Seborrheic Keratosis: Clinical Description
Seborrheic keratosis (SK) is the most common benign epidermal tumor in humans, presenting as well-demarcated, "stuck-on" waxy or keratotic lesions that typically appear in individuals over age 50. 1, 2
Clinical Appearance
Key morphological features include:
- "Stuck-on" appearance with symmetric, waxy, or keratotic surface texture that appears as if it could be easily peeled off 2, 3
- Well-demarcated borders with variable pigmentation ranging from flesh-colored to tan, brown, or black 1, 3
- Surface characteristics may show a verrucous (warty) texture with visible keratin plugs or horn cysts that give a characteristic appearance 1, 4
- Size variation from a few millimeters to several centimeters in diameter 1
Distribution and Epidemiology
Location patterns:
- Most commonly affects the face and upper trunk, though can occur anywhere except palms and soles 1
- Multiple lesions are typical, with 33% of patients having more than 15 lesions and 67% having 15 or fewer 2
- Affects approximately 83 million Americans, making it among the most common cutaneous lesions 2
Risk factors include:
- Age over 50 years with increasing incidence with advancing age 1, 4
- Genetic predisposition plays a role in development 1
- Ultraviolet radiation exposure is thought to contribute, though the relationship is less clear than with other skin lesions 1, 4
Clinical Variants
Several subtypes exist based on clinical and histologic features:
- Common SK with typical stuck-on, waxy appearance 1
- Flat (plaque-type) SK with minimal elevation 1
- Pedunculated SK on a stalk 1
- Irritated SK showing inflammation, often with itching 5
Important Diagnostic Considerations
A critical pitfall is that SK can mimic melanoma or other malignancies, and conversely, skin cancers can mimic SK or arise within SK lesions. 3 This is particularly relevant because:
- SK lesions are commonly referred in Britain, especially when they itch, which can lead to confusion with melanoma 5
- Dermoscopy improves diagnostic accuracy and should be used when available to differentiate SK from malignant lesions 3
- When in doubt, biopsy is warranted to exclude malignancy, particularly for atypical presentations 3
Symptomatic Features
Most SK lesions are asymptomatic, but may present with:
- Pruritus (itching) is common and was historically overemphasized as a melanoma warning sign, leading to excessive SK referrals 5
- Irritation or trauma from clothing or jewelry rubbing against lesions 2
- Cosmetic concerns are the primary reason patients seek treatment, with 61% of women attempting to conceal lesions with hairstyles, makeup, or clothing 6
Benign Nature
SK is purely benign with no malignant potential, despite recent identification of oncogenic mutations (such as FGFR3 and PIK3CA mutations) that do not carry risk of malignant transformation 4. The natural history involves gradual increase in size, thickness, and pigmentation over time 2.