Causes and Risk Factors of Longitudinal Melanonychia

Melanonychia is a medical term used to describe the black or brownish pigmentation of the nail plate. It is common in people of African descent but can also be caused by nail trauma or systemic disease. There are two main types of melanonychia, known as traverse melanonychia and longitudinal melanonychia. Transverse melanonychia is rare and typically associated with medications and radiation therapy.

Longitudinal melanonychia is arguably more concerning in that it sometimes indicates serious disease. Causes include nail infections, trauma, infection, and cancer. As per its name, longitudinal melanonychia is recognized by the appearance of a pigmented stripe, either partial or complete, running length-wise along the nail bed.

The color or 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 collection of blood) but also activate local melanocytes in the nail bed. When this happens, melanin will collect in the disrupted nail cells (called 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 nail fold (cuticle). There are many reasons why this might occur, some of which are totally benign.

Benign longitudinal melanonychia is generally recognized by the appearance of light to dark brown stripes that are parallel and regular in color, thickness, and spacing. The border would be clearly defined and be less than 3 millimeters (roughly 1/10th of an inch) in width.

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

Common causes of longitudinal melanonychia 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 in nature)
• Chronic paronychia (a bacteria or fungal infection occurring where the side of a nail meets the skin)
• Pyogenic granuloma (blood-red skin growths that sometimes occur with pregnancy or when taking 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)

Longitudinal melanocytosis is a feature of several rare genetic disorders in which skin hyperpigmentation is a common symptom. 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 the 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 digits)
• Touraine syndrome (a rare, non-progressive disorder characterized by sparse body hair, brittle teeth, a reduced ability to sweat, and hyperpigmented lesions)

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

The latter is especially concerning given that melanocytes are prone to genetic errors when they multiply, leading 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 remains the most serious concern when investigating the possible causes of longitudinal melanonychia. The malignancy 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 that cancer is the cause, including:

• 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 millimeters
• 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.

Subungual melanoma can only be definitively diagnosed with a nail biopsy. People with nail matrix melanoma tend to have poor outcomes. 

Longitudinal melanonychia can also develop when pigments other than melanin are introduced into the nail fold. Examples include tar deposits from chain-smoking, hair dyes, henna, ink, potassium permanganate (a topical disinfectant), and silver nitrate used to heal burns and wounds. These can be absorbed into the nail fold and underlying onychocytes and be carried along as the nail plate grows.

An examination of the cuticle along with a review of the medical history can often differentiate these external causes from other more serious conditions. In most cases, the streak will not go far past the lunula (the whitish crescent at the base of the nail). There may also be discoloration immediately beneath the nail fold and the rim of skin surrounding the nail.