What is electromyography (EMG) indicated for?

What is Electromyography (EMG) For?

Electromyography (EMG) is indicated to diagnose and differentiate neuromuscular diseases by detecting abnormalities in muscle electrical activity, identifying motor neuron diseases, diagnosing peripheral neuropathies, and distinguishing between myopathic and neuropathic conditions. 1

Core Diagnostic Applications

EMG detects three primary categories of neuromuscular pathology:

• Myopathies (muscle diseases) are identified by analyzing motor unit action potentials showing characteristic short-duration, low-amplitude potentials with early recruitment at rest and during contraction 1, 2
• Motor neuron diseases are diagnosed through patterns of denervation (fibrillation potentials, positive sharp waves, fasciculations) and reinnervation (long-duration, large-amplitude motor unit action potentials) 1, 3
• Peripheral neuropathies are distinguished by differentiating axonal patterns (reduced recruitment with high-frequency firing) from demyelinating patterns, including conditions like Guillain-Barré syndrome, polyneuropathy of critical illness, and lower motor neuron involvement with spinal cord injury 1, 3

Specific Clinical Scenarios Where EMG is Essential

Respiratory muscle weakness:

• Diaphragm EMG diagnoses neuromuscular disease presenting with respiratory muscle weakness and assesses diaphragmatic function in patients with breathing difficulties 1
• Esophageal electrodes provide artifact-free signals for diaphragm assessment unaffected by chest wall configuration 1

Neuromuscular junction disorders:

• EMG with repetitive nerve stimulation (RNS) differentiates botulism, myasthenia gravis, and Lambert-Eaton myasthenic syndrome 1, 2
• In myasthenia gravis, RNS shows a decrement in compound muscle action potential amplitude with 2-5 Hz stimulation 1, 2
• If RNS is normal but suspicion persists, single-fiber EMG should be performed in at least one symptomatic muscle, with abnormality defined as 10% of fiber pairs exceeding normal jitter or showing impulse blocking 1

Vocal fold paralysis:

• Laryngeal EMG differentiates neurological causes from mechanical limitations in patients with vocal fold paralysis or paresis 1
• Serial examinations track disease progression or recovery over time 1

Severity Assessment and Prognostic Value

EMG quantifies disease severity objectively:

• Provides objective measures of the degree of nerve or muscle damage 1
• Reduced recruitment of motor units with acute or chronic spontaneous activity indicates a poor prognosis 1
• Serial examinations monitor disease progression or response to treatment 1

Critical Timing Considerations and Pitfalls

Common pitfall: Testing too early in the disease course

• Early in disease processes, EMG results may be normal or near-normal, particularly in conditions like botulism or early nerve compression 1
• Denervation changes typically appear 2-3 weeks after nerve injury, so EMG performed before this window may miss pathology 1
• This timing limitation is crucial for clinical decision-making and may require repeat testing if initial EMG is negative but clinical suspicion remains high 1

Technical Implementation for Optimal Results

Electrode selection depends on the clinical question:

• Monopolar or concentric needle electrodes minimize cross-talk from adjacent muscles for intramuscular recordings and are preferred for precise localization 1
• Surface electrodes assess global respiratory muscle activity but have more cross-talk and are affected by factors like innervation zones, muscle temperature, and thoracic wall configuration 4, 1
• Esophageal electrodes provide signals free from artifacts not affected by chest wall configuration, particularly valuable when respiratory weakness is the primary manifestation 4, 1

Distinguishing Myopathic from Neuropathic Patterns

Key EMG pattern differences:

• Neuropathic pattern: Reduced recruitment with high-frequency firing, long-duration large-amplitude motor unit action potentials, fibrillation potentials, positive sharp waves, and fasciculations 4, 1, 3
• Myopathic pattern: Early recruitment with many small units, short-duration low-amplitude motor unit action potentials 4, 1, 2
• Single fiber and motor unit analysis identifies whether weakness stems from nerve pathology versus primary muscle disease 1

Practical Limitations

EMG is operator-dependent and requires:

• Specialized training and equipment 2
• Up to 2 hours to complete 2
• Expert interpretation of results 2
• The test should be treated as an extension of clinical examination, not a replacement, and correlates well with clinical signs when done adequately 5