What are the differential diagnoses and initial management for a rash that spares the chest wall?

Differential Diagnosis for Rash Sparing the Chest Wall

When a rash notably spares the chest wall, prioritize herpes zoster (shingles) as the primary diagnosis, which characteristically presents in a unilateral dermatomal distribution that may spare the anterior chest while affecting other body regions. 1

Primary Diagnostic Consideration

Herpes Zoster

• Herpes zoster presents with pain in a dermatomal distribution that is triggered by touch, followed by a characteristic unilateral rash that follows dermatome boundaries 1
• The rash does not cross the midline and can spare the chest wall if the affected dermatome does not include this region 1
• Pain typically precedes the rash by several days, and the distribution is strictly unilateral 1
• This is a critical diagnosis to make early, as antiviral therapy is most effective when initiated within 72 hours of rash onset

Other Important Differential Diagnoses

Adult-Onset Still's Disease (AOSD)

• The characteristic salmon-pink, evanescent maculopapular rash of AOSD predominantly affects the proximal limbs and trunk, with rare involvement of the face and distal limbs 1
• The rash occurs in 72.7% of patients and is often accompanied by high-spiking quotidian fevers exceeding 39°C 1
• Look for the classic triad: high-spiking fevers, characteristic rash, and arthritis/arthralgias affecting knees, wrists, and ankles 1
• The rash may demonstrate a Koebner phenomenon and is often mildly pruritic 1
• Associated findings include sore throat (68%), myalgias (75%), lymphadenopathy (54%), and splenomegaly (14%) 1

Atopic Dermatitis Distribution Patterns

• In older children and adults, atopic dermatitis lesions are typically located in flexural areas (antecubital and popliteal fossae) along with head and neck, potentially sparing the chest wall 2
• The diaper area in infants is rarely affected by atopic dermatitis, which can help distinguish distribution patterns 2
• Chronic lesions feature lichenification with accentuated skin markings and hyperpigmentation 2

Drug-Induced Rashes in Cancer Patients

• EGFR inhibitors cause acneiform rash in 75-90% of patients, typically developing within the first days to weeks of therapy 3
• MEK inhibitors cause papulopustular eruption in 74-85% of patients 3
• These rashes may have variable distribution patterns that could spare certain body regions 3
• Critical pitfall: Do not confuse drug-induced rash with Stevens-Johnson syndrome or toxic epidermal necrolysis, which require immediate hospitalization 3

Asbestos-Related Pleural Disease

• While primarily a pleural condition, asbestos exposure can cause skin manifestations in the context of systemic disease 1
• However, this is not a primary dermatologic diagnosis and would not present as isolated rash sparing the chest wall

Initial Management Approach

Immediate Assessment

• Document the exact distribution of the rash, noting whether it follows a dermatomal pattern (suggests herpes zoster) or affects proximal limbs and trunk (suggests AOSD) 1
• Assess for fever: high-spiking quotidian fevers point toward AOSD, while low-grade or absent fever is more consistent with herpes zoster 1, 4
• Evaluate for pain characteristics: dermatomal pain triggered by touch strongly suggests herpes zoster 1
• Check for systemic symptoms: arthritis, sore throat, and lymphadenopathy suggest AOSD 1

Key Physical Examination Findings

• Examine for unilateral dermatomal distribution that does not cross midline (herpes zoster) 1
• Assess rash morphology: vesiculobullous lesions suggest herpes zoster, while salmon-pink maculopapular eruption suggests AOSD 1, 5
• Palpate for joint tenderness, particularly in knees, wrists, and ankles if AOSD is suspected 1
• Check for lymphadenopathy and splenomegaly in febrile patients 1

Diagnostic Testing

• For suspected herpes zoster: viral culture or PCR of vesicular fluid for definitive diagnosis 1
• For suspected AOSD: complete blood count, inflammatory markers (ESR, CRP), ferritin (markedly elevated), liver function tests, and rheumatoid factor (typically negative) 1
• Consider skin biopsy if diagnosis remains unclear after initial evaluation 6, 5

Treatment Considerations

• For herpes zoster: initiate antiviral therapy (acyclovir, valacyclovir, or famciclovir) within 72 hours of rash onset 1
• For AOSD: NSAIDs for mild disease, corticosteroids for moderate-to-severe disease, with consideration of disease-modifying agents for refractory cases 1
• Avoid empiric treatment without establishing diagnosis, as management differs significantly between conditions 4, 5

Critical Clinical Pitfalls

• Do not assume chest wall sparing excludes serious systemic disease—herpes zoster can affect any dermatome, and AOSD has characteristic but variable distribution 1
• Herpes zoster pain may precede rash by several days, leading to diagnostic confusion if only pain is present initially 1
• AOSD is a diagnosis of exclusion requiring careful evaluation to rule out infection, malignancy, and other rheumatologic conditions 1
• Drug history is essential, as medication-induced rashes can have atypical distributions and may require immediate discontinuation 3, 4