Differential Diagnosis

When differentiating between nummular eczema, psoriasis, tinea corporis (ringworm), granuloma annulare, fixed drug eruption, and localized morphea, consider the following categories:

• Single Most Likely Diagnosis
  • Nummular eczema: This is often the first consideration due to its common presentation of coin-shaped patches of eczema, which can be very similar to the other conditions listed but is frequently seen in individuals with a history of atopic diseases.
• Other Likely Diagnoses
  • Psoriasis: Characterized by well-delineated, erythematous, scaly plaques. It's a common condition that can mimic the appearance of nummular eczema or tinea corporis, especially if the scales are not prominent.
  • Tinea corporis (ringworm): A fungal infection that causes circular, itchy patches on the skin, often with a clear center, making it a likely differential for nummular eczema and granuloma annulare.
  • Granuloma annulare: Presents as a ring of small, firm bumps over the backs of the forearms, hands, or feet, which can be confused with the ring-like appearance of tinea corporis or nummular eczema.
  • Fixed drug eruption: A condition that occurs as a reaction to medication, causing round, flat, itchy patches that can resemble nummular eczema or tinea corporis, especially if the patient has recently started a new drug.
• Do Not Miss Diagnoses
  • Localized morphea: A form of scleroderma that causes patches of skin to become thick and hard, which can be mistaken for chronic eczema or psoriasis. It's crucial not to miss this diagnosis due to its potential for significant morbidity if left untreated.
  • Fixed drug eruption: Although listed under other likely diagnoses, it's also a "do not miss" due to its association with medication use and the potential for severe reactions if the offending drug is not discontinued.
• Rare Diagnoses
  • Other forms of dermatitis or skin conditions that could mimic the listed conditions, such as pityriasis rosea, which can sometimes present with a herald patch followed by a secondary eruption that might be confused with nummular eczema or psoriasis.
  • Sarcoidosis: A condition that can cause skin lesions, among other systemic symptoms, which might rarely be considered in the differential diagnosis of these skin conditions, especially if there are other systemic signs or symptoms present.

Each diagnosis requires careful consideration of the patient's history, physical examination, and sometimes additional diagnostic tests such as skin scrapings for fungal infections, biopsy for granuloma annulare or morphea, and patch testing for contact dermatitis or drug reactions.