Pictorial Case Study: Groin rash

The case: This 69-year-old obese diabetic man presented to the surgery with a long history of a rash affecting his groin. The rash was present bilaterally and extended onto his upper thighs and scrotum. It was occasionally itchy but most of the time did not bother him.

Erythrasma: predisposing factors include obesity and diabetes
Erythrasma: predisposing factors include obesity and diabetes

Assuming it was a fungal infection, the like of which he had in his younger, rugby playing days, he had treated it with topical clotrimazole for a number of weeks, but this had failed to help.

His skin was otherwise dry and both axillae and his groin had a number of skin tags. Closer inspection revealed an area of well demarcated red-brown macules with a wrinkled appearance to the skin and a fine scale.

Diagnosis and management
The diagnosis is erythrasma, a bacterial infection caused by Corynebacterium minutissimum. This is part of the normal skin flora which proliferates in affected areas. Predisposing factors include excessive sweating, obesity, diabetes, poor hygiene, advanced age and other conditions which affect immunity.

Diagnosis can be made by shining a Wood's light on the affected areas, which reveals a coral-red fluorescence caused by porphyrins produced by the bacteria. Note these may not be present if the patient has bathed recently.

Treatment can be with either oral or topical antibiotics. Although resistance may occur, antibiotics, such as penicillin, erythromycin, cephalosporins and tetracyclines, are usually effective. The prognosis is usually excellent but recurrence is common if underlying factors are not also addressed.

Possible differential diagnoses

  • Tinea cruris
  • Acanthosis nigricans
  • Seborrhoeic dermatitis
  • Intertrigo
  • Plaque psoriasis
  • Candidiasis
  • Contact dermatitis

Differential diagnosis
Tinea cruris

  • A very common condition, especially in men.
  • Usually associated with pruritus and less hyperpigmentation (unlike erythrasma).
  • Most common causative organisms are Trichophyton rubrum and Epidermophyton floccosum.
  • In the acute stages the rash may be moist with exudate and inflamed erythematous papules.
  • The rash may extend to the buttocks and perineum but the penis is usually spared.
  • Treat with a topical antifungal, such as clotrimazole. More resistant cases may require oral antifungals.
Contributed by Dr Nigel Stollery, a GP in Kibworth, Leicestershire, and clinical assistant in dermatology at Leicester Royal Infirmary

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