How should I clinically test for axillary nerve damage?

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Clinical Testing for Axillary Nerve Damage

Test for axillary nerve damage by assessing deltoid muscle strength (shoulder abduction against resistance), checking for sensory loss over the lateral shoulder ("regimental badge" area), and confirming with electrodiagnostic studies including nerve conduction studies and electromyography. 1, 2

Physical Examination Findings

Motor Testing

  • Assess shoulder abduction strength by having the patient raise their arm laterally against resistance, comparing side-to-side symmetry 1
  • Look for visible deltoid muscle atrophy, which becomes apparent within 3-4 weeks of injury and indicates significant axonal loss 3, 1
  • Test the teres minor muscle (external rotation with arm at side), as it is also innervated by the axillary nerve 4
  • Grade muscle strength using manual muscle testing or isometric testing, documenting whether strength is good, fair, poor, or absent 1

Sensory Testing

  • Check sensation over the lateral aspect of the shoulder in the "regimental badge" distribution, which is the cutaneous territory of the superior lateral brachial cutaneous nerve (terminal sensory branch of the axillary nerve) 4, 1
  • Compare sensation bilaterally using light touch and pinprick testing 1

Inspection and Palpation

  • Inspect for deltoid muscle wasting by comparing the contour of both shoulders, particularly the middle deltoid prominence 5-7 cm distal to the acromion 1, 2
  • Palpate the deltoid muscle for tone and bulk asymmetry 1

Electrodiagnostic Confirmation

Nerve Conduction Studies

  • Perform axillary motor nerve conduction studies bilaterally to compare compound muscle action potentials (CMAPs) between sides 2
  • Place the active electrode over the most prominent portion of the middle deltoid, approximately 5-7 cm distal to the acromion, with the reference electrode over the acromion 2
  • Stimulate supramaximally at Erb's point 2
  • Asymmetry of >40% in CMAPs between symptomatic and asymptomatic sides has 95.2% sensitivity and 96.6% specificity for detecting axillary nerve lesions 2
  • Normal values for males: CMAP ≥7.6 mV, onset latency ≤5.0 ms; for females: CMAP ≥6.5 mV, onset latency ≤3.5 ms 2

Electromyography

  • Perform EMG of the deltoid and teres minor muscles to assess for denervation potentials (fibrillations, positive sharp waves) and motor unit action potential changes 3, 1
  • EMG findings help determine the severity of axonal loss and timing of injury 3
  • Wait 3-4 weeks after injury before performing EMG, as denervation changes take time to develop 3

Imaging Considerations

Ultrasound

  • Ultrasound can directly visualize the axillary nerve and assess for structural abnormalities, though it requires thorough knowledge of local anatomy 5
  • Sonographic findings correlate well with surgical exploration for determining severity and location of nerve injury 5
  • Ultrasound serves as a first-line imaging tool for axillary nerve trauma when performed by experienced operators 5

MRI

  • MRI can define the site and extent of nerve injury, though imaging of the axillary nerve is technically challenging due to its complex course through multiple shoulder compartments 5

Common Pitfalls and How to Avoid Them

  • Pitfall: Failing to test bilaterally - Always compare the affected side to the unaffected side for both motor strength and nerve conduction studies, as side-to-side comparison is more sensitive than comparing to population norms 1, 2
  • Pitfall: Performing electrodiagnostic studies too early - Wait at least 3-4 weeks after injury before EMG testing to allow denervation changes to develop; nerve conduction studies can be performed earlier 3, 1
  • Pitfall: Missing associated injuries - Axillary nerve injury commonly occurs with shoulder dislocation or direct trauma, so assess for concurrent bony or ligamentous injuries 1
  • Pitfall: Assuming isolated sensory loss rules out axillary nerve injury - Some patients may have predominantly motor involvement with minimal sensory changes 4
  • Pitfall: Delaying surgical exploration - If no recovery is observed by 3-4 months following injury on serial electrodiagnostic testing, surgical exploration is indicated 1

References

2
Research

Axillary motor nerve conduction study: Description of technique and provision of normative data.

Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology, 2018

3
Research

Iatrogenic axillary neuropathy after intramuscular injection of the deltoid muscle.

American journal of physical medicine & rehabilitation, 2007

4
Research

Axillary and musculocutaneous neuropathies.

Handbook of clinical neurology, 2024

5
Research

Ultrasound imaging of the axillary nerve and its role in the diagnosis of traumatic impairment.

Ultraschall in der Medizin (Stuttgart, Germany : 1980), 2014

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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