OVERVIEW

What is a nail matrix nevus?

A nail matrix nevus (also called melanocytic nevus of the nail matrix) is a mole that grows in the nail matrix.

In most cases, nail matrix nevi are similar in nature to moles on the skin, known as "junctional nevi," differing only in their location. The nevus grows forward with the nail, eventually forming a longitudinal black line called melanonychia striata, which is the most common symptom of nail matrix nevus.

The nail matrix, often referred to simply as the matrix, is located at the root of the nail and is responsible for nail growth. The cells produced by the matrix form the nail plate (the visible part of the nail).

Nail matrix nevi can occur at any age but are relatively more common in children and adolescents. They may be present at birth or appear later in life. Studies show that nail matrix nevi can develop between ages 0 and 52, suggesting they may be congenital (present at or shortly after birth) or acquired (appearing during childhood or later).

Diagnosis of a nail matrix nevus requires evaluation by a medical professional, who can distinguish it from conditions like nail melanocyte activation.

Observation is often recommended for nail matrix nevi, especially in children. If treatment is needed, surgical excision can provide a cure. No isolation is required during the condition, and patients' quality of life is generally not significantly affected.

SYMPTOMS

What are the symptoms of nail matrix nevus?

• Nail matrix nevus does not cause itching or pain.
• The most common manifestation of nail matrix nevus is longitudinal melanonychia, also called linear melanonychia. Longitudinal melanonychia occurs when melanin leaks from the nail matrix at the base of the nail into the nail plate and moves forward with nail growth toward the edge.
• Longitudinal melanonychia appears as a band, with varying widths—ranging from as narrow as 2 mm to covering the entire nail.
• The color of the band may vary in intensity, commonly appearing as brown, dark brown, or black. Most cases persist, though some may show fading, darkening, or even spontaneous disappearance over time. However, some believe this is due to reduced pigment production rather than the nevus regressing.
• In children, longitudinal melanonychia may noticeably widen or darken.
• Nail matrix nevus usually affects only one nail, more commonly fingernails than toenails, with nearly half occurring on the thumb.
• Additionally, the skin around the affected nail may appear darkened—not due to actual pigmentation but from the underlying pigment visible through the translucent periungual skin, known as pseudo-Hutchinson's sign.

Can nail matrix nevus be diagnosed based on longitudinal melanonychia?

Longitudinal melanonychia does not necessarily indicate nail matrix nevus, as it can result from other conditions.

• Among adults with longitudinal melanonychia, nail matrix nevus accounts for about 12% of cases.
• Among children with longitudinal melanonychia, nail matrix nevus accounts for about 48% of cases.

Thus, nail matrix nevus is the most common cause of longitudinal melanonychia in children.

Is nail matrix nevus a benign condition?

Most nail matrix nevi are benign, with an extremely low risk of malignancy, especially in children.

Generally, benign nail matrix nevi produce longitudinal melanonychia with a width of less than 4 mm. If the condition remains stable without significant changes, monitoring is sufficient, and excessive concern is unnecessary.

CAUSES

What causes a nail matrix nevus?

A nail matrix nevus is a mole that grows in the nail matrix, caused by an increase in melanin produced by the normal melanocytes in the nail matrix.

As for the types of nail matrix nevi, almost all are "junctional nevi," and in rare cases, they may be "compound nevi," "blue nevi," or "Spitz nevi."

Is a nail matrix nevus contagious?

A nail matrix nevus is not an infectious disease, so it cannot be transmitted through direct or indirect contact.

Is a nail matrix nevus hereditary?

A nail matrix nevus is not related to genetics. Developing a nail matrix nevus is not associated with whether one's parents have it.

DIAGNOSIS

What tests are needed for nail matrix nevus?

Common tests include dermatoscopy and pathological examination.

• Dermatoscopy: A special magnifying tool that helps observe pigment components and deeper tissue lesions without causing trauma.
• Pathological examination: A minor surgical procedure performed in a medical institution, where the lesion is excised, processed into a specimen, sliced, and examined under a microscope. This method is invasive. A punch biopsy is generally recommended, as it causes relatively minor trauma, leaves no significant scars, and aids in diagnosing melanocyte activation (the most common cause of nail melanonychia). The biopsy can be done in an outpatient setting, with results available in a few weeks. Risks include pain, bleeding, and uneven regrowth of the nail.

Whether to perform tests and which tests to use should be decided by a professional dermatologist after comprehensive evaluation.

What does nail matrix nevus look like under dermatoscopy?

The dermatoscopic features of nail matrix nevus differ between children and adults.

• In children: The lines are usually irregular, with variations in color and thickness, and often non-parallel.
• In adults: Dermatoscopy typically shows longitudinal, regular parallel lines on a brown background.

Is a pathological examination always necessary for diagnosing nail matrix nevus?

Diagnosis mainly relies on pathological examination, though it is not performed for all suspected cases.

Some experts recommend pathological examination for adults with nail melanonychia in the following cases, primarily to rule out nail melanoma:

• Single nail melanonychia with no clear cause;
• A very dark, rapidly growing band wider than 4–6 mm;
• Irregular lines on dermatoscopy (color, width, or spacing);
• Nail atrophy, widening of the melanonychia, and involvement of the proximal nail fold (Hutchinson's sign).

Unfortunately, these adult criteria do not apply well to children. Some "abnormal" features in children may even indicate benign changes rather than melanoma.

Given the rarity of nail melanoma in children (about 0.8 per million in those under 10), pathological examination is not routinely recommended. Follow-up observation is advised unless parents explicitly request treatment.

What conditions can be confused with nail matrix nevus? How to differentiate them?

Nail melanonychia is a common symptom of nail matrix nevus, but not all nail melanonychia indicates nevus.

Other causes include melanocyte activation, lentigo, onychomycosis, and nail melanoma, requiring professional differentiation.

• Melanocyte activation: The most common cause. Features increased melanin production without an increase in melanocyte count. It can occur in healthy individuals, especially those with darker skin, or due to trauma, medications, or inflammation. Pathologically, normal melanocyte numbers and their location above the basal layer support this diagnosis.
• Lentigo: Rare, also called melanocyte hyperplasia. Shows increased melanocytes (2–3 times normal) forming a brown band, typically in young people. Unlike nevus, melanocytes are not nested.
• Onychomycosis: Caused by fungal infection (e.g., dematiaceous fungi). Fungal microscopy or culture confirms diagnosis. Pathology with PAS staining can also help.
• Nail melanoma: A highly malignant tumor, rare (0.7%–3.5% of melanomas). Most common in middle-aged/elderly (50–70 years) thumbs, rarely in children. Single-nail melanonychia in children is almost always nevus.

Nail melanoma usually arises from normal skin, not pre-existing nevi.

As mentioned, signs suggesting melanoma (in adults) include:

• Very dark, rapidly growing bands >4–6 mm;
• Irregular lines on dermatoscopy;
• Nail atrophy, widening melanonychia, and Hutchinson's sign (proximal nail fold involvement), indicating tumor spread.

Pathological examination remains the gold standard for diagnosis.

TREATMENT

Which department should I visit for a nail matrix nevus?

Dermatology.

How is nail matrix nevus treated?

In children, nail matrix nevi generally do not require surgical treatment but should be monitored regularly for changes. It is usually recommended to have a follow-up examination every six months. If significant changes in size, shape, color, or number of the nevus are observed during monitoring, prompt medical evaluation is advised.

For those requiring treatment or suspected of malignant transformation, surgical intervention is recommended. The procedure typically involves local excision of the lesion, but a professional doctor should conduct a comprehensive assessment.

DIET & LIFESTYLE

What should be noted in the diet for nail matrix nevus?

Nail matrix nevus is unrelated to diet. Diet does not worsen nail matrix nevus nor does it help improve it. A normal, healthy diet is sufficient.

What should be noted in daily life for nail matrix nevus?

In daily life, it is important to monitor changes in the nail's dark line and schedule regular follow-ups with a dermatologist. If suspicious lesions appear, prompt medical attention is recommended.

Generally, the following situations should raise caution, as they may signal malignant transformation of nail matrix nevus:

• Significant widening of the nail's dark line;
• New dark moles appearing around the nail;
• Irregular edges of the mole;
• Uneven or darker color;
• Accompanied by abnormal symptoms such as skin ulceration.

PREVENTION

Can nail matrix nevus be prevented?

Nail matrix nevus is not related to lifestyle factors, it develops spontaneously, and there is no effective prevention method.