This 12 year old boy has developed an itchy, scaly rash on both feet - a not uncommon presentation in clinical practice. Given his history of allergic rhinitis and conjunctivitis, eczema (i.e. atopic dermatitis) is a differential diagnosis which naturally springs into mind. Looking at the positive family history, psoriasis is also worth considering, despite the fact that 12 years is usually well below the typical age of onset of the disease, while the rash is in an unlikely site. Although the history does not provide specific details, contact dermatitis (particularly due to footwear) is not improbable either. The other possibility in this case is that he has acquired a dermatological infection, possibly from the communal facilities he likely shares as a member of the track and field team. Examination reveals erythematous, scaly lesions in the interdigital spaces; this is characteristic of a fungal (tinea pedis) infection. The brittle, thickened toenail with white striae complies with the typical clinical picture of a tinea unguium infection. However, the diagnosis of a fungal infection cannot be confirmed via clinical findings alone; this patient requires further evaluation; this is provided by the presence of branching septate hyphae in the KOH test. While cultures of skin scrapings can be used to confirm the presence of a dermatophyte infestation, identification of the specific group of fungi does not impact the subsequent management. In addition, cultures are time consuming and expensive, and probably not justified given the clear clinical picture of a tinea infection. However, they might need to be considered down the line, if the patient proves to be unresponsive to therapy, and the diagnosis is second-guessed. While patient's FBC reveals eosinophilia, this is not surprising, given the history of allergic rhinitis and conjunctivitis. Allergen specific IgE testing is not rational when there is no solid evidence in favour of an allergic etiology. While topical antifungal agents are adequate to treat tinea pedis, this patient is also likely to benefit from concomitant use of an oral agent due to the involvement of his toenail. While short term use of an antifungal/steroid combination is justifiable if the inflammation is severe, long term steroid use is harmful due to the many potential complications and systemic side effects. Phototherapy would be a relevant treatment option had this patient's diagnosis been psoriasis.
Tinea infections (dermatophytosis) are superficial fungal infections caused by a group of fungi (dermatophytes) including Epidermophyton, Microsporum and Trichophyton genera. These organisms have achieved the ability to metabolize dead keratin, creating characteristic lesions consisting of a central clear area and an advancing erythematous, scaly border, producing a ring like pattern (hence the name 'ringworm'). Both immunocompetent and immunocompromised patients are affected - although the disease can bring forth disastrous consequences in the latter group. The lifetime risk of acquiring the infection varies from 10 to 20 percent. Tinea pedis and tinea unguium (onychomycosis) respectively refer to foot and nail infections caused by dermatophyte fungi. Note, from here onwards the discussion is focussed on these conditions only. The incubation periods of tinea pedis and unguium are variable, with a median duration of 2 to 3 weeks. Tinea pedis (also known as athlete's foot) more commonly affects adults than children and men than women. It is spread via direct contact with infected persons, or indirectly through contaminated fomites. The most common presentation of tinea pedis is toe-web maceration, while in some cases a dermatophytid reaction known as the 'id' reaction may occur with small vesicles and pustules. Since fungi prefer warm and humid environments, closely tied occlusive footwear is unsurprisingly a common risk factor. Tinea unguium affects toe nails more frequently than fingernails; three clinical forms of infection exist: Distal Lateral Subungual Onychomycosis (DLSO), Proximal Subungual Onychomycosis (PSO) and Superficial White Onychomycosis (SWO); in DLSO and PSO there is involvement of the structures underneath the nail plate. Although patients with tinea unguium are more concerned about the cosmetic aspect of the disease, it may cause recurrent tinea of other sites and cellulitis of the lower leg, serving as a port of entry for Streptococci and S. Aureus. Note that subsequent limitation of mobility and reduction of peripheral circulation may lead to worsening of pre-existing conditions such as venous stasis and diabetic foot ulcers. The diagnosis of tinea infections require the demonstration of organisms by microscopy or culture of the samples obtained from the suspected lesions.This is particularly important in onychomycosis, which must be differentiated from other nail dystrophies and especially psoriasis of the nails. In microscopy, material scraped from the active edge of the lesion is placed in a drop of potassium hydroxide (KOH), hence the name 'KOH test'. Note that the KOH test is highly sensitive and specific for dermatophytes, and has the advantage of being minimally invasive. Cultures are rarely used now, unless in special circumstances such as when the diagnosis is doubtful, or when the patient requires long term drug therapy. Although most dermatophytes do not fluoresce, when positive, Wood's light examination can be used to determine the the extent of infection and to evaluate the response to therapy. Tinea pedis usually responds to topical antifungal agents except for the "mocassin" hyperkeratotic form which requires oral antifungal agents to resolve. In contrast, tinea unguium usually requires an oral antifungal agent, although the superficial white form of tinea unguium (SWO) may respond to topical antifungals. From the two principal pharmacological agents, namely azoles and allylamines, the former should be avoided in pregnancy. Clotrimazole and miconazole are some commonly used topical agents while terbinafine and ketoconazole can be used orally. Ketoconazole is not preferred in tinea unguium as the prolonged therapy needed may increase the likelihood of hepatic side effects. The duration of therapy varies depending on the site of infection; which usually amounts to 4 weeks in case of tinea pedis, 6 to 12 weeks for fingernails and 3 to 6 months for toenails. Regular weekly debridement of the affected nails is a rule. In DLSO complete removal of the hyperkeratotic nail is indicated. This can also be achieved pharmacologically with the application of special topical formulations of keratolytics (40% urea) under dressing. Patients who have symptomatic dermatophytosis with pruritus and erythema may benefit from the judicious use of an antifungal/steroid combination. Note that despite the rapid relief provided by the steroid component, it may cause potential complications if used indiscriminately on long term. As with most infections, simple hygienic measures such as maintaining appropriate skin and nail care, wearing open toed shoes and changing socks regularly are helpful in prevention of the disease.