What are the causes of brown discoloration along the lateral edges of the fingernails?

Brown Discoloration on the Lateral Edges of Nails

Brown coloring on the side of nails is most commonly caused by distal and lateral subungual onychomycosis (DLSO), a fungal infection that invades through the lateral or distal margins of the nail, causing thickening and discoloration. 1

Primary Causes to Consider

Fungal Infections (Most Common)

Distal and Lateral Subungual Onychomycosis (DLSO) is the predominant cause of brown discoloration along nail edges. 1

• The fungus penetrates the nail bed through the lateral or distal margins, causing the affected nail to become thickened and discolored 1
• Toenails are more commonly affected than fingernails 1
• The causative organism is typically Trichophyton rubrum 1
• The nail plate shows varying degrees of onycholysis (separation from the nail bed) 1

Bacterial Pigmentation

Pseudomonas aeruginosa or Proteus species can cause lateral edge discoloration, though typically with a greenish or grayish hue rather than pure brown. 1, 2

• The discoloration is often confined specifically to the lateral edge of the nail 2
• This represents bacterial colonization rather than true infection in many cases 1

Melanocytic Causes (Critical to Rule Out)

Longitudinal melanonychia presents as brown to black streaks and requires careful evaluation to exclude malignancy. 2, 3, 4

• Subungual melanoma should be suspected if there is abrupt onset after middle age, rapid growth, bandwidth >3 mm, irregular borders, or Hutchinson's sign (periungual pigment spread) 3
• Benign causes include ethnic melanonychia (in darker-skinned individuals), trauma, medications, or melanocytic nevi 3, 4
• Red flags for malignancy: single digit involvement, proximal widening of the band, nail dystrophy, personal/family history of melanoma 3

Trauma-Related Discoloration

Subungual hematoma from single heavy trauma or repeated microtrauma can cause brown discoloration. 2

• Commonly found on the medial aspect of the great toe from repetitive microtrauma 2
• Usually oval in shape but does not form a neat longitudinal streak 2

Exogenous Pigmentation

Contact exposure to elemental iron can cause orange-brown chromonychia on toenails. 5

• Typically occurs from exposure to iron-rich water sources 5
• Resolves over 6 weeks once exposure is eliminated 5

Diagnostic Approach

Clinical Evaluation

Examine for features distinguishing fungal from other causes:

• Fungal infections cause soft, friable nail surfaces with subungual hyperkeratosis 1
• Bacterial pigmentation has greenish/grayish hue confined to lateral edges 2
• Melanonychia presents as longitudinal streaks with defined borders 3, 4
• Trauma-related discoloration is typically oval and may have history of injury 2

Laboratory Confirmation

For suspected fungal infection, obtain nail specimens for microscopy and culture before initiating treatment. 1

• Material should be taken from discolored, dystrophic, or brittle parts 1
• Cut the affected nail as far back as possible through entire thickness 1
• Calcofluor white staining enhances microscopic detection 1

When to Biopsy

Nail matrix biopsy is indicated when melanoma cannot be excluded clinically. 2, 3

• Perform biopsy for: abrupt onset after age 50, bandwidth >3-5 mm, irregular borders, rapid growth, Hutchinson's sign, or single digit involvement 2, 3
• In children, melanonychia is almost always benign; a wait-and-see approach is appropriate 3

Common Pitfalls

• Do not assume all lateral nail discoloration is fungal - bacterial colonization and melanoma must be excluded 1, 2
• Do not ignore melanonychia in a single digit - this increases suspicion for melanoma compared to multiple digit involvement 3
• Do not treat empirically without confirmation - obtain microscopy and culture to confirm fungal infection before starting systemic antifungals 1
• Do not dismiss pigmentation in darker-skinned individuals as automatically benign - while ethnic melanonychia is common, melanoma still occurs and requires the same vigilance 3