How should I evaluate and manage a patient with a rash limited to the trunk?

Evaluation and Management of Truncal Rash

Initial Assessment: Morphology and Distribution

Begin by determining whether the rash is maculopapular or petechial/purpuric, as this distinction fundamentally alters your diagnostic approach and urgency of intervention. 1

Maculopapular Rash on Trunk

• Viral exanthems are the most common cause, particularly enteroviral infections that present with trunk and extremity involvement while characteristically sparing palms, soles, face, and scalp 1
• Drug eruptions present as fine reticular maculopapular rashes or broad, flat erythematous macules and patches, typically appearing 1-8 weeks after medication initiation 1, 2
• Human herpesvirus 6 (roseola) causes macular rash following high fever resolution 1
• Epstein-Barr virus produces maculopapular rash, especially pronounced if the patient received ampicillin or amoxicillin 1

Critical Red Flags Requiring Immediate Action

If the patient has fever + truncal rash + headache + any tick exposure or residence in an endemic area, initiate doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation. 1

• Rocky Mountain Spotted Fever initially presents with small (1-5 mm) blanching pink macules on ankles, wrists, or forearms 2-4 days after fever onset, then spreads centrally to trunk while sparing the face 1
• Up to 40% of RMSF patients report no tick bite history, and up to 20% never develop a rash, making absence of these features unreliable for exclusion 1, 3
• The case-fatality rate for RMSF is 5-10%, with 50% of deaths occurring within 9 days of illness onset 1, 3

Diagnostic Workup Algorithm

Step 1: Assess for Life-Threatening Causes

• Check for systemic toxicity: fever, tachycardia, confusion, hypotension, or altered mental status indicating life-threatening infection 3
• Examine palms and soles carefully: involvement indicates advanced RMSF, secondary syphilis, meningococcemia, bacterial endocarditis, or drug reactions 1, 4, 3
• Look for petechiae or purpura: non-blanching lesions require immediate consideration of meningococcemia or advanced RMSF 4, 3

Step 2: Obtain Targeted History

• Recent medications within past 2-8 weeks: anticonvulsants (phenytoin, carbamazepine, phenobarbital, lamotrigine) cause hypersensitivity syndrome with fever, rash, and internal organ involvement 2
• Outdoor exposure during April-September in grassy/wooded areas: increases RMSF probability regardless of recalled tick bite 1, 3
• Recent viral upper respiratory infection: supports viral exanthem diagnosis 1
• Ampicillin/amoxicillin use: suggests EBV-related drug eruption 1

Step 3: Immediate Laboratory Testing (if RMSF/ehrlichiosis suspected)

• Complete blood count with differential: look for thrombocytopenia (present in 94% of RMSF), leukopenia (53% of RMSF), or increased immature neutrophils 1
• Comprehensive metabolic panel: check for hyponatremia (present in 53% of RMSF) and elevated hepatic transaminases 1
• Acute serology for Rickettsia rickettsii, Ehrlichia chaffeensis, and Anaplasma phagocytophilum 1
• Peripheral blood smear: examine for morulae within granulocytes 1

Management Algorithm

If RMSF Cannot Be Excluded:

• Start doxycycline 100 mg twice daily immediately, even in children <8 years old, as mortality risk outweighs dental staining concerns 1, 3
• Expect clinical improvement within 24-48 hours; lack of response should prompt consideration of alternative diagnoses or coinfection 1
• Do not wait for the classic triad of fever, rash, and tick bite—it is present in only a minority at initial presentation 1, 3

If Drug Hypersensitivity Suspected:

• Discontinue the suspected offending drug immediately 2
• Obtain liver transaminases, complete blood count, urinalysis, and serum creatinine as minimum laboratory battery 2
• Administer systemic corticosteroids if symptoms are severe or internal organ involvement is present 2
• Avoid all aromatic anticonvulsants (phenytoin, phenobarbital, carbamazepine) due to 75% cross-reactivity rate 2

If Viral Exanthem Most Likely:

• Supportive care is appropriate for self-limited viral causes 1, 5
• Monitor for progression or development of systemic symptoms that would prompt reassessment 6

Common Pitfalls to Avoid

• Do not exclude RMSF based on absence of tick bite history—40% of patients have no recall of tick exposure 1, 3
• Do not exclude RMSF based on absence of rash—20% never develop rash, and absence is associated with increased mortality 1, 4
• Do not wait for palm/sole involvement to treat RMSF—this indicates advanced disease with higher mortality 1, 3
• In darker-skinned patients, petechial rashes may be difficult to recognize, increasing risk of delayed diagnosis 4
• Do not assume benign viral cause in patients with fever + rash + any systemic symptoms—obtain laboratory studies to assess for thrombocytopenia and hyponatremia 1

Special Considerations

EGFR Inhibitor Therapy Patients:

• Acneiform rash occurs in 45-100% of patients on EGFR inhibitors, typically within first 2-4 weeks of treatment 7
• Lesions are papular or pustular, mainly on face and upper trunk in sebaceous-rich areas 7
• This is a sterile eruption without comedones—the term "acneiform rash" should be used, not "acne" 7
• Rarely life-threatening but significantly impacts quality of life and may require dose modification 7

Psoriasis Patients:

• Guttate psoriasis can present with truncal involvement and should be managed by general practitioners for stable disease 7
• Certain drugs worsen psoriasis: beta-blockers, NSAIDs, lithium, chloroquine, and mepacrine may cause severe deterioration 7