Causes and Risk Factors of Longitudinal Melanonychia

Melanonychia is a medical term used to describe the black or brownish pigmentation of the nail plate. The most common type is longitudinal melanonychia, characterized by the appearance of a pigmented stripe (either partial or complete) running length-wise along the nail bed.

Longitudinal melanonychia is common in people of African descent but can also be caused by nail trauma or systemic disease. Other causes include nail infections and cancer.

A rare form of melanonychia, called transverse melanonychia, is recognized by a darkened line that runs side to side along the nail plate. Transverse melanonychia is typically associated with certain medications and radiation therapy.

Common Causes

The color of your nails, hair, and skin are produced by cells known as melanocytes that secrete a pigment called melanin. Although the melanin in the nail plate is generally evenly distributed, it can sometimes become irregular.

For example, if you hit a nail with a hammer, it can not only cause a hematoma (a bruise) but also activate local melanocytes in the nail bed. When this happens, melanin will collect in the disrupted nail cells (onychocytes), causing blackish or grayish discoloration. It is not unlike how freckles darken when UV rays from the sun activate melanocytes in the skin.

With longitudinal melanonychia, melanocytes at the base of the nail will transfer melanin to local onychocytes. As the nail grows outward, the melanin will be carried along, creating longitudinal (length-wise) stripes from the cuticle (nail fold). There are many reasons why this might occur, some of which are totally benign.

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Common causes include:

• Nail trauma (usually starting on or near the base of the nail)
• Subungual hematomas (blood under the nail)
• Subungual verruca (a wart under the nail plate)
• Onychomycosis (a common fungal nail infection)
• Nail psoriasis (an autoimmune disorder)
• Lichen planus (an inflammatory skin condition believed to be autoimmune)
• Chronic paronychia (a bacterial or fungal infection occurring where the side of a nail meets the skin)
• Pyogenic granuloma (blood-red skin growths that sometimes occur in pregnancy or as a side effect of certain drugs)
• Addison's disease (insufficiency of the adrenal glands caused by autoimmune disease, cancer, infection, or pituitary tumors)
• Subungual exostosis (a bony overgrowth of the fingertips or toes, possibly caused by persistent bone irritation

How to Tell If the Cause Is Benign

Benign longitudinal melanonychia is recognized by 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.

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 is a feature of several rare genetic disorders in which skin hyperpigmentation is a common symptom. Many of these conditions are autosomal dominant, meaning that you only need to inherit the gene mutation from one parent to manifest with the disease.

Examples 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 characterized by hyperpigmentation of the mouth, lips, digits, and nails as well as the high risk of cancer)
• Peutz-Jeghers syndrome (a rare disorder manifesting with 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

The causes of longitudinal melanonychia are divided into two categories: melanocytic activation (in which melanocytes produce extra melanin) and melanocytic hyperplasia (in which melanocytes multiply abnormally).

The latter is especially concerning given that melanocytes are prone to genetic errors when they multiply, which can lead to the development of benign or cancerous skin growths known as neoplasms.

Among the neoplasms that can cause longitudinal melanonychia are:

• 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)
• Myxoid cysts (small, benign lumps that occur near a nail)
• Subungual melanoma (the most deadly form of skin cancer occurring under the nail plate)

Subungual Melanoma

Subungual melanoma mainly affects people over 50 and is considered rare, accounting for only 0.7% to 0.35% of all 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 digit rather than several. Other symptoms may include longitudinal streaks on the affected finger or toe as well as the darkening of the palms or soles.

One of the key indications of subungual melanoma is the "Hutchinson's sign." 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 definitively diagnosed with a nail biopsy. People with nail matrix melanoma tend to have poor outcomes. 

Environmental Causes

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

Common examples include:

• Ink
• Tar deposits from chain-smoking
• Hair dyes or henna, ink,
• Potassium permanganate (a topical disinfectant)
• Silver nitrate used to heal burns and wounds.

An examination of the cuticle along with a review of the medical history can help differentiate these environmental causes from other more serious conditions.

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 the 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.

The cause of melanonychia can be usually diagnosed by a dermatologist. If there are suspicious signs, the doctor may take a small sample of the nail for evaluation in the lab. If a problem is found, early diagnosis and treatment almost invariably leads to better outcomes.