Pitted keratolysis

What is pitted keratolysis, and how common is it?

Pitted keratolysis (also called keratolysis plantare sulcatum) is a bacterial infection of the soles. It stays in the thick outer layer of skin, called the stratum corneum, and makes small, crater-like pits that aren't inflamed¹. Corynebacterium bacteria are the main cause².

It's described as a common infection, and it has been reported in many countries, in people who wear shoes and people who don't²³. How many people have it overall isn't clear yet.

What causes it, and who gets it?

The infection is caused by bacteria, mainly Corynebacterium, that multiply in the thick skin of the sole². Raised skin pH, heavy sweating and long periods in enclosed footwear let the bacteria multiply⁴. They then break down keratin (the tough protein in your outer skin), which leaves the pits⁵. Heavy sweating (hyperhidrosis) is described as the most important predisposing factor¹.

It's reported more often in people who spend long periods in wet, enclosed shoes, such as soldiers, sailors and industrial workers, and in barefoot labourers and farmers. Active feet can also be affected, because sport makes feet hot and wet⁶. It's also described as more common in warm tropical and subtropical countries¹, and men whose work needs closed footwear are commonly affected⁵. These are descriptions of who is often affected. They don't show how much each factor raises the risk.

What does it look and feel like, and can it be serious?

The main sign is a cluster of small pits in the skin of the sole. They usually start on the pressure-bearing areas: the pads of the toes, the ball of the foot and the heel. The spaces between the toes are next most often affected⁷. Both soles are often involved².

Most people also have sweaty feet. The skin can feel slimy, and there's often a distinctive smell⁷, which is attributed to sulphur compounds made by the bacteria⁸. Itching and foot pain are common⁵. In more serious cases the pits merge into larger, irregular erosions or grooves, with or without redness⁸.

How is it diagnosed?

Sweaty, irritated soles with many small pits are described as the classic sign of pitted keratolysis⁴, so it can often be recognised from the look of the skin and the sweating and smell. Foot infections can look different on different skin tones, which can make diagnosis harder⁹. Other skin problems of the sole, such as athlete's foot or plantar warts, can look similar.

Can it become serious?

Pitted keratolysis itself is a surface infection. Common foot skin infections in general can potentially progress to deep abscesses and bone infection¹⁰. These signs can mean infection in any foot wound or sore skin:

• A cut, blister or wound that isn't healing, or skin that turns red, warm or painful¹¹.
• Increasing redness, swelling and pain, red streaking or pain moving up the leg, or sore lymph nodes (behind the knees, in the groin, neck or armpits). These can mean a spreading infection that, untreated, could lead to sepsis¹².
• Feeling cold, shivery, shaky and unwell with a foot wound, which can mean sepsis is developing¹³.
• Unexplained heat, a sudden increase in pain, or tight, painful swelling in the foot¹⁴. A rapidly swollen, painful foot with feeling unwell is also described as a warning sign¹⁵.

Foot problems can be more serious with diabetes. Nerve damage can mean a blister or a pebble in the sock goes unnoticed, and reduced blood flow can make sores and infections slow to heal¹¹.

What helps first?

Reducing moisture on the feet through foot hygiene is the starting point of treatment⁹.

A topical antibiotic is the treatment usually tried first. Reviews describe it as the recommended first step, with oral antibiotics kept for cases that don't respond to topical treatment⁸. Antibiotics applied to the skin are reported to work well, to be easy to use and to suit patients⁶. They're often combined with hygiene advice and antiperspirants³.

The evidence for all of this is limited. It's mostly case reports and small case series, and there's no agreement on the single best approach³. Benzoyl peroxide, another skin treatment, has also been used for the infection and for foot smell in naval cadets¹⁶.

What helps if that isn't enough?

No best next step has been agreed. Combining an antimicrobial treatment with extra measures such as antiperspirants appears to work best³. Oral antibiotics are described as being for cases that don't respond to topical antibiotics⁸. Because heavy sweating makes the bacteria more likely to multiply, controlling the sweating is a sensible focus¹⁷. Which particular sweat treatments help pitted keratolysis isn't clear yet.

Foot infections that don't respond to treatment are taken seriously, because of the risk of deeper infection described above⁹.

How long does it take, and how can I stop it coming back?

The outlook is described as excellent. The condition is treatable, non-contagious and non-inflammatory, and the pits and smell are reported to clear within 1 month of effective antibiotic treatment⁸.

The bacteria thrive when skin pH rises and when feet sweat and stay enclosed for long periods⁴. Heavy sweating on the soles is also linked with a higher risk of skin infections in general¹⁸. Cutting down on long stretches of sweaty, enclosed feet targets those conditions, for example by giving your feet breaks from enclosed shoes. How well this prevents the infection coming back isn't clear yet.