What is the diagnosis and treatment for a flat reddish circular rash with a lighter colored center on the back of the wrist that is not pruritic (itchy) or warm?

Diagnosis: Erythema Migrans (Lyme Disease) vs. Granuloma Annulare

The most critical diagnosis to rule out immediately is erythema migrans from Lyme disease, which requires prompt antibiotic treatment to prevent serious complications including cardiac, neurologic, and arthritic manifestations. 1, 2

Immediate Diagnostic Approach

Key Clinical Features to Assess

History:

• Recent tick exposure or outdoor activities in endemic areas (within past 3-30 days) 2
• Travel to wooded or grassy areas 3
• Timing of rash onset and progression pattern 3
• Any systemic symptoms: fever, headache, fatigue, muscle/joint aches 1, 3
• Medication history to exclude drug-induced eruptions 4

Physical Examination:

• Measure the lesion diameter (erythema migrans typically ≥5 cm and expands centrifugally) 2
• Assess for multiple lesions (suggests disseminated Lyme disease) 2
• Check for central induration or vesiculation 2
• Examine for scaling at borders (absence favors Lyme disease or granuloma annulare over tinea corporis) 2, 5
• Palpate for firmness (granuloma annulare has firm papules at borders) 5

Differential Diagnosis

Primary Considerations:

Erythema Migrans (Lyme Disease):

• Annular, erythematous patch with central clearing that expands outward 2
• Non-pruritic and non-painful (matches your presentation) 2
• Recognizing this lesion is critical for initiating prompt antibiotics 2
• Treatment: Immediate doxycycline 100 mg twice daily for 10-21 days (or amoxicillin if doxycycline contraindicated) 2

Granuloma Annulare:

• Ring of flesh-colored to erythematous papules with central clearing 2, 5
• Asymptomatic, non-scaly, non-itchy (matches your presentation perfectly) 5
• Benign and self-limiting, typically resolves within 2 years 5
• Associated with diabetes mellitus in some cases 5
• Treatment: Usually none needed; high-potency topical steroids or intralesional steroids for cosmetic concerns 5

Tinea Corporis:

• Less likely given absence of scaling, which is typically present at the advancing border 2
• Would require potassium hydroxide examination of scrapings for diagnosis 2

Erythema Annulare Centrifugum:

• Similar appearance but typically has trailing scale 2

Recommended Diagnostic Testing

If Lyme disease suspected:

• Two-tier serologic testing (ELISA followed by Western blot if positive) 1
• However, do not delay antibiotic treatment while awaiting serology if clinical suspicion is high, as early Lyme disease may be seronegative 1

If diagnosis unclear:

• Punch biopsy of the lesion border to differentiate granuloma annulare from other conditions 5
• Potassium hydroxide preparation if tinea corporis considered 2
• Fasting glucose or HbA1c if granuloma annulare suspected (to assess for diabetes association) 5

Critical Clinical Pitfalls

• Do not wait for systemic symptoms to treat Lyme disease—erythema migrans alone is sufficient for diagnosis and treatment 2
• Do not rely solely on tick bite history—many patients with Lyme disease do not recall a tick bite 3
• In darker-skinned patients, erythematous rashes may be difficult to recognize, increasing risk of delayed diagnosis 6, 7
• Absence of pruritus does not exclude infectious causes—both Lyme disease and granuloma annulare are characteristically non-pruritic 2, 5

Management Algorithm

1. Assess for Lyme disease risk factors (tick exposure, endemic area, expanding lesion ≥5 cm) 2

   • If present: Start doxycycline immediately 2

2. If Lyme disease unlikely:

   • Observe for 2-4 weeks as granuloma annulare is self-limited 5
   • Consider biopsy if diagnosis uncertain or lesion persists beyond 2 months 5
   • Screen for diabetes mellitus 5

3. If lesion has scaling: Perform potassium hydroxide examination to rule out tinea corporis 2