Can pneumonia cause axillary lymph node enlargement?

Pneumonia and Axillary Lymph Node Enlargement

Pneumonia can rarely cause axillary lymph node enlargement, primarily in cases with severe infection, atypical pathogens, or when there is chest wall involvement. 1

Relationship Between Pneumonia and Lymphadenopathy

Typical Lymph Node Involvement in Pneumonia

• Pneumonia most commonly causes enlargement of mediastinal and hilar lymph nodes
• According to the 2020 rapid advice guideline for COVID-19 pneumonia, mediastinal lymph node enlargement is considered an atypical finding, occurring in only a small percentage of cases 1
• Axillary lymph node involvement is not a common manifestation of typical pneumonia

When Axillary Lymph Nodes May Enlarge in Pneumonia

• In cases with chest wall invasion from severe pneumonia 2
• With certain atypical pathogens that can cause more widespread lymphadenopathy
• As part of a pronounced immune response, particularly in younger, otherwise healthy patients 3

Diagnostic Approach for Axillary Lymphadenopathy

When encountering axillary lymphadenopathy in a patient with suspected pneumonia:

1. Complete clinical evaluation to assess for other sites of adenopathy and potential non-pneumonia etiologies 1
2. Age-appropriate diagnostic imaging:
   • Ultrasound with mammogram for patients ≥30 years
   • Ultrasound alone for patients <30 years 1
3. Consider lymph node characteristics that suggest malignancy:
   • Short axis diameter >10 mm
   • Cortical thickness >3 mm
   • Loss of fatty hilum
   • Irregular or spiculated margins
   • Heterogeneous enhancement 4

Alternative Causes to Consider

When axillary lymphadenopathy is present with respiratory symptoms, consider these alternative diagnoses:

• Malignancy: Primary lung cancer with axillary metastasis (rare, occurs in <1% of lung cancer cases) 2
• Atypical infections:
  • Chlamydia pneumoniae (can cause hilar and occasionally more widespread lymphadenopathy) 5
  • Tuberculosis
• Autoimmune conditions: Sarcoidosis, connective tissue diseases
• Recent vaccination: COVID-19 mRNA vaccines commonly cause transient axillary lymphadenopathy 6
• Rare conditions: Kikuchi-Fujimoto disease (histiocytic necrotizing lymphadenitis) 7

Clinical Pearls and Pitfalls

• Pearl: Always inquire about recent vaccinations, especially COVID-19 mRNA vaccines, which are a common cause of transient axillary lymphadenopathy 6
• Pitfall: Assuming axillary lymphadenopathy with respiratory symptoms is always due to pneumonia - consider broader differential diagnosis
• Pearl: Persistent axillary lymphadenopathy after appropriate antibiotic treatment for pneumonia warrants further investigation
• Pitfall: Missing lung cancer with axillary metastasis - check for mediastinal involvement, which typically precedes axillary spread 2

Management Approach

1. Treat the underlying pneumonia appropriately
2. If axillary lymphadenopathy persists after pneumonia treatment:
   • Perform ultrasound evaluation of the lymph nodes
   • Consider core needle biopsy for suspicious lymph nodes 1
   • Follow up with repeat imaging in 3-6 months for benign-appearing nodes 4

Remember that while pneumonia can occasionally cause axillary lymphadenopathy, persistent or prominent axillary lymph node enlargement should prompt consideration of alternative diagnoses.