What are the indications for ordering an Electromyogram (EMG) in patients with suspected neuromuscular disorders?

Indications for Ordering Electromyography (EMG)

EMG should be ordered when you suspect a neuromuscular disorder and need to differentiate between neuropathic, myopathic, or neuromuscular junction pathology, or when you need to confirm the anatomic localization and severity of peripheral nerve or muscle disease.

Primary Diagnostic Indications

Suspected Neuromuscular Junction Disorders

• Order EMG with repetitive nerve stimulation (RNS) when evaluating for myasthenia gravis or Lambert-Eaton myasthenic syndrome, particularly when patients present with fluctuating weakness, ptosis, diplopia, or bulbar symptoms 1, 2, 3.
• RNS demonstrates decremental response in compound muscle action potential amplitude with low-frequency stimulation (2-3 Hz) in myasthenia gravis, reflecting postsynaptic receptor blockade 2, 3.
• If RNS is normal but clinical suspicion remains high, proceed to single-fiber EMG (SFEMG), which is more sensitive than standard RNS 3.
• In Lambert-Eaton syndrome, look for incremental response with high-frequency RNS (30-50 Hz) 1.

Peripheral Neuropathy Evaluation

• Order EMG when you need to distinguish between axonal versus demyelinating neuropathy, determine severity, or identify the distribution pattern (length-dependent, multifocal, or asymmetric) 4, 5.
• EMG is particularly valuable when the clinical pattern is atypical, when considering inflammatory neuropathies (like Guillain-Barré syndrome), or when planning specific treatments 1.
• For patients with motor and/or sensory peripheral neuropathy, obtain combined nerve and muscle biopsy over nerve biopsy alone to increase diagnostic yield, but ensure you sample clinically affected tissue 1.
• A critical pitfall: Not all neuropathy patients require EMG—avoid ordering it for straightforward cases of distal symmetric polyneuropathy with clear etiology (diabetes, B12 deficiency) where the diagnosis is clinically obvious and EMG won't change management 4.

Suspected Myopathies

• Order EMG when evaluating for inflammatory myopathies (polymyositis, dermatomyositis), muscular dystrophies, or metabolic myopathies where you need to confirm myopathic changes before proceeding to muscle biopsy or genetic testing 5, 6, 7.
• EMG shows characteristic myopathic features: short-duration, low-amplitude motor unit potentials with early recruitment 1.
• In extraocular muscle involvement, specialized EMG of extraocular muscles has 83% diagnostic accuracy for myopathic diseases and 88% for muscular dystrophy, making it valuable when ocular manifestations predominate 7.

Neuromuscular Disorders with Cardiac Involvement

• For patients with neuromuscular disorders including muscular dystrophies, annual follow-up is recommended even in asymptomatic phases with normal ECG, as cardiac involvement may develop 1.
• EMG helps establish the neuromuscular diagnosis that then guides cardiac monitoring and ICD placement decisions 1.

Acute Weakness in Hospitalized Patients

• Order EMG for inpatients with unexplained weakness to identify critical illness polyneuropathy, Guillain-Barré syndrome, or other acute neuromuscular conditions 8.
• In the inpatient setting, EMG confirms clinical diagnosis in 53% of cases and provides new, clinically relevant diagnoses in 13% of cases that alter management 8.
• For critically ill patients with suspected diaphragm weakness, needle EMG of the diaphragm can diagnose polyneuropathy of critical illness, though this requires specialized expertise 1.

Specific Clinical Scenarios

Botulism Evaluation

• EMG with RNS may help distinguish botulism from other causes of descending paralysis (Guillain-Barré, myasthenia gravis), particularly in sporadic cases where early diagnosis guides antitoxin versus plasmapheresis decisions 1.
• Look for incremental response with high-frequency RNS (30-50 Hz), decreased motor unit recruitment, and decreased motor-evoked amplitude 1.
• Important caveat: Early in botulism, electrodiagnostic studies are often normal, so negative results don't exclude the diagnosis 1.

Monitoring Disease Activity

• For patients with established peripheral motor neuropathy, use serial neurologic examinations rather than repeated EMG every 6 months to monitor disease activity, as repeated EMG is invasive and unnecessary with stable symptoms 1.
• Repeat EMG only when there's uncertainty about new or worsening neurologic processes 1.

Technical Considerations and Limitations

When EMG May Not Be Helpful

• EMG is operator-dependent, requires specialized training and equipment, takes up to 2 hours to complete, and results require expert interpretation 1.
• The procedure can be painful, particularly RNS at high frequencies 1.
• EMG should be viewed as an extension of clinical examination, not a replacement—always correlate findings with clinical, epidemiologic, and laboratory data 1, 6.

Cost-Effectiveness Considerations

• EMG is a major driver of healthcare costs in neuropathy diagnosis 4.
• Before ordering, confirm that EMG results will actually change management—if the diagnosis is clinically clear and treatment won't differ, EMG may not be cost-effective 4, 5.
• EMG provides cost-effective diagnostic workup before ordering expensive genetic testing panels for suspected hereditary neuromuscular diseases 5.