What are the differential diagnoses for keratotic lesions on the sole of the foot in an elderly female patient with flat feet (pes planus)?

Differential Diagnosis of Keratotic Lesions on the Sole in an Elderly Female with Flat Feet

The primary differential diagnoses for keratotic lesions on the sole of the foot in an elderly female with flat feet include calluses (diffuse hyperkeratosis), corns (heloma durum), seed corns (heloma miliare), plantar warts (verruca plantaris), porokeratosis plantaris discreta, and less commonly, neurovascular hyperkeratosis or focal palmoplantar keratoderma. 1

Primary Mechanical Keratotic Lesions (Most Common in Flat Feet)

Calluses (Diffuse Hyperkeratosis)

• Calluses appear as diffuse thickened areas without a central core, are generally less painful than corns, and enlarge with continuing friction as a direct hyperproliferative response of keratinocytes. 1
• In flat foot deformity, pain under the medial midfoot combined with visible callus formation strongly suggests flatfoot-related mechanical overload, characterized by hard, thickened, yellowish areas at characteristic pressure points. 2
• The collapsed arch in pes planus creates abnormal pressure distribution, particularly under the medial midfoot and metatarsal heads, predisposing to callus formation. 2

Corns (Heloma Durum)

• Corns present as hard, yellow lesions with a central nucleus or plug of keratin, causing pain described as "walking on a small stone or pebble," and are typically located on the tips of toes or underneath the metatarsal heads on weight-bearing surfaces. 1
• If the skin is hard and yellow with a nucleus or plug of keratin, it is called a corn or helom, and will appear red if inflamed. 3
• The central core of a corn extends downwards in a cone-shaped point and can cause notable pain and discomfort. 3

Seed Corns (Heloma Miliare)

• Seed corns are smaller, multiple discrete hyperkeratotic lesions with keratin plugs, frequently seen with toe and foot deformities, and are less painful than heloma durum but can be numerous. 1
• These are often encountered in patients with foot deformities due to increased pressure and friction on the skin. 1

Infectious Keratotic Lesions

Plantar Warts (Verruca Plantaris)

• Plantar warts are HPV-associated lesions on the sole of the foot that can present with hyperkeratotic appearance and must be distinguished from mechanical hyperkeratosis. 3
• Warts can persist for years with little or no sign of inflammation, and persistence for 5-10 years in adults is not uncommon. 3
• Paring of the lesion reveals characteristic pinpoint bleeding (thrombosed capillaries), which distinguishes warts from corns or calluses. 3

Less Common but Important Differentials

Porokeratosis Plantaris Discreta (PPD)

• PPD is a localized plantar hyperkeratosis that is resistant to therapy, often mistaken for a wart, and should be considered in the differential diagnosis of keratotic lesions on the sole. 4
• This condition presents as discrete keratotic plugs and requires biopsy for definitive diagnosis. 4

Neurovascular Hyperkeratosis

• Neurovascular hyperkeratosis is a rare variant (<1% of cases) where nerve endings and blood vessels become prominent in the epidermis, presenting as sensitive, painful areas that bleed with normal debridement, and usually affects skin overlying calcaneum and hallux. 1
• This results from long-standing gross hyperkeratosis and is an important consideration before performing aggressive debridement. 1
• Aggressive debridement should not be performed without first ruling out neurovascular hyperkeratosis, as this causes severe pain and bleeding. 1

Focal Palmoplantar Keratoderma

• Focal palmoplantar keratoderma is an inherited or acquired disorder causing localized hyperkeratosis, distinguished by distribution pattern and family history. 1

Critical Diagnostic Considerations

When to Suspect Malignancy

• Before performing intralesional steroid injection for resistant hyperkeratotic areas, it is recommended to exclude malignancy by biopsy. 1
• Biopsy is mandatory before aggressive debridement of suspicious lesions, such as squamous cell carcinoma, to exclude malignancy. 1
• Failure of an individual lesion to respond to physical therapy indicates a need for further evaluation, which could include formal excision. 3

Key Distinguishing Features

• Diagnosis of hyperkeratotic lesions requires careful examination and sometimes paring of the lesion, and hyperkeratosis needs to be distinguished from other conditions such as actinic keratoses, knuckle pads, squamous cell carcinoma, or focal palmoplantar keratoderma. 1
• Keratotic lesions develop beneath an osseous prominence and can result in substantial disability, making it imperative to establish the etiology among many possibilities and then address the specific pathology accordingly. 5

Biomechanical Context in Flat Feet

In elderly females with flat feet, the biomechanical abnormality creates predictable pressure points that favor callus and corn formation, particularly under the medial midfoot, first metatarsal head, and medial heel. 2 The collapsed arch redistributes weight abnormally, leading to chronic friction and pressure that drives hyperkeratotic lesion development. 2