Differential Diagnosis for Small Patch of Erythema on Leg
- Single most likely diagnosis
- Erythema migrans (Lyme disease): This is a strong consideration given the description of a well-demarcated patch of erythema. Although it's non-pruritic, which aligns with erythema migrans, the incidence being more common before 1990 in the US could suggest a period when Lyme disease was becoming more recognized or had higher incidence rates in certain areas.
- Other Likely diagnoses
- Contact dermatitis: Although the lesion is described as non-pruritic, some forms of contact dermatitis can present without significant itching, especially in the initial stages. The well-demarcated nature could suggest an allergic or irritant reaction to something that came into contact with the skin.
- Fixed drug eruption: This condition presents with well-demarcated patches of erythema that can appear at the site of drug administration or elsewhere on the body. It's less common but should be considered, especially if the patient has started any new medications.
- Nummular dermatitis: While typically pruritic, nummular dermatitis can sometimes present with minimal itching, especially in early stages. The well-demarcated, coin-shaped patches could fit this description.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Erythema nodosum: Although it typically presents with tender nodules, there can be variations in presentation. It's associated with several systemic conditions, including infections and autoimmune diseases, making it crucial not to miss.
- Cellulitis: Early stages of cellulitis can present with erythema that might seem well-demarcated. It's critical to distinguish this from other causes due to the potential for rapid progression and severity.
- Cutaneous lymphoma (e.g., mycosis fungoides): While rare, cutaneous lymphomas can present with patches of erythema. Given the potential severity of these conditions, they should not be overlooked.
- Rare diagnoses
- Erythema multiforme: This condition can present with target lesions but sometimes appears as patches of erythema. It's often associated with infections or medications.
- Sweet syndrome (acute febrile neutrophilic dermatosis): Characterized by fever, neutrophilia, and tender erythematous skin lesions, Sweet syndrome is rare but should be considered in the appropriate clinical context.
- Granuloma annulare: Typically presents with annular lesions but can sometimes appear as patches of erythema. It's more common in children but can occur at any age.