What is the recommended initial evaluation for suspected brachial plexus disorders using Electromyography (EMG) studies?

EMG Studies for Brachial Plexus Disorder Evaluation

EMG/nerve conduction studies (NCS) are essential for the initial evaluation of suspected brachial plexus disorders and should be performed in conjunction with clinical examination to localize the lesion, determine severity, assess pathophysiology (axonal vs demyelinating), and provide prognostic information. 1, 2, 3

Role of Electrodiagnostic Studies

EMG/NCS serves as the cornerstone diagnostic procedure for brachial plexus evaluation, providing critical information that imaging alone cannot deliver:

• Localization: Distinguishes preganglionic (root avulsion) from postganglionic lesions, differentiates plexopathy from radiculopathy or peripheral nerve lesions 1, 4, 3
• Severity assessment: Determines extent of axonal loss versus demyelination, which directly impacts prognosis 2, 3
• Pathophysiology: Identifies whether injury involves axonal degeneration, demyelinating conduction block, or mixed patterns 3
• Prognostic value: Predicts recovery potential based on the pattern and severity of nerve injury 4, 3

Recommended EMG/NCS Protocol Components

The electrodiagnostic evaluation requires a systematic, comprehensive approach because the brachial plexus cannot be assessed by a single test:

Sensory Nerve Conduction Studies (Most Critical for Localization)

• Sensory NCS are the most useful component for localizing brachial plexus lesions 3
• Measure ulnar, median, radial, and lateral antebrachial cutaneous sensory nerve action potentials (SNAPs) 1, 2
• Key principle: Preserved SNAPs with absent motor responses suggest preganglionic (root avulsion) injury, since the dorsal root ganglion remains intact 1, 4
• Absent or reduced SNAPs indicate postganglionic plexus or peripheral nerve involvement 1, 3

Motor Nerve Conduction Studies

• Assess compound muscle action potentials (CMAPs) in multiple distributions (median, ulnar, radial, musculocutaneous, axillary) 2, 3
• Determine severity of motor involvement and identify demyelinating conduction blocks 3
• Motor NCS abnormalities exceeding sensory abnormalities suggest intraspinal canal lesions like root avulsions 3

Needle Electromyography (NEE)

• Needle EMG is especially helpful in traumatic and idiopathic brachial plexopathy 1
• Sample muscles from multiple myotomes and plexus distributions to map the extent of involvement 2, 3
• Include paraspinal muscles to help distinguish radiculopathy from plexopathy, though this has limited value in multiple root lesions due to innervation overlap 4
• Identify denervation patterns (fibrillations, positive sharp waves) and reinnervation changes 2, 3

Supplementary Studies (Limited Utility)

• F-wave responses and somatosensory evoked potentials (SEPs) have more limited utility in most cases 1
• SEPs can help guide management in traumatic plexopathy and confirm proximal lesion sites when peripheral SNAPs are normal 1

Timing Considerations

A critical pitfall: EMG/NCS should ideally be performed at least 3-4 weeks after acute injury to allow wallerian degeneration to develop, enabling detection of denervation potentials on needle examination 4, 2

Integration with Clinical Diagnosis

The American College of Radiology emphasizes that diagnosis of inflammatory and immune-mediated brachial plexus disorders (Parsonage-Turner syndrome, chronic inflammatory demyelinating polyradiculoneuropathy, multifocal motor neuropathy) is typically based on clinical and electrodiagnostic evaluation, as imaging features overlap considerably 5

Diagnostic Accuracy and Limitations

• A validated EMG/NCS protocol provides reliable analysis of traumatic brachial plexus lesions in 84% of patients 4
• Important limitation: Extraforaminal lesions may mask root avulsion or central lesions 4
• Normal SNAPs can occasionally occur in extraforaminal lesions 4
• EMG/NCS can identify lesions outside the brachial plexus not expected on clinical grounds in 21% of patients 4

Complementary Role with MRI

While EMG/NCS is essential, it should be complemented by MRI of the brachial plexus:

• MRI provides additional diagnostic information beyond clinical evaluation and electrodiagnostic studies in 45% of patients 6
• MRI is superior for identifying structural lesions (tumors, masses, nerve sheath tumors) that EMG/NCS cannot visualize 5, 6
• EMG/NCS provides functional information about nerve integrity that MRI cannot assess 6, 2
• Correlation between abnormal intraneural signal on MRI and active radiculopathy on EMG strengthens diagnostic confidence 7

Common Pitfalls to Avoid

• Don't rely on paraspinal muscle sampling alone in multiple root lesions, as innervation overlap limits diagnostic value 4
• Don't perform EMG/NCS too early after acute trauma; wait 3-4 weeks for denervation changes to develop 4, 2
• Don't interpret median sensory NCS for plexus localization if concomitant carpal tunnel syndrome is present, as this negates its utility 3
• Don't assume normal EMG/NCS excludes all pathology; some microstructural nerve injuries may not demonstrate electrodiagnostic abnormalities despite clinical dysfunction 6