How to diagnose nerve compression or damage?

Diagnostic Approach to Nerve Compression vs. Nerve Damage

The most effective way to differentiate between nerve compression and nerve damage is through a combination of electrodiagnostic studies (EMG/NCS), targeted imaging, and specific clinical tests that evaluate nerve function and response to stimuli. 1

Clinical Assessment

Key Symptoms to Evaluate

• Distribution of symptoms (follows specific nerve territory?)
• Timing of symptom onset (acute vs. gradual)
• Character of symptoms:
  • Compression: Paresthesia, numbness, positional worsening
  • Damage: Persistent weakness, atrophy, sensory loss

Physical Examination Tests

• Motor testing: Assess for weakness in specific muscle groups
• Sensory testing:
  • Pinprick sensation
  • Thermal sensitivity
  • Vibration perception (using 128-Hz tuning fork)
  • Pressure sensation (using 10-g monofilament) 2
• Reflex testing: Evaluate deep tendon reflexes
• Provocative tests: Tinel's sign, compression tests

Diagnostic Testing Algorithm

First-Line Testing

1. Electrodiagnostic Studies

   • EMG (Electromyography): Evaluates muscle electrical activity
     • Nerve damage: Shows fibrillation potentials, positive sharp waves
     • Compression: May show normal or reduced recruitment patterns
   • NCS (Nerve Conduction Studies): Measures nerve signal transmission
     • Compression: Shows focal slowing, conduction block at compression site
     • Damage: Shows reduced amplitude throughout nerve course 3

2. MRI Imaging

   • Optimal for soft tissue visualization
   • Use specific sequences:
     • T1-weighted: Anatomical detail
     • T2-weighted with fat suppression: Inflammatory changes
     • Fluid-sensitive sequences: Edema around nerves 1
   • Timing: Recommended after 6 weeks of persistent symptoms 1

Second-Line Testing

1. CT Myelography

   • When MRI is contraindicated or inconclusive
   • Particularly useful for nerve root compression 2

2. Diagnostic Nerve Blocks

   • Can help localize the site of nerve involvement
   • Limited specificity (approximately 65%)
   • Not definitive for distinguishing between compression and damage 1

Interpretation of Results

Nerve Compression Indicators

• Focal slowing of nerve conduction at compression site
• Normal distal nerve function
• Improvement with positional changes
• Reversible symptoms with decompression
• MRI showing external compression without intrinsic nerve abnormality

Nerve Damage Indicators

• Wallerian degeneration on EMG (appears 3-7 days after injury) 2
• Reduced amplitude throughout nerve course
• Fibrillation potentials and positive sharp waves on EMG
• No improvement with positional changes
• MRI showing intrinsic nerve abnormality or disruption

Important Considerations

• Timing matters: Electrodiagnostic studies become stable and informative approximately 7 days after symptom onset; testing before this may be misleading 2
• Testing beyond 14-21 days may be less reliable 2
• In cases of complete nerve paralysis, ENoG (Electroneuronography) can quantify the extent of nerve damage:
  • Response amplitude >10% of contralateral side suggests good recovery potential
  • Response amplitude <10% indicates poorer prognosis 2

Treatment Response as Diagnostic Tool

• Nerve compression typically responds better to anti-inflammatory medications and decompression
• Neuropathic pain from nerve damage typically responds better to anticonvulsants and antidepressants 1

By following this systematic diagnostic approach, clinicians can effectively differentiate between nerve compression and nerve damage, allowing for appropriate treatment planning and prognostication.