Summary

Seborrhoeic Keratosis (SK) is the most common benign tumour of the skin, originating from keratinocytes in the stratum basale. Often described as the "barnacles of ageing," they typically appear after age 30 and increase in number and size with age. They vary widely in appearance but characteristically present as "stuck-on," waxy, hyperpigmented papules or plaques with a verrucous (wart-like) surface. They have zero malignant potential themselves but can clinically mimic melanoma or pigmented basal cell carcinoma.

Key Facts

• Definition: Benign epidermal neoplasm caused by clonal expansion of mutated keratinocytes.
• Prevalence: Near universal (>90%) in population over age 60.
• Mortality/Morbidity: Strictly benign - cosmetic concern or irritation only.
• Key Management: Reassurance is first-line. Removal via cryotherapy or curettage if irritating.
• Critical Threshold: Sudden eruption of hundreds of lesions (Sign of Leser-Trélat) requires cancer screening.
• Key investigation: Dermoscopy (cerebriform pattern, milia-like cysts) is usually diagnostic.

Clinical Pearls

The "Stuck-On" Appearance: SKs often look like a piece of wax or clay has been pressed onto the skin. You can often peel or pick at the edge, distinguishing them from melanocytic naevi which are part of the skin.

Dermatosis Papulosa Nigra (DPN): A variant seen in darker skin types (Fitzpatrick IV-VI), presenting as multiple small, dark papules on the face and neck (think Morgan Freeman). These are histologically SKs but smaller.

The "Ugly Duckling" in Reverse: While a melanoma is the "ugly duckling" (different from other moles), an SK is often the "ugly duckling" in a field of moles because it looks so disordered and warty, yet is perfectly safe.

Why This Matters Clinically

The primary importance of SK lies in the differential diagnosis. A deeply pigmented SK can look identical to a nodular melanoma to the naked eye. Recognizing diagnostic features of SK allows clinicians to confidently reassure patients and avoid unnecessary excision of benign lesions, whilst ensuring true malignancies are not dismissed as "just a wart."

Structured Approach

General:

• Total Body Skin Exam: SKs are rarely solitary. Look for the "company they keep" - finding multiple typical SKs elsewhere supports the diagnosis of a questionable lesion.

Specific Lesion Examination:

• Palpation: Runs finger over it. It should feel elevated and rough compared to surrounding skin.
• Pick Test: Gentle picking at the edge often lifts a greasy scale (unlike melanocytic naevi).

Dermoscopy (The Gold Standard)

Dermoscopy increases diagnostic accuracy significantly.

Natural History

• SKs persist indefinitely. They do not spontaneously resolve.
• They tend to grow slowly thicker and darker over years.
• New lesions appear continuously.

Outcomes with Treatment

Prognostic Factors

Not applicable for benign lesions.

Key Guidelines

1. British Association of Dermatologists (BAD) — Guidelines for the management of Seborrhoeic Keratoses. Emphasizes reassurance and non-intervention.
2. DermNet NZ — Seborrhoeic Keratoses. Gold standard online resource for clinical features.

Clinical Distinctions (The "Collision" Lesion)

• Collision Tumour: Occasionally, a Melanoma or BCC can grow inside or colliding with an SK.
• Evidence: Histopathological studies show this is rare but possible. This is why "blind" destruction (Cryo) of atypical lesions is discouraged. if not 100% sure, BIOPSY.

Evidence Strength

What is a Seborrhoeic Keratosis?

It is a harmless skin growth that is incredibly common as we get older. We often call them "senile warts" or "wisdom spots." It is basically a build-up of skin cells that pile up on the surface like a barnacle on a ship. They are NOT caused by the sun, and they are NOT skin cancer.

Why does it matter?

They don't matter for your health, but they can be annoying. They can catch on bra straps or belts, become itchy, or bleed if scratched. The main reason to see a doctor is just to double-check it isn't a bad mole, especially if it is dark black.

How is it treated?

Most of the time, we leave them alone. If one is bothering you or looks unsightly:

1. Freezing (Cold Spray): We spray it with liquid nitrogen. It creates a scab and falls off in a week or two.
2. Scraping: We numb the skin and gently scrape it off. This usually heals very well.

What to expect

• If we remove it, you might be left with a white mark or a small scar.
• You will likely get more of these spots as you get older. This is normal.
• They do not turn into cancer.

When to seek help

• If a spot bleeds without being scratched.
• If it changes shape or colour very quickly.
• If you suddenly get dozens of them appearing all at once over a few weeks.