What are the possible causes of a dark linear streak (melanonychia) on a fingernail or toenail and when does it require urgent evaluation?

Dark Line on Nail: Evaluation and Management

Immediate Priority: Rule Out Melanoma

A dark linear streak on a nail requires urgent evaluation to exclude nail unit melanoma, particularly in adults with a single affected digit, irregular pigmentation, or involvement of the thumb or great toe. 1

Diagnostic Algorithm

Step 1: Assess for Hemorrhage First

• Subungual hemorrhage is a common mimic of melanonychia and should be excluded initially through clinical examination and dermoscopy 2
• Hemorrhage typically appears as non-continuous pigmentation, while true melanonychia presents as continuous longitudinal streaks 3

Step 2: Age-Based Risk Stratification

• Adults: Nail unit melanoma occurs almost exclusively in adults and most commonly affects the thumb or great toe, requiring high suspicion for any new pigmented band 2, 1
• Children: Benign longitudinal melanonychia in pediatric patients may exhibit clinical features resembling adult melanoma, but a conservative approach with close follow-up is recommended rather than immediate biopsy 1

Step 3: Number of Nails Involved

• Single nail involvement: Higher suspicion for melanoma, particularly if irregular or expanding 2
• Multiple nails: More likely benign causes including physiologic pigmentation, systemic conditions, or drug-induced changes 3

Step 4: Examine the Free Edge and Periungual Tissue

• Hutchinson's sign (pigment extending onto the periungual skin) is highly suspicious for melanoma and mandates urgent biopsy 1
• Examine the free edge dermoscopically for specific pigmentation patterns 2

Step 5: Evaluate Color Characteristics

• Brown pigmentation typically indicates melanin-derived melanonychia 1
• Green or black discoloration may indicate bacterial infection (Pseudomonas aeruginosa) 4, 5
• Orange discoloration with nail ridging suggests Candida infection 5

Step 6: Assess Band Width and Regularity

• Bands >3mm wide, irregular borders, or rapidly expanding lesions require urgent evaluation for melanoma 2, 1
• Regular, narrow bands (<3mm) are more consistent with benign melanocytic activation 2

Common Benign Causes

Fungal Infections

• Fungal melanonychia can present as a solitary black linear streak, though this is rare 6
• Candida-associated longitudinal melanonychia has been reported, typically with associated paronychia (swollen, erythematous nail folds) and history of moisture exposure 5, 6
• Dermatophyte infections more commonly cause nail thickening, yellow-brown discoloration, and friable texture rather than true linear melanonychia 7, 4

Bacterial Infections

• Green Nail Syndrome (Pseudomonas aeruginosa) presents with green-black discoloration 4, 5
• Treatment includes keeping the area dry and applying topical povidone iodine 2% twice daily 4, 5

Inflammatory Conditions

• Lichen planus causes nail thinning, longitudinal ridging, and occasional pigmentation 4, 5
• Psoriasis may produce nail changes with associated pitting and onycholysis 5

Diagnostic Testing

Initial Evaluation

• Onychoscopy (nail dermoscopy) is a useful initial diagnostic tool that can help differentiate benign from malignant causes and aid in biopsy planning 1
• Histopathologic examination of nail clippings can provide diagnostic information without requiring matrix biopsy 1

When Fungal Infection is Suspected

• Obtain mycological confirmation before initiating treatment, as 50% of dystrophic nails are non-fungal despite similar appearance 4, 5
• Potassium hydroxide (KOH) preparation with microscopy is first-line screening 4
• Fungal culture on Sabouraud's glucose agar identifies specific pathogen species 4
• Collect specimens from the most proximal part of the infection using adequate material 5

When Melanoma Cannot be Excluded

• Nail matrix tangential excisional biopsy is the gold standard for diagnosing or ruling out nail unit melanoma 1
• For suspicious lesions, tangential excision is recommended; for high-likelihood invasive melanoma, longitudinal excision provides information on tumor extension 1

Treatment Based on Confirmed Etiology

Fungal Melanonychia (Candida-associated)

• Topical treatment: 5% amorolfine hydrochloride nail lacquer or modified Castellani paint with 1% clotrimazole cream 6
• Oral itraconazole as monotherapy or combined with topical amorolfine for more extensive involvement 6
• Keep affected area dry and use protective gloves during wet work 5

Bacterial Infection

• Keep area dry and apply topical povidone iodine 2% twice daily 4, 5

Critical Red Flags Requiring Urgent Referral

• Single digit involvement in an adult, especially thumb or great toe 2, 1
• Hutchinson's sign (periungual pigment spread) 1
• Band width >3mm or rapidly expanding 2
• Irregular borders or color variegation 2
• New onset in adult without clear benign cause 1

Common Pitfalls to Avoid

• Never assume fungal infection based on appearance alone—laboratory confirmation is essential as 50% of dystrophic nails are non-fungal 4, 5
• Do not delay evaluation of suspicious pigmented bands in adults—early melanoma has subtle features that can be missed 1
• Avoid over-aggressive biopsy in children—benign pediatric melanonychia can mimic adult melanoma; close follow-up is preferred unless highly suspicious features present 1
• Do not overlook bacterial superinfection—Candida nail infections frequently have concurrent bacterial involvement contributing to dark discoloration 5