A mole, or nevus, is a common skin growth that develops from clusters of melanocytes, the cells responsible for producing pigment, or melanin. These pigmented spots can appear anywhere on the body, but their presence on specific locations, such as the thumb or under the nail, often prompts concern. While most moles are harmless, a small percentage can evolve into melanoma, a serious form of skin cancer. Understanding the typical appearance of moles in this unique location is important for knowing when to seek professional medical advice.
Understanding Moles on Acral Skin
Moles appearing on the palms, soles, fingers, and toes are called acral nevi, as this area is known as acral skin. These nevi often look different from moles elsewhere on the body. Acral nevi are generally benign and are not primarily driven by sun exposure. They commonly present as small, dark brown, or black spots on the thumb skin, or as linear streaks under the nail plate (subungual nevi).
The unique structure of acral skin, with its ridges and furrows, causes melanocytes to cluster in different patterns. Benign acral nevi frequently show pigment concentrated in the furrows, or valleys of the skin lines. These moles are common, especially in individuals with darker skin tones. Genetics and general cell activity, not ultraviolet radiation, are considered the primary factors influencing their development.
Most acral nevi are stable, appearing during childhood or young adulthood and remaining unchanged. However, any new or growing pigmented lesion in an adult requires careful attention. While acral nevi are rarely precursors to melanoma, new melanomas frequently arise spontaneously in this area. The growth pattern, rather than the initial appearance, is the most important characteristic for distinguishing a benign mole from a malignancy.
Warning Signs for Lesions on the Thumb
Acral lentiginous melanoma (ALM) is a rare but aggressive subtype of melanoma that is a concern for lesions on the thumb. When evaluating a pigmented spot, the standard ABCDE criteria apply. The “E” for “Evolving” is significant, meaning any rapid change in size, shape, color, elevation, or symptoms like itching or bleeding warrants attention. Lesions larger than 6 millimeters should also raise suspicion, as acral melanomas are generally larger than 7 millimeters when diagnosed.
A pigmented streak under the thumbnail is called longitudinal melanonychia and requires vigilance. The most specific sign for subungual melanoma is Hutchinson’s sign. This is the spread of pigment from the nail streak onto the adjacent skin, including the cuticle and the proximal or lateral nail folds. This extension indicates the radial growth of potentially malignant melanocytes beyond the nail plate.
Other signs of a suspicious nail lesion include changes in the nail plate, such as distortion, thinning, or partial destruction. Bleeding or ulceration within the pigmented area, or an irregular, non-uniform band of color wider than 3 millimeters, also indicate potential malignancy. Although Hutchinson’s sign is strongly associated with melanoma, its presence warrants immediate professional evaluation, as other benign conditions can mimic this appearance.
Medical Assessment and Management
If a lesion exhibits suspicious characteristics, the next step is a medical assessment by a dermatologist. The initial examination uses a dermoscope, a device that illuminates the skin to visualize subsurface structures. Dermoscopy helps distinguish benign patterns, such as pigment concentrated in the furrows of the skin lines, from worrisome patterns characteristic of melanoma, like pigment concentrated in the ridges.
For lesions under the nail, dermoscopy assesses the pigmented band for features like loss of parallelism and heterogeneous coloring, which suggest malignancy. If findings are inconclusive or highly suspicious, a biopsy is necessary for a definitive diagnosis. For skin lesions on the thumb, an excisional biopsy is often recommended to remove the entire lesion for analysis.
A suspicious lesion in the nail unit requires a specialized nail matrix biopsy. Longitudinal excision biopsy is often the preferred technique for nail lesions because it provides the best architectural overview for the pathologist. If the biopsy confirms melanoma, the standard management is surgical removal with clear margins to prevent cancer spread.