What are the characteristic EMG (Electromyography) and NCV (Nerve Conduction Velocity) findings that differentiate between Guillain-Barré Syndrome (GBS) and Critical Illness Myopathy (CIM)?

Characteristic EMG/NCV Findings to Differentiate Between GBS and CIM

The most distinctive EMG/NCV findings that differentiate Guillain-Barré Syndrome (GBS) from Critical Illness Myopathy (CIM) are the presence of demyelinating features in GBS (reduced conduction velocities, prolonged distal latencies, conduction blocks, and abnormal F-waves/H-reflexes) versus preserved nerve conduction with decreased muscle compound action potentials in CIM.

Guillain-Barré Syndrome (GBS) EMG/NCV Characteristics

Early Findings (First Week)

• H-reflex abnormalities are the most sensitive early indicator, absent in up to 97% of GBS cases 1
• Abnormal F-waves occur in 84% of early GBS cases, making them one of the earliest detectable abnormalities 1
• Upper extremity sensory nerve action potentials (SNAPs) are frequently abnormal (61% of cases) within the first week 1
• "Sural sparing pattern" is characteristic - normal sural SNAP with abnormal median/ulnar SNAPs (67% of patients under 60 years) 1, 2
• Prolonged distal motor latencies appear early, seen in 55% of patients within the first 4 days 3

Progressive/Later Findings

• Reduced conduction velocities develop in 52% of cases 1
• Temporal dispersion of motor responses occurs in 58% of patients 1
• Partial motor conduction blocks develop in 13% of early cases, increasing in frequency later 1
• Reduced compound muscle action potential (CMAP) amplitudes are seen in 71% of cases 1
• Albumino-cytological dissociation in CSF (elevated protein with normal cell count) supports the diagnosis but may be normal in 30-50% of patients in the first week 2

Electrophysiological Subtypes

• AIDP (Acute Inflammatory Demyelinating Polyneuropathy): Shows prominent demyelinating features 2
• AMAN (Acute Motor Axonal Neuropathy): Shows primarily axonal motor involvement 2
• AMSAN (Acute Motor and Sensory Axonal Neuropathy): Shows axonal degeneration affecting both motor and sensory fibers 2
• About one-third of GBS patients may be initially classified as "equivocal" or "inexcitable" 2

Critical Illness Myopathy (CIM) EMG/NCV Characteristics

• Normal or near-normal sensory nerve conduction studies (key differentiating feature from GBS) 4
• Preserved motor nerve conduction velocities despite reduced CMAP amplitudes 4, 5
• No significant conduction blocks or temporal dispersion (unlike GBS) 4
• Normal F-wave latencies (often abnormal in GBS) 4
• Abnormal spontaneous activity (fibrillation potentials, positive sharp waves) on needle EMG 4
• Low-amplitude, short-duration, polyphasic motor unit potentials on voluntary contraction 4
• Early recruitment pattern of motor units 4

Key Differentiating Features

• Sensory nerve conductions: Abnormal in GBS (especially the characteristic sural sparing pattern); normal in CIM 2, 4
• Motor nerve conduction velocity: Slowed in GBS (especially AIDP subtype); normal or minimally affected in CIM 2, 4
• F-waves and H-reflexes: Frequently abnormal or absent in GBS; typically preserved in CIM 1, 4
• Conduction blocks and temporal dispersion: Present in GBS; absent in CIM 1, 4
• Needle EMG: Shows neuropathic pattern in GBS; myopathic pattern in CIM 4

Timing Considerations

• Early GBS may show minimal abnormalities, with H-reflex and F-wave being the most sensitive early indicators 1, 3
• Repeat studies 3-8 weeks after symptom onset may help classify initially unclassifiable GBS cases 2, 6
• In GBS, a second EMG/NCV measurement allows additional successful classification in 14-19% of patients 6
• Clinical severity correlates with successful electrophysiological classification in GBS 6

Common Pitfalls and Caveats

• Normal electrophysiological findings early in GBS course don't rule out the diagnosis 2
• Single-fiber EMG with measurement of jitter may be more sensitive for early diagnosis but requires specialized training 2
• Comparing conduction velocities from proximal versus distal muscle recordings can help differentiate axonal from demyelinating neuropathies 5
• Reversible conduction failure can be found in approximately 7% of GBS patients, potentially causing misclassification 6
• Miller Fisher variant of GBS can have similar electrodiagnostic findings to botulism, requiring careful clinical correlation 2

By systematically evaluating these EMG/NCV patterns, clinicians can more accurately differentiate between GBS and CIM, leading to appropriate treatment decisions and improved patient outcomes.