Brown Discoloration on the Lateral Edges of Nails
The most common cause of brown discoloration along the sides of nails is distal and lateral subungual onychomycosis (DLSO), a fungal infection where dermatophytes invade the nail by penetrating the distal or lateral margins, causing thickening and discoloration. 1
Primary Infectious Causes
Fungal Infections (Most Common)
DLSO represents the most frequent presentation of dermatophyte nail infection, with the fungus specifically invading through the lateral or distal margins of the nail, leading to brown discoloration, thickening, and varying degrees of nail plate separation from the nail bed 1
Toenails are affected more commonly than fingernails, and the infection typically begins at the lateral edges before progressing 1
Nondermatophyte mould infections can also cause lateral brown discoloration, accounting for approximately 5% of cases in the UK and 20% in North America, particularly in older individuals with previously traumatized nails 1
Bacterial Infections
Pseudomonas aeruginosa infection typically produces greenish or grayish (not brown) discoloration, often confined to the lateral edge of the nail 2
Bacterial pigmentation can coexist with fungal infections and may require separate treatment 1
Non-Infectious Causes
Melanocytic Pigmentation
Longitudinal melanonychia presents as brown to black streaks due to melanin production by matrix melanocytes 2
Critical warning signs for malignancy include: width >5mm, Hutchinson's sign (pigment extending to nail fold), nail dystrophy, or bleeding mass 2
Benign melanonychia in children is very rare for malignant transformation, but subungual melanoma must be excluded in adults 2, 3
Trauma-Related
Subungual hematoma from repeated microtrauma (commonly on the medial aspect of the great toe) can cause brown discoloration, though typically oval-shaped rather than forming a neat lateral streak 2
Repetitive trauma leads to distal onycholysis with secondary colonization and discoloration 1
Exogenous Pigmentation
Occupational exposures (such as artichoke handling) can cause brown nail and skin pigmentation through enzymatic browning mechanisms 4
Exogenous pigmentations usually lack the streaky appearance or regular borders seen in endogenous causes 2
Diagnostic Approach
When evaluating lateral brown nail discoloration, prioritize these steps:
Examine for fungal infection characteristics: thickening, onycholysis, subungual hyperkeratosis, and involvement of multiple nails (especially toenails) 1
Obtain nail specimens for laboratory confirmation: cut the affected nail as far back as possible through the entire thickness, including any crumbly material from discolored areas 1
Laboratory testing should include: direct microscopy with calcofluor white (more sensitive than potassium hydroxide) and fungal culture, with results typically available in 2-6 weeks 1
Assess for melanoma risk factors: single nail involvement, width >5mm, irregular borders, Hutchinson's sign, age >50 years, or recent changes in appearance 2, 3
Differentiate from trauma: examine the nail bed after clipping; a normal nail bed with intact longitudinal epidermal ridges suggests trauma rather than infection 1
Common Pitfalls
Do not assume all lateral brown discoloration is benign fungal infection—subungual melanoma can present similarly and requires early recognition for favorable outcomes 2, 5
Bacterial infection (particularly Pseudomonas) produces green or black discoloration, not brown, helping differentiate from fungal causes 1
Candidal nail infections typically begin proximally with paronychia, not laterally, making them less likely for isolated lateral brown discoloration 1
Consider systemic conditions: psoriasis, lichen planus, and other inflammatory conditions can cause nail dystrophy with secondary discoloration 1