What are causes of dark underarms

Dark underarms is a long-term top skin infection involving layers of the  skin caused by bacteria Corynebacterium minutissimum, also known as erythrasma ¹. The bacteria,  Corynebacterium minutissimum, responsible for erythrasma are gram-positive, non-spore-forming, aerobic or facultative bacilli ².

Erythrasma affects mostly adults, especially those with diabetes and those living in the tropics. Erythrasma is most common in the foot, where it causes scaling, cracking, and breakdown of the skin between the 4th and 5th toes. This infection is also common in the groin, where it causes irregularly shaped pink or brown patches and fine scaling especially where the thighs touch the scrotum (in men). The armpits, skinfolds under the breasts or on the abdomen, and the area between the vaginal opening and the anus (perineum) are prone to this infection, particularly among people with diabetes and among obese middle-aged women. In some people, the infection spreads to the torso and anal area.

• Erythrasma may coexist with or be confused with other causes of intertrigo (rashes in the skin folds) including fungal infections such as tinea or Candida albicans (thrush).

Although erythrasma may be confused with a fungal infection, doctors can easily diagnose erythrasma because skin infected with Corynebacterium glows coral-red under an ultraviolet light (Wood’s lamp).

An antibiotic given by mouth, such as erythromycin or tetracycline, can eliminate the infection. Antibacterial soaps, such as chlorhexidine, may also help. Drugs applied directly to the affected area (topically), such as erythromycin and clindamycin, are also effective. Antifungal creams such as miconazole may be helpful if yeast or fungus is present in the affected areas as well. Erythrasma may recur, necessitating a second treatment.

Who gets erythrasma ?

Erythrasma affects males and females, but it is thought to be more common in the groin of males and between the toes of females.

It is reported to be more prevalent in the following circumstances:

• Warm climate
• Excessive sweating
• Skin of color
• Diabetes
• Obesity
• Poor hygiene
• Advanced age
• Other immunocompromised states

What are the clinical features of erythrasma ?

Erythrasma presents as well-defined pink or brown patches with fine scaling and superficial fissures. Mild itching may be present.

Common sites for erythrasma are armpits, groin and between the toes. Intergluteal fold, submammary, and periumbilical skin may also be affected. Widespread infections are associated with diabetes mellitus.

Erythrasma is classified into 3 types according to location.

• Interdigital erythrasma: between 3rd, 4th and 5th toe web spaces
• Intertriginous erythrasma: in armpits, groin, under the breasts and umbilicus
• Generalised/disciform erythrasma: on the trunk

Symptoms of underarm erythrasma

The main symptoms are reddish-brown slightly scaly patches with sharp borders. They may itch slightly. The patches occur in moist areas such as the groin, armpit, and skin folds.

The patches often look similar to other fungal infections, such as ringworm.

Exams and Tests

The health care provider will check your skin and ask about the symptoms.

These tests can help diagnose erythrasma:

• Lab tests of scrapings from the skin patch
• Examination under a special lamp called a Wood lamp
• A skin biopsy

Figure 1. Underarm erythrasma

Note: Erythrasma is displayed as slowly enlarging brown or pink, rough areas in body folds.

Figure 2. Underarm erythrasma

Note: Erythrasma often has a very sharp border

Figure 3. Underarm erythrasma under Wood’s lamp

Note: This image shows the pink-red fluorescence of corynebacteria, an infection of the body folds known as erythrasma, shown under a “Woods lamp” in a dark room.

What are the complications of erythrasma ?

Erythrasma is usually self limiting. It can be complicated by contact dermatitis, lichenification, postinflammatory pigmentation, and coinfection with other bacteria, yeasts, and dermatophytes.

Serious complications are very rare. Corynebacteria have been reported to causes abscess, cellulitis, cutaneous granuloma, endocarditis, pyelonephritis, endophthalmitis, arteriovenous fistula infection and meningitis.

How is the diagnosis of erythrasma made ?

Erythrasma has a typical clinical appearance. Diagnosis may be supported by the following investigations.

• Wood’s lamp skin examination: long wavelength ultraviolet radiation causes erythrasma to fluoresce a coral-pink colour due to coproporphyrin III released by the bacteria.
• Swab or skin scrapings: microscopy may reveal gram-positive filamentous rods. Methylene blue also stains C minutissimum.

Treatments Your Doctor May Prescribe for Erythrasma

Erythrasma can be treated with antiseptic or topical antibiotic.

Once the diagnosis of erythrasma is established, the doctor may try one of the following treatments:

• Topical antibiotic lotions such as erythromycin or clindamycin
• Whitfield’s ointment (a mixture of benzoic acid and salicylic acid)
• Aluminum chloride solution to inhibit sweating and moisture
• Oral antibiotics such as erythromycin or clarithromycin.

Treatment is a single dose of clarithromycin 1 gram orally. One to two treatments (80 J/cm2) of broadband red light (635 nm) have been successful in a small case series. Topical erythromycin or clindamycin is also effective. Recurrence is common ³.

Home remedies for prevention of dark underarms

Recurrence of erythrasma is common. Antibacterial soap can be used to prevent recurrence. Treatment can be repeated if necessary.

If you are overweight, it may help to lose weight and often change your body position.

Other things you can do are:

• Separate skin folds with dry towels.
• Blow a fan on moist areas.
• Wear loose clothing and moisture wicking fabrics.
• Keep skin cool and dry.
• Do not wear tight shoes or clothing. Wear a bra that has good support.
• Wear clothes made with absorbent fabrics, such as cotton. Avoid nylon or other synthetic (manmade) fibers.
• After exercising, shower and dry off completely. Use a hair dryer with a cool setting to dry areas that can trap wetness, such as under your arms or breasts.
• Gently cleanse the affected areas daily with mild soap substitutes.
• Antibacterial soap. Antibacterial soaps for household use generally contain the active ingredient triclosan at concentrations between 0.1% and 0.45% weight/volume. Triclosan has varying effectiveness across bacterial and fungal species, and is less effective against viruses. A related compound, triclocarban, is used in antibacterial bar soaps.Laboratory-based studies have shown triclosan at high concentration (1.0% weight/volume or higher) can reduce bacterial counts on the hands compared with plain soap.

  However, community-based studies have shown that triclosan, at usual household concentrations of 0.1%-0.45% weight/volume, is generally no more effective than plain soap in reducing bacterial levels on the hands or in reducing infectious illnesses. Interestingly, even in studies conducted in areas with high rates of infectious illnesses, such as squatter settlements in Pakistan, antibacterial soaps offered little benefit compared with plain soap.

  As most community infections are viral, the lack of a clear benefit for antibacterial household soap is not unexpected. However, even when examining the incidence of bacterial infections, such as impetigo, triclosan provided no benefit over plain soap.

1. Erythrasma. Medline Plus. https://medlineplus.gov/ency/article/001470.htm[↩]
2. Erythrasma. https://www.dermnetnz.org/topics/erythrasma/[↩]
3. Erythrasma. Merck Manual. https://www.merckmanuals.com/professional/dermatologic-disorders/bacterial-skin-infections/erythrasma[↩]