Melanonychia

Melanonychia (also called longitudinal melanonychia or melanonychia striata) is a brown to black pigmented streak that runs lengthwise along the nail, from the base to the free edge. It is caused by melanin — the same pigment that gives skin and hair their color — being deposited into the nail plate as it grows.

Melanonychia is extremely common in individuals with darker skin tones, where it occurs naturally and affects multiple nails. In lighter-skinned individuals it is less frequent, and when it appears — particularly on a single nail in an adult — it requires careful evaluation to distinguish harmless causes from nail melanoma.

In most cases, melanonychia is benign. The pigmented band is produced by melanocytes (pigment-producing cells) in the nail matrix (the tissue at the base of the nail where the nail plate is formed). These cells may become more active without multiplying (melanocytic activation) or they may increase in number (melanocytic hyperplasia), as seen in nail moles (nevi), freckles (lentigines), and rarely, melanoma.

• Appearance: A longitudinal (vertical) band of pigment running from the nail base to the tip
• Color: Ranges from light brown or gray to dark brown or black
• Width: Can be a narrow line (1–2 mm) or a broad band (>3 mm); very wide bands require medical evaluation
• Number: May involve one nail or multiple nails simultaneously
• Borders: Benign bands tend to have regular, well-defined edges; irregular or blurry borders are concerning
• Nail surface: The nail itself may be smooth and normal, or it may show ridging, splitting, or dystrophy in more concerning cases

• Fingernails: Most commonly the thumbnail, followed by the index finger
• Toenails: The great toenail is most frequently affected
• Single nail vs. multiple nails: A single pigmented band in a light-skinned adult warrants attention; multiple bands on several nails are common in individuals with darker skin and are usually physiologic
• Periungual skin: Pigment that extends beyond the nail onto the surrounding skin (Hutchinson sign) may indicate melanoma

• In physiologic melanonychia: Multiple uniform, thin bands on several nails; no nail damage; common in darker skin types; increases with age
• In trauma-related melanonychia: Dark spots or streaks associated with nail biting, tight shoes, or repetitive pressure; may include tiny blood spots visible on dermoscopy
• In nail moles or freckles: Single-digit band, usually narrow (<5 mm), homogeneous color; the nail surface is intact
• In nail melanoma: A single, broad (>3 mm) band with irregular borders, color variation within the band, progressive widening, nail plate damage, and possible pigmentation of surrounding skin

Seek medical evaluation for:

• A new pigmented band on a single nail, especially if you are an adult with lighter skin
• A band that is wider than 3 mm or that is getting wider over time
• A pigmented band with uneven color (mixed light and dark areas) or irregular borders
• Pigment spreading from the nail onto the surrounding skin (cuticle, lateral nail folds)
• A pigmented nail that also shows cracking, splitting, or thickening
• Any change in a pre-existing pigmented band — darkening, widening, or blurring of edges
• A family history of melanoma combined with a new pigmented nail band

Longitudinal melanonychia (LM) is a pigmented, brown to black, longitudinal streak of the nail plate caused by increased melanin deposition from melanocytes in the nail matrix. The melanin is incorporated into the growing nail plate and is visible as a band running from the proximal nail fold to the distal free edge. LM may result from melanocytic activation (increased pigment production without an increase in melanocyte number) or melanocytic hyperplasia (increased number of melanocytes), which includes benign lesions (lentigo, nevus) and malignant lesions (melanoma).

By pathogenesis

• Melanocytic activation (functional melanonychia): Increased melanin production by existing melanocytes without proliferation. The most common cause of LM overall. Includes physiologic melanonychia (darkly pigmented skin), pregnancy-related, trauma-related (frictional melanonychia), drug-induced, inflammation-associated, and systemic disease-associated forms.

• Melanocytic hyperplasia: Increased number of melanocytes in the nail matrix. Includes:

  • Lentigo: Slight to moderate increase in individually aligned basal melanocytes (10–31 per mm); no significant atypia
  • Nevus: Melanocyte hyperplasia with nest formation; common in children
  • Melanoma: Atypical melanocyte proliferation with nuclear atypia and pagetoid spread

By etiology

• Physiologic: Common in individuals with darkly pigmented skin (Fitzpatrick phototypes IV–VI); prevalence >70% in some populations. Multiple nails affected; bands increase in number and width with age.
• Traumatic (frictional): From repetitive microtrauma (nail biting, ill-fitting footwear, occupational)
• Drug-induced: Chemotherapeutic agents (cyclophosphamide, doxorubicin, 5-fluorouracil, hydroxyurea), antiretrovirals (zidovudine), antimalarials, minocycline, psoralens
• Inflammatory: Nail psoriasis, nail lichen planus, onychomycosis, chronic paronychia
• Systemic: Addison disease, Cushing syndrome, Peutz-Jeghers syndrome, Laugier-Hunziker syndrome, HIV
• Neoplastic: Nail lentigo, melanocytic nevus, melanoma in situ, invasive melanoma

Epidemiology

The prevalence of LM varies dramatically by skin phototype:

• Lightly pigmented skin: approximately 1%
• Chinese populations: 0.6% (age 20–29) to 1.7% (age ≥50)
• Japanese populations: approximately 11%
• Brazilian populations with phototypes IV–VI: approximately 12%
• Darkly pigmented skin: >70%

Nail melanoma accounts for 1–3% of all melanomas in lightly pigmented populations and 15–30% in darkly pigmented populations. Two-thirds of nail melanomas present as longitudinal melanonychia.

General features

LM presents as one or more longitudinal pigmented bands running from the proximal nail fold to the distal free edge. Color varies from gray to light or dark brown to black. Bands occur more frequently on the thumbnail or great toenail. The width, color homogeneity, border regularity, and number of digits involved are key features in clinical assessment.

Melanonychia due to melanocytic activation

The most common type in both adults and children. Presents as regular, homogeneous, thin bands of brown color. In darkly pigmented individuals, multiple nails are affected and the finding is physiologic. Drug-induced and inflammatory causes typically involve multiple nails. A pseudo-Hutchinson sign (periungual pigment visible through a thin cuticle) may be present and does not indicate malignancy.

Melanonychia due to melanocytic hyperplasia

Nail lentigo and nevus: Present as single-digit bands, usually <5 mm wide, light brown to dark brown or black. Lentigines are more common in adults; nevi are the most common cause of LM in children. The thumbnail is most frequently affected. In children, nail nevi may show features (irregular lines, pseudo-Hutchinson sign) that mimic melanoma on dermoscopy.

Nail melanoma: Rare but important. Accounts for 1–3% of melanomas in light-skinned and 15–30% in dark-skinned populations. Presents in two-thirds of cases as a brown to black longitudinal band on a single nail, generally wider than 3 mm, with proximal widening, irregular or blurred lateral borders, and color variegation. May be associated with nail plate dystrophy. In one-third of cases, nail melanoma is amelanotic and presents as a nail bed mass or nail plate abnormality rather than a pigmented band.

Hutchinson sign and pseudo-Hutchinson sign

The Hutchinson sign is the periungual spread of pigment into the proximal or lateral nail folds. Historically considered pathognomonic for subungual melanoma, but now recognized to also occur in benign conditions.

The pseudo-Hutchinson sign refers to periungual pigmentation seen in benign conditions: melanonychia in darkly pigmented skin, nail nevi (pigment visible through a thin cuticle), Laugier-Hunziker syndrome, Peutz-Jeghers syndrome, and drug-induced melanonychia. In children with nail matrix nevi, cuticle pigmentation may be illusory (pigment showing through the transparent cuticle rather than true periungual spread).

Melanocytes in the nail unit are fewer than in normal skin. Melanonychia originates in the distal nail matrix because melanocytes in the proximal matrix are dormant, while approximately half of those in the distal matrix can become activated.

Melanocytic activation: Various stimuli (inflammation, hormones, drugs, UV exposure, trauma) trigger dormant distal matrix melanocytes to produce melanin, which is incorporated into the growing nail plate. The number of melanocytes remains unchanged.

Melanocytic hyperplasia: An increase in the number of melanocytes in the nail matrix epithelium. In lentigines, individually aligned melanocytes increase in the basal layer. In nevi, melanocytes form nests. In melanoma, atypical melanocytes proliferate with nuclear atypia, pagetoid spread, and eventually dermal invasion.

Histopathologic examination of a nail matrix biopsy is the gold standard for diagnosis. However, distinguishing early melanoma from benign melanocytic lesions can be challenging even for expert pathologists.

• Melanocytic activation: Melanic pigmentation of the matrix epithelium without increased melanocyte number. Melanocytes with pigmented dendrites in the suprabasal layer of the proximal matrix and basal layer of the distal matrix. Pigmented keratinocytes present. Fontana-Masson stain helpful. No cytologic atypia.

• Lentigo: Slight to moderate increase in melanocytes (10–31 per mm), individually aligned in the basal layer without confluence. Absent or mild atypia. Pigmentation usually limited to the lower third of the nail epithelium. Fine melanin granularity on Fontana-Masson stain.

• Nevus: Melanocyte hyperplasia with nest formation. Lentiginous pattern at the center of the lesion. Suprabasal pagetoid spread tendency, mild nuclear pleomorphism, and nail plate involvement may be present.

• Melanoma in situ: Increased melanocytes (39–136 per mm) with predominance of single melanocytes and a few nests. Nuclear atypia, pagetoid spreading, large hyperchromatic pleomorphic nuclei, prominent nucleoli, increased mitoses, and long branching dendrites. Dermal lymphoid infiltrate. Melanocytes in the nail plate corresponding to the matrix keratogenous zone is an important finding for malignancy.

• Invasive melanoma: Atypical melanocytes invading the dermis. Most common histogenic subtype is acral lentiginous, followed by nodular and desmoplastic.

Common mimics

• Splinter hemorrhages: Thin, dark red-brown lines caused by extravasated blood in nail bed capillaries; do not grow with the nail in a longitudinal band pattern
• Fungal melanonychia: Dark pigment produced by certain dermatophytes or molds; confirmed with KOH and culture
• Exogenous discoloration: Tobacco staining, tar, dyes, henna; usually affects multiple nails and can be partially scraped off
• Longitudinal erythronychia: Red (not brown/black) longitudinal band; different etiology

Must-not-miss diagnoses

• Subungual melanoma: A single, broad, irregularly pigmented band with Hutchinson sign, nail dystrophy, or rapid change; biopsy mandatory
• Pigmented onychomatricoma: Rare benign nail matrix tumor that may produce a pigmented band; diagnosed on biopsy

By number of digits

• Single digit: Lentigo, nevus, or melanoma — requires careful evaluation; biopsy if suspicious features present
• Multiple digits: Physiologic, drug-induced, systemic disease, or inflammatory cause — usually benign; address underlying condition

Frequently associated

• Darkly pigmented skin: Physiologic melanonychia in >70% of individuals with Fitzpatrick IV–VI
• Drug exposure: Chemotherapy (cyclophosphamide, doxorubicin, hydroxyurea), zidovudine, minocycline, antimalarials
• Pregnancy: Hormonally driven, reversible
• Nail trauma: Chronic friction, nail biting

Occasionally associated

• Psoriasis: Nail involvement with melanocytic activation
• Lichen planus: May cause longitudinal melanonychia along with other nail changes
• Onychomycosis: Fungal infection triggering melanocytic activation or producing fungal pigment
• Addison disease: Diffuse hyperpigmentation including nails
• Peutz-Jeghers syndrome: Mucocutaneous pigmentation including nail bands
• Laugier-Hunziker syndrome: Acquired benign macular hyperpigmentation of lips, oral mucosa, and nails

Clinical examination and history

Essential assessment:

• Number of involved nails: Single (higher suspicion) vs. multiple (usually benign)
• Color and width of band(s): Broad (>3 mm), variegated bands are more concerning
• Periungual pigmentation: True Hutchinson sign vs. pseudo-Hutchinson sign
• Nail plate integrity: Dystrophy, fissuring, or splitting raise suspicion
• Patient age: New melanonychia in an adult is more concerning than in a child
• Skin phototype: Physiologic melanonychia is expected in darker skin
• Drug and medical history: Identify drug-induced or systemic causes
• Onset and progression: Rapid change is concerning
• Family history: Melanoma or dysplastic nevus syndrome

ABCDEF mnemonic for melanoma suspicion

• A — Age: Peak incidence fifth to seventh decade; African-American, Asian, Native American ancestry
• B — Breadth: Band wider than 3 mm; Border: irregular or blurred
• C — Change: Rapid increase in size or growth rate
• D — Digit: Thumb > great toe > index finger > single digit > multiple digits
• E — Extension: Pigment extends to proximal or lateral nail fold (Hutchinson sign) or free edge
• F — Family history: Personal or family history of melanoma or dysplastic nevus syndrome

Dermoscopic examination (onychoscopy)

Dermoscopy of the nail plate (onychoscopy) helps differentiate benign from potentially malignant lesions:

• Melanocytic activation: Gray background with thin, gray, regular, parallel lines. In trauma-related activation, tiny dark red to brown spots (blood extravasation) may be present.
• Lentigo: Homogeneous longitudinal thin gray or brown lines on a gray or light brown background
• Nevus: Band of regular lines of light brown to black on a brown background (more common in children)
• Melanoma: Irregular longitudinal lines (in color, spacing, thickness, and parallelism); micro-Hutchinson sign (dermoscopic cuticle pigmentation not visible to the naked eye)

Important caveat: In children, nail matrix nevi may exhibit irregular dermoscopic features that mimic adult melanoma. A conservative approach with close follow-up is generally recommended.

When to biopsy

Biopsy not required: Stable LM involving multiple nails with a clear cause (physiologic, drug-related, systemic).

Low threshold for biopsy when:

• Single digit LM in an adult, especially on thumb, index finger, or great toe
• Band wider than 3 mm with variegated pigmentation or proximal widening (triangular shape)
• Pre-existing band becoming darker, wider, or developing blurred borders
• LM associated with nail plate fissuring, splitting, or dystrophy
• Pigment extending to the nail folds (Hutchinson sign)
• Rapidly enlarging band

Children: Nail melanoma is exceedingly rare in children. Most LM in children is due to nail matrix nevi. Biopsy is generally avoided unless the band enlarges or darkens rapidly or involves the entire nail.

Benign presentations

• Physiologic melanonychia in darkly pigmented skin — normal variant, no intervention needed
• Drug-induced melanonychia — reversible upon discontinuation in most cases
• Pregnancy-related melanonychia — resolves after delivery
• Stable childhood melanonychia — most commonly nevus; monitor clinically

Warning signs

• New single-digit band in an adult with lighter skin
• Band wider than 3 mm with irregular borders or color variegation
• Hutchinson sign (periungual pigmentation)
• Nail dystrophy accompanying the pigmented band
• Rapid change in width, color, or borders

Emergency presentations

• Nail melanoma confers a higher mortality rate compared with other cutaneous melanomas, often due to delayed diagnosis. Any suspicious nail pigmentation warrants prompt dermatologic evaluation and biopsy when indicated.

Professional documentation should include:

• Digit involved: Specify which finger or toe
• Band characteristics: Width (mm), color, border regularity, homogeneity
• Periungual pigmentation: Present or absent; true Hutchinson vs. pseudo-Hutchinson
• Nail plate changes: Dystrophy, fissuring, splitting, surface irregularity
• Dermoscopic findings: Line regularity, color pattern, micro-Hutchinson sign
• Photography: Essential for monitoring; include dermoscopic images when possible

Example: "Longitudinal melanonychia of the right thumbnail, 4 mm wide, dark brown with lighter brown borders, irregular line pattern on dermoscopy, no Hutchinson sign, no nail dystrophy. Biopsy recommended."

• Physiologic melanonychia: Reassurance; no treatment needed; document for future comparison
• Drug-induced: Review causative medication; band may fade after discontinuation (takes months as the nail grows out)
• Inflammatory/infectious: Treat the underlying condition (psoriasis, lichen planus, onychomycosis)
• Lentigo/nevus (confirmed on biopsy): Observation with periodic clinical and dermoscopic monitoring
• Suspicious for melanoma: Nail matrix biopsy with appropriate technique; if melanoma confirmed, wide surgical excision (melanoma in situ) or digit amputation (invasive melanoma)
• Atypical melanocytic hyperplasia: Adults — consider complete resection with margin control; children — clinical and dermoscopic surveillance may be reasonable given extreme rarity of pediatric nail melanoma
• Monitoring: Clinical and dermoscopic examination every 6 months for subtle lesions not requiring immediate biopsy; baseline photography essential

Key points for patient education:

• A pigmented band on the nail is very common and usually harmless, especially in individuals with darker skin
• If a new dark band appears on a single nail, especially in adulthood, it should be evaluated by a dermatologist
• Changes in width, color, or borders of an existing band should be reported promptly
• Nail melanoma is rare but treatable when caught early — early evaluation saves lives
• Photographing your nails periodically can help track changes over time
• Do not delay evaluation out of concern about the biopsy procedure — nail matrix biopsies are routine and generally well tolerated