, sometimes very high.
• Muscle biopsy: invasion of myocytes by CD8 T cells.

Dermatomyositis

• Similar EMG findings to polymyositis.
• Plus skin manifestations (heliotrope rash, Gottron’s papules).
• Biopsy: perifascicular atrophy, perivascular B/CD4 cell infiltrates.
• Association with malignancy (especially in adults).

Inclusion Body Myositis (IBM)

• Older patients (>50 years).
• Distinctive pattern: distal upper extremity (FDP, FCU) AND proximal lower extremity (quadriceps).
• Asymmetric.
• EMG:
  • Mixed myopathic AND chronic neurogenic features (large polyphasic MUAPs).
  • Fibrillations and PSWs.
  • Reduced recruitment in severely affected muscles.
• Biopsy: rimmed vacuoles, intracellular inclusions, CD8+ T cell invasion.
• Resistant to immunotherapy.

Necrotizing Autoimmune Myopathy

• Often paraneoplastic or statin-induced.
• Severe proximal weakness.
• EMG: prominent fibs and PSWs; small polyphasic MUAPs.
• Antibodies: anti-HMGCR (statin-induced), anti-SRP.
• Biopsy: necrotic fibers without inflammatory infiltrate.

Antisynthetase Syndrome

• Anti-Jo1 (most common) or other anti-tRNA synthetase antibodies.
• Inflammatory myopathy + interstitial lung disease + mechanic’s hands.
• EMG similar to polymyositis.

Muscular Dystrophies

Duchenne and Becker Muscular Dystrophy

• X-linked dystrophin mutations.
• Duchenne: severe, early onset (3–5 years).
• Becker: milder, later onset.
• EMG findings:
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Spontaneous activity less prominent than inflammatory myopathies.
  • Pseudohypertrophy (calves) but muscle replaced by fat — reduced insertional activity.
• Genetic confirmation: DMD mutations.
• EMG less commonly used now; genetics drives diagnosis.

Limb-Girdle Muscular Dystrophy (LGMD)

• Multiple subtypes (LGMD1, LGMD2 with subdivisions).
• Proximal weakness, often beginning in pelvic girdle.
• EMG: myopathic pattern.
• Subtype-specific findings less distinctive.
• Genetic confirmation now standard.

Facioscapulohumeral Dystrophy (FSHD)

• Autosomal dominant.
• Distinctive distribution: face, shoulders, then proximal lower extremity.
• Asymmetric weakness common.
• EMG: myopathic in affected muscles.
• Genetic: DUX4 region on chromosome 4q35.

Myotonic Dystrophy Type 1 (DM1)

• Autosomal dominant.
• Multisystem: weakness + myotonia + cataracts + cardiac conduction defects + insulin resistance + frontal balding.
• EMG findings:
  • Myotonic discharges: stereotyped trains of muscle action potentials waxing and waning in amplitude and frequency.
  • “Dive-bomber” or “motorcycle revving” sound.
  • Distinctive of myotonic disorders.
  • Distribution: widespread, including distal muscles.
• Plus myopathic MUAPs (small polyphasic).
• Genetic: CTG expansion in DMPK gene.

Myotonic Dystrophy Type 2 (DM2)

• Autosomal dominant.
• Proximal predominant weakness (vs distal in DM1).
• EMG: myotonic discharges similar to DM1.
• Less severe phenotype overall.
• Genetic: CCTG expansion in CNBP gene.

Metabolic Myopathies

Pompe Disease (Glycogen Storage Disease Type II)

• α-glucosidase deficiency.
• Infantile, juvenile, adult onset.
• EMG: myopathic + sometimes myotonia-like discharges.
• Treatment: enzyme replacement therapy.

Acid Maltase Deficiency (Adult Onset)

• Adult variant of Pompe.
• Proximal weakness + respiratory involvement.
• EMG: myotonic discharges may be present (uncommon in other myopathies).

Mitochondrial Myopathy

• Variable presentations: progressive external ophthalmoplegia, fatigable weakness, ragged red fibers.
• EMG: myopathic pattern; sometimes neurogenic features in mixed encephalomyopathies.
• Lab: lactate, pyruvate, genetic testing.

Lipid Storage Myopathy

• Carnitine deficiency, CPT2 deficiency.
• Exercise intolerance, episodic weakness, rhabdomyolysis.
• EMG: variable.

Channelopathies (Muscle)

Myotonia Congenita

• Chloride channel mutation (CLCN1).
• Myotonia, often warmed up by exercise (“warm-up phenomenon”).
• EMG: prominent myotonic discharges.
• No or mild weakness.

Paramyotonia Congenita

• Sodium channel mutation (SCN4A).
• Paradoxical myotonia (worsens with exercise/cold).
• EMG: myotonic discharges; paradoxical myotonia testing (worsens with cooling).

Hyperkalemic Periodic Paralysis

• Sodium channel mutation (SCN4A).
• Episodes of paralysis triggered by K-rich food, cold, or rest after exercise.
• EMG: myotonic discharges between attacks; electrical silence during attacks.

Hypokalemic Periodic Paralysis

• Calcium channel (CACNA1S) or sodium channel (SCN4A) mutation.
• Episodes of paralysis triggered by carbohydrate, exercise, K-low foods.
• EMG: variable; can show myopathic.

Andersen-Tawil Syndrome

• Kir2.1 K⁺ channel mutation.
• Episodic paralysis + cardiac arrhythmia + dysmorphic features.
• EMG: myotonia in some.

Myasthenia Gravis (Neuromuscular Junction, Not Pure Myopathy)

• Postsynaptic NMJ disease.
• Routine EMG often normal at rest.
• Decrement on repetitive nerve stimulation (RNS): diagnostic.
• Single-fiber EMG: increased jitter (most sensitive).
• Covered in dedicated NMJ page.

Critical Illness Myopathy

• Common in ICU patients exposed to neuromuscular blockers and corticosteroids.
• EMG: myopathic pattern, often with fibs (acute denervation in muscle from inflammation).
• Often coexists with critical illness neuropathy.
• Distinguish from primary muscle disease by clinical context.

Drug-Induced Myopathies

Statin-Induced

• Asymptomatic CK elevation common.
• Symptomatic myopathy: pain, weakness, elevated CK.
• EMG often myopathic.
• Severe variant: necrotizing autoimmune myopathy (anti-HMGCR positive).
• Usually resolves with discontinuation.

Steroid Myopathy

• Chronic corticosteroid use.
• Proximal weakness without significant CK elevation.
• EMG often normal or shows myopathic pattern without fibs.
• Reversible with dose reduction.

Antimalarial-Induced

• Chloroquine, hydroxychloroquine.
• Rare; often subclinical.

Colchicine-Induced

• Myopathy + neuropathy.
• Reversible.

Endocrine Myopathies

• Hypothyroid myopathy: proximal weakness, hyporeflexia, muscle stiffness, “mounding” phenomenon. EMG often relatively unremarkable.
• Hyperthyroid myopathy: proximal weakness, fasciculations sometimes. EMG variable.
• Cushing syndrome: proximal weakness similar to steroid myopathy.
• Hyperparathyroidism, Addison disease, acromegaly: variable myopathic features.

Distinguishing Inflammatory from Dystrophic Myopathy

Combined Patterns

• Critical illness neuropathy + myopathy: both neurogenic and myopathic features in ICU patient.
• IBM: chronic mixed myopathic + neurogenic in older patient.
• Long-standing dystrophy: secondary reinnervation in selectively damaged muscles.

🔍 Did You Know?

The recognition that inclusion body myositis (IBM) is the most common acquired muscle disease in patients over 50 has reshaped how electrodiagnosticians approach proximal weakness in older adults. IBM has a distinctive pattern that distinguishes it from polymyositis and other inflammatory myopathies: distal upper extremity weakness (finger flexors, FDP, FCU) combined with proximal lower extremity weakness (quadriceps), often asymmetric. Patients often present with difficulty climbing stairs and a tendency for falls — and they may have been carrying a misdiagnosis of “ALS” or “polymyositis” for years before IBM is recognized. The EMG finding is particularly distinctive: mixed myopathic AND chronic neurogenic features in the same muscle, reflecting both myocyte injury and secondary reinnervation. The combination of large polyphasic MUAPs (typically thought of as neurogenic) alongside small polyphasic MUAPs is the hallmark. Muscle biopsy is confirmatory: rimmed vacuoles in myocytes are pathognomonic. The clinical importance is significant — IBM is resistant to immunotherapy, unlike polymyositis and dermatomyositis. Treating IBM as polymyositis with steroids and immunosuppressants exposes the patient to long-term steroid side effects without disease modification. For the electrodiagnostician, IBM should be considered in any older patient with proximal weakness + distal upper extremity weakness + EMG showing mixed neurogenic/myopathic features. For practicing neurologists, the take-home is to specifically ask about finger flexion difficulty (e.g., grip strength, dropping objects) and quadriceps weakness (stairs, falls) in older patients with apparent inflammatory myopathy before committing to chronic immunotherapy.

Pitfalls and Pearls

• Myopathic MUAPs: small, brief, polyphasic; early recruitment.
• Active inflammatory myopathy: increased insertional + fibs/PSWs + small polyphasic MUAPs.
• Polymyositis: proximal weakness; high CK; immunotherapy responsive.
• Dermatomyositis: + skin manifestations; check for malignancy.
• Inclusion body myositis (IBM): distal upper + proximal lower; mixed neurogenic/myopathic EMG; rimmed vacuoles; immunotherapy-resistant.
• Necrotizing autoimmune myopathy: anti-HMGCR (statin), anti-SRP; necrosis without infiltrate.
• Muscular dystrophy: small polyphasic + early recruitment; reduced insertional (fibrofatty replacement); minimal spontaneous.
• Myotonic dystrophy: myotonic discharges + myopathic MUAPs; multisystem.
• Channelopathies: myotonia congenita (Cl⁻), paramyotonia (Na⁺), periodic paralyses.
• Pompe disease: myopathic + sometimes myotonic discharges.
• Critical illness myopathy: ICU patient; common in steroid + NMJ blocker exposure.
• Statin myopathy: discontinuation usually reverses; rare severe necrotizing form.
• Steroid myopathy: proximal weakness without CK elevation; EMG often unremarkable.
• Endocrine myopathies: check thyroid, parathyroid, cortisol.
• Genetic testing: increasingly drives muscular dystrophy diagnosis.
• Muscle biopsy: still essential for inflammatory myopathies and IBM.
• Distribution + EMG features + lab + biopsy: complete picture for muscle disease.

References

1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
2. Dalakas MC. Inflammatory muscle diseases. N Engl J Med. 2015;372(18):1734-1747.
3. Greenberg SA. Inclusion body myositis: clinical features and pathogenesis. Nat Rev Rheumatol. 2019;15(5):257-272.
4. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle. 4th ed. Oxford University Press; 2013.
5. Lundberg IE, Tjärnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies. Ann Rheum Dis. 2017;76(12):1955-1964.
6. Selva-O’Callaghan A, Pinal-Fernandez I, Trallero-Araguás E, et al. Classification and management of adult inflammatory myopathies. Lancet Neurol. 2018;17(9):816-828.