What are electrophysiologic studies for carpal tunnel syndrome, specifically nerve conduction studies (NCS) and electromyography (EMG)?

Electrophysiologic Studies for Carpal Tunnel Syndrome

Electrophysiologic studies, including nerve conduction studies (NCS) and electromyography (EMG), are the reference standard for confirming carpal tunnel syndrome diagnosis and evaluating its severity.1

Nerve Conduction Studies (NCS)

NCS are the primary electrophysiologic tests for carpal tunnel syndrome (CTS) and involve:

• Methodology: Providing an electrical stimulus to a nerve and recording the electrical response from a sensory nerve or muscle 2

• Key measurements:

  • Median distal sensory latencies (DSLs)
  • Sensory nerve action potentials (SNAPs)
  • Distal motor latencies (DMLs)
  • Compound muscle action potentials (CMAPs)
  • Forearm median nerve conduction velocities (FMCVs)
  • Wrist-palm motor conduction velocities (W-P MCVs) 3

• Most sensitive comparative tests:

  • Comparison of median and radial distal sensory latencies in digit I (thumb)
  • Comparison of median and ulnar distal sensory latencies in digit IV (ring finger)
  • Combined, these tests yield abnormal results in 51% of hands with clinically suspected CTS but normal standard NCS 4
  • Comparison of median (index finger) to ulnar (little finger) sensory amplitude is abnormal in 73% of CTS cases 5

Electromyography (EMG)

EMG complements NCS in the diagnosis of CTS:

• Methodology: Involves inserting a needle electrode into a muscle (typically abductor pollicis brevis) and recording electrical activity at rest and during muscle contraction 2
• Key findings: Records motor unit potentials or motor unit action potentials 2
• Clinical value:
  • Detects axonal damage (denervation) in more severe cases
  • Helps differentiate CTS from other neuromuscular disorders 1
  • Particularly important when CMAP amplitudes are between 2.1 mV and the lower normal limit (4.9 mV) 3

Diagnostic Value and Interpretation

• Predictive value of NCS parameters:

  • Abnormal median CMAP amplitudes are the strongest predictors of spontaneous EMG activity (OR 36.58) 3
  • If median CMAP amplitude is ≤2.1 mV, the positive predictive rate for spontaneous EMG activity is >95% 3
  • If median CMAP amplitude is normal (>4.9 mV), the negative predictive rate for spontaneous EMG activity is >94% 3

• Combined diagnostic yield:

  • A combination of various electrodiagnostic tests yields a positive result in 91% of CTS cases 5
  • The diagnostic yield increases with longer disease duration 5

Classification Systems

Two neurophysiological classification systems are available for CTS severity grading:

• Bland system: Based solely on nerve conduction study findings 1
• GEHS system: Includes both NCS and EMG findings, providing evidence of myelinopathy and/or axonopathy 1

Clinical Considerations and Limitations

• Physician confidence: Low physician confidence in the clinical diagnosis of CTS is highly predictive of normal electrodiagnostic testing (sensitivity 97%, specificity 40%, accuracy 87%) 6

• Technical challenges:

  • Tests are operator-dependent
  • Require specialized training and equipment
  • Take approximately 2 hours to complete 2

• Patient factors:

  • Examination can cause discomfort, especially repetitive nerve stimulation 2
  • Patient cooperation is required for optimal testing 2
  • "Puzzling patient factors" (heightened illness concern, disproportionate complaints, vague symptoms) are highly specific (96%) predictors of normal electrodiagnostic testing 6

• Interpretation complexity: Results require expert interpretation and should always be considered in the context of clinical findings 2