How do you rule out a median nerve injury?

Ruling Out Median Nerve Injury

The most effective approach to rule out median nerve injury is through a combination of clinical examination, electrodiagnostic studies, and targeted imaging with MR neurography when necessary. 1, 2

Clinical Examination

• Assess motor function by testing:

  • Thumb opposition (opposition of thumb to little finger) 1
  • Thumb abduction (perpendicular movement of thumb away from palm) 1
  • Flexion of the distal phalanx of the index and middle fingers (anterior interosseous nerve function) 1, 3

• Evaluate sensory function by testing:

  • Light touch and two-point discrimination in the palmar aspect of the thumb, index, middle finger, and radial half of the ring finger 1
  • Compare with the unaffected side for reference 2

• Look for specific clinical signs:

  • "Ape hand" deformity (flattening of the thenar eminence) in chronic cases 4
  • Positive Tinel's sign at the site of injury or compression 1, 3
  • Weakness in pinch strength 4

Electrodiagnostic Studies

• Nerve conduction studies (NCS) and electromyography (EMG) are the gold standard for confirming median nerve injury 1, 2

  • Measure distal motor latency, conduction velocity, and amplitude 1
  • Compare with contralateral side and normative values 1
  • EMG can detect denervation changes in median-innervated muscles 1

• Timing considerations:

  • Perform initial studies within 7-10 days of injury 2
  • Repeat studies after 3-4 weeks if initial results are equivocal 3
  • Fibrillation potentials may not appear until 2-3 weeks after injury 3

Imaging Studies

• MR neurography is the preferred imaging modality for evaluating median nerve injuries 1, 2

  • High-resolution 3T imaging provides excellent soft-tissue contrast 1
  • Can identify nerve discontinuity, neuromas, and perineural edema 1
  • Particularly useful when clinical and electrodiagnostic findings are inconclusive 1

• Ultrasound can be a useful adjunct for:

  • Identifying compression, hematoma, or pseudoaneurysm 2
  • Evaluating the cross-sectional area of the median nerve 1
  • Real-time dynamic assessment of nerve movement 1

• CT and conventional MRI have limited roles in direct nerve assessment 1

Special Considerations

• For suspected high median nerve injuries (above elbow):

  • Focus on pronator teres, flexor carpi radialis, and anterior interosseous nerve function 5, 3
  • Assess for Martin-Gruber anastomosis (median-ulnar communication) that may mask deficits 6

• For suspected distal injuries (wrist level):

  • Differentiate from carpal tunnel syndrome through provocative tests 1
  • Assess thenar muscle atrophy 4

• Pitfalls to avoid:

  • Relying solely on sensory testing (may be preserved due to overlapping innervation) 5, 6
  • Missing partial injuries (incomplete loss of function) 3, 6
  • Delaying diagnosis (outcomes worsen with prolonged time to repair) 7

• When examining patients under anesthesia:

  • Intraoperative nerve monitoring can help identify nerve injury not visible to the naked eye 1
  • Periodic assessment of upper extremity position is recommended to prevent iatrogenic injury 1

Remember that early diagnosis and intervention lead to better functional outcomes, particularly for motor recovery of the median nerve 6, 7.