What is the diagnosis and treatment for an adult patient with no significant medical history presenting with erythematous rashes in the axilla, inguinal area, and neck?

Differential Diagnosis and Management of Erythematous Rashes in Axilla, Inguinal Area, and Neck

The most critical immediate consideration is Rocky Mountain Spotted Fever (RMSF) or other tickborne rickettsial diseases, which require urgent empiric treatment with doxycycline to prevent mortality, as these infections can be rapidly fatal if untreated. 1

Immediate Life-Threatening Considerations

Tickborne Rickettsial Diseases (RMSF, Ehrlichiosis)

• Initiate empiric doxycycline 2.2 mg/kg orally twice daily immediately if clinical suspicion exists, without waiting for serologic confirmation 1
• Up to 40% of RMSF patients report no tick bite history 1
• Ticks preferentially attach in difficult-to-observe locations including scalp, axillae, and inguinal regions 1
• Early presentation (first 2-4 days) typically shows nonspecific fever, headache, and myalgia before rash appears 1
• Rash in RMSF typically begins on wrists/ankles but can involve axillae and groin 1
• Consider concurrent ceftriaxone administration if meningococcemia cannot be excluded 1
• RMSF can cause death within 9 days of illness onset if untreated 1

Meningococcemia

• Can present with erythematous rash progressing to petechiae in similar distribution 1
• Requires immediate blood cultures and empiric antibiotics 1

Common Dermatologic Diagnoses

Histiocytic Neoplasms (If Chronic Presentation)

Langerhans Cell Histiocytosis (LCH)

• Presents as erythematous papular rash in groin, abdomen, chest, or back 1
• Can be misdiagnosed as seborrheic dermatitis when scalp involved 1
• Requires tissue biopsy for definitive diagnosis 1

Erdheim-Chester Disease (ECD)

• Affects axillary and inguinal folds with yellow-brown xanthomatous papulonodules 1
• Lesions range from few millimeters to 1.5 cm 1
• Pruritus may be present 1
• Requires comprehensive systemic evaluation including cardiac and pulmonary assessment 1

Rosai-Dorfman Disease (RDD)

• Presents with red-to-brown macules or papules in 50% of patients 1
• Can manifest as panniculitis with tender subcutaneous nodules 1

Fungal Infections

Tinea Cruris with Extension

• Well-demarcated erythematous plaques with scaling borders 2
• Prominent advancing border with central clearing 2
• Satellite lesions help differentiate from inverse psoriasis 2
• Diagnose with KOH preparation or fungal culture from lesion scrapings 2
• Treat with topical antifungals and ensure complete drying of skin folds after bathing 2

Inflammatory Dermatoses

Contact Dermatitis (Allergic or Irritant)

• Large erythematous plaques without scaling but with pruritus 3
• Intertriginous areas have increased corticosteroid absorption risk 3
• Use only low-potency hydrocortisone 1% on axillae, groin, and neck to prevent skin atrophy 3
• Add oral antihistamines: cetirizine 10 mg, loratadine 10 mg, or fexofenadine 180 mg daily 3
• Identify and discontinue causative agents immediately 3

Inverse Psoriasis

• Well-demarcated minimally scaling plaques in intertriginous areas 2
• Lacks satellite lesions unlike fungal infections 2

Diagnostic Algorithm

Step 1: Assess for Systemic Illness

• If fever, headache, myalgia present: Start doxycycline immediately for presumed RMSF 1
• Query recent outdoor activities, tick exposure, travel to endemic areas 1
• Check for concurrent illness in family members or pets 1

Step 2: Characterize the Rash

• Petechial/purpuric: Consider RMSF, meningococcemia, vasculitis 1, 4
• Erythematous with scaling borders: Likely fungal (obtain KOH prep) 2
• Erythematous without scaling: Consider contact dermatitis or early inflammatory process 3
• Xanthomatous papulonodules: Consider histiocytic neoplasms (requires biopsy) 1

Step 3: Laboratory Evaluation (If No Acute Infection Suspected)

• Complete blood count, comprehensive metabolic panel, thyroid function 5
• Skin scrapings for KOH preparation and fungal culture 2
• Consider skin biopsy if diagnosis uncertain after 2 weeks of appropriate therapy 3

Treatment Approach

For Suspected Tickborne Disease

• Doxycycline 2.2 mg/kg orally twice daily for minimum 5 days 1
• Do not delay treatment waiting for serologic confirmation 1
• Obtain convalescent serology 2-4 weeks later 1

For Inflammatory Dermatoses

• Low-potency hydrocortisone 1% for intertriginous areas 3
• Oral antihistamines for pruritus 3
• Avoid high-potency steroids on face, genitalia, or skin folds 3
• Taper steroids gradually rather than abrupt discontinuation 3

For Fungal Infections

• Topical antifungals to affected areas 2
• Keep skin folds completely dry 2
• Daily changes of clothing 2
• Treat concurrent tinea pedis if present 2

Critical Pitfalls to Avoid

• Never delay doxycycline if RMSF suspected—mortality increases dramatically with delayed treatment 1
• Never use high-potency corticosteroids in axillae or groin due to increased absorption and atrophy risk 3
• Never assume absence of tick bite excludes RMSF—40% have no bite history 1
• Do not apply potent steroids to large body surface areas for extended periods 3

When to Refer to Dermatology

• No response to appropriate first-line therapy after 2 weeks 3
• Diagnosis remains uncertain despite initial evaluation 3
• Development of systemic symptoms (fever, malaise, joint pain) 3
• Lesions become painful or suggest vasculitis 3
• Suspected histiocytic neoplasm requiring tissue diagnosis 1