When should an electromyogram (EMG) be ordered?

When to Order Electromyography (EMG)

EMG should be ordered when there is clinical suspicion of neuromuscular disease, particularly when diagnosis is uncertain, with specific indications including suspected myopathies, neuropathies, neuromuscular junction disorders, and radiculopathies that cannot be confirmed by clinical examination alone.

Primary Indications for EMG

Suspected Neuromuscular Disorders

• Myopathies and Neuropathies
  • When clinical presentation suggests muscle weakness but diagnosis is uncertain 1
  • To differentiate between neurogenic and myopathic processes 2
  • For patients with suspected inflammatory myopathies (e.g., dermatomyositis, polymyositis) 3

Specific Clinical Scenarios

• Peripheral Neuropathy Evaluation

  • For patients with motor and/or sensory deficits to characterize the pattern and severity 4
  • When combined nerve and muscle biopsy is being considered (EMG can guide biopsy site selection) 4

• Neuromuscular Junction Disorders

  • For suspected myasthenia gravis when repetitive nerve stimulation testing is normal but clinical suspicion remains high 4
  • To detect abnormal jitter or impulse blockade in symptomatic muscles 4

• Laryngeal Disorders

  • For evaluation of vocal fold paralysis/paresis when the cause is uncertain 4
  • To differentiate between neurological and mechanical causes of vocal fold impairment 4

Timing Considerations

1. Acute Setting:

   • For rapidly progressive weakness to distinguish between urgent neurological conditions
   • When considering surgical intervention for radiculopathies or entrapment neuropathies

2. Subacute/Chronic Setting:

   • At least 3 weeks after onset of symptoms to allow development of denervation changes in neurogenic conditions 4
   • For monitoring disease progression in chronic neuromuscular disorders

EMG as Part of a Diagnostic Algorithm

1. First-line Diagnostic Steps:

   • Clinical assessment of muscle strength using validated tools (e.g., Medical Research Council scale)
   • Basic laboratory testing (CK, electrolytes, thyroid function, inflammatory markers)

2. When to Order EMG:

   • After initial assessment suggests neuromuscular pathology
   • Before invasive procedures like muscle biopsy to guide site selection 5
   • When distinguishing between neurogenic and myopathic causes is necessary

3. EMG in Conjunction with Other Tests:

   • With nerve conduction studies (NCS) for comprehensive electrodiagnostic assessment
   • Before MRI in non-acute cases to help direct imaging focus
   • To complement serological testing for myositis-specific antibodies

Special Considerations

Pediatric Patients

• For juvenile dermatomyositis, EMG may be used when diagnosis is uncertain, but MRI is often preferred 4
• Consider age-appropriate normative values when interpreting results

Respiratory Muscle Assessment

• When respiratory muscle weakness is suspected but not confirmed by pulmonary function tests 4
• For patients with neuromuscular disorders showing signs of respiratory compromise

Follow-up EMG

• Not recommended for routine monitoring of disease activity in patients with stable symptoms 4
• Should be considered only when there is uncertainty about new or worsening symptoms 4

Diagnostic Yield and Limitations

• EMG has approximately 87% sensitivity and 65% specificity for detecting abnormalities confirmed by muscle biopsy 5
• False negatives can occur in:
  • Early stages of disease
  • Certain myopathies (e.g., mitochondrial myopathies, central core myopathy) 5
  • Patchy or focal muscle involvement

Common Pitfalls to Avoid

1. Ordering EMG too early after acute nerve injury (before denervation changes develop)
2. Relying solely on EMG without clinical correlation
3. Failure to recognize that normal EMG does not exclude all neuromuscular disorders
4. Repeating EMG unnecessarily in patients with stable symptoms 4

EMG remains a valuable diagnostic tool when used appropriately as part of a comprehensive evaluation of neuromuscular symptoms, but should be ordered judiciously based on specific clinical indications rather than as a routine screening test.