Definition: no basic elements, too few to constitute a pattern, or the visible structures cannot be assigned to one of the five basic elements.
Lesions that only have a structureless pattern are difficult to diagnose using dermatoscopy and often require histopathology.
One color predominates over all others
- Black + structureless: presence of hemoglobin (not melanin) and its degradation products, e.g. hemorrhagic crusts, hemorrhages in the epidermis and thrombosed vessels.
- Differential diagnoses: heavily pigmented melanocytic lesions, e.g. Reed nevus, Clark nevus or melanoma.
- Blue + structureless: "blue nevi" of all types.
- Differential diagnoses: melanomas melanoma metastases, structureless blue pigmented basal cell carcinomas.
- Brown + structureless: solar lentigo, flat seborrheic keratosis, pigmented Bowen’s disease, melanocytic nevus of "superficial" or "superficial and deep" congenital type.
- Red + structureless: recent hemorrhage in the stratum corneum. This will become a black structureless lesion as the hemoglobin degrades, before it entirely disappears due to transepidermal elimination.
More than one color
Difficult to decide whether clods or structureless. Clods: well circumscribed and always occur in numbers. Structureless: one large contiguous area.
- Colors of keratin (yellow and orange): keratinizing lesions such as seborrheic keratosis.
- Differential diagnoses: basal cell carcinoma
- Colors of hemoglobin (red and purple): hemorrhage, or hemorrhage in a pre-existing lesion such as a nevus.
- Black zones in structureless lesion: thrombosis.
- Differential diagnoses: black should be attributed to blood when it appears together with red or purple and attributed to melanin when it appears together with brown, blue or gray.
- Symmetrical distribution of melanin: nevus.
- Asymmetrical distribution of melanin: melanoma, melanoma metastasis, seborrheic keratosis.
- Differential diagnosis: pigmented basal cell carcinoma or a dermatofibroma.