Section 05 of 09
Variants
Not every seborrhoeic keratosis looks like the textbook example. The classic features described above — milia-like cysts, comedo-like openings, cerebriform surface — are most prominent in the typical raised, warty seborrhoeic keratosis. But several common variants may show fewer or different features.
Understanding variants matters because they are the seborrhoeic keratoses most likely to cause uncertainty — and most likely to be referred unnecessarily or, conversely, to be confused with something more concerning.
Flat seborrhoeic keratosis
Flat variants lack the raised, warty surface of classic seborrhoeic keratosis. They sit flush with the skin and may resemble a solar lentigo or even a flat melanocytic lesion.
What to look for:
- Milia-like cysts may still be present — look carefully, as they can be subtle against a flat background
- Comedo-like openings may be sparse or small
- The fissure-and-ridge pattern is typically absent (no raised surface to create it)
- Sharp demarcation is usually preserved — the edges remain well-defined
- The moth-eaten border pattern is common in flat variants
In flat seborrhoeic keratoses, milia-like cysts and comedo-like openings become even more important because the cerebriform surface is absent. If you can see even a few bright cysts or dark plugs in a well-demarcated flat lesion, that strongly supports seborrhoeic keratosis.
Irritated seborrhoeic keratosis
Seborrhoeic keratoses that have been scratched, rubbed, or otherwise traumatised can become inflamed. This changes their appearance significantly.
What to look for:
- Redness and crusting may obscure the classic features
- Milia-like cysts and comedo-like openings may be harder to identify through the inflammation
- Vascular structures may become prominent — hairpin or glomerular vessels can appear across the surface
- The sharp border may become less distinct due to surrounding erythema
Irritated seborrhoeic keratoses are among the most common mimickers of malignancy. If the classic dermoscopic features are not clearly visible through the inflammation, do not assume benign — treat as uncertain and consider further evaluation.
Clonal seborrhoeic keratosis
Clonal seborrhoeic keratosis is a histological variant — meaning the distinction is primarily made under the microscope. However, it has dermoscopic implications because the nests of cells it contains can create patterns that mimic basal cell carcinoma.
What to look for:
- Well-defined dark nests or globules within the lesion
- These nests can resemble the blue-grey ovoid nests of pigmented BCC
- Classic seborrhoeic keratosis features (cysts, openings) may still be visible alongside the nests
- The overall architecture tends to remain well-demarcated
If you see dark nests in what otherwise looks like a seborrhoeic keratosis, look carefully for the reassuring classic features. If the cysts and openings are present alongside the nests, the balance of evidence favours seborrhoeic keratosis. If the nests dominate and classic features are absent, the lesion needs further evaluation.
You are examining a flat, well-demarcated brown lesion on the trunk of an older patient. There are no fissures or ridges, but you can see several bright white dots and a few dark plugged openings. What is the most likely interpretation?