Image source from DermNet (https://dermnetnz.org/)

Patients often come to the dermatologist when they spot a dark line or streak on their nails. Thanks to the power of health education, many are now aware that they need to get this checked for possible cancer.

A dark line on the nail is known as longitudinal melanonychia. Although some causes of longitudinal melanonychia are benign (e.g. injury, moles, fungal nail infection, hormonal disorders, medications), certain cases can be due to an aggressive type of skin cancer known as melanoma.

What are the clues that a dark line on the nail could be cancerous?

1. Only one nail is involved
2. There is pigment on surrounding skin (this is known as Hutchinson’s sign)
3. Wide band of pigment (>2-3 mm), especially in older patients (aged >65)
4. Streaks of normal background nail colour within pigmented band
5. Older patients
6. Triangle sign (pigment is broader proximally than distally)
7. Irregular lines

Dermatologists may use tools such as dermoscopy and, in some cases, nail biopsy may be required to determine the cause.

Early diagnosis can make a major difference. If you notice a changing dark streak on your nail, it’s best to get it assessed as soon as possible.