Melanonychia is the term for dark pigmentation of the fingernails or toenails. The discoloration can be a natural occurrence in people with dark skin, but it may sometimes indicate health issues.

Melanonychia is a nail condition in which melanin is present in the nail plate. It can present as a single dark band or streak or involve the entire nail. There are many possible causes, so an accurate diagnosis is vital.

This article discusses the different types of melanonychia and their risk factors and treatment options. It also looks at the potential complications.

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Melanonychia refers to the discoloration of the nail plate on a finger or toe. The discoloration is normally brown or black, and it can occur in one or more digits.

The nail plate is usually semitransparent because of a lack of melanin, the substance responsible for skin pigmentation. However, under some circumstances, melanin can cause nail discoloration, leading to melanonychia.

There are three main types of melanonychia:

  • Longitudinal melanonychia: Also called melanonychia striata, this is the most common form of melanonychia. It presents as a long band along the length of the nail plate.
  • Diffuse or total melanonychia: This form causes discoloration over the entire nail plate.
  • Transverse melanonychia: This type appears as a band of color that runs along the width of the nail plate.

Melanocytes are the cells that produce melanin. If they increase in number or produce more melanin than usual, this can lead to melanonychia.

There are two ways in which this can happen:

Melanocytic activation

In melanocyte activation, there is no change in the number of melanocytes, but these cells increase the production and distribution of melanin in the nail.

Melanocytic hyperplasia

Melanocyte hyperplasia refers to an increased number of melanocytes in the nail. In some cases, it can be harmless, or benign, but in others, it can be malignant.

There are many possible causes of melanonychia via melanocytic activation, as well as factors that can make it more likely. Causes and risk factors include:

  • Racial differences: Melanonychia is more prevalent in people with dark skin. For example, it affects up to 20% of Japanese people and 77–100% of African American people. In comparison, only 1% of white people will experience it.
  • Pregnancy: Longitudinal melanonychia may happen during pregnancy, when it may affect several fingernails or toenails.
  • Fungal infection: Certain fungi can produce melanin that becomes incorporated into the nail plate. This occurs more frequently in males than in females and involves the toenails.
  • Bacterial infection: People with a weakened immune system can be more at risk of infection with bacteria that can cause melanonychia, such as Pseudomonas aeruginosa.
  • Viral infection: People living with HIV may develop longitudinal and transverse bands in their fingernails and toenails.
  • Skin conditions: Inflammatory skin conditions, such as psoriasis, lichen planus, and amyloidosis, can lead to melanocyte activation and melanonychia.
  • Trauma: Nail biting, pulling, chewing, and friction can lead to diffuse or longitudinal bands.
  • Tumors: Both benign and malignant tumors, such as Bowen’s disease or squamous cell carcinoma, can lead to melanocytic activation.
  • Systemic diseases: People with diseases such as Addison’s disease and Cushing’s syndrome may develop melanonychia in several fingernails or toenails. The condition usually presents as diffuse melanonychia or in the form of multiple streaks.
  • Drugs: Drugs typically cause transverse melanonychia in multiple nails.
  • Medical treatment-related: People who are receiving chemotherapy or have undergone X-rays may experience melanonychia. The length of exposure to the agent may determine whether or not melanonychia affects a person.

The possible causes of melanonychia via melanocytic hyperplasia include:

  • Nail matrix nevus: This benign condition presents as a light brown to black longitudinal band, about 3–5 millimeters (mm) wide. It is the most common cause of melanonychia in children. The presence of melanocyte “nests,” or collections of cells, distinguishes this condition.
  • Lentigo: This benign condition occurs without the presence of melanocyte nests but with similar-sized 3–5 mm bands. It is more common in adults than in children.
  • Nail unit melanoma (NUM): This condition is rare but malignant. Asian countries, including Japan, China, and Korea, report a higher incidence. It tends to affect the big toes, thumbs, and middle fingers.

Melanonychia can affect individuals of all sexes and ages. However, there is an increased incidence of melanonychia in People of Color.

Environments that expose people to bacteria, viruses, or fungi also increase melanonychia risk. For example, people whose work involves prolonged or repeated contact with water or wet substances are more likely to develop bacterial melanonychia.

People who participate in activities that can cause friction or trauma to the nail plate may develop melanonychia. A 2016 study involving 275 Korean people with melanonychia found that trauma was one of the three most common causes.

The treatment options for melanonychia will depend on the primary cause. If the cause is benign, the individual may not need treatment.

Some cases of melanonychia may fade after addressing the cause. For example, if drugs are the cause, melanonychia should fade within 6–8 weeks of stopping the treatment.

Where the cause is malignant, such as in NUM, a doctor may suggest surgery. A surgeon may remove the entire nail unit or amputate the end of the toe or finger.

A doctor will usually diagnose the cause of melanonychia by taking a medical history and examining the nails. They will determine the number of affected digits and observe the pattern and color of the melanonychia.

In some cases, the doctor may require further tests to aid diagnosis. They may examine the affected nail under a specific type of microscope called a dermatoscope. This process is called onychoscopy.

Onychoscopy can help differentiate melanonychia from other conditions. For example, a splinter hemorrhage presents as a reddish-brown longitudinal streak under the nail plate and looks like a wood splinter. Onychoscopy would help distinguish this from melanonychia.

Similarly, a closer examination can help the doctor determine whether the condition is benign or malignant. With NUM, the band or streak on the affected nail may change by:

  • getting wider over weeks to months
  • becoming more irregular in color
  • extending and affecting the skin bordering the nail
  • bleeding
  • causing the nail plate to deform and change in shape

The doctor may order a biopsy for further confirmation. This procedure involves removing a part of the nail and looking at the cells of the streak more closely. This analysis remains the gold standard for an accurate diagnosis.

The complications of melanonychia will depend on the underlying cause. For example, if the melanonychia is malignant, it could spread and affect other parts of the body, leading to severe health issues.

Some causes of melanonychia can lead to the nail becoming deformed, which people may find concerning or bothersome.

Melanonychia is a nail pigmentation condition that has several underlying causes. It occurs because of an increase in the number of melanocytes in the nail plate or an increase in melanin production.

An individual may not need treatment unless the melanonychia is potentially malignant.

If a person suspects that they have melanonychia, they should seek medical advice to ensure an accurate diagnosis and avoid the risk of complications.