The appearance of one or multiple black or brown bands on the nails is referred to as “nail black lines.” There are various reasons for the formation of these lines, including nail bed bleeding, melanonychia striata, subungual melanoma, normal variation, or increased activity of melanocytes due to trauma, medication, or inflammation.
Subungual bleeding:
Subungual bleeding occurs when the nail is injured or compressed, leading to bleeding beneath the nail and the formation of black lines. Simple subungual bleeding usually does not require treatment and will gradually disappear as the blood moves towards the distal edge.
Activation of melanocytes:
Normal nails have a white, translucent appearance. However, abnormal deposition of melanin in the nails can cause localized darkening, often affecting multiple nails with a grayish color and blurred edges. This condition can be associated with pregnancy, inflammation (such as chronic paronychia), fungal infections (such as onychomycosis), trauma (such as nail biting or shoe pressure), systemic diseases, or the use of certain medications. Longitudinal melanonychia caused by activated melanocytes usually does not require excessive treatment and may naturally resolve after removing the underlying cause.
Proliferation of melanocytes:
This refers to an increase in the number of melanocytes within the nail. Black pigmentation caused by this condition can be benign, such as nail matrix nevus or lentigo-like melanocytic hyperplasia, or malignant, such as subungual melanoma, which is the most severe condition. Nail matrix nevus is essentially a pigmented mole that grows within the nail, similar to moles found elsewhere on the body. It is commonly seen on the thumbs, big toes, index fingers, and other areas. Nail matrix nevi usually have a slow and uniform increase in width. However, there is a risk of malignant transformation into subungual melanoma, which has a high rate of malignancy and mortality.
There are many different causes of nail black lines, and it is difficult to determine the exact cause without professional medical evaluation and diagnosis.