What is the differential diagnosis (DD) for a lesion presenting as a keratin plug at the sole of the foot?

Differential Diagnosis of Keratin Plug Lesion on Sole of Foot

The most likely diagnosis is a corn (heloma durum), which characteristically presents as a hard, yellow lesion with a central nucleus or plug of keratin on weight-bearing surfaces of the sole. 1

Primary Differential Diagnoses

Corns (Heloma Durum) - Most Common

• Hard corns present as localized hyperkeratotic lesions with a central keratin plug extending downward in a cone-shaped point 1
• Typically located on tips of toes or underneath metatarsal heads on weight-bearing surfaces 1
• Patients describe the sensation as "walking on a small stone or pebble" due to the central keratin core 2
• Pain is a distinguishing feature compared to calluses 1

Seed Corns (Heloma Miliare)

• Smaller, multiple discrete hyperkeratotic lesions with keratin plugs 3
• Frequently seen with toe and foot deformities 3
• Less painful than heloma durum but can be numerous 3

Porokeratosis Plantaris Discreta

• Discrete, exquisitely painful plantar lesions with a cutaneous groove filled by a keratin plug (cornoid lamella) 4
• Histologically shows focal absence of granular layer underlying the keratin plug 4
• Relatively uncommon but important to recognize due to severe pain 4

Secondary Differential Diagnoses

Calluses (Tyloma)

• Diffuse thickened areas without a central keratin core or plug 1, 5
• Generally less painful than corns 1
• Enlarge with continuing friction as hyperproliferative response 1

Neurovascular Hyperkeratosis

• Rare variant (<1% of cases) where nerve endings and blood vessels become prominent in the epidermis 3, 1
• Presents as sensitive, painful areas that bleed with normal debridement 1
• Usually affects skin overlying calcaneum and hallux 3, 1
• Results from long-standing gross hyperkeratosis 3

Verruca Plantaris (Plantar Wart)

• Can present with hyperkeratotic surface and central keratin plug 6
• Distinguished by pinpoint bleeding when pared (thrombosed capillaries) 6
• May have viral origin rather than purely mechanical 5

Focal Palmoplantar Keratoderma

• Inherited or acquired disorder causing localized hyperkeratosis 1
• Distinguished by distribution pattern and family history 1

Critical Diagnostic Considerations

When to Suspect Malignancy

• Before performing aggressive debridement or intralesional steroid injection, exclude malignancy by biopsy 1
• Squamous cell carcinoma can present with hyperkeratotic appearance 1
• Biopsy is mandatory for suspicious lesions before aggressive treatment 1

Special Populations

• In patients with epidermolysis bullosa, heloma miliare (seed corns) are frequently encountered due to foot deformity 3
• Diabetic patients require more conservative approach due to neuropathy and vascular disease 7

Diagnostic Approach

Clinical Examination Features to Assess

• Paring the lesion reveals whether a central keratin core is present (corn) versus diffuse thickening (callus) 1, 8
• Location on weight-bearing surfaces suggests mechanical etiology 8, 9
• Presence of pain with pressure indicates corn rather than callus 1
• Check for bleeding with debridement (suggests neurovascular hyperkeratosis or wart) 1, 4

Key Pitfalls to Avoid

• Do not perform aggressive debridement without first ruling out neurovascular hyperkeratosis, as this causes severe pain and bleeding 1
• Avoid chemical plasters for removal, especially in diabetic patients 2, 7
• Do not assume all hyperkeratotic lesions are benign; biopsy suspicious lesions 1