A dark spot on a nail can cause anxiety, though the cause is often benign. Discoloration under the nail plate signals activity in the nail bed beneath it. Understanding the source of the pigment is necessary to determine if the spot is harmless or requires medical evaluation.
The Primary Cause: Subungual Hematoma (Nail Trauma)
The most frequent reason for a black or dark spot on a nail is a subungual hematoma, simply a bruise under the nail plate. This occurs when an injury breaks blood vessels in the nail bed, trapping blood between the nail and the underlying tissue. The discoloration often appears deep purple, reddish-brown, or black, typically resulting from a crush injury, such as slamming a finger or dropping a heavy object.
Severe, throbbing pain is common because the pooled blood creates pressure beneath the rigid nail plate. The spot remains until the nail grows out, which takes four to six months for a fingernail and up to a year for a toenail. If the hematoma is large or causes intense pain, a doctor may drain the blood through a small hole in the nail plate, a procedure called trephination, to relieve the pressure.
Benign Pigmentation: Nail Matrix Nevi and Melanonychia
Some dark spots are caused by melanin, the natural pigment responsible for skin and hair color, rather than blood. This discoloration, called melanonychia, originates from melanocyte cells in the nail matrix at the base of the nail. Longitudinal melanonychia presents as a brown or black stripe running lengthwise along the nail.
In individuals with darker skin tones, this is a common and usually harmless condition, sometimes called racial or functional melanonychia. A Nail Matrix Nevus, essentially a mole, is another benign cause of a pigmented stripe. These benign spots are stable over time, do not change rapidly in size, shape, or color, and often involve multiple nails.
When Pigmentation is Dangerous: Subungual Melanoma
The most serious, though rare, cause of a dark nail spot is subungual melanoma, a form of skin cancer arising from pigment cells in the nail unit. Early detection is important, and characteristics of the pigmented band provide clues summarized by the ABCDEF criteria. The stripe often has a width of three millimeters or more and may have irregular borders.
A significant warning sign is a rapid change in the band, such as increasing in size or darkening in color over weeks or months. The most concerning feature is the extension of the pigment onto the skin surrounding the nail, known as Hutchinson’s sign. This spread indicates that the abnormal melanocytes are migrating beyond the nail bed.
Subungual melanoma often affects a single digit, most commonly the thumb or the great toe, and can present with nail plate destruction, splitting, or a non-healing ulcer. This cancer is not always pigmented; in some cases, it can be amelanotic, appearing as a pink or reddish mass, making diagnosis challenging. Any pigmented stripe that develops in an adult without trauma, particularly if it is solitary and changing, should raise immediate suspicion.
Other Medical and Systemic Reasons for Dark Spots
Dark spots not due to trauma or primary melanocyte activity can signal other medical conditions or external factors. Splinter hemorrhages are tiny, thin lines of blood that run vertically under the nail, resembling a wooden splinter. While often caused by minor injury to the nail bed vessels, their appearance on multiple nails without trauma can be associated with systemic diseases.
These thin lines may be linked to conditions like infective endocarditis, vasculitis, or certain autoimmune disorders, which cause microemboli or damage to vessel walls. Additionally, various medications can induce nail discoloration as a side effect. Chemotherapy agents, certain antimalarial drugs, and some antibiotics are known to cause brown or black pigmentation.
Severe or chronic fungal infections (onychomycosis), although typically white or yellow, can also lead to a dark brown or black appearance. This discoloration is often due to the buildup of debris or pigment produced by the fungus itself. The appearance of dark spots without a clear cause requires a careful medical history review, including recent changes in medication or signs of underlying infection.
Deciding When to See a Specialist
While most dark nail spots are harmless bruises that will grow out, professional evaluation is necessary for any suspicious lesion. A visit to a dermatologist is warranted if a dark spot appears suddenly without trauma, or if a pre-existing stripe begins to change. Concerning features include a pigmented stripe that is rapidly widening, has irregular borders, or has varying shades of color.
Any sign of the pigment spreading beyond the nail plate onto the surrounding skin, known as Hutchinson’s sign, is a significant red flag requiring urgent attention. Similarly, any associated nail destruction, such as splitting, cracking, or a non-healing ulcer, should prompt a consultation. When subungual melanoma is suspected, a nail matrix biopsy remains the gold standard procedure.