Diagnosis: Tinea corporis (ringworm)
The patient and physician initially considered the possibility of allergic contact dermatitis due to nickel because of the history of redness, scaling, and itching underneath the watch when it was worn on the left wrist, and then when it was worn on the right wrist. Nickel dermatitis is common and it is easy to attribute the cause of a condition like this to the most obvious diagnosis without considering a more complete differential diagnosis.1
However, there were clues that prompted us to suspect tinea corporis (ringworm). The red, scaly rash spread centrifugally over several weeks, and fomites, such as a watch, can spread infectious diseases. Also, our patient had a few erythematous papular lesions, and the presence of papules in addition to scaling rings is typical of fungal infections involving hair follicles (Majocchi’s granuloma).
A positive potassium hydroxide (KOH) preparation confirmed the diagnosis and eliminated the need for nickel patch testing.2
Warmth and moisture could explain tinea on the wrists
Dermatophytes are fungi that can cause infections in the skin, hair, and nails. They are classified by where they are found—anthropophilic (humans), geophilic (soil), or zoophilic (animals). Anthropophilic and zoophilic dermatophytes from the genera Trichophyton, Microsporum, and Epidermophyton are primarily responsible for human fungal infections.3 It is estimated that superficial fungal infections affect up to a quarter of the world’s population.3
Tinea corporis mainly occurs in prepubertal children, presenting as a red, annular, scaly, pruritic patch with central clearing and an active border.4 Tinea corporis includes all superficial dermatophyte infections of the glabrous skin and is particularly common in areas of excessive heat and moisture.5 Patients can pick up tinea corporis via fomites at the gym, through soil in the garden, or by touching a pet’s fur or a child’s scalp when either has the fungal infection.
The wrists are not a common place for tinea corporis, but the condition can occur anywhere on the body. This patient may well have contracted tinea from his own interdigital tinea pedis. Warmth and moisture under the watch could also explain the predilection for fungus to grow on the wrists.
Distinguish between contact dermatitis and tinea corporis
The differential diagnosis for tinea corporis includes allergic contact dermatitis, granuloma annulare, annular elastolytic granuloma, and erythema chronicum migrans.
Allergic contact dermatitis is caused by an allergy to a substance, such as the metal nickel. A preliminary diagnosis of contact dermatitis could easily be made in error if one were to assume that a patient was having a type IV hypersensitivity response to nickel from a watch.6
Granuloma annulare produces slowly expanding annular plaques that are not itchy and do not scale. This commonly occurs over the joints and is of unknown etiology.7
Annular elastolytic granuloma is a variant of granuloma annulare that occurs on skin that has been exposed to the sun. It presents with a red, ring-like pattern and is associated with little scaling or pruritus.8
Erythema chronicum migrans produces annular lesions at the site of a tick bite and is the primary sign of Lyme disease. The tick must be in place for 24 hours for infection to occur.9 (Our patient did not notice a tick attached at either site.)
In this case, a KOH preparation of skin scrapings identified septate hyphae, which supported our diagnosis of tinea corporis.2 A history of red, scaly, itchy, and expanding round/oval patches or plaques and evidence of “athlete’s foot” can also help one to make the diagnosis.