What could be the cause of dark long lines in two fingernails bilaterally?

Dark Long Lines in Two Fingernails Bilaterally

This presentation of bilateral longitudinal pigmented bands (longitudinal melanonychia) requires urgent evaluation to exclude subungual melanoma, particularly if the patient is middle-aged or older, with dermoscopy and possible biopsy being essential next steps.

Immediate Diagnostic Priorities

The bilateral symmetric presentation suggests either a benign systemic/physiologic cause or, less commonly, synchronous malignancies. The most critical task is distinguishing benign melanonychia from subungual melanoma, as early diagnosis dramatically impacts mortality. 1, 2

High-Risk Features Requiring Urgent Dermatology Referral and Biopsy

• Abrupt onset after age 50 with no prior history of nail pigmentation 2
• Bandwidth >3 mm in any affected nail 2
• Hutchinson sign: pigment spreading onto the cuticle, proximal nail fold, or lateral nail folds 1, 2
• Irregular borders that are blurred or poorly defined 2
• Variegated pigmentation with multiple shades of brown, black, or gray within the band 2
• Rapid growth or darkening of pre-existing bands 2
• Associated nail plate dystrophy, elevation, or splitting 2
• Personal or family history of melanoma 2

Benign Features Suggesting Observation May Be Appropriate

• Age <20 years: melanonychia in children is almost always benign 2
• Dark skin ethnicity: ethnic melanonychia is the most common cause in individuals with Fitzpatrick skin types IV-VI 2
• Multiple digits affected bilaterally with symmetric appearance 2
• Narrow bands <3 mm with regular, sharp borders 2
• Stable appearance over months to years without change 2

Systematic Diagnostic Approach

Step 1: Detailed Clinical Assessment

• Document exact characteristics: measure bandwidth in millimeters, photograph all affected nails, note color variation (uniform tan/brown vs. variegated black/gray) 1, 2
• Examine all 20 nail units: multiple nail involvement suggests systemic/benign cause rather than melanoma 3, 2
• Assess for Hutchinson sign: carefully inspect cuticles and nail folds under magnification 1, 2
• Check for trauma history: chronic friction or trauma can cause melanocytic activation 2

Step 2: Dermoscopic Examination

Dermoscopy is essential before deciding on biopsy, as it can reveal patterns suggestive of malignancy versus benign causes. 1 While specific dermoscopic criteria are not detailed in the provided guidelines, this examination should be performed by a dermatologist experienced in nail dermoscopy.

Step 3: Rule Out Non-Melanocytic Causes

Fungal infection can occasionally present with dark discoloration, though typically with additional features:

• Nail thickening, friability, and onycholysis are characteristic 4
• Obtain KOH preparation and fungal culture if texture changes are present 5
• Note: 50% of dystrophic nails are non-fungal despite similar appearance 5, 6

Bacterial infection (Green Nail Syndrome from Pseudomonas):

• Produces green-black discoloration, not true longitudinal bands 5, 6
• Associated with moisture exposure and paronychia 4

Medication-induced melanonychia:

• Common culprits include chemotherapy agents, antimalarials, AZT, and minocycline 2
• Review complete medication list including supplements 2

Systemic conditions:

• Endocrine disorders (Addison's disease, hyperthyroidism) 2
• Nutritional deficiencies (vitamin B12, folate) 2
• Peutz-Jeghers syndrome, Laugier-Hunziker syndrome 2

Step 4: Biopsy Decision Algorithm

Immediate biopsy indicated if:

• Any high-risk feature present (see above) 1, 2
• Age >50 with new-onset melanonychia 2
• Single digit involvement with irregular features 2
• Patient anxiety warranting definitive diagnosis 1

Biopsy technique (if performed):

• Complete nail plate elevation to visualize the matrix lesion precisely 1
• Longitudinal excisional biopsy for lesions ≤3 mm (minimal scarring) 1
• Incisional or tangential "shave" biopsy for wider lesions 1
• Critical pitfall: inadequate biopsy sampling can miss melanoma; ensure matrix tissue is obtained 1

Observation with close follow-up if:

• Child or adolescent with benign features 2
• Dark-skinned individual with multiple symmetric bands 2
• Clear benign etiology identified (trauma, medication) 2
• Follow-up protocol: photograph and measure every 3-6 months; biopsy if any change occurs 2

Management Based on Diagnosis

If Melanoma Confirmed

Immediate complete excision of the entire nail unit with appropriate margins is required; this is beyond the scope of this discussion but demands urgent surgical oncology referral. 1

If Benign Melanocytic Lesion (Nevus/Lentigo)

• Continued surveillance with photography every 6-12 months 2
• Patient education on warning signs requiring urgent return 2
• Consider prophylactic excision if patient preference or ongoing anxiety 1

If Ethnic/Physiologic Melanonychia

• Reassurance with explanation of benign nature 2
• Baseline photography for future comparison 2
• Annual examination sufficient unless changes develop 2

Critical Pitfalls to Avoid

• Never assume bilateral presentation excludes melanoma: while uncommon, synchronous melanomas can occur 2
• Do not delay biopsy in adults with new-onset melanonychia: the "wait-and-see" approach is appropriate for children but risky in adults over 50 2
• Avoid inadequate biopsy technique: superficial sampling or nail plate biopsy alone will miss matrix pathology 1
• Do not overlook subtle Hutchinson sign: use magnification and good lighting to detect early periungual pigment spread 1, 2
• Never diagnose fungal infection based on appearance alone: 50% of dystrophic nails are non-fungal; always confirm with KOH and culture before treating 5, 6