ConditionSkin, hair & nails
Melanonychia (Dark Streak in a Nail)
Melanonychia is brown or black pigmentation of a fingernail or toenail, most often a vertical band running from the cuticle to the tip (longitudinal melanonychia). It is very common and usually harmless in people with darker skin, where several nails often have bands. Other causes include injury, medicines, fungal infection and moles in the nail matrix. A single, new, widening or irregular dark band, especially in a thumb or big toe, or pigment spreading onto the skin, needs a dermatologist to rule out nail melanoma.
Also called: Longitudinal melanonychia, nail pigmentation, pigmented nail band, dark nail streak, melanonychia striata
At a glance
- What it is
- Brown or black pigment in the nail, usually a vertical band.
- Common causes
- Normal pigmentation in darker skin, injury, friction, medicines, pregnancy, fungal infection, nail moles.
- Serious cause
- Nail (subungual) melanoma, uncommon but important.
- Warning signs
- One nail, new in adulthood, widening, irregular colors, wider than 3 mm, pigment on the surrounding skin.
- Diagnosis
- Dermoscopy of the nail, and a nail matrix biopsy when suspicious.
When to get help
See a doctor within a day or two
- A dark band that is widening over weeks to months, or has blurred or irregular edges
- Pigment spreading from the nail onto the cuticle or fingertip skin (Hutchinson's sign)
- A nail that is splitting, bleeding or ulcerating along a dark band, or a lump under the nail
Book a routine appointment
- A new single dark band in one nail in an adult, especially the thumb, index finger or big toe
- A band wider than 3 mm, or with multiple shades of brown and black
- A dark mark under a nail after an injury that is not moving forward as the nail grows
- Dark bands appearing in several nails at once with tiredness, weight loss or skin darkening
Usually safe to manage at home
- Several thin, even, light-brown bands in multiple nails that have been stable for years, especially with darker skin (photograph and monitor)
Why nails become pigmented
The nail grows from the matrix, under the skin at the base of the nail. Melanocytes (pigment cells) in the matrix are normally inactive. When they are activated or multiply, they deposit melanin into the growing nail, creating a band that runs the length of the nail.
- Melanocyte activation: more pigment from normal cells, as in ethnic pigmentation, friction or medicines
- Melanocyte proliferation: more pigment cells, as in a nail mole (nevus), lentigo or melanoma
Non-melanin causes, such as blood or fungus, can mimic melanonychia.
Causes
| Cause | Typical features |
|---|---|
| Ethnic (racial) pigmentation | Several nails, thin even bands, darker skin types. Very common, increasing with age |
| Repeated friction or trauma | Toenails rubbing in shoes, nail biting, picking |
| Medicines | Chemotherapy (hydroxyurea, cyclophosphamide, doxorubicin), zidovudine, minocycline, antimalarials. Often several nails |
| Pregnancy | Temporary bands, may fade after delivery |
| Medical conditions | Addison's disease, HIV, B12 deficiency, hyperthyroidism, Laugier-Hunziker syndrome. Usually several nails |
| Nail matrix nevus (mole) | Single band, often in children, regular brown color |
| Fungal infection | Usually a darker band wider at the tip, with thickening and debris. See onychomycosis |
| Blood under the nail | Red-purple to black, follows injury, moves toward the tip as the nail grows. See nail trauma |
| Subungual melanoma | Single nail, widening, irregular, may spread to skin, often thumb or big toe |
Nail melanoma warning signs: the ABCDEF rule
Doctors use this checklist for nail melanoma:
- Age: most common between 50 and 70. Also, African American, Asian and Native American people have a higher proportion of melanomas in nails
- Band: brown to black, width 3 mm or more, irregular or blurred borders
- Change: rapid increase in width or darkness, or nail changes despite treatment
- Digit: thumb, big toe and index finger are most often affected, usually a single digit
- Extension: pigment extending onto the nail fold skin (Hutchinson's sign)
- Family or personal history of melanoma or atypical moles
Nail melanoma is often diagnosed late because it is mistaken for a bruise or fungus. It is not linked to sun exposure in the way skin melanoma is.
Diagnosis
- History: when it appeared, any injury, medicines, other nails, family history
- Examination of all fingernails, toenails, skin and mucous membranes
- Dermoscopy (onychoscopy): magnified examination of the band's lines, colors and borders. Regular, parallel lines of uniform color are reassuring. Irregular lines and colors raise concern
- Photographs to track changes over time
- Fungal testing if infection is possible
- Nail matrix biopsy: the only way to confirm or rule out melanoma, done by a dermatologist or hand surgeon under local anesthetic. It may leave a permanent nail ridge or split
Treatment
Treatment depends on the cause:
- benign causes: no treatment, with monitoring every 3 to 6 months and photographs
- medicine-related: usually fades slowly after stopping the drug, if appropriate
- friction: well-fitting shoes, stopping nail biting
- blood under the nail: grows out over months
- fungal infection: antifungal treatment
- nail matrix nevus: monitoring or removal
- nail melanoma: surgery, ranging from removing the nail unit for early (in situ) melanoma to partial amputation of the digit for invasive melanoma, sometimes with lymph node assessment and further treatment
Frequently asked questions
What causes a dark line on a fingernail?
The most common causes are normal ethnic pigmentation, injury or friction, medicines, pregnancy, fungal infection and harmless nail moles. Less commonly, a dark line is caused by nail melanoma, which is why new or changing single bands should be checked.
Is a black line on my nail cancer?
Usually not. Most nail bands are benign. Warning signs of melanoma are a single band that is new in adulthood, widening, wider than 3 mm, irregular in color or border, or with pigment on the surrounding skin. A dermatologist can assess it.
How can I tell a bruise from melanoma under the nail?
A bruise usually follows an injury, is reddish-purple to black, and moves toward the tip as the nail grows, eventually growing out. Melanoma forms a band that starts at the cuticle and does not grow out. If unsure, see a dermatologist.
Is melanonychia normal in darker skin?
Yes. Longitudinal melanonychia is very common in people with darker skin, especially with age, and often affects several nails. A single band that changes or looks different from the others should still be checked.
How is melanonychia diagnosed?
A dermatologist examines the nails with dermoscopy to look at the pattern of lines and colors and may photograph the nail for comparison. If anything looks suspicious, a biopsy of the nail matrix is the only way to confirm the diagnosis.
Sources
This guide was written from current patient and clinical guidance, including:
- American Academy of Dermatology: Nail melanoma and nail changes
- Skin Cancer Foundation: Subungual melanoma
- DermNet: Melanonychia
- American Osteopathic College of Dermatology: Melanonychia striata
- MedlinePlus (US National Library of Medicine): Nail abnormalities