What is the differential diagnosis and appropriate workup for chronic lateral axillary shooting pain in an adult without known systemic illness?

Chronic Lateral Axillary Shooting Pain: Differential Diagnosis and Workup

For chronic shooting pain in the lateral axillary area lasting one year, the primary concern is brachial plexopathy, and you should obtain dedicated MRI of the brachial plexus as your initial imaging study.

Differential Diagnosis

The shooting nature and chronicity of this pain points toward a neurologic etiology. Based on the anatomic location and pain characteristics, consider:

Neurologic Causes (Most Likely)

• Brachial plexopathy - inflammatory, immune-mediated, or compressive etiologies 1
  • Parsonage-Turner syndrome (neuralgic amyotrophy/brachial plexitis)
  • Chronic inflammatory demyelinating polyradiculoneuropathy
  • Nerve sheath tumors (schwannomas, neurofibromas)
  • Desmoid tumors or lipomas compressing the plexus
• Upper thoracic radiculopathy - particularly important as axillary pain can be a heralding sign of neoplasm involving upper thoracic roots 2
• Axillary nerve injury - though typically presents more acutely 3
• Thoracic outlet syndrome - if symptoms worsen with arm positioning 4

Structural/Mass Lesions

• Malignancy - lymphoma, metastatic disease (lung/breast), or Pancoast tumor invading lower brachial plexus 1
• Axillary lymphadenopathy - infectious, inflammatory, or neoplastic 5, 6

Less Likely Given Chronicity

• Referred pain from cervical spine pathology
• Muscular causes (though these typically don't produce shooting pain)

Recommended Workup Algorithm

Step 1: Clinical Assessment

Focus your history on:

• Pain characteristics: Distribution along specific dermatomes or nerve territories
• Motor weakness: Which specific muscles are affected
• Sensory changes: Exact distribution of numbness/tingling
• Red flags: Fever, night sweats, unintentional weight loss, progressive symptoms
• Risk factors: History of cancer, recent trauma, occupational repetitive movements, immunosuppression
• Functional limitations: Specific activities that provoke symptoms

Physical examination should identify:

• Specific muscle weakness patterns
• Sensory distribution abnormalities
• Lymph node characteristics (size >2cm, hard, matted, supraclavicular location suggest malignancy) 6
• Provocative maneuvers for thoracic outlet syndrome

Step 2: Initial Imaging - MRI Brachial Plexus

MRI of the brachial plexus is the gold standard initial imaging study 1. This must be a dedicated brachial plexus protocol, NOT a routine neck or chest MRI. The study should include:

• Orthogonal views through oblique planes of the plexus
• T1-weighted, T2-weighted, and fat-saturated T2-weighted sequences
• Fat-saturated T1-weighted post-contrast sequences
• High-resolution T2-weighted MR neurography sequences

MRI provides superior soft-tissue contrast and spatial resolution to define intraneural anatomy and localize pathologic lesions when electrodiagnostic findings are nonspecific 1.

Step 3: Complementary Studies

Electrodiagnostic testing should be obtained within 4 weeks if not already done, with follow-up at 12 weeks 3. This helps:

• Localize the lesion (root, plexus, or peripheral nerve)
• Assess severity and chronicity
• Guide prognosis

Ultrasound of the axilla may be appropriate if:

• A palpable mass is present 5
• Lymphadenopathy is suspected clinically
• US-guided biopsy may be needed

Step 4: Additional Imaging Based on MRI Findings

If MRI suggests mass lesion or malignancy:

• CT chest/abdomen/pelvis to evaluate for primary malignancy or metastatic disease
• Consider chest imaging specifically to rule out Pancoast tumor if lower plexus involvement 1

If MRI is negative but clinical suspicion remains high for upper thoracic pathology:

• MRI cervical and upper thoracic spine - remember that axillary pain can be the presenting symptom of upper thoracic root neoplasm 2

Critical Pitfalls to Avoid

1. Don't order routine neck or chest MRI - these lack the specialized sequences needed to properly evaluate the brachial plexus 1

2. Don't dismiss chronic axillary pain as musculoskeletal - the shooting quality and one-year duration warrant neurologic investigation. Severe or progressive axillary pain may indicate malignant pathology 2

3. Don't wait too long for surgical consultation if nerve injury is identified - optimal surgical outcomes occur within 3-6 months of injury 3

4. Don't forget upper thoracic roots - these can present with axillary pain and are easily missed if imaging focuses only on the cervical spine 2

5. Avoid empiric corticosteroids before tissue diagnosis if lymphadenopathy is present, as they can mask lymphoma or other malignancy 6

When to Escalate

Refer urgently if:

• Constitutional symptoms (fever, night sweats, weight loss)
• Progressive motor weakness
• Hard, fixed, or supraclavicular lymph nodes
• No improvement on electrodiagnostic testing at 12 weeks 3