Diagnosing a case of tinea incognito

Patients often take it upon themselves to self-diagnose and self-treat, says Dr W Iain F Henderson

Mr Jones was a 52-year-old patient who had been treated for psoriasis — he had chronic plaque psoriasis that also affected his scalp, flexures and nails.

He had recently developed a groin rash to which he had applied clobetasone butyrate. This initially eased the rash, but each time he stopped the clobetasone it worsened.

Next he tried mometasone cream, which he had been given to use on his plaque psoriasis in combination with calcipotriol cream.

Once again it gave temporary relief, but the rash kept recurring when he stopped using the cream. The area of the rash had also slowly increased.

Examination
On examination, the rash was present in both groin flexures and on the upper inner thighs. There were a few pustules around the periphery, which itself was flat and not particularly inflamed. It was not significantly itchy or scaly.

Examination of the rest of his body revealed some psoriatic plaques on his knees, elbows and scalp.

He also had nail changes in fingers and toes, with pitting, onycholysis and subungual hyperkeratosis.

The rash in the groin did not have the classical appearance of flexural psoriasis, which is smooth, shiny, red and well demarcated.

Scrapings were taken from the edge of the rash and sent for mycology; these proved positive for Trichophyton rubrum.

Nail clippings with some subungual debris were taken, because he also had signs of tinea pedis in his toe web spaces; approximately 20 per cent of tinea pedis can go on to affect toenails. The clippings and debris also proved positive for T rubrum.

Superficial dermatophyte infections can usually be treated with topical antifungals such as terbinafine cream daily for 10 days, or any topical imidazole for 14 days.

Fungal nail infections (onychomycosis), however, need oral antifungals.

Oral terbinafine 250mg a day for three months cleared up both his groin rash and the infection in his nails, although the psoriatic nail changes persisted.

Tinea incognito
Tinea incognito is often called steroid-modified tinea. Because of the application of steroids the rash loses the classical appearance of tinea infections.

It usually presents with scaling and a prominent peripheral rim. As the infection spreads there is commonly a central clearing.

In tinea incognito there is loss of scaling and the prominent rim, with a reduction in itch.

There can be pustules and papules, and it can be more extensive due to a damping down of the inflammation and immune response by the steroid cream.

Because of the non-classical appearance, the diagnosis may be missed.

If topical steroids have been applied recently it may be difficult to obtain sufficient scrapings for mycology due to the lack of scale and prominent rim.

If sufficient scrapings are obtained a few days after the steroid has been stopped, the decreased immune response resulting from steroid use may cause more fungal elements to be seen on microscopy and more rapid growth in culture.

Differential diagnoses
Differential diagnoses of groin rashes would include flexural psoriasis, erythrasma and candidiasis. The appearance of psoriasis is mentioned above. There are usually signs of psoriasis elsewhere.

Candidiasis can also have peripheral pustules and papules (satellite lesions) but tends to be redder and cause more soreness than tinea incognito.

It may aggravate intertrigo or flexural psoriasis and can also affect the toe web spaces. Swabs and scrapings will distinguish it from dermatophyte infections.

Erythrasma is caused by Corynebacterium minutissimum. It can affect the groin, axillae and web spaces.

The rash is reddish/brown with fine wrinkles and scales and is not usually itchy. It fluoresces coral pink under a Wood’s light due to the porphyrins released by the bacteria.

The spread of infection
I recently heard an interesting theory from a colleague on why the vast majority of cases of tinea cruris are in males.

He put it down to males drying themselves from the feet up when coming out of a shower or bath, which helps the spread of tinea from the feet to the groin. Females dry themselves from the top down.

Differential diagnoses of the dermatophyte nail deformities would be psoriasis and candida. Psoriasis and dermatophyte infection can co-exist and the skin of the feet can also be affected by both.

T rubrum is the most common cause of onychomycosis followed by T interdigitale.

It is advisable to get a positive mycology result before contemplating antifungal therapy as treatment is not free of potential adverse events.

Dermatophyte onychomycosis is more common in toenails whereas candidiasis, often with accompanying paronychia, is more common in fingernails. Oral itraconazole is the treatment of choice for chronic candidal nail infection.

Dr Henderson is a GP in Scotstoun, Glasgow, a hospital practitioner in dermatology, Western Infirmary, Glasgow, and a Scottish representative of the Primary Care Dermatology Society

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