Erythrasma and Pitted Keratolysis
On this page
Direct answer
Erythrasma is a superficial Corynebacterium minutissimum infection of the stratum corneum in skin folds, producing well-demarcated, scaly light brown patches that fluoresce coral-red under a Wood lamp. Pitted keratolysis is a related bacterial infection of the weight-bearing soles that digs shallow pits into the stratum corneum, typically in sweaty feet inside occlusive footwear. Both respond to topical antibacterials and drying measures, and both are often mistaken for fungal disease.
What you must remember
- Erythrasma organism: Corynebacterium minutissimum, which produces porphyrins that give the characteristic fluorescence.
- Erythrasma sites: intertriginous areas such as the axillae, groins, inframammary folds, interdigital toe webs and perineum.
- Erythrasma morphology: sharply demarcated, light brown to reddish patches with fine scale, usually asymptomatic or mildly itchy.
- Wood lamp colours: coral-red fluorescence in erythrasma, and yellow to yellow-green fluorescence in pityriasis versicolor from Malassezia.
- Pitted keratolysis: multiple discrete, shallow, punctate pits on weight-bearing plantar surfaces, often with a foul odour, favoured by sweating and occlusion.
- Predisposing factors: diabetes, obesity, hyperhidrosis and occlusive clothing or footwear; both conditions recur while these persist.
- Treatment: topical erythromycin, clindamycin or fusidic acid for erythrasma, with oral erythromycin for extensive disease; topical clindamycin, benzoyl peroxide or aluminium chloride for pitted keratolysis.
- Prevention: keep intertriginous and plantar skin dry, change socks daily, rotate footwear and use antiperspirants where sweating is excessive.
Two superficial bacterial infections compared
Both conditions are superficial bacterial infections of the stratum corneum that look deceptively benign, and the comparison below separates them by site, lesion and treatment. Candidates should memorise the site first, because the site usually points directly to the diagnosis.
| Feature | Erythrasma | Pitted keratolysis |
|---|---|---|
| Typical site | Intertriginous folds | Weight-bearing soles |
| Lesion | Light brown patches with fine scale | Shallow, punctate pits in the stratum corneum |
| Organism | Corynebacterium minutissimum | Kytococcus sedentarius and related corynebacteria |
| Odour | Usually absent | Often foul |
| Key treatment | Topical antibacterials, oral erythromycin if extensive | Topical clindamycin, benzoyl peroxide or aluminium chloride |
The two infections share a moist, occluded environment, so the same hygiene advice applies to both. Pitted keratolysis is most common in people whose work or footwear keeps the soles wet and enclosed for long periods. In practice, the site, the lesion and a Wood lamp examination resolve most cases, and a potassium hydroxide preparation is reserved for genuine diagnostic doubt.
Where the exam catches students
Most traps arise from mislabelling a superficial bacterial infection as a fungal one.
- Trap: labelling every scaly groin patch as tinea cruris Correct: erythrasma is sharply demarcated, lacks the raised vesicular border of tinea and fluoresces coral-red under a Wood lamp.
- Trap: confusing erythrasma with pityriasis versicolor Correct: Malassezia gives yellow to yellow-green fluorescence, whereas Corynebacterium minutissimum gives coral-red fluorescence.
- Trap: prescribing antifungals alone for pitted keratolysis Correct: the causative organisms are bacteria, so topical antibacterials and reduction of sweat and occlusion are the logical treatment.
- Trap: treating recurrent erythrasma without screening Correct: diabetes and hyperhidrosis predispose to recurrence, so the underlying cause must be checked.
Frequently asked questions
What does erythrasma look like under a Wood lamp?
Erythrasma shows coral-red fluorescence under a Wood lamp because Corynebacterium minutissimum produces porphyrins in the stratum corneum. This bedside clue helps separate erythrasma from fungal intertrigo and from pityriasis versicolor.
Which fungal infection does erythrasma most resemble?
Erythrasma most resembles tinea cruris in the groin, but it lacks the raised, vesicular advancing border of tinea and fluoresces coral-red. A Wood lamp examination and a clear demarcation of the lesion usually distinguish the two conditions.
What causes pitted keratolysis?
Pitted keratolysis is caused by bacteria such as Kytococcus sedentarius and certain corynebacteria acting on the plantar stratum corneum. Sweaty, occluded feet favour it, and it is managed with topical antibacterials together with reduction of sweating and occlusion.
How is erythrasma treated?
Localised erythrasma responds to topical erythromycin, clindamycin or fusidic acid, while extensive disease is treated with oral erythromycin. Keeping the intertriginous skin dry and controlling any diabetes or hyperhidrosis reduces the chance of erythrasma recurring.
Why does erythrasma recur in some patients?
Erythrasma recurs when the moist, occluded intertriginous environment persists, especially in obesity, hyperhidrosis and diabetes. Antibacterial treatment alone does not correct these factors, so drying measures and control of the underlying condition are essential.