A harmless condition called melanonychia results in black lines on fingernails, as a result of increased pigmentation. Melanonychia is common in dark-skinned people and the elderly. It is rarely found in children.
Melanonycia is also seen following radiation therapy, under ultraviolet light exposure, during pregnancy and while taking certain medications. Moles in the nails also appear as black lines.
The lines are also present in certain underlying medical conditions. Black lines should be seen by a physician, as they may indicate a type of skin cancer called subungual melanoma. The dark lines, called Hutchinson's sign, are helpful in a cancer diagnosis. Generally, only one nail is affected by subungual melanoma. The dark line may change in appearance, becoming wider or darker over time. The skin surrounding the affected nail may also become discolored.
A biopsy of the nail measures how far the cancer has spread, and the depth it has reached. A lymphangiogram is an X-ray of the lymph node and vessels that reveals in more detail whether the cancer has traveled elsewhere in the body. Chemotherapy, radiation and surgery are common treatments. If the melanoma involves the nail and the surrounding skin area, or nail fold, the nail may be removed. In extreme cases, amputation is required.