Causes and Risk Factors of Longitudinal Melanonychia

Longitudinal melanonychia is the most common type of melanonychia, a medical term used to describe a black or brownish pigmentation of the nail plate (the hard part of toenails or fingernails).

Longitudinal melanonychia can look like a partial or complete stripe running length-wise (longitudinal) along the nail.

It can have many causes that range from harmless pigment deposits or growths to infections, body-wide diseases, or cancer.

This article will explore the many causes of longitudinal melanonychia, what happens in nail cells, and how it's diagnosed.

Types of Longitudinal Melanonychia

The color of your nails, hair, and skin are produced by cells known as melanocytes that produce a pigment called melanin.

Cases of longitudinal melanonychia can divided based on how they occur:

• Melanocytic activation: Melanocytes produce extra melanin.
• Melanocytic hyperplasia: Melanocytes multiply abnormally.
  

Melanocytic hyperplasia is especially concerning given that melanocytes are prone to genetic errors when they multiply. This can lead to the development of benign (noncancerous) or cancerous skin growths known as neoplasms.

Common Causes and Risk Factors

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

Although the melanin in the nail plate is generally evenly distributed, it can sometimes become irregular.

With longitudinal melanonychia, pigment cells at the base of the nail will transfer melanin to nail cells.

As the nail grows outward, the melanin will be carried along, creating length-wise stripes from the nail fold, or cuticle. There are many reasons why this might occur, some of which are totally harmless.

Longitudinal melanonychia due to excess melanin is most common in people of African descent. It can also occur with nail trauma, body-wide disease, or nail infections.

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

Nail Trauma

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

For example, if you hit your nail with a hammer, it can not only cause a bad bruise from a collection of blood, but also "turn on" melanocytes in the nail bed.

When this happens, melanin will collect in the nail cells, causing blackish or grayish discoloration. This process is a type of melanocyte activation.

It is not unlike how freckles darken when ultraviolet (UV) rays from the sun activate melanocytes in the skin.

Infections

Fungal, viral, or bacterial infections can also cause the streaks. These include:

• Subungual verruca, a wart under the nail plate that is 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

Conditions that affect the skin and nails can cause longitudinal melanonychia. Autoimmune disorders, in which the immune system attacks the body's own tissues, can as well.

These conditions include:

• 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 (growths in the pituitary gland)
  
• Lichen planus, an inflammatory skin condition that may be caused by an autoimmune reaction
  

Benign Growths

When nail cells multiply abnormally it is often noncancerous.

Common types of benign growths that can cause longitudinal melanonychia include:

• Pyenic granuloma, which are 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, or small, benign lumps that occur near a nail

How to Tell If the Cause Is Benign

Non-cancerous longitudinal melanonychia typically has the appearance of light to dark brown stripes that are parallel and regular in color, thickness, and spacing.

The border will be clearly defined and less than 3 millimeters (mm) in width, or roughly 1/10th of an inch.

Another sign that longitudinal melanonychia is benign is the yellowish fading of color along the outer edges of the stripe. Melanonychia caused by a serious disease doesn't typically fade.

Moreover, 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.

Genetic Causes

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

Genetic disorders occur due to genes within your cells.

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 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 the high 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

Cancer

Sometimes the neoplasms, or growths, that cause longitudinal melanonychia are cancerous or potentially cancerous. This can include:

• 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—can also cause longitudinal melanonychia. The cause of keratoacanthoma is unknown, but it's often considered a type of squamous cell carcinoma, a type of skin cancer.

Subungual Melanoma

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

Subungual melanoma, also called nail matrix melanoma, is a type that mainly affects people over 50 and is considered rare, accounting for only 0.7% to 3.5% of all melanoma skin cancers.

When performing a physical examination of the nail, the doctor will look for certain tell-tale 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 more often involves a single finger or toe rather than several. Other symptoms may include lengthwise streaks on the affected digit as well as the darkening of the palms or soles.

Hutchinson's sign is one of the key indications of subungual melanoma. This is when a streak extends 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. If there are suspicious signs, your healthcare provider may take a small sample of the nail for evaluation in the lab.

If a problem is found, early diagnosis and treatment leads to better outcomes.

Environmental Causes

Longitudinal melanonychia can also develop when pigments other than melanin are introduced into the nail fold.

These can be absorbed by the cuticle and underlying nail cells and be carried along as the nail plate grows.

Examples include:

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

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

An examination of the cuticle along with a review of the medical history can help a healthcare provider identify environmental causes.

Summary

Longitudinal melanonychia can result from an overproduction of melanin (skin pigment) or an increase in melanocytes (pigment cells).

These increases in melanin or melanocytes can occur for many reasons ranging from harmless growths to injuries, infections, or cancer.

A Word From Verywell

The appearance of a darkened streak on the nail bed can be distressing, but it is doesn't necessarily mean that you have a disease or are at risk of illness.

At the same time, it isn't something you should ignore, particularly if the condition is persistent, affects a large part of a nail, or is associated with bleeding.

Have it looked at by a healthcare provider. An early diagnosis can make a big difference if it due to a serious condition.