What is the differential diagnosis and management for erythematous skin in the axilla?

Differential Diagnosis of Raised Erythematous Streak in Axilla

The most likely diagnoses for a raised, erythematous streak in the axilla without detergent changes include erythrasma, tinea cruris (with axillary extension), hidradenitis suppurativa (early stage), erythema migrans (Lyme disease), and less commonly, necrotizing fasciitis or drug-induced eruptions.

Primary Diagnostic Considerations

Erythrasma

• Most common bacterial infection of intertriginous areas, including axillae, caused by Corynebacterium minutissimum 1
• Presents as well-demarcated patches with central hypopigmentation and peripheral hyperpigmented scaling in warm, moist skin folds 1
• Definitive diagnosis: Wood lamp examination reveals bright coral-pink fluorescence 1
• Not typically raised unless chronic with secondary changes 1

Tinea Cruris (Dermatophyte Infection)

• Presents with well-demarcated, erythematous plaques with scaling borders and prominent advancing border with central clearing 2
• Satellite lesions are a key clinical clue for dermatophyte infection 2
• Axillary involvement can occur, though groin is more typical 2
• Diagnosis confirmed by KOH preparation or fungal culture from skin scrapings 2
• Unlike candidal infections, tinea has more prominent scaling and less maceration 2

Hidradenitis Suppurativa (Early Stage)

• Presents with painful nodules, abscesses, sinus tracts, or bridged scars in axillae, groin, perineum, and other flexural areas 3
• Requires chronic and recurrent presentation for diagnosis 3
• Can present with erythematous papules, pustules, or nodules before abscess formation 3
• Associated with comedones (characteristically paired) 3

Erythema Migrans (Lyme Disease)

• Erythema migrans lesions often occur at sites that would be highly unusual for community-acquired bacterial cellulitis, specifically including the axilla 3
• Lesions can be homogeneously erythematous or have central clearing with target-like appearance 3
• An erythematous skin lesion present while an Ixodes tick is still attached or within 48 hours of detachment is likely tick bite hypersensitivity (<5 cm, urticarial, resolves in 24-48 hours) rather than erythema migrans 3
• True erythema migrans increases in size over 1-2 days; mark borders with ink to observe progression 3
• Not scaly unless long-standing and fading 3

Critical Differentials Requiring Urgent Evaluation

Necrotizing Fasciitis

• Initial presentation mimics cellulitis but progresses with distinctive wooden-hard feel of subcutaneous tissues 3
• Cellulitis or erythema (90%), edema (80%), skin discoloration or gangrene (70%), and anesthesia of involved skin are key features 3
• A broad erythematous tract in the skin along the route of infection as it advances is often observable 3
• Systemic toxicity with high fever, disorientation, and lethargy 3
• Requires immediate surgical consultation if suspected 3

Drug-Induced Eruptions

• Anticancer agents and other medications can cause toxic erythema resembling various conditions 3, 4
• Eczematous dermatitis from medications (e.g., telaprevir) affects neck and axillary creases particularly, with erythema, xerosis, and vesicular lesions 3
• Grade 3 rash (>50% body surface) or suspicion of DRESS/Stevens-Johnson syndrome requires immediate discontinuation of offending agent and emergency hospitalization 3

Diagnostic Algorithm

Initial Assessment

1. Examine lesion characteristics:

   • Size and borders (well-demarcated vs. ill-defined) 3, 1
   • Presence of scaling, central clearing, or target appearance 3, 2
   • Texture: raised nodules vs. flat patches 3, 1
   • Associated features: satellite lesions, comedones, pustules 3, 2

2. Palpate subcutaneous tissues:

   • Yielding tissues suggest cellulitis/erythrasma; wooden-hard feel indicates necrotizing fasciitis requiring emergency intervention 3

3. Mark lesion borders with ink and observe for 24-48 hours:

   • Expansion suggests erythema migrans or progressive infection 3
   • Resolution suggests hypersensitivity reaction 3

Diagnostic Testing

• Wood lamp examination: coral-pink fluorescence confirms erythrasma 1
• KOH preparation or fungal culture for suspected dermatophyte infection 2
• Skin biopsy if diagnosis uncertain, particularly to exclude cutaneous T-cell lymphoma or other serious conditions 3, 5
• Consider tick exposure history and serologic testing for Lyme disease if epidemiologically appropriate (though serology is insensitive in first 2 weeks) 3

Management Approach

For Erythrasma

• Topical 2% mupirocin ointment twice daily leads to resolution 1
• Alternative: oral erythromycin or topical clindamycin 1

For Tinea Cruris

• Topical antifungal agents (azoles, allylamines) 2
• Complete drying of axillary folds after bathing is essential 2
• Daily changes of clothing to reduce fungal burden 2

For Hidradenitis Suppurativa (Early)

• Topical or systemic antibiotics depending on severity 3
• Pain management 3
• Dermatology referral for chronic management 3

For Erythema Migrans

• Treatment based on clinical findings without waiting for serologic confirmation 3
• Oral doxycycline, amoxicillin, or cefuroxime for 10-21 days 3

Symptomatic Management

• Topical hydrocortisone 1% applied to affected area 3-4 times daily for itching and inflammation 6
• Avoid in suspected fungal or bacterial infections until diagnosis confirmed 6

Critical Pitfalls to Avoid

• Do not dismiss a raised erythematous streak in the axilla as simple irritant dermatitis without proper evaluation, as this location is atypical for common cellulitis and may indicate Lyme disease 3
• Do not delay surgical consultation if necrotizing fasciitis is suspected based on wooden-hard subcutaneous tissues or rapid progression 3
• Do not rely on serologic testing for early Lyme disease diagnosis; treat based on clinical presentation 3
• Do not confuse erythema multiforme (which presents with target lesions on extremities) with simple axillary erythema 4, 7
• In immunocompromised patients, consider broader differential including disseminated fungal infections, which can present with erythematous nodules in axilla and other sites 3