What could cause a bump in the armpit of an adult or adolescent with no known medical history?

Armpit Bump: Differential Diagnosis and Management

An armpit bump in an adolescent or adult with no known medical history most commonly represents reactive lymphadenopathy from infection, recent vaccination, or less commonly, a skin/soft tissue infection such as a furuncle or abscess, though malignancy and other systemic conditions must be excluded based on specific clinical features. 1

Initial Clinical Assessment

Key Historical Features to Elicit

• Recent vaccination history (particularly COVID-19 vaccine within 6 weeks), as vaccine-related adenopathy is now a common cause and typically resolves spontaneously 1
• Timing and growth pattern: Rapidly enlarging masses over days to weeks raise concern for infection or malignancy, while stable or slowly growing lesions suggest benign processes 2
• Associated symptoms: Fever, night sweats, weight loss (suggesting systemic disease or malignancy); pain and erythema (suggesting infection); pruritus (suggesting dermatologic conditions) 1
• History of breast implants in females, as silicone migration to axillary lymph nodes can present as adenopathy 1
• History of malignancy, particularly breast, melanoma, lymphoma, or gastric cancer, as these can metastasize to axillary nodes 1, 3
• Skin trauma, insect bites, or injection drug use that could lead to localized infection 1

Critical Physical Examination Findings

• Lymph node characteristics: Size (>1 cm warrants further evaluation), consistency (hard suggests malignancy, fluctuant suggests abscess), mobility (fixed nodes suggest malignancy), and tenderness (painful suggests infection or inflammation) 1, 2
• Skin changes: Erythema, warmth, fluctuance, or a "wooden-hard" feel of subcutaneous tissues (the latter suggesting necrotizing fasciitis, a surgical emergency) 1
• Presence of a visible skin lesion: Furuncle, carbuncle, or evidence of cellulitis tracking along fascial planes 1
• Bilateral vs. unilateral: Unilateral adenopathy ipsilateral to recent vaccination is expected; bilateral adenopathy suggests systemic disease 1
• Examination of other lymph node regions and assessment for hepatosplenomegaly to evaluate for systemic lymphadenopathy 2

Primary Differential Diagnoses

Reactive Lymphadenopathy (Most Common)

• Post-vaccination adenopathy: Occurs within days to weeks after vaccination (especially COVID-19), typically resolves within 6 weeks, and is ipsilateral to the injection site 1
• Infectious causes: Upper respiratory infections, skin infections, or systemic infections can cause reactive adenopathy 1

Skin and Soft Tissue Infections

• Furuncle/carbuncle: Caused by Staphylococcus aureus, presents as a painful, erythematous nodule with central pustule formation 1
• Cellulitis: Diffuse spreading infection with erythema, warmth, and edema; may have associated lymphangitis 1
• Abscess: Fluctuant, tender mass requiring incision and drainage 1

Critical pitfall: A "wooden-hard" feel to subcutaneous tissues with systemic toxicity suggests necrotizing fasciitis, which requires immediate surgical consultation and is a medical emergency 1

Malignancy-Related

• Metastatic disease: Breast cancer (most common in females), melanoma, lymphoma, or rarely gastric cancer can present as axillary adenopathy 1, 3
• Primary lymphoma: Painless, firm, rubbery lymph nodes that progressively enlarge 2

Other Causes

• Silicone adenitis: In patients with breast implants, free silicone can migrate to axillary lymph nodes, creating a "snowstorm" appearance on ultrasound 1
• Hidradenitis suppurativa: Chronic inflammatory condition affecting apocrine glands, presents with recurrent painful nodules and abscesses 1
• Lipoma or other benign soft tissue tumors: Soft, mobile, non-tender masses 2

Diagnostic Algorithm

For Patients with Recent Vaccination (<6 weeks)

• Expectant management without imaging is appropriate for small (<2 cm), mobile, non-tender nodes in low-risk patients 1
• Short-interval ultrasound follow-up at 6 weeks if the adenopathy persists or if there is higher clinical concern 1
• Immediate evaluation if nodes are hard, fixed, rapidly enlarging, or associated with systemic symptoms 1

For Patients Without Recent Vaccination or High-Risk Features

Initial imaging recommendation:

• Ultrasound is the first-line imaging modality for evaluating axillary masses, as it can distinguish between lymph nodes, abscesses, and solid masses, and can identify morphologically abnormal lymph nodes 1
• Ultrasound can diagnose silicone adenitis (in implant patients), identify abscesses requiring drainage, and guide fine-needle aspiration if needed 1

Age-specific considerations:

• Patients <30 years: Ultrasound alone is typically sufficient initially; mammography/tomosynthesis reserved for suspicious ultrasound findings 1
• Patients 30-39 years: Ultrasound with consideration for diagnostic mammography or digital breast tomosynthesis if suspicious findings identified 1
• Patients ≥40 years: Diagnostic mammography or digital breast tomosynthesis with complementary ultrasound to evaluate for occult breast malignancy 1

When to Pursue Tissue Diagnosis

Biopsy is indicated when:

• Lymph nodes are morphologically abnormal on ultrasound (loss of fatty hilum, irregular cortex, round shape) 1
• High risk of metastatic disease based on cancer history 1
• Nodes persist beyond 6 weeks post-vaccination without decrease in size 1
• Clinical suspicion for lymphoma (painless, progressive enlargement, systemic symptoms) 2
• Suspected abscess that requires culture-directed antibiotic therapy 1

Fine-needle aspiration or core biopsy can be performed under ultrasound guidance for solid masses or abnormal lymph nodes 1, 3

Management Based on Etiology

Infectious Causes

For furuncles/carbuncles:

• Incision and drainage for fluctuant lesions 1
• Oral antibiotics covering S. aureus (including MRSA if prevalent in community): Options include trimethoprim-sulfamethoxazole, doxycycline, or clindamycin 1
• For recurrent furunculosis: Nasal mupirocin ointment twice daily for first 5 days each month, or clindamycin 150 mg daily for 3 months 1

For cellulitis:

• Oral antibiotics for mild cases: Cephalexin, dicloxacillin, or clindamycin 1
• Intravenous antibiotics for severe cases with systemic toxicity 1
• Immediate surgical consultation if necrotizing fasciitis suspected (wooden-hard tissues, rapid progression, systemic toxicity) 1

Post-Vaccination Adenopathy

• Reassurance and expectant management without routine follow-up imaging for typical presentations 1
• Follow-up ultrasound at 6 weeks if adenopathy persists or if patient has higher baseline cancer risk 1

Malignancy-Related

• Multidisciplinary discussion involving oncology, surgery, and radiology 1
• MRI with contrast if biopsy confirms metastatic disease from occult primary breast cancer 1
• Appropriate staging and treatment based on primary malignancy 1, 3

Silicone Adenitis

• Evaluation for implant rupture with dedicated breast imaging 1
• Surgical consultation for implant removal or replacement if rupture confirmed 1

Red Flags Requiring Urgent Evaluation

• Wooden-hard subcutaneous tissues with systemic toxicity (necrotizing fasciitis) 1
• Rapidly enlarging mass over days to weeks 2
• Hard, fixed, non-tender lymph nodes (malignancy) 2
• Constitutional symptoms: Fever, night sweats, unintentional weight loss 2
• History of malignancy with new adenopathy 1, 3

Prevention of Recurrent Issues

For patients with history of cellulitis or lymphedema:

• Prophylactic antibiotics: Monthly intramuscular benzathine penicillin 1.2 MU or oral penicillin V 1 g twice daily or erythromycin 250 mg twice daily 1
• Weight loss for overweight/obese patients to reduce lymphedema risk 1
• Skin care: Treat interdigital maceration, maintain skin hydration, compression stockings for edema 1

For patients undergoing cancer screening:

• Schedule mammography either before vaccination or at least 6 weeks after completion to avoid false-positive findings 1