Black discoloration beneath or within a fingernail is medically known as chromonychia, or melanonychia when the pigment is melanin. While alarming, the color black is a symptom, not a diagnosis, resulting from various underlying processes. These causes range widely in severity, from minor injury to serious systemic disease. Understanding the specific presentation is the first step in determining the origin.
Immediate Causes: Trauma and External Staining
The most frequent reason for a sudden black area under the nail is physical trauma, leading to a condition known as a subungual hematoma. This occurs when a direct blow or crushing injury causes small blood vessels beneath the nail plate to rupture and bleed. The trapped blood clots, resulting in the dark, purplish-black color that is visible through the translucent nail.
The intensity of the pain often correlates with the size of the hematoma, as blood pressure builds up in the confined space. Unlike a bruise, the discoloration cannot be absorbed quickly. The area will remain until the damaged section grows out completely with the nail plate, which takes six to nine months for a fingernail, or up to a year for a toenail.
Another common, non-medical cause is external staining, where dyes or chemicals interact directly with the nail keratin. Substances like certain hair dyes, heavy metals, or specific topical medications can leave a dark residue on the nail surface or soak into the plate. This type of discoloration usually presents as diffuse staining, often affecting multiple nails, and can sometimes be buffed away or slowly grows out.
Pigmentation Patterns: Longitudinal Melanonychia
When the black or brown discoloration appears as a distinct band or stripe running from the cuticle to the tip, it is classified as longitudinal melanonychia (LM). This pattern is caused by an increase in melanin pigment production by melanocytes located in the nail matrix, the tissue where the nail plate forms. The melanocytes become stimulated and deposit pigment into the growing nail plate, resulting in the characteristic linear streak.
Longitudinal melanonychia is a common finding, especially among individuals with darker skin tones, where it is often a completely benign, inherited variation. In these cases, the condition may affect multiple digits and typically appears during childhood or early adulthood, remaining stable over time. It represents a hyperactivity of pigment-producing cells rather than an abnormal proliferation.
The appearance of the stripe warrants observation, as subtle changes can indicate a shift in cellular activity. Benign bands are typically narrow, uniform in color, and have sharp, stable borders. If the pigmented band starts to widen or the color becomes non-uniform, it suggests a more significant process is occurring. This localized pigmentation represents a biological process within the nail unit itself.
Underlying Health Conditions and Medications
Black nail discoloration can sometimes be a manifestation of internal systemic dysfunction, separate from localized injury or benign pigmentation. Certain endocrine disorders, such as Addison’s disease, can lead to increased melanocyte-stimulating hormone levels, causing diffuse pigmentation in the skin and often the nails. Similarly, nutritional deficiencies, including a lack of Vitamin B12, have been linked to generalized nail darkening or dark streaks.
In cases of chronic kidney failure, metabolic byproducts can accumulate and sometimes deposit in the nail bed, resulting in a dark, often brownish discoloration. These systemic causes usually affect several or all nails simultaneously and present as a more generalized change rather than a single band.
Pharmaceutical agents are another source of drug-induced chromonychia, where the chemical is deposited into the nail matrix or plate. Common culprits include certain chemotherapy drugs, antimalarial medications, and antibiotics like minocycline. The discoloration caused by medications typically resolves slowly once the drug is discontinued, as the pigmented portion grows out with the nail.
Recognizing Subungual Melanoma
The most serious, though rare, cause of black nail discoloration is subungual melanoma (SM), a form of skin cancer originating from the melanocytes in the nail matrix. This condition requires immediate attention because it is an aggressive malignancy that is often diagnosed later than other melanomas due to its hidden location. Longitudinal melanonychia is the most common initial clinical presentation of subungual melanoma.
The ABCDEF rule provides a framework for differentiating a suspicious pigmented band from a benign one. ‘A’ stands for Age, typically affecting individuals between 40 and 70 years old, or specific ethnicities like African Americans or Asians. ‘B’ refers to the Breadth of the band, where a width over three millimeters is a concerning feature.
ABCDEF Criteria for Melanoma
The remaining criteria focus on the characteristics of the band and the affected digit.
- ‘C’ focuses on Change, specifically a rapidly growing lesion or a lack of symmetry in color or border definition, which is often irregular or blurred.
- ‘D’ addresses the Digit involved, with the thumb or the great toe being the most frequently affected sites.
- ‘E’ denotes the Extension of the pigment onto the proximal or lateral nail fold skin (Hutchinson’s sign), indicating malignant cells have migrated outside the nail unit.
- ‘F’ covers Family history of melanoma or a Failure of the lesion to improve or stabilize over time.
Any pigmented lesion that exhibits these malignant characteristics should prompt an immediate medical consultation.