What does a vertical black line on the nail indicate?

Vertical Black Line on Nail: Clinical Significance and Management

A vertical black line on the nail (longitudinal melanonychia) requires immediate clinical evaluation to exclude subungual melanoma, particularly in adults over 40 years with new-onset pigmentation, though most cases are benign. 1, 2

Differential Diagnosis

Benign Causes (Most Common)

• Ethnic/physiologic melanonychia: Most common in dark-skinned individuals due to melanocytic activation without hyperplasia 2
• Nail matrix nevus or lentigo: Benign melanocytic hyperplasia 1
• Trauma-induced: Chronic repetitive trauma causing melanocyte activation 2
• Drug-induced: Various medications can trigger melanin production 1, 2
• Systemic conditions: Endocrine disorders, Peutz-Jeghers syndrome, Laugier-Hunziker syndrome 2

Malignant Cause (Critical to Exclude)

• Subungual melanoma: Presents as longitudinal melanonychia in two-thirds of cases, carries higher mortality than other cutaneous melanomas due to delayed diagnosis 1, 3

Other Causes to Consider

• Bacterial infection: Pseudomonas can cause green or black discoloration 4, 5
• Fungal infection: Can present with discoloration, though typically with thickening and friable texture 4, 5
• Squamous cell carcinoma in situ: Longitudinal melanonychia has been reported as a presenting feature 4

Red Flags Suggesting Melanoma (Require Urgent Biopsy)

High-risk features mandating nail matrix biopsy include: 1, 2, 3

• Age: Abrupt onset after age 40-50 years
• Band width: >3 mm width
• Hutchinson sign: Periungual pigment spread onto cuticle, proximal or lateral nail folds
• Progression: Rapid growth or darkening of existing band
• Morphology:
  • Blurry or irregular lateral borders
  • Pigment variegation (multiple colors)
  • Proximal widening of the band
  • Associated nail plate dystrophy
• Distribution: Single digit involvement (vs. multiple digits)
• History: Personal or family history of melanoma

Diagnostic Approach

Initial Evaluation

• Clinical examination: Assess all features listed above 1, 2
• Dermoscopy (onychoscopy): Useful initial diagnostic tool, though not definitive; helps with biopsy planning and patient triage 1, 6
• Nail clipping examination: Histopathologic examination can be useful initial tool before proceeding to matrix biopsy 1

When to Biopsy

Perform nail matrix biopsy if ANY red flag features are present. 1, 7, 3

In children: Conservative approach with close follow-up is recommended, as benign pediatric longitudinal melanonychia may exhibit features resembling adult melanoma 1

In adults with suspicious features: Do not delay biopsy, as early diagnosis is critical for survival 3

Biopsy Technique

Nail matrix excisional biopsy is the gold standard for diagnosis. 1, 7

Technique Selection Based on Clinical Suspicion:

• For suspicious lesions (moderate concern): Tangential excisional ("shave") biopsy of nail matrix is recommended 1, 7
• For high-likelihood invasive melanoma: Longitudinal excision provides information on tumor extension 1, 7
• For lesions ≤3 mm: Longitudinal resection biopsy can be performed without significant nail dystrophy 7
• For extensive lesions: Incision or tangential biopsy acceptable without impairing prognosis 7
• For presentations strongly suggestive of melanoma: Immediate complete resection of entire nail unit may be proposed 7

Complete elevation of the nail plate enables precise lesion localization. 7

Management Based on Diagnosis

If Melanoma Confirmed

• Refer immediately to surgical oncology/dermatologic surgery
• Management typically involves local excision, Mohs micrographic surgery, or distal phalanx amputation depending on extent 4

If Benign Etiology Confirmed

• Observation with prolonged follow-up is mandatory for early detection of possible malignant changes 2
• Treat underlying cause if identified (discontinue offending medication, manage systemic condition, etc.)

If Bacterial Infection (Green/Black Nail Syndrome)

• Keep affected area dry 5
• Apply topical povidone iodine 2% twice daily 8, 5

If Fungal Infection Confirmed

• Confirm diagnosis with KOH preparation and fungal culture before treatment 4, 5
• Treat with appropriate antifungal agents per guidelines 4, 5

Critical Pitfalls to Avoid

• Never assume benign etiology based on appearance alone - melanoma can mimic benign conditions 1, 2
• Do not delay biopsy in adults with new-onset pigmentation - delayed diagnosis significantly worsens melanoma prognosis 3
• Avoid "watch and wait" in adults with red flag features - this approach is only appropriate for children or adults with clearly benign features 1, 2
• Do not perform inadequate biopsy - ensure sufficient tissue for definitive histopathologic diagnosis 1, 7