Tinea and fungal skin infection — branching viva

Opening — framing the problem

The examiner begins: a five-year-old has a three-week history of an itchy, scaly bald patch on the scalp with broken hair stubs, black dots flush with the skin and an enlarged occipital node. Talk me through your approach. [1]

I would frame this as tinea capitis. The broken hair stubs and black dots are hair broken flush with the scalp surface, which is the endothrix pattern of Trichophyton tonsurans, and the enlarged occipital node favours a fungal scalp infection over a non-infectious dermatosis. I would confirm with potassium hydroxide microscopy of a plucked hair looking for branching septate hyphae and arthroconidia, and send fungal culture to identify the species. [1]

Branch A — the treatment principle

How would you treat this, and why? [2]

The principle is that tinea capitis always needs an oral antifungal, because the fungus lives inside the hair shaft and a topical cream cannot penetrate the follicle. So I would start an oral agent — griseofulvin or terbinafine, guided by the species — rather than a cream. A topical alone would fail, the hair loss would continue, and the child would keep shedding spores to siblings and classmates. [2]

Branch B — the species-guided drug choice

The culture returns Trichophyton tonsurans and the child weighs 22 kg. Which drug, what dose, and why not griseofulvin? [4]

I would choose terbinafine, because the meta-analyses show terbinafine is superior for Trichophyton species and griseofulvin is superior for Microsporum species. For a 22 kg child the terbinafine dose is 125 mg daily, using the weight bands of 62.5 mg daily under 20 kg, 125 mg daily for 20 to 40 kg, and 250 mg daily over 40 kg, given for 4 weeks. Griseofulvin would be the better choice if the organism were Microsporum, but for Trichophyton tonsurans terbinafine gives a shorter, more effective course. [4] [5]

Branch C — preventing recurrence

After the course the scalp clears, but three weeks later it returns. What did you miss? [10]

I missed the household reservoir. Recurrence after an adequate oral course almost always reflects untreated asymptomatic scalp carriage in family contacts and an untreated pet. I would screen and treat the household contacts, use an antifungal shampoo such as ketoconazole to reduce spore shedding, and have a veterinarian examine and treat any pets. For a zoophilic organism I would specifically ask about kittens and puppies as the source. [10]

Branch D — the pivot to kerion

Now a four-year-old presents with a painful, boggy, purulent, inflamed scalp mass with crusting and enlarged nodes, two weeks after the family acquired a kitten. What is this and how do you manage it? [1]

This is a kerion, an intense delayed hypersensitivity reaction to a zoophilic dermatophyte, most likely Microsporum canis from the kitten. I would start an oral antifungal immediately and add a short tapering course of oral corticosteroid to quiet the inflammation and reduce the risk of permanent scarring alopecia. I would explicitly avoid incision and drainage, because the pus is inflammatory rather than a drainable bacterial abscess. I would add an antistaphylococcal antibiotic only if there were genuine signs of secondary bacterial infection. [1] [2]

Closing — the steroid cream

A parent asks whether to keep using a combined corticosteroid, antifungal and antibiotic cream on an itchy annular forearm rash. What do you advise? [6]

I would advise stopping it. A topical corticosteroid applied to an undiagnosed tinea lesion produces tinea incognito: it suppresses the inflammatory border and the itch, so the child feels better, but the fungus is unopposed and spreads, the classic ring is lost, and the lesion becomes atrophic and pustular. I would stop the cream, confirm tinea with potassium hydroxide microscopy of the active border, and treat with a plain topical antifungal such as terbinafine or clotrimazole, reserving oral therapy for extensive disease. The rule is never to use a steroid-containing cream on an unrecognised scaly annular lesion. [6]