What could be causing multiple sore knots on my arm, underarm, and face?

Multiple Sore Knots on Arm, Underarm, and Face

Immediate Life-Threatening Causes to Exclude First

If you have fever, systemic symptoms, or recent tick exposure, seek immediate medical attention as this could represent Rocky Mountain spotted fever, which has 5-10% mortality if untreated. 1, 2

• Look specifically for fever >39°C (102.2°F), headache, malaise, and whether the lesions started as pink macules evolving to vesicles 2
• Check for recent outdoor activities or tick bites in the past 2 weeks 1
• If present with fever and systemic illness, empiric doxycycline treatment should begin immediately without waiting for confirmation 2

Most Likely Diagnoses Based on Clinical Presentation

Erythema Nodosum (Most Common Benign Cause)

Erythema nodosum presents as raised, tender, red or violet subcutaneous nodules 1-5cm in diameter, commonly affecting the arms and can involve the trunk. 3

• These nodules are painful and firm to touch 3
• Often associated with systemic symptoms including fatigue and joint pain 3
• Diagnosis is made clinically without biopsy in typical cases 3
• Treatment focuses on identifying and treating the underlying trigger; systemic corticosteroids are required in severe cases 3
• For relapsing or resistant forms, immunomodulators like azathioprine or anti-TNF agents (infliximab, adalimumab) may be used 3

Skin and Soft Tissue Infections

Furuncles (boils) appear as inflammatory nodules with overlying pustules, caused by Staphylococcus aureus, and can occur anywhere on hairy skin. 3

• Multiple furuncles suggest furunculosis, which may occur in outbreaks 3
• Small furuncles respond to moist heat application; larger lesions require incision and drainage 3
• Systemic antibiotics are unnecessary unless extensive surrounding cellulitis or fever is present 3
• Recurrent furunculosis may require eradication of nasal S. aureus carriage with antibacterial agents 3

Cutaneous Abscesses

Cutaneous abscesses are painful, tender, fluctuant red nodules often surrounded by erythematous swelling. 3

• Treatment requires incision, thorough evacuation of pus, and probing to break up loculations 3
• Simply covering with dry dressing is usually sufficient; packing or suturing may be used 3
• Gram stain, culture, and systemic antibiotics are rarely necessary unless multiple lesions, extensive cellulitis, or systemic symptoms are present 3

Critical Red Flags Requiring Urgent Evaluation

• Rapidly progressive lesions (increasing >1cm to >10cm in <24 hours) suggest ecthyma gangrenosum or necrotizing fasciitis 3
• Violaceous edges with central necrosis developing within 24 hours indicates possible pyoderma gangrenosum or ecthyma gangrenosum 3
• Fever with neutropenia (if on chemotherapy or immunosuppressed) requires immediate broad-spectrum antibiotics targeting Pseudomonas aeruginosa 3
• Hemodynamic instability with skin lesions mandates emergency evaluation for sepsis 3

Specific Examination Findings to Document

• Measure the exact size of each nodule (1-5cm suggests erythema nodosum; >5cm suggests other pathology) 3
• Check for fluctuance (present in abscesses, absent in erythema nodosum) 3
• Assess for central pustule or hair follicle involvement (suggests furuncle) 3
• Look for violaceous borders or central necrosis (suggests pyoderma gangrenosum or ecthyma gangrenosum) 3
• Palpate for regional lymphadenopathy 3
• Check temperature and vital signs 1, 2

When to Biopsy

• Biopsy is not routinely needed for typical erythema nodosum presentation 3
• Consider biopsy if lesions have atypical features, violaceous borders, or rapid progression to rule out pyoderma gangrenosum or vasculitis 3
• Biopsy from the periphery of ulcerating lesions if pyoderma gangrenosum is suspected 3

Common Pitfalls to Avoid

• Do not wait for the classic triad (fever, rash, tick bite) before treating suspected Rocky Mountain spotted fever—only a minority present with all three initially, and delayed treatment increases mortality 1
• Do not assume all nodules are infectious—erythema nodosum is inflammatory, not infectious, and does not respond to antibiotics alone 3
• Do not prescribe antibiotics for simple furuncles unless extensive cellulitis or fever is present—incision and drainage is the primary treatment 3
• Do not confuse erythema nodosum with metastatic Crohn's disease—the latter shows non-caseating granulomas on histology 3