Causes of Lines on Toenails
A line on a toenail can result from fungal infection (particularly superficial white onychomycosis presenting as linear bands), trauma (causing Beau lines or longitudinal ridges), melanonychia (pigmented bands requiring melanoma exclusion), or underlying systemic disease—with fungal infection being the most common cause in toenails.
Primary Diagnostic Categories
Fungal Infections (Most Common in Toenails)
Superficial white onychomycosis (SWO) specifically presents as linear bands emerging from the proximal nail fold 1. These superficial infections can show deep penetration and won't respond well to topical therapy. This pattern is particularly common in children and typically caused by T. interdigitale 1.
Other fungal patterns include:
- Proximal subungual onychomycosis (PSO): Originates at the proximal nail fold with linear progression distally. In immunocompromised patients (especially HIV/AIDS), this can be a marker of immune dysfunction 1
- Distal-lateral subungual onychomycosis (DLSO): Most common overall fungal pattern, though typically presents with thickening and discoloration rather than distinct lines 1
Trauma-Related Lines
Beau lines are transverse grooves caused by temporary cessation of nail matrix activity 2, 3. These mark the timing of severe illness, major surgery, infection, or significant trauma. The line moves distally as the nail grows, providing a timeline of the insult.
Longitudinal ridges result from chronic repetitive trauma and are dystrophic changes 3. These are localized conditions without systemic significance.
Melanonychia (Pigmented Lines)
Longitudinal brown-black bands arise from melanocyte activation or proliferation in the nail matrix 4. This is critical because:
- Melanoma presents as longitudinal pigmented bands in approximately 70% of cases 4
- In dark-skinned individuals, subungual melanomas account for 50% of all melanomas 3
- Any new, changing, or irregular pigmented band requires dermoscopy and consideration for biopsy 4
Systemic Disease Markers
Lines can indicate underlying conditions:
- Muehrcke lines (transverse white bands): Associated with hypoalbuminemia 3
- Multiple Beau lines: Suggest recurrent severe systemic illness 2
Critical Diagnostic Approach
- Determine line orientation: Longitudinal vs. transverse
- Assess color: White/opaque, pigmented (brown/black), or normal nail color
- Evaluate texture: Superficial crumbling (fungal), groove (Beau line), or smooth ridge
- Location: Proximal origin suggests matrix involvement; distal suggests nail bed pathology
High-Risk Features Requiring Urgent Evaluation
- Pigmented bands with irregular borders, color variation, or rapid change (melanoma concern) 4
- Proximal white bands in immunocompromised patients (PSO as HIV marker) 1
- Linear bands with nail destruction (deep fungal infection requiring systemic therapy) 1
Common Pitfalls
The British Association of Dermatologists guidelines emphasize that linear fungal infections emerging from the proximal nail fold are superficial but can show deep penetration—these will not respond to topical therapy alone 1. This is frequently missed, leading to treatment failure.
Laboratory confirmation is essential before treating presumed fungal infections, as many non-infectious conditions mimic onychomycosis including psoriasis, lichen planus, chronic trauma, and malignancy 1. The nail bed examination after clipping helps differentiate trauma (intact longitudinal ridges) from infection 1.
For melanonychia in any patient, dermoscopy should be performed to triage between observation and biopsy 4. The threshold for biopsy should be lower in adults with new pigmented bands and in any patient with concerning dermoscopic features.