Pits on the soles of the feet

Diagnosis: Pitted keratolysis

Pitted keratolysis (PK), also known as keratolysis plantare sulcatum, is a skin disorder characterized by pits and collarettes from bacterial infection. PK is a superficial infection, confined to the stratum corneum.

Micrococcus sedentarius, a Gram-positive Staphylococcus-related bacterium, Dermatophilus congolensis, a Gram-positive facultative anaerobic Actinomyces species, and several Corynebacterium species have all been identified as causative agents of PK. These bacteria make proteinases that destroy the stratum corneum and open small tunnels and pits in the skin.^1-4

Clinical picture of PK

The plantar aspects of the feet are most commonly affected by PK, pressure-bearing areas such as the ventral aspect of the toes and the ball of the foot in particular. Some patients develop lesions on the interdigital surfaces. The localized absence of the stratum corneum leads to a punched-out appearance of the skin.

Prolonged time of occlusion and hyperhidrosis often lead to increased skin surface pH. This triggers bacterial infections, resulting in PK. Malodor is common, presumed to be due the production of sulfur-compound byproducts such as thiols, sulfides, and thioesters.^2-4 Often asymptomatic, a patient with PK may develop varying degrees of discomfort, ranging from mild burning sensation to severe tenderness and limitation of function.

The diagnosis is often clinical and seldom poses a challenge; skin biopsy is rarely performed. In recent reports, the use of transmission electron microscopy and scanning electron microscopy showed bacteria in the stratum corneum with typical transversal septations. Tunnel-like spaces were built inside the stratum corneum, where the bacteria exhibited a hairy surface.

The differential diagnosis for PK may include the following, especially when the soles are involved: candidal infections, basal cell nevus syndrome, and keratolysis exfoliativa.