What are the possible causes and recommended evaluation for new darkening of the nail beds?

Darkening of the Nail Beds: Evaluation and Management

New darkening of the nail beds requires urgent evaluation to exclude subungual melanoma, particularly when presenting as a longitudinal pigmented band in a single digit, with features such as width >3 mm, irregular borders, or periungual pigmentation (Hutchinson sign). 1, 2, 3

Immediate Red Flags Requiring Biopsy

The following features mandate urgent nail matrix biopsy to rule out melanoma:

• Hutchinson sign: Pigmentation extending onto the cuticle, proximal nail fold, or lateral nail folds 3
• Single digit involvement in adults, especially after age 40 3
• Band width >3 mm with irregular borders or color variegation 3
• Proximal widening of the pigmented band (broader proximally than distally suggests proliferative process) 4, 3
• Rapid growth or darkening of a pre-existing band 3
• Associated nail dystrophy, ulceration, bleeding, or nodule formation 1, 3
• Personal or family history of melanoma 3

When suspicious nail lesions require biopsy, the nail matrix must be sampled by a practitioner skilled in nail apparatus biopsy, as melanoma arises in the nail matrix and improper technique can cause permanent nail dystrophy. 1, 2

Differential Diagnosis by Clinical Pattern

Longitudinal Melanonychia (Brown/Black Bands)

Benign causes:

• Ethnic melanonychia: Most common in dark-skinned individuals, typically involves multiple nails 3
• Medication-induced: Chemotherapy agents (taxanes, capecitabine, doxorubicin), antimalarials, minocycline, zidovudine, hydroxyurea 5, 3, 6
• Trauma: Chronic friction or injury to nail matrix 3
• Inflammatory conditions: Lichen planus (causes nail thinning with longitudinal ridging), psoriasis 1, 5, 7
• Systemic conditions: Pregnancy, endocrine disorders, Addison's disease, Peutz-Jeghers syndrome, Laugier-Hunziker syndrome 3

Malignant cause:

• Subungual melanoma: Presents as longitudinal melanonychia, often with irregular features described above 1, 2, 3

Diffuse Nail Darkening

Green-black discoloration:

• Pseudomonas infection (Green Nail Syndrome): Keep area dry and apply topical povidone-iodine 2% twice daily 5, 7
• Bacterial superinfection: May coexist with fungal or Candida infection 5

Yellow-brown with thickening:

• Fungal infection (onychomycosis): Characterized by thickening, yellow-brown discoloration, friable texture, and subungual hyperkeratosis 5, 7, 8

Blue discoloration:

• Single nail: Consider glomus tumor, blue nevus, or melanoma 6
• Multiple nails: Consider medications (minocycline, zidovudine), silver exposure, HIV/AIDS, systemic lupus erythematosus 6

Diagnostic Algorithm

Step 1: Assess for Melanoma Risk Features

• Examine for Hutchinson sign, band width, color variegation, single vs. multiple digit involvement 3
• If ANY melanoma red flags present → urgent referral for nail matrix biopsy 1, 2

Step 2: Evaluate Infectious Causes

For suspected fungal infection (thickening, yellow-brown color, friable texture):

• Obtain mycological confirmation BEFORE treatment: 50% of dystrophic nails are non-fungal despite similar appearance 5, 7, 8
• Collect subungual debris from proximal infection site using dental scraper 5
• Perform KOH preparation with microscopy AND fungal culture on Sabouraud's glucose agar 5, 7, 8
• Consider calcofluor white staining to enhance visualization 7, 8

For suspected Candida (proximal involvement with paronychia, occupational moisture exposure):

• Candida begins proximally with nail fold swelling/erythema, unlike dermatophytes which start distally 5, 7
• More common in fingernails with wet occupations 5
• Keep area dry, use protective gloves, apply topical antifungals 5

For suspected bacterial infection (green-black color):

• Keep area dry, apply topical povidone-iodine 2% twice daily 5, 7

Step 3: Consider Inflammatory and Systemic Causes

Examine for associated features:

• Nail pitting, oil drop sign: Suggests psoriasis 8
• Nail thinning with longitudinal ridging: Suggests lichen planus 1, 5, 7
• Multiple nails with occupational exposure: Consider chronic paronychia or contact dermatitis 5

Screen for systemic conditions when appropriate:

• Thyroid function tests and complete blood count for brittle nails with darkening 5
• Review medication list for chemotherapy agents, antimalarials, minocycline 5, 3, 6

Step 4: Pediatric Considerations

• In children, melanonychia is usually benign, especially when involving multiple digits 7, 3
• Wait-and-see approach is acceptable for benign-appearing bands in children 3
• However, examine family members for fungal infections if onychomycosis suspected, as household transmission is common 7

Critical Pitfalls to Avoid

• Never assume fungal infection without laboratory confirmation: Clinical appearance alone is unreliable, and 50% of dystrophic nails are non-fungal 5, 7, 8
• Never delay biopsy of suspicious melanonychia: Subungual melanoma has poor prognosis due to delayed diagnosis and increased risk of nodal disease 2
• Do not assume subungual hemorrhage excludes malignancy: Bleeding does not rule out melanoma 4
• Do not assume fungal infection excludes melanoma: Both can coexist 4
• Inadequate specimen collection leads to false negatives: Scrape from proximal subungual debris, not superficial nail plate 5
• Improper biopsy technique causes permanent nail dystrophy: Refer to experienced practitioner for nail matrix sampling 1, 2