What are the causes of axillary lymphadenopathy?

Causes of Swollen Lymph Nodes Under the Armpit

Swollen axillary lymph nodes have a broad range of causes including infections, inflammatory conditions, malignancies (particularly breast cancer and lymphoma), and benign reactive processes, with the specific etiology requiring clinical correlation and often imaging or biopsy for definitive diagnosis 1.

Primary Etiologic Categories

Malignant Causes

• Breast cancer metastases - the most common malignant cause when axillary lymphadenopathy is present 1
  • Can present as occult breast cancer (primary tumor not visible on imaging) in approximately 5 cases per study cohort 2
  • Metastatic breast cancer accounts for approximately 26% of lymphadenopathy cases in screening populations 3
• Lymphoma - particularly non-Hodgkin's lymphoma and chronic lymphocytic leukemia/well-differentiated lymphocytic lymphoma (17% of cases) 3
• Metastases from non-breast primary malignancies 2, 3
• Leukemia 3, 4

Infectious/Inflammatory Causes

• Reactive lymphadenopathy - the most common benign cause (29% of cases), often from nonspecific inflammation 3
• Bacterial lymphadenitis (acute infections) 2
• Tuberculous lymphadenitis - may show characteristic macrocalcifications on imaging 2
• HIV-related lymphadenopathy 3
• Mastitis-associated reactive changes 3

Autoimmune/Systemic Causes

• Collagen vascular diseases (rheumatoid arthritis, lupus) 2, 3
• Sarcoidosis 3
• Amyloidosis 4

Vaccine-Related Causes

• COVID-19 vaccination - extremely common side effect (0.3-53% incidence) with average duration exceeding 100 days 5, 6
• Other vaccinations (influenza, etc.) 6

Other Benign Causes

• Accessory breast tissue lesions (fibroadenoma, lactational changes) 1, 7
• Soft tissue tumors (lipomas, hemangiomas, lymphangiomas, nerve sheath tumors) 7
• Post-surgical complications (seroma, lymphocele, hematoma) 7
• Epidermal cysts 3

Key Diagnostic Considerations

High-Risk Features Suggesting Malignancy

• Size >33 mm (97% specificity for malignancy) 3
• Hard, matted, or fused nodes 8
• Ill-defined or spiculated margins 3
• Loss of fatty hilum 4
• Epitrochlear or supraclavicular location 8
• Persistence >4 weeks with systemic symptoms (fever, night sweats, unintentional weight loss) 8

Important Clinical Pitfalls

Do not delay breast imaging for recent vaccination - robust evidence shows near-zero malignancy risk in asymptomatic patients with isolated ipsilateral axillary lymphadenopathy after COVID-19 vaccination when breast imaging is normal 5. However, always document vaccination history (vaccine type, date, injection site) 5, 6.

Benign and malignant lymph nodes often cannot be distinguished by imaging alone - in screening populations with lymphadenopathy as the sole finding, 52-62% ultimately have malignancy 4. Therefore, persistent, progressive, or suspicious nodes require tissue diagnosis regardless of vaccination status 6.

Recommended Evaluation Algorithm

For vaccine-related lymphadenopathy: If persists >3 months post-vaccination, perform sonographic follow-up at 6 months total. Biopsy if nodes are progressive, persistent beyond 6 months, or morphologically suspicious 5, 6.

For non-vaccine-related lymphadenopathy:

• Ultrasound is the appropriate initial imaging modality 1
• If breast cancer is suspected but mammography is negative, MRI detects occult primary in >66% of cases 1
• Fine-needle aspiration or core biopsy is indicated for nodes that are suspicious on imaging, >2 cm with loss of fatty hilum, or persistent >4 weeks with concerning features 4, 8

Critical exception: In patients with known or suspected breast cancer requiring staging, do not delay biopsy for follow-up intervals - suspicious nodes should be sampled immediately to avoid delaying appropriate oncologic therapy 5, 6.