What is pitted keratolysis

Pitted keratolysis is a descriptive term for a superficial bacterial skin infection that affects the soles of the feet, and less often, the palms of the hands ¹⁾. Pitted keratolysis is one of the causes of smelly feet. Pitted keratolysis is characterized by whitish skin and clusters of punched-out pits. Pitted keratolysis is common among athletes and individuals in professions with greater use of occlusive footwear ²⁾. Patients may experience hyperhidrosis (excessive sweating), foot odor and sometimes itching or burning while walking, although most cases are asymptomatic ³⁾. The lesions tend to be multiple, superficial rounded depressions, 0.5 to 7 mm in diameter, affecting mainly weight-bearing areas of the soles. Palms are less commonly involved. Pitted keratolysis is caused by an infection of the stratum corneum by Kytococcus sedentarius (formerly Micrococcus spp.), though Dermatophilus congolensis and Corynebacterium spp. have also been implicated ⁴⁾. Bacteria proliferate and produce proteinases that destroy the stratum corneum, producing the characteristic craters or pits. The odor linked to pitted keratolysis is due to the production of sulphur compounds.

The diagnosis of pitted keratolysis is clinical ⁵⁾. Cultures, if taken, show gram-positive coccobacilli or bacilli. Examination with a Wood light is not reliable for diagnosis and may be negative or show coral-red fluorescence when Corynebacterium spp. is implicated ⁶⁾. Dermoscopy usually reveals abundant pits with well-marked walls that sometimes show the bacterial colonies ⁷⁾. A potassium hydroxide examination of the lesions can exclude tinea pedis ⁸⁾.

Pitted keratolysis has an excellent prognosis; effective treatment clears both the lesion and the odor in 3-4 weeks. Topical antibiotics are effective, easy to use and accepted by patients. Recommendations include twice daily application of erythromycin solution or gel ⁹⁾, 1% clindamycin hydrochloride solution ¹⁰⁾, fusidic acid cream and mupirocin cream ¹¹⁾.

Figure 1. Pitted keratolysis feet

Figure 2. Skin structure

Figure 3. Skin epidermis

Is pitted keratolysis contagious?

Yes. More than the half of the female patients (58.8%) with pitted keratolysis had a history of regular pedicure and foot care in a spa salon ¹²⁾. Moisture and inappropriate hygiene may be the predisposing factors in such patients.

Pitted keratolysis is caused by a cutaneous infection with Kytococcus sedentarius ¹³⁾, Dermatophilus congolensis ¹⁴⁾ and the Corynebacterium species ¹⁵⁾. Under appropriate conditions (i.e. prolonged occlusion, hyperhidrosis, increased skin surface pH), these bacteria proliferate and produce proteinases that destroy the stratum corneum, creating pits. Kytococcus sedentarius has been found to produce two keratin-degrading enzymes, protease P1 and P2 respectively ¹⁶⁾. The malodor associated with pitted keratolysis is presumed to be due to the production of sulfur-compound by-products, such as thiols, sulfides and thioesters ¹⁷⁾.

What is the cause of pitted keratolysis?

Pitted keratolysis is caused by several bacterial species, including corynebacteria, Dermatophilus congolensis, Kytococcus sedentarius, actinomyces and streptomyces.

The bacteria proliferate in moist conditions. The pitting is due to destruction of the horny cells (stratum corneum) by protease enzymes produced by the bacteria.

The bad smell is due to sulfur compounds produced by the bacteria: thiols, sulfides and thioesters.

Who’s at risk of pitted keratolysis?

Pitted keratolysis is much more common in males than in females. Occupations at risk include:

• Farmers
• Athletes
• Sailors or fishermen
• Industrial workers
• Military personnel

Females offering pedicure and foot care in a spa salon may also be affected by pitted keratolysis.

Theoretically, both males and females should be affected; however, most written case reports or studies have involved male patients ¹⁸⁾. Adult males with sweaty feet are most susceptible to pitted keratolysis (97% of the cases) ¹⁹⁾. Few cases have been reported in the elderly ²⁰⁾. Pitted keratolysis is reported to be more common among barefooted laborers/farmers, marine workers, soldiers and industrial workers wearing occluded shoes for prolonged periods.

Factors that lead to the development of pitted keratolysis include:

• Hot, humid weather
• Occlusive footwear, such as rubber boots or vinyl shoes
• Excessive sweating of hands and feet (hyperhidrosis)
• Thickened skin of palms and soles (keratoderma)
• Diabetes mellitus
• Advanced age
• Immunodeficiency

How can pitted keratolysis be prevented?

Pitted keratolysis will quickly recur unless the feet are kept dry. The following precautions should be taken to prevent recurrences:

• Wear boots for as short a period as possible
• Wear socks which effectively absorb sweat, i.e., cotton and/or wool
• Wear open-toed sandals whenever possible
• Wash feet with soap or antiseptic cleanser twice daily
• Apply antiperspirant to the feet at least twice weekly
• Do not wear the same shoes two days in a row — dry them out
• Do not share footwear or towels with others.

Pitted keratolysis symptoms

The result is very smelly feet, due to infection of the soles. Either the forefoot or the heel or both become white with clusters of punched-out pits. The appearance is more dramatic when the feet are wet. Very rarely, the fingers are similarly affected. There is a variant of pitted keratolysis where there are more diffuse red areas on the soles.

• Mostly affects the soles, forefoot, the heel or both. Palms are rarely infected.
• Results in a whitish skin surface with clusters of multiple, fine punched-out pits.
• Pits often join together (coalesce) to form larger, crater-like lesion.
• A variant of pitted keratolysis presents with diffuse red areas on the soles.

The most commonly reported symptom is malodor. The pits themselves are usually asymptomatic but may cause soreness or itching when walking.

Pitted keratolysis treatment

Changing footwear and socks regularly as well as airing out or rotating shoes is key to management. Avoiding use of occlusive footwear, reduction of foot friction with properly fitting footwear, using absorbent cotton socks, wearing open toed sandals whenever possible, washing feet with soap or antibacterial cleanser twice a day, and avoiding sharing of footwear or towels. In some cases it may be helpful to reduce any associated hyperhidrosis with the application of a roll-on antiperspirant or 20% aluminum chloride solution ²¹⁾.

Topical antibiotics are the first line of medical treatment ²²⁾.

Pitted keratolysis can be successfully treated with topical antibiotics and antiseptics including:

• Erythromycin
• Clindamycin
• Mupirocin
• Fusidic acid
• Benzoyl peroxide

Oral erythromycin is also helpful, may shorten the treatment period ²³⁾.

Twice-daily application of erythromycin 1% (solution or gel), 1% clindamycin hydrochloride solution, fusidic acid or mupirocin 2% are recommended ²⁴⁾. Botulinum toxin or iontophoresis for localized treatment of sweating may be useful ²⁵⁾. There is also evidence for adding benzoyl peroxide to the antibiotics treatment ²⁶⁾. Prognosis is excellent, and lesions usually resolve in three to four weeks ²⁷⁾.