Brown Line on Nails: Vitamin Deficiencies and Other Medical Causes

A brown line on the nail is called longitudinal melanonychia or longitudinal striata. It is the most common type of melanonychia, a medical word for a black or brownish pigmentation of the nail plate (the hard part of toenails or fingernails).

Longitudinal melanonychia can have many causes, from harmless pigment deposits or growths to infections, body-wide diseases, or melanoma (skin cancer).

This article will cover the causes of longitudinal melanonychia, what happens in nail cells that cause the black or brown lines, the symptoms of melanonychia, as well as how the nail condition is diagnosed and treated.

Types of Longitudinal Melanonychia

Your nails, hair, and skin are colored by cells (melanocytes) that make a pigment called melanin. If you have a brown line on your nail, it’s related to how much melanin is being made in your body. 

Melanonychia striata can look like a partial or complete stripe running length-wise (longitudinal) along the nail. Causes of longitudinal melanonychia are categorized according to how they happen:

• Melanocytic activation: Melanocytes make extra melanin.
• Melanocytic hyperplasia: Melanocytes copy themselves (multiply) abnormally.

Each type can have a range of causes: 

Melanocytic Activation

Traumatic or infectious causes:

• Injury (e.g., nail-biting, stubbed toes, wearing shoes that do not fit, slamming your finger in a door)
• Infections 
  

Medical condition-related causes:

• Lichen planus
• Lupus
• Psoriasis
• Amyloidosis
• Hyperthyroidism
• Addison’s disease
• Skin cancer
• HIV/AIDS
• Cushing’s syndrome
  

Health-related causes:

• Sun sensitivity (photosensitivity) 
• Iron overload
• Nutrient deficiencies
• Pregnancy
• Growth hormone problems
• Warts

Treatment-related causes: 

• Phototherapy
• X-rays
• Radiation
• Chemotherapy
• Medications to treat malaria

Melanocytic Hyperplasia

• Growths or lesions (such as birthmarks and moles) that are not cancer (benign)
• Nail cancer
• Bacteria that normally live on the skin/nails
• Silver nitrate 
• Hair dye, henna tattoo ink 
• Tobacco products 
  

Common Causes and Risk Factors

Many common causes of longitudinal melanonychia are related to the extra production of melanin.

Melanin in the nail plate is usually evenly spread out, but it can become irregular. With longitudinal melanonychia, pigment cells at the bottom of the nail transfer melanin to nail cells. As the nail grows out, the melanin gets carried with it, creating length-wise stripes from the nail fold (cuticle).

Many causes of the brown lines on the nails are harmless and nothing to worry about. However, some of the causes are serious and need medical attention. 

Longitudinal melanonychia that’s caused by extra melanin is most common in people of African descent. It can also happen if there is nail trauma, body-wide (systemic) disease, or nail infections.

Broadly speaking, longitudinal melanonychia can be caused by:

• Nail trauma/injury 
• Infections of the nail
• Environmental triggers
• Autoimmune diseases
• Skin conditions
• Non-cancerous (benign) growths
• Cancer
• Genetic disorders 

Nail Trauma

Nail trauma (especially starting on or near the base of the nail) and blood under the nail (subungual hematomas) are common causes of longitudinal melanonychia.

For example, hitting your nail with a hammer could cause blood to pool under it and "turn on" the melanocytes in the nail bed.

When this happens, melanin will collect in the nail cells and cause blackish or grayish discoloration. This process is a type of melanocyte activation. It’s similar to why freckles get darker when ultraviolet (UV) rays from the sun activate melanocytes in the skin.

Infections

Fungal, viral, or bacterial infections can also cause streaks on the nails. 

• Subungual verruca (a wart under the nail plate caused by a virus)
• Onychomycosis (a common fungal nail infection)
• Chronic paronychia (a bacterial or fungal infection occurring where the side of a nail meets the skin)
  

Autoimmune and Skin Conditions

Skin and nail conditions can cause longitudinal melanonychia. Autoimmune disorders can also cause nail discoloration. 

• Nail psoriasis (an autoimmune disorder that causes nail changes)
• Addison's disease (an insufficiency of the adrenal glands caused by autoimmune disease, cancer, infection, or pituitary tumors)
• Lichen planus (an inflammatory skin condition that may be caused by an autoimmune reaction)
  

Benign Growths

When nail cells multiply abnormally, it is often not cancerous. Common benign growths that can cause longitudinal melanonychia include:

• Pyenic granuloma (blood-red skin growths that sometimes occur in pregnancy or as a side effect of certain drugs)
  
• Subungual exostosis (a bony overgrowth of the fingertips or toes, possibly caused by persistent bone irritation)
• Myxoid cysts (small, benign lumps that occur near a nail)

How to Tell if the Cause Is Benign

Longitudinal melanonychia that's not cancer is typically light to dark brown stripes. The lines are parallel and regular in color, thickness, and spacing. The border will be clearly defined and less than 3 millimeters (mm) in width—roughly 1/10th of an inch.

Another sign that longitudinal melanonychia is not cancer is the yellowish fading of color along the outer edges of the stripe. Melanonychia caused by a serious disease does not typically fade.

If a blackish mark is caused by an injury, it will tend to move toward the tip as it grows, leaving behind an unblemished nail.

Cancer

Sometimes the growths that cause longitudinal melanonychia are cancerous or could become cancerous.

• Subungual melanoma (a dangerous form of skin cancer occurring under the nail plate)
• Glomus tumor (a rare and potentially deadly tumor found mainly under the nail, on the fingertip, or at the end of a toe)
• Keratoacanthoma (a low-grade, dome-shaped tumor usually found on sun-exposed skin); the cause is unknown, but it's often considered a type of squamous cell carcinoma, a type of skin cancer).

Nail Apparatus Melanoma

Melanoma accounts for about 1% of skin cancers, but it is also the most dangerous.

Nail apparatus melanoma (subungual melanoma) is a rare subtype of acral lentiginous melanoma that accounts for only 0.7% to 3.5% of all melanoma skin cancers. While melanoma cannot be "cured," studies have shown that treatment often leads to a good outlook for patients. Usually, it requires surgery to remove the cancer.

When doing a physical examination of your nail, a provider will look for certain telltale signs of cancer:

• Involvement of more than two-thirds of the nail plate 
• Grey or black colors mixed with brown
• Irregular brown and granular pigmentation
• Variations in the color and thickness of the stripe
• Blurred borders larger than 3 mm
• Distortion of the nail plate
• Recurrent, spontaneous bleeding at the same site

Subungual melanoma usually involves a single finger or toe rather than more than one. Other symptoms include lengthwise streaks on the affected digit and darkening of the palms of the hands or soles of the feet. 

Hutchinson's sign is a key indicator of subungual melanoma. This happens when a streak goes from the tip of the nail all the way down to the nail bed and into the cuticle.

Subungual melanoma can only be diagnosed with a nail biopsy. Your provider may take a small sample of your nail and the tissue underneath to be checked in a lab. If a problem like cancer is found, early diagnosis and treatment lead to better outcomes.

Environmental Causes

Longitudinal melanonychia can also develop when pigments other than melanin get into the nail fold. These pigments can be absorbed by the cuticle and underlying nail cells and be carried along as the nail plate grows.

Examples of pigments that can get absorbed include:

• Ink
• Tar deposits from smoking
• Hair dyes or henna ink
• Silver nitrate (used to heal burns and wounds)
• Potassium permanganate (a disinfectant sometimes applied to skin)
  

If the cause is environmental, the streak will usually not go far past the whitish crescent at the base of the nail (lunula). There can also be discoloration beneath the nail fold and the surrounding rim of the skin.

Carefully looking at your cuticles and reviewing your medical history can help a healthcare provider identify environmental causes of melanocytosis.

Genetic Causes

Longitudinal melanocytosis can occur with several rare genetic disorders in which skin hyperpigmentation (excess pigmentation) is a common symptom.

Genetic disorders occur because you have certain mutations in your genes. Many of these conditions are autosomal dominant, meaning that you only need to inherit the gene mutation from one parent to have the disease.

Examples of genetic causes include:

• Familial (hereditary) amyloidosis (a rare and potentially life-threatening disease that occurs when a protein called amyloid builds up in organs and tissues)
• Laugier-Hunziker syndrome (a rare disorder with hyperpigmentation of the mouth, lips, fingers, and nails as well as a higher risk of cancer)
• Peutz-Jeghers syndrome (a rare disorder that leads to benign polyps in the gastrointestinal tract and hyperpigmented lesions on the mouth, lips, nails, and fingers)
• Touraine syndrome (a rare, non-progressive disorder characterized by sparse body hair, brittle teeth, a reduced ability to sweat, and hyperpigmented lesions)

What to Do

Having brown lines on your nails is not always a problem, but if you’re worried about the discoloration, it’s important to tell your provider. There are many causes of longitudinal melanonychia, and you won’t know whether or not it needs to be treated until you see a provider who can tell you why you have it. 

The cause of melanonychia can be usually diagnosed by a dermatologist—a provider who specializes in treating skin, hair, and nails.

If the reason for the lines is something harmless, you may not need to do anything other than keep an eye on your nails to make sure they don’t change in other ways. If there’s a specific cause that can be treated (like a skin disorder or cancer), your provider will discuss next steps.  

Summary

Longitudinal melanonychia can happen when there is an overproduction of melanin or an increase in melanocytes. These increases in melanin or melanocytes can be caused by harmless growths to injuries, infections, or cancer.

A darkened streak on the nail bed does not necessarily mean that you have a disease, but if it’s persistent, affects a large part of a nail, or is associated with bleeding, you should have your provider take a look. Many of the more serious conditions that can cause a brown line on your nail can be treated, but it’s important to be diagnosed early.