Tinea Korporis

01Pathophysiology

Tinea corporis is a superficial dermatophyte infection of the glabrous (non-hair-bearing) skin of the body. The causative organisms belong to three genera: Trichophyton, Microsporum, and Epidermophyton. These fungi produce keratinases, enzymes that degrade keratin and allow the organism to colonize and invade the stratum corneum, the outermost layer of the epidermis.

The infection begins when fungal spores (arthroconidia) are deposited on the skin through direct contact with an infected person (anthropophilic transmission), an infected animal (zoophilic transmission, classically kittens or puppies carrying M. canis), or contaminated fomites (soil, gym mats, shared towels). The spores germinate and the fungal hyphae grow radially outward through the keratinized tissue, digesting keratin as fuel.

The characteristic annular morphology with central clearing is a direct consequence of the host immune response interacting with this centrifugal growth pattern. At the advancing peripheral border, active fungal invasion triggers a robust inflammatory response: complement activation, neutrophil chemotaxis, and a cell-mediated (Th1) immune reaction produce erythema, edema, scaling, and sometimes vesiculation. In the center, the immune system has already cleared the organisms, so inflammation subsides and the skin appears relatively normal. This is why the lesion looks like a "ring." If the patient is immunocompromised, the central clearing may be absent or the presentation may be atypical because the immune response is blunted.

A critical concept is Majocchi granuloma, which occurs when dermatophytes penetrate beyond the stratum corneum into the hair follicle and dermis. This happens most commonly after application of topical corticosteroids to a misdiagnosed lesion (suppressing the local immune response and allowing deeper invasion) or in immunocompromised patients. Majocchi granuloma presents as perifollicular papules, pustules, or nodules rather than the classic ring, and it requires systemic antifungal therapy because topical agents cannot reach the dermal depth of infection.

Another important phenomenon is tinea incognito, in which a dermatophyte infection has been treated with topical corticosteroids (often prescribed empirically for a "rash"). The steroid suppresses inflammation, masking the classic annular morphology and the raised scaly border. The lesion may appear as a poorly defined, atypical erythematous patch that continues to enlarge despite "treatment." Recognizing tinea incognito depends on suspecting fungal infection when a rash worsens or fails to resolve with steroids and then performing a KOH preparation.

02Classification and Clinical Manifestation

Dermatophyte infections are classified by the anatomical site involved. Tinea corporis refers to the body (trunk and extremities), but on an exam, you must be able to distinguish it from infections at other sites because the management differs:

Within tinea corporis itself, clinical patterns can vary:

03Diagnostic Workup

KOH Preparation is the cornerstone of diagnosis and the best initial test. You scrape scales from the active, advancing border of the lesion (not the cleared center, which has few organisms), place them on a slide with 10% to 20% potassium hydroxide, and examine under light microscopy. The KOH dissolves the keratinocytes, leaving behind the fungal elements. The finding of septate, branching hyphae confirms a dermatophyte infection. This is the test you order when a vignette describes an annular scaly plaque and asks "what is the next step in diagnosis" or "what test confirms the diagnosis." It is rapid, inexpensive, and has good sensitivity when performed correctly.

Fungal culture on Sabouraud dextrose agar is the most accurate test for identifying the exact species. However, it takes 2 to 4 weeks for colonies to grow, so it is not practical as a first-line diagnostic step. Culture is ordered when species identification is clinically important (e.g., determining zoophilic vs. anthropophilic source for infection control) or when KOH is negative but clinical suspicion remains high.

Wood lamp examination has limited utility in tinea corporis. It detects fluorescence produced by certain Microsporum species but does not detect Trichophyton, which is the most common cause of tinea corporis. A negative Wood lamp does not rule out dermatophyte infection. Do not select Wood lamp as the best initial test for tinea corporis.

Skin biopsy with periodic acid-Schiff (PAS) staining is reserved for cases where the diagnosis is unclear after KOH and culture, or when the clinical picture is atypical (e.g., immunocompromised patient, suspected Majocchi granuloma). PAS staining highlights fungal cell walls within the tissue.

04Management and Treatment

Localized disease is managed with topical antifungals alone. Terbinafine 1% cream is the preferred topical agent because it is fungicidal (kills the organism rather than merely inhibiting growth) and requires a shorter treatment course (1 to 2 weeks vs. 2 to 4 weeks for azoles). Clotrimazole and ketoconazole are fungistatic alternatives that require longer treatment. Apply the antifungal to the affected area and 1 to 2 cm beyond the visible border to ensure coverage of subclinical spread. Continue treatment for at least one week after clinical resolution to prevent relapse.

Extensive or refractory disease requires systemic therapy. Oral terbinafine 250 mg daily is the first-line systemic agent. It accumulates in keratinized tissue and achieves high concentrations in the stratum corneum. Oral itraconazole is an alternative, particularly when Microsporum is suspected (terbinafine has somewhat lower activity against Microsporum compared to Trichophyton). Before starting oral antifungals for prolonged courses, obtain baseline liver function tests, as both terbinafine and itraconazole carry a risk of hepatotoxicity.

Majocchi granuloma always requires oral antifungal therapy regardless of the number or size of lesions. Topical agents cannot penetrate to the depth of the hair follicle and dermis where the infection resides. This is a frequently tested concept: if the vignette describes perifollicular papulonodules or mentions a patient who used topical steroids and now has deeper-appearing lesions, the answer is oral terbinafine, not a topical antifungal.

Key contraindications and interactions:

• Terbinafine: hepatotoxicity (monitor LFTs); avoid in severe liver disease

• Itraconazole: negative inotrope (avoid in congestive heart failure); inhibits CYP3A4 (numerous drug interactions); requires acidic gastric pH for absorption (avoid with proton pump inhibitors)

• Griseofulvin: induces CYP enzymes (reduces efficacy of oral contraceptives and warfarin); teratogenic (contraindicated in pregnancy); requires fatty food for absorption

• Ketoconazole (oral): no longer recommended for dermatophytosis due to hepatotoxicity and adrenal suppression risk; topical formulation remains safe

One critical management pearl: never prescribe topical corticosteroids alone for an undiagnosed annular rash. If a question stem describes a patient whose "rash improved initially with steroid cream but then got worse or spread," the diagnosis is tinea incognito. The next best step is to stop the steroid, perform KOH, and start antifungal therapy.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get questions on tinea corporis wrong falls into three categories.

First, confusing annular lesions. Exam writers rely on your ability to distinguish between annular lesions that look similar on first glance. The key differentiator is always the quality of the border: tinea corporis has a scaly, raised, active border with central clearing, while granuloma annulare has a smooth, non-scaly border, and erythema migrans is a flat, non-scaly, expanding macule. If you see "annular" in the stem, your reflex should be to look for scale and KOH findings.

Second, the steroid trap. A vignette describing a rash that "improved with hydrocortisone but is now worse" or "changed in appearance after steroid use" is testing whether you recognize tinea incognito. The wrong answer is to prescribe a stronger steroid. The right answer is to perform KOH, confirm hyphae, and switch to antifungal therapy. A related trap is Majocchi granuloma: once the vignette mentions perifollicular nodules or deeper invasion after steroid use, topical antifungals are no longer sufficient, and the answer shifts to oral antifungals.

Third, choosing the wrong diagnostic test. Students sometimes select Wood lamp as the best initial test. Remember that Wood lamp fluorescence is only positive for Microsporum species and does not detect Trichophyton rubrum, which is the most common cause. The best initial test is always KOH preparation. Similarly, do not select fungal culture as the best initial test because it takes weeks to result. Culture is the most accurate test, but KOH is the first step.

The core competency being tested with tinea corporis is pattern recognition of annular lesions, appropriate use of KOH as a diagnostic tool, and understanding the consequences of mismanagement with topical steroids. When you see an annular scaly plaque in a vignette, think dermatophyte until proven otherwise, and always confirm with KOH before treating.