What are the differential diagnoses for a hyperkeratotic, adherent, scaly plaque on the anterior tibia and a hyperpigmented, scaly, punched‑out plaque on the lateral malleolus?

Differential Diagnosis of Hyperkeratotic Lower Leg Lesions

The differential diagnosis for these distinct lower leg lesions should prioritize chronic friction-related hyperkeratosis, actinic keratosis, hypertrophic lichen planus, porokeratosis, and squamous cell carcinoma in situ (Bowen's disease), with the punched-out appearance at the lateral malleolus raising particular concern for malignancy or porokeratosis.

Key Distinguishing Features to Assess

The clinical presentation describes two morphologically different lesions requiring separate consideration:

Anterior Tibia Lesion (Hyperkeratotic, Adherent, Scaly Plaque)

Primary considerations:

• Chronic hyperkeratosis/callus formation: The anterior tibia is a pressure point susceptible to friction-related hyperkeratosis, particularly with repetitive trauma or ill-fitting footwear 1. However, this typically occurs on weight-bearing surfaces rather than the anterior shin.

• Hypertrophic actinic keratosis: These present as adherent, hyperkeratotic plaques on sun-exposed areas. The anterior tibia receives significant UV exposure. Hypertrophic variants can develop thick scale and may progress to squamous cell carcinoma 2, 3.

• Hypertrophic lichen planus: Presents as hyperkeratotic plaques, often on the shins, with characteristic violaceous color and Wickham's striae, though these may be obscured by thick scale 4.

• Psoriasis: Can present as well-demarcated, hyperkeratotic plaques with silvery scale, though typically more widespread 5.

Lateral Malleolus Lesion (Punched-Out, Scaly, Hyperpigmented)

The punched-out morphology is particularly concerning and narrows the differential significantly:

• Porokeratosis: Characteristically presents as well-demarcated plaques with an atrophic center and raised, scaly border, creating a "punched-out" appearance. Importantly, porokeratosis has malignant transformation potential and can mimic squamous cell carcinoma 6.

• Squamous cell carcinoma in situ (Bowen's disease): Presents as scaly, erythematous or hyperpigmented plaques. The punched-out appearance could represent early ulceration or atrophy. This requires urgent biopsy 7.

• Invasive squamous cell carcinoma: Hyperkeratotic variants can present with adherent scale and central depression. The lateral malleolus location is atypical but possible 7.

• Neurovascular hyperkeratosis: A rare form of callus where nerve endings and blood vessels become prominent, typically painful and sensitive. Usually affects the calcaneum and hallux but could theoretically occur at the lateral malleolus with chronic trauma 1.

Critical Management Algorithm

Immediate action required:

1. Biopsy both lesions separately - The punched-out morphology at the lateral malleolus mandates histological examination to exclude malignancy. Any persistent hyperkeratotic lesion unresponsive to conservative management requires biopsy 8, 7.

2. Assess for:

   • Sun exposure history: Extensive UV exposure suggests actinic keratosis or SCC
   • Trauma/friction history: Chronic rubbing, ill-fitting shoes, or occupational factors suggest hyperkeratosis
   • Pain/bleeding: Neurovascular hyperkeratosis is painful; bleeding suggests malignancy
   • Duration and progression: Rapid growth or ulceration indicates malignancy
   • Immunosuppression status: Increases risk of SCC and aggressive behavior 3

3. Dermoscopy evaluation: Can help distinguish between benign hyperkeratosis, actinic keratosis, and malignancy before biopsy 9.

Common Pitfalls to Avoid

• Do not assume friction-related hyperkeratosis without biopsy confirmation, especially with the punched-out morphology. The lateral malleolus is not a typical pressure point for callus formation.

• Do not debride these lesions before obtaining tissue diagnosis. Debridement of malignant lesions delays diagnosis and may cause bleeding, particularly if neurovascular hyperkeratosis is present 1.

• Do not treat empirically with keratolytics until malignancy is excluded. Persistent lesions despite appropriate topical therapy warrant biopsy 8.

• The hyperpigmentation is non-specific and occurs in chronic inflammation, post-inflammatory changes, and can be seen in Bowen's disease 7.

Most Likely Diagnoses Based on Location and Morphology

For the anterior tibia plaque: Hypertrophic actinic keratosis or hypertrophic lichen planus are most likely, given the hyperkeratotic nature and shin location.

For the lateral malleolus punched-out lesion: Porokeratosis or squamous cell carcinoma in situ are the primary concerns given the distinctive morphology. The punched-out appearance with hyperpigmentation and scale is highly characteristic of porokeratosis, but SCC in situ must be excluded given its similar presentation and the malignant potential of both conditions 6, 7.

Biopsy is non-negotiable for both lesions - the punched-out morphology combined with hyperkeratosis and hyperpigmentation creates sufficient concern for malignancy or premalignant conditions that tissue diagnosis is mandatory before any treatment intervention.