Melanonychia is a brown or black stain in your fingernail or toenail. You can have a dark line on one nail, a few nails, or all of your nails. The line is caused by the natural color in your body called "melanin." It's the substance that colors your skin and hair.
Because your nails are usually clear, the cells in your nail bed don't often make melanin. But some conditions can activate your melanin cells or cause them to multiply and form a light to dark nail stain.
The shape and color of your melanonychia can be a clue to its cause. The reason for the discoloration can range from being harmless to a potentially serious type of cancer.
But with treatment, you can avoid complications. Find out what causes this dark line in your nails, when to get medical care, and how melanonychia is treated.
What Does Melanonychia Look Like?
Your melanonychia can look like a:
- Faint shadow
- Brown line
- Black streak of marker down your nail
- Bruise at the cuticle that runs in a line to the tip of your nail
- Line that's more spread out (diffuse melanonychia)
- Grey line that runs to the tip of your nail
Melanonychia can look different depending on your skin color and the cause. The term melanonychia means black nail. That's because the colors can vary from solid to shadow-like brown to black shades.
The line can run across your nail or the entire length of it. And the dark line can be thick or thin. Your doctor can sometimes diagnose your medical condition based on the color, size, and shape of your melanonychia.
What Are the Patterns of Melanonychia?
The different patterns of the line are a way to group melanonychia. The pattern can:
- Run lengthwise from your nail bed to the its tip
- Be solid or like a shadow (diffuse) to color your whole nail
- Run across your nail from side to side
Longitudinal melanonychia or melanonychia striata
If the band runs from your nail bed to the tip of your nail, it's called longitudinal melanonychia or melanonychia striata. Striata is a term that means line. It's usually a black, brown, or grey line that runs the length of your nail.
Diffuse or total melanonychia
The color can turn your entire nail dark or spread out (be diffused) to shade your whole nail.
Transverse melanonychia
The colored band runs across (transverse) the width of your nail. The shaded line appears to run crosswise, from one side of your nail to the other.
How Does Melanonychia Happen?
Melanonychia can happen if certain conditions inflame and "wake up" your melanocytes. After the melanin cells are active, the number of cells can stay the same or multiply, depending on the cause.
Active melanocytes
Once active, melanocytes make melanin inside your nails to form colored bands in patterns:
- Straight line. The most common pattern where the line runs from your nail bed to the tip.
- Total melanonychia. The line covers your entire nail.
- Transverse melanonychia. Runs from one side to the other.
Functional melanonychia happens if the number of melanin cells stays the same — the cells just make more color now. But some conditions can multiply the number of melanocytes in your nail.
Higher number of melanocytes
If the melanocytes multiply, you'll have a higher number of cells in your nail than before.
These melanin cells form the colored line by multiplying, also called proliferation or melanocytic hyperplasia. The term hyperplasia means an organ or tissue gets bigger because there's more cells than before.
Melanocytic hyperplasia can be benign (noncancerous) or cancerous.
There are three ways this can happen:
- Nevus forms from your nail bed; but this dark, line-like mole is more common in children.
- Lentigo also grows out of your nail bed as a flat, tan, or brown freckly line that can be slightly raised.
- Melanoma is a serious type of cancer, usually found on the nail of your big toes, index fingers, or thumbs.
What Causes Melanonychia Due to Active Melanocytes?
Active nail melanocytes can happen for many reasons, including:
- Normal body feature such as skin tone, race, or pregnancy
- Infection
- Nail inflammation
- Medication and treatments
- Poor nutrition
- Nail injury (trauma)
- Hormone disorders
- Skin conditions
- Systemic conditions
- Tumors
Skin tone
If you have a darker skin tone, your melanocytes naturally make more melanin (pigment). And with higher amounts of melanin, melanonychia is more common in people of color.
Only 1 in 100 people with lighter skin tones have melanonychia, but 70 in 100 with darker skin tones have it.
Some studies report melanonychia is commonly found in:
- About 20 in 100 people from Japan
- About 1.7 in 100 people from China (50 years or older)
- About 12 in 100 people from Brazil
And nearly all people with Afro-Caribbean heritage can get a line on their nail by the time they're 50 years old.
You may notice a few bands running from your nail bed to the tip of your nail. And it's more commonly found in more than one fingernail. The bands can get wider as you get older, too.
Pregnancy
Skin color changes, especially darker colored skin (hyperpigmentation), are common during pregnancy --- even melanonychia. Often, you'll get melanonychia in more than one finger or toe, and it goes away after you deliver your baby.
Infections
If a germ triggers your melanonychia, the infection can be:
- Fungal. Cause color changes because the germ itself has pigment, such as longitudinal melanonychia from athlete's foot (Trichophyton rubrum).
- Bacteria. Discolored nails in longitudinal or diffuse patterns can happen, especially if you work in a lab or your immune system is weakened.
- Viral. HIV and human papillomavirus (HPV) can discolor your nails. And HPV can cause verruca vulgaris melanonychia, a rare wart that turns your nail brown to black.
Inflammation
Certain health conditions, such as amyloidosis, can inflame your nail bed or the skin around your nail, causing melanonychia.
Medicines
Although transverse melanonychia is uncommon, it's usually a sign of melanonychia caused by medication such as:
- Chemotherapy
- Antivirals for HIV/AIDS
- Malaria medicines
The band colors vary and can be brown, diffuse black, or blue.
Poor nutrition
If your body can't get enough nutrients, you can get melanonychia. Lack of certain nutrients can lead to active melanin cells in your nails. So, you might get melanonychia due to:
- Lack of protein
- Low vitamin D
- Vitamin B12 deficiency (usually a diffuse bluish, longitudinal line)
Trauma
Sudden injury or repeated friction can irritate and inflame your nail bed to activate your melanin cells. Trauma causing melanonychia is usually linked to ridges and grooves, too.
Nail injury can happen because you have:
- Poor-fitting or tight shoes, which often cause discoloration (usually brown) of part or all of your fourth or fifth toenail
- Nail-biting habit or compulsive nail biting (onychophagia), which usually forms diffuse or longitudinal grey bands
- Nail bed injury from work
- Compulsive picking or pulling on your nails (onychotillomania)
Skin conditions
Some skin conditions can activate the melanin cells in your nails because of inflammation. With melanonychia, you may also have:
- Psoriasis
- Lichen planus
- Chronic radio dermatitis
- Chronic paronychia
Systemic disease
Although rare, this type of melanonychia often shows up on more than one fingernail or toenail. And you'll likely notice many diffuse bands, too. When it does happen, you usually have another condition such as:
- Endocrine disorders, such as Addison's disease, Cushing's syndrome, and hyperthyroidism
- Lupus
- Metabolic disorders, such as alcaptonuria
- Hyperbilirubinemia (in which you can't break down bilirubin)
- Rare genetic problems, such as porphyria, Touraine syndrome, or Laugier-Hunziker syndrome
Tumors
Active melanocytes can form longitudinal melanonychia because of noncancerous and cancerous tumors. Some common tumors include:
- Onychomatricoma, which causes noncancerous, thick yellow nails
- Bowen's disease (an early form of squamous cell cancer)
- Squamous cell carcinoma
- Basal cell carcinoma
- Subungual fibrous histiocytoma, which impacts the tissues under your nail
- Subungual keratosis, which leads to thicker tissue under your nails
What Causes Melanonychia Due to Melanocytic Proliferation?
Melanocytic proliferation means the dark line happens because there's a higher number of melanin cells in your nail.
The number of cells can get higher because you can have:
- A nevus, a flat mole on your nail formed by noncancerous cells nesting together
- A lentigo, a noncancerous, flat mole on your nail
- Subungual melanoma, a very serious type of cancer on your nail
How is melanoma of the nail diagnosed?
Your dermatologist is a doctor who specializes in nails and skin. They'll ask about your medical history and do a nail exam to check your nail line for:
- Color
- How even and regular the color looks
- Line width and shape
- Blurry or fuzzy borders on the band
- Hutchinson's sign, a dark band that spreads to the nearby skin around your nail
Your doctor can use the "ABCDEF rule" for nail melanoma to check for any abnormal longitudinal melanonychia bands on your nails:
A. Age — the peak range being from 50 to 70 years old (But nail melanoma also commonly occurs in people who have darker skin tones, such as African Americans, Asians, and Native Americans.)
B. Band color that is brown-black; breadth more than 3 millimeters; blurry borders
C. Change in size or growth rate of the band, or no change if you've had treatment
D. Digit with the band — usually the thumb of your dominant hand
E. Extent of color spreading to nearby tissues
F. Family or personal history of melanoma or unusual moles
Your dermatologist may also perform tests such as:
- Dermoscopy (onychoscopy), in which your doctor uses an instrument called a dermatoscope to closely look at your nail under a microscope and take detailed pictures of your nail streaks
- Nail biopsy with histopathology, in which a small piece of your nail is taken to test for cancer
How Is Melanonychia Treated?
Your treatment depends on what causes your melanonychia. With noncancerous conditions, your doctor will often suggest waiting. They'll monitor your melanonychia to see if it gets better on its own.
Your doctor will talk to you about follow-up visits and retest your nail to make sure the band goes away after you:
- Treat your underlying condition
- Stop or change your medicine
- Avoid nail injury
- Treat your infection
- Correct your diet and vitamin or mineral deficiency
- Deliver your baby
Treatment for nail melanoma
With nail melanoma, your doctor may suggest surgery. It's rare; only about 0.7 to 3.5 in 100 people get subungual melanoma.
It can depend on your cancer stage, but your doctor may suggest removing your:
- Nail plate, nail bed, and nail matrix, followed by a skin graft to repair your digit
- Digit to keep the melanoma from traveling up your finger
Often, a single digit with melanoma prompts quick treatment than many.
Takeaways
Melanonychia is a brown-black line on your nails. The shape, color, and size of the line can give your doctor a clue about the cause. Often noncancerous, the dark line usually goes away once you treat the underlying medical condition. It's rare, but if the line changes shape, spreads to your cuticle, has an ulcer, or bleeds, see your doctor about nail melanoma to avoid other problems.
Melanonychia FAQs
When should I be concerned about a dark line on my nail?
Contact your doctor if the dark line on your nail:
- Changes shape or color (wider, darker, uneven, or fuzzy)
- Colors the skin under your nail plate
- Runs into your cuticle
- Is only on your thumb, index finger, or big toe
- Is new and you have fair skin
- Is only on one digit (although common in darker skin tones, a single digit is uncommon)
- Has an ulcer or fold, or it bleeds
What does cancerous melanonychia look like?
With melanonychia that is cancerous, it can look like your nail has:
- Uneven colors in the dark band
- Swelling and inflammation
- Come away from the nail bed
- Bleeding, an ulcer, or developed a bump (nodule)
- Discolored the nearby skin (Hutchinson's sign)
- Cracked, split, or changed shape
What STDs cause melanonychia?
Some viral infections, such as HIV and HPV, can inflame your nail, which activates your melanin cells, causing a discolored band in your nail. Sometimes, HPV can cause verruca vulgaris melanonychia, a rare brown-black wart.
