What are the causes of melanonychia?

Melanonychia Causes

Primary Etiologic Categories

Melanonychia results from either melanin deposition (endogenous) or non-melanin pigmentation (exogenous), with melanin-derived causes subdivided into melanocytic activation versus melanocytic hyperplasia. 1

Exogenous (Non-Melanin) Pigmentation

• External pigments from dyes, chemicals, or occupational exposures can stain the nail plate brown-black 1
• Blood/hemorrhage beneath the nail plate creates dark discoloration and should be ruled out first in any pigmented band 2
• Bacterial infections, particularly Pseudomonas aeruginosa (Green Nail Syndrome), produce green-black discoloration 3, 4
• Fungal infections cause thickening, discoloration, and friable texture, though typically yellow-brown rather than true melanonychia 5, 3

Melanocytic Activation (Normal Melanocyte Number, Increased Melanin Production)

Physiologic Causes

• Ethnic/racial melanonychia is the most common cause in dark-skinned individuals, often involving multiple nails 6, 7
• Pregnancy induces melanocytic activation through hormonal changes 6

Local Causes

• Chronic trauma from repetitive friction or pressure stimulates melanin production 6, 2
• Infections of the nail matrix can trigger reactive melanocytic activation 6

Systemic and Endocrine Disorders

• Endocrine disorders including Addison's disease and hyperthyroidism cause diffuse melanonychia 6, 7
• Hemochromatosis produces nail pigmentation through iron deposition and melanin stimulation 6
• Porphyria causes photosensitivity-related melanocytic activation 6
• Alkaptonuria leads to ochronotic pigmentation 6

Dermatologic Conditions

• Lichen planus produces nail thinning with longitudinal ridging and may include pigmentation 5, 4
• Psoriasis can present with nail dystrophy and occasional pigmentation 3, 8

Iatrogenic and Drug-Induced

• Chemotherapeutic agents including taxanes (docetaxel, paclitaxel), capecitabine, etoposide, cytarabine, cyclophosphamide, and doxorubicin cause dose-dependent nail pigmentation 4
• Targeted therapies such as mTOR inhibitors, EGFR inhibitors, and MEK inhibitors produce nail alterations including melanonychia 4
• Antimalarials, tetracyclines, and zidovudine are well-documented causes of drug-induced melanonychia 6, 7

Syndromic Causes

• Peutz-Jeghers syndrome presents with characteristic perioral pigmentation and longitudinal melanonychia 6, 7
• Laugier-Hunziker syndrome manifests as mucosal and nail pigmentation without systemic involvement 6, 7
• Graft-versus-host disease can trigger melanocytic activation in transplant recipients 6

Melanocytic Hyperplasia (Increased Melanocyte Number)

Benign Proliferations

• Nail matrix nevus represents a benign melanocytic proliferation, more common in children and often stable over time 1, 6
• Nail matrix lentigo is a benign melanocytic lesion with increased melanocytes along the basal layer 1, 6

Malignant Proliferation

• Nail unit melanoma (NUM) or subungual melanoma is the most critical diagnosis to exclude, presenting as longitudinal melanonychia with specific warning features 1, 9, 6

Critical Diagnostic Red Flags for Malignancy

Nail apparatus melanoma should be suspected when melanonychia demonstrates abrupt onset after middle age, rapid growth, bandwidth >3 mm, irregular borders, pigment variegation, proximal widening, or Hutchinson sign (periungual pigment spread). 6, 2

• Age matters: Melanoma occurs almost exclusively in adults, while pediatric melanonychia is benign in the vast majority of cases 1, 2
• Single digit involvement (especially thumb or great toe) raises suspicion compared to multiple nail involvement 6, 2
• Personal or family history of melanoma significantly increases risk 6
• Associated nail plate dystrophy with elevation or destruction suggests invasive disease 6
• Hutchinson sign (pigmentation extending onto proximal or lateral nail folds) is highly concerning for melanoma 6, 7

Common Diagnostic Pitfalls

• Do not diagnose based on appearance alone—approximately 50% of dystrophic nails are non-fungal despite similar clinical presentation, and benign pediatric melanonychia can mimic adult melanoma 3, 8, 1
• Hemorrhage must be excluded first in any pigmented band, as subungual hematoma is a common melanonychia mimic 2
• Avoid unnecessary biopsies in children—benign pediatric melanonychia may exhibit clinical features resembling adult melanoma, warranting conservative management with close follow-up rather than immediate biopsy 1
• Onychoscopy of the nail plate and free edge significantly improves diagnostic accuracy and should precede biopsy decisions 1, 2
• Nail matrix excisional biopsy remains the gold standard for definitive diagnosis when melanoma cannot be excluded clinically 1