What is the differential diagnosis and treatment for a skin rash on the buttocks?

Differential Diagnosis of Buttock Rash

The differential diagnosis for a buttock rash depends critically on whether fever is present, the morphology of the lesion, and recent travel or environmental exposures, with life-threatening tickborne illnesses requiring immediate empiric treatment if systemic symptoms exist.

Immediate Life-Threatening Considerations

If fever, headache, or systemic symptoms are present with a buttock rash, initiate doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation 1. This covers:

• Rocky Mountain Spotted Fever (RMSF): Presents with small blanching pink macules on ankles/wrists that progress to maculopapular rash with central petechiae, spreading to trunk and extremities with 5-10% mortality 1
• Human Monocytic Ehrlichiosis: Rash occurs in only 30% of adults, appearing later (median 5 days) with 3% mortality 1

Critical red flags requiring immediate doxycycline include: fever + rash + headache + tick exposure, or presence of thrombocytopenia/hyponatremia 1. Obtain complete blood count, comprehensive metabolic panel, and acute serology for R. rickettsii, E. chaffeensis, and A. phagocytophilum immediately 1. Expect clinical improvement within 24-48 hours of starting treatment 1.

Parasitic/Tropical Infections (Travel-Related)

Onchocerciasis

• Presentation: Diffuse, pruritic dermatitis predominantly over legs and buttocks, developing into "leopard skin" hypopigmented patches in chronic cases 2
• Exposure: Recent travel near fast-flowing rivers in Africa, Central/South America, or Arabian peninsula 2
• Incubation: 8-20 months 2
• Diagnosis: Blood microscopy taken within 2 hours of midnight and serology 2
• Treatment: Diethylcarbamazine under specialist guidance 2

Larva Currens (Strongyloides)

• Presentation: Itchy, linear, urticarial rash that moves several millimeters per second, most commonly around trunk, upper legs, and buttocks 2
• Mechanism: Subcutaneous larval migration 2
• Treatment: Ivermectin 200 mcg/kg once daily for 2 days or albendazole 400 mg once daily for 3 days 2

Cutaneous Larva Migrans

• Presentation: Characteristic migratory rash with eosinophilia 2, 3
• Treatment: Ivermectin 200 mcg/kg single dose or albendazole 400 mg once daily for 3 days 2
• Alternative in pregnancy: Local application of liquid nitrogen instead of oral anthelmintics 3

Inflammatory Dermatoses

Contact Dermatitis (Irritant or Allergic)

• Presentation: Erythematous, pruritic rash from chronic cumulative exposure to detergents, soaps, or other irritants 4
• Diagnosis: Patch testing essential to distinguish allergic from irritant type 4
• Treatment: Hydrocortisone 2.5% applied to affected area 3-4 times daily 5

Psoriasis (Inverse/Flexural)

• Presentation: Erythematous inflammatory plaques, may lack typical silvery scale in intertriginous areas 4
• Note: Difficult to distinguish from chronic allergic contact dermatitis 4

Drug-Induced Eruptions

Mild Drug Eruption (Grade 1-2)

• Presentation: Fine reticular maculopapular rash or broad, flat erythematous macules covering <30% body surface area 1
• Management:
  • Alcohol-free moisturizers with 5-10% urea twice daily 2
  • Low-potency topical corticosteroids (hydrocortisone 2.5% or alclometasone 0.05%) twice daily 2
  • Oral tetracyclines (doxycycline 100 mg twice daily or minocycline 100 mg once daily) for at least 6 weeks 2

Severe Drug Eruption (Grade 3-4)

If skin sloughing, vesicles, or mucosal involvement present with >30% body surface area involvement, suspect Stevens-Johnson syndrome/toxic epidermal necrolysis and discontinue all medications immediately 6. This requires:

• Emergency hospitalization to burn unit or intensive care 6
• IV methylprednisolone 1-2 mg/kg 6
• Immediate dermatology consultation 6
• Punch biopsy and clinical photography 6

Algorithmic Approach

1. Assess for fever/systemic symptoms: If present → start doxycycline immediately and obtain CBC, CMP, serology 1
2. Evaluate travel history: Recent tropical travel → consider onchocerciasis, strongyloides, cutaneous larva migrans 2, 3
3. Assess morphology and extent:
   • Migratory linear lesion → larva currens 2
   • Diffuse pruritic dermatitis → onchocerciasis 2
   • Erythematous plaques → psoriasis vs. contact dermatitis 4

   • 30% BSA with sloughing → SJS/TEN (emergency) 6

4. Check medication history: Recent drug initiation (especially after 6 weeks) → drug eruption 2, 6
5. Initiate empiric treatment: Topical corticosteroids for inflammatory conditions 2, 5

Critical Pitfalls to Avoid

• Do not exclude RMSF based on absence of tick bite history, as 40% do not report exposure 4
• Do not wait for serologic confirmation before starting doxycycline if fever + rash present 1
• Do not use topical antibiotics prophylactically; reserve only for documented superinfection 6
• Do not delay dermatology consultation if Grade 2 or higher drug reaction suspected 6