What are the primary indications for Electromyography (EMG)?

Primary Indications for Electromyography (EMG)

EMG is primarily indicated to elucidate the etiology of muscle weakness by differentiating between neurological and muscular causes, including myopathies, motor neuron diseases, and peripheral neuropathies. 1

Core Diagnostic Applications

Neuromuscular Disease Diagnosis

• EMG detects abnormalities in muscle diseases (myopathies) by analyzing motor unit action potentials at rest and during contraction 1
• Motor neuron diseases can be identified through characteristic patterns of denervation and reinnervation 1
• Peripheral neuropathies are diagnosed by distinguishing between axonal and demyelinating patterns, including Guillain-Barré syndrome, polyneuropathy of critical illness, and lower motor neuron involvement with spinal cord injury 2

Specific Clinical Scenarios

Respiratory Muscle Dysfunction:

• EMG of the diaphragm is particularly useful for diagnosing neuromuscular disease presenting with respiratory muscle weakness 2
• The technique helps assess diaphragmatic function in patients with breathing difficulties and can detect neuropathic processes like Guillain-Barré syndrome 2, 1

Laryngeal and Voice Disorders:

• Laryngeal EMG (LEMG) differentiates neurological causes from mechanical limitations (such as cricoarytenoid joint fixation) in patients with vocal fold paralysis or paresis 2, 1
• LEMG identifies peripheral nerve injuries affecting the recurrent laryngeal nerve or external branch of the superior laryngeal nerve 2
• The test helps distinguish between vocal fold immobility due to neural/muscular pathology versus mechanical fixation 2

Critical Care Settings:

• EMG detects critical illness polyneuropathy/myopathy in ICU patients before clinical examination is possible in sedated or uncooperative patients 1
• This early detection capability is crucial for prognostication and management decisions in critically ill patients 1

Severity Assessment and Prognosis

• EMG quantifies the degree of nerve or muscle damage, providing objective measures of disease severity 1
• Serial EMG examinations can track disease progression or recovery over time 2
• In cervical spine surgery candidates, EMG has mixed utility for predicting surgical outcomes, though it may help evaluate atypical symptoms 2

Differential Diagnosis Capabilities

Neuromuscular Junction Disorders:

• EMG with repetitive nerve stimulation differentiates between botulism, myasthenia gravis, and Guillain-Barré syndrome 1
• In botulism, EMG shows increment in compound motor nerve action potential amplitude with repetitive stimulation at 30-50 Hz 1
• Myasthenia gravis demonstrates characteristic decremental responses on fatigability testing 1

Distinguishing Neuropathic from Myopathic Processes:

• Single fiber and motor unit analysis identifies whether weakness stems from nerve pathology versus primary muscle disease 2
• The pattern of spontaneous activity (fibrillation potentials, positive sharp waves) indicates denervation 2
• Motor unit action potential characteristics (duration, amplitude, recruitment pattern) distinguish myopathic from neuropathic conditions 2

Important Caveats and Limitations

Timing Considerations:

• Early in disease processes, EMG results may be normal or near-normal, particularly in conditions like botulism or early nerve compression 1, 3
• Denervation changes (fibrillation potentials) typically appear 2-3 weeks after nerve injury, so EMG performed too early may miss pathology 2

Technical Limitations:

• Small fiber neuropathy produces normal EMG findings because standard techniques only assess large myelinated fibers, not small C and Aδ fibers responsible for pain and temperature sensation 3
• Early or mild nerve compression frequently shows normal results despite clinical symptoms 3
• The examination is operator-dependent and requires specialized training for accurate interpretation 1

Patient Factors:

• Full EMG examination requires patient cooperation, which may be difficult in severely ill or pediatric patients 1
• The procedure can be painful, especially repetitive nerve stimulation at high frequencies 1
• There is a small risk of pneumothorax with intercostal or diaphragm recordings, plus risks of bleeding and bruising 2

When EMG Should NOT Be the Primary Test

• For suspected cubital tunnel syndrome with 4th and 5th digit numbness, plain radiographs followed by MRI neurography are more appropriate when EMG is normal, as early nerve compression may not show electrodiagnostic abnormalities 3
• In suspected cardiac tumors or cardiovascular disease, EMG has no role (this was included in the evidence but is clearly not relevant to neuromuscular EMG indications) 2

Practical Implementation

Electrode Selection:

• Monopolar or concentric needle electrodes minimize cross-talk from adjacent muscles for intramuscular recordings 4
• Surface electrodes are useful for global respiratory muscle activity assessment but have more cross-talk 2
• Esophageal electrodes provide artifact-free signals for diaphragm assessment unaffected by chest wall configuration 2

Signal Analysis:

• Time domain analysis evaluates motor unit action potential amplitude, duration, and recruitment patterns 2
• Frequency domain analysis assesses muscle fiber conduction velocity and can detect respiratory muscle fatigue 2
• Multiple sampling sites (2-3 locations) within a muscle are necessary to adequately assess for spontaneous activity and recruitment patterns 4