Dermatology MCQ - Inflammatory Dermatoses - Erythema chronicum migrans

A 45-year-old man presents with a solitary, expanding, erythematous patch on his right thigh. He returned from a hiking trip in Wisconsin one week ago. The lesion is warm to the touch but non-tender and not pruritic. He reports no systemic symptoms. On examination, there is a large, 12 cm in diameter, annular plaque with central clearing, giving a "bull's-eye" appearance. There is no scale or vesiculation. The most likely diagnosis is:

A. Tinea corporis
B. Granuloma annulare
C. Erythema multiforme
D. Erythema chronicum migrans
E. Fixed drug eruption

Correct Answer: D. Erythema chronicum migrans

Answer & Explanation

Explanation:

The presentation is classic for Erythema Chronicum Migrans (ECM), the pathognomonic cutaneous manifestation of the early localized stage of Lyme disease.

• Key Diagnostic Clues:

  • Epidemiology: History of potential tick exposure in an endemic area (Wisconsin is a high-risk state).

  • Temporal Course: Appearance days to a week after exposure.

  • Morphology: A rapidly expanding, annular, erythematous patch or plaque.

  • "Bull's-eye" or "Target" Appearance: Characterized by central clearing, which can be partial or complete. It is crucial to distinguish this from the true target lesion of erythema multiforme, which has a central dusky purpura or blister.

  • Lack of Surface Change: The lesion is typically devoid of scale, crust, or vesiculation.

  • Symptoms: The lesion is often asymptomatic but can be mildly burning or pruritic.

The other options are incorrect:

• A. Tinea corporis: This also presents as an annular, expanding plaque. However, it almost always has scale at the advancing border, which is absent in ECM. It also tends to expand more slowly.

• B. Granuloma annulare: This presents as annular plaques, but they are composed of firm, skin-colored or erythematous papules. They lack central clearing and are not associated with a tick bite or systemic illness.

• C. Erythema multiforme: The target lesions of EM are true "targets" with a central dusky, purpuric, or blistered area, not simple clearing. EM is also often acral and multifocal, whereas ECM at this stage is typically a solitary lesion at the site of the inoculating tick bite.

• E. Fixed drug eruption: This presents as a solitary or few, round, dusky red to violaceous plaques that recur in the exact same location upon re-challenge with the offending drug. They do not expand rapidly with central clearing and often leave post-inflammatory hyperpigmentation.

Key Associations for Erythema Chronicum Migrans

• Pathophysiology: ECM is caused by the dissemination of Borrelia burgdorferi sensu lato complex spirochetes from the site of a tick bite (Ixodes species). The inflammation is a reaction to the spirochetes in the skin.

• Associated Systemic Symptoms: While the patient in this vignette is asymptomatic, early disseminated Lyme disease can present with multiple secondary ECM lesions, fever, malaise, headache, myalgias, arthralgias, lymphadenopathy, and neurologic or cardiac manifestations.

• Differential Diagnosis: The main differentials include other annular erythemas (e.g., erythema annulare centrifugum, which has trailing scale), cellulitis (which is tender, hot, and not annular), and an insect bite reaction.

• Histopathology: Not routinely used for diagnosis. It may show a superficial and deep perivascular and interstitial infiltrate of lymphocytes and plasma cells. Special stains (e.g., Warthin-Starry) may rarely demonstrate spirochetes, but this is insensitive.

• Prognosis: Excellent if treated promptly. If left untreated, the infection can disseminate, leading to arthritis, carditis, or neuroborreliosis.

• Management:

  • Diagnosis: Primarily clinical in a classic presentation from an endemic area. Serologic testing (ELISA with Western Blot confirmation) is not recommended for diagnosis of early localized disease as it may be negative; it is used for confirmation in later stages.

  • Treatment: First-line treatment is a course of oral antibiotics, such as doxycycline, amoxicillin, or cefuroxime axetil. Doxycycline has the advantage of also covering other potential tick-borne illnesses like anaplasmosis.