A black or discolored nail, medically known as melanonychia, signifies a change in the color of the nail plate or the tissue underneath it. While the sight of a darkened nail can be alarming, the cause is frequently a benign condition that is easily identifiable. The nail plate is typically translucent, which allows the underlying nail bed to show through. Discoloration occurs when pigment, either blood or melanin, is deposited into the nail plate or trapped beneath it. Distinguishing between a collection of blood, an increase in natural pigment, or a cancerous change is the first step toward understanding the cause and determining the appropriate course of action.
Subungual Hematoma: The Cause Related to Injury
The most common reason a nail turns black is the direct result of trauma, creating what is known as a subungual hematoma. This describes bleeding that occurs when capillaries in the nail bed rupture and trap blood underneath the rigid nail plate. Common causes include acute crushing injuries, such as slamming a finger in a door or dropping a heavy object on a toe. The discoloration appears rapidly as a dark red, purplish, or black bruise beneath the nail surface.
A subungual hematoma is often accompanied by significant, throbbing pain due to the pressure buildup from the trapped blood. Repetitive microtrauma, such as that experienced by runners wearing ill-fitting shoes, can also cause this bruising, particularly on the toenails. In these cases, the discoloration will slowly grow out with the nail plate over several months, which confirms its benign, traumatic origin.
When the pressure causes severe pain, a medical procedure called trephination may be necessary to drain the hematoma. This involves a healthcare provider creating a small, sterile hole through the nail plate to allow the blood to escape, which immediately relieves the painful pressure. Attempting this drainage at home is not recommended, as it carries a high risk of introducing infection and causing further damage to the nail bed.
Pigmentation and Non-Traumatic Discoloration
Not all black nails are caused by acute injury; many result from the deposition of the natural pigment melanin into the nail plate. This often appears as a longitudinal band or streak running from the base to the tip of the nail.
Benign Longitudinal Melanonychia
Benign longitudinal melanonychia is particularly common in individuals with darker skin tones, such as those of African or Asian descent, where it is often a normal variant, affecting multiple nails and appearing in childhood. In these instances, the melanocytes in the nail matrix are simply more active, producing more pigment that gets incorporated into the growing nail.
Systemic Causes and Medications
Discoloration can also be a side effect of various systemic conditions or medications, which trigger melanocyte activation. Certain drugs, including some chemotherapy agents and antimalarials, may cause a diffuse or banded darkening of the nails. Additionally, underlying health issues like chronic kidney disease, Addison’s disease, or thyroid disorders can manifest as changes in nail pigmentation.
Fungal Infections
Fungal infections (onychomycosis) less commonly present as a solid black nail, but they can contribute to a dark appearance. Fungal debris, dirt, or secondary bacterial infections that occur under the nail can combine to give the nail a dark brown or black hue. The discoloration from these non-traumatic causes, unlike a hematoma, does not typically have a clear, sudden onset and may be accompanied by nail thickening or crumbling.
Recognizing Subungual Melanoma
While most black nail changes are harmless, a rare but serious cause is subungual melanoma, a form of skin cancer arising from the pigment-producing cells in the nail matrix. It is crucial to recognize the specific features that differentiate it from benign pigmentation or a bruise. Clinicians often use a modified version of the ABCDE rule to assess concerning nail lesions:
- A: Age, with most cases occurring in adults between 50 and 70 years old.
- B: Band, typically wide, often wider than three millimeters, with blurred or irregular borders.
- C: Change in the band, such as rapid growth, color variation, or a failure to resolve over time.
- D: Digit most commonly involved, which is often the thumb or the great toe.
- E: Extension of pigment onto the skin surrounding the nail, known as the Hutchinson sign.
This spread of black or brown color onto the cuticle or the lateral nail folds is a strong indicator of melanoma. Unlike a subungual hematoma, which moves distally as the nail grows, the pigment from melanoma will remain fixed and will not grow out with the nail plate.
When Immediate Medical Attention is Necessary
The appearance of a black nail warrants professional evaluation if certain warning signs are present, particularly when a clear traumatic event cannot be recalled. Any instance of persistent, unexplained discoloration that does not visibly move with the growth of the nail plate should be examined by a healthcare provider. This is because a bruise will resolve completely as the nail grows out, while a fixed stripe or patch suggests a continuous source of pigment.
Immediate attention is necessary if there are signs of infection, such as increasing pain, severe swelling, pus draining from the area, or warmth and redness spreading beyond the nail.
The presence of the Hutchinson sign, where the black or brown pigment extends onto the skin of the cuticle or nail fold, requires urgent medical assessment to rule out subungual melanoma. Simple external stains from things like hair dye or certain chemicals should also be considered; these are superficial, usually wipe off, or are confined to the surface of the nail plate, making them easy to differentiate from internal causes.