What is the cause of a longitudinal line across the nail bed that is not dark and causes the nail to break when it grows long?

Longitudinal Nail Ridge with Splitting: Causes and Management

A non-dark longitudinal line across the nail that causes splitting when the nail grows long is most commonly caused by age-related changes in the nail matrix, though inflammatory conditions like lichen planus or psoriasis, chronic trauma, or fungal infection should be considered based on associated clinical features. 1

Primary Differential Diagnosis

Age-Related Nail Changes (Most Common)

• Vertical ridges running from cuticle to nail tip are often a normal sign of aging as the nail matrix's ability to produce smooth nails diminishes 1
• Brittle nails are characterized by longitudinal ridging with splitting or brittleness of the nail plate, which may indicate systemic conditions, medication side effects, or nail matrix damage 1
• The nail becomes friable and prone to breaking at the distal free edge when grown long 1

Inflammatory Dermatologic Conditions

• Lichen planus produces thinning of the nail plate with subungual hyperkeratosis and longitudinal ridging 1
• Psoriasis can cause nail thinning and longitudinal ridging, particularly when affecting the nail matrix 1
• These conditions typically affect multiple nails and present with additional findings like pitting, onycholysis, or subungual hyperkeratosis 1

Fungal Infection (Must Rule Out)

• Fungal infections present with thickening, discoloration, and friable texture of nails 1
• The key distinguishing feature is that the nail surface becomes soft and friable with yellow-brown discoloration in fungal infections, unlike the hard, brittle texture of non-infectious causes 2
• Subungual hyperkeratosis (debris under nail) is typically prominent in fungal infections 2
• Toenails are predominantly affected (80% of dermatophyte infections) 2

Diagnostic Algorithm

Clinical Examination Features to Assess

• Examine for soft, friable texture suggesting fungal infection versus hard, brittle texture of non-infectious causes 1
• Assess for paronychia (nail fold swelling/erythema) which suggests Candida infection, particularly with occupational moisture exposure 1
• Look for associated nail changes such as pitting, onycholysis, subungual hyperkeratosis, or discoloration that would suggest inflammatory disease rather than simple aging 1
• Evaluate nail color: yellow-brown discoloration indicates fungal infection; normal color suggests age-related or inflammatory causes 2

Laboratory Confirmation When Indicated

• Direct microscopy with potassium hydroxide (KOH) preparation and fungal culture on Sabouraud's glucose agar are essential for confirming diagnosis of fungal infection 3
• Calcofluor white staining enhances visualization of fungal elements 3
• Do not assume fungal infection without laboratory confirmation—50% of nail dystrophy cases are non-fungal despite similar clinical presentation 3

Management Approach

For Age-Related Brittle Nails with Ridging

• Apply daily topical emollients to periungual folds, nail matrix, and nail plate to maintain hydration 1
• Use protective nail lacquers to limit water loss from the nail plate 1
• File nail surfaces gently with an emery board after softening in warm water 1
• Avoid trauma, nail biting, use of nails as tools, prolonged water soaking, and exposure to harsh chemicals 1

For Inflammatory Causes (Psoriasis/Lichen Planus)

• For inflammatory causes affecting less than 3 nails, intralesional triamcinolone acetonide 5-10 mg/cc is recommended for nail matrix involvement 1
• Topical steroids with or without vitamin D analogs for nail bed involvement 1
• Daily application of urea-based keratolytic cream to reduce nail thickness if needed 1

For Confirmed Fungal Infection

• Treat confirmed fungal infections with systemic antifungal agents, such as terbinafine or itraconazole, only after mycological confirmation 1
• Terbinafine 250 mg daily for 6 weeks (fingernails) or 12 weeks (toenails) with 70% mycological cure rate 2
• Itraconazole pulse therapy: 5 mg/kg/day for 1 week per month for 2 pulses (fingernails) or 3 pulses (toenails) 2

Critical Pitfalls to Avoid

• Do not assume fungal infection without laboratory confirmation—the nail surface in non-infectious conditions typically doesn't become soft and friable as in fungal infection 1
• Neglecting occupational counseling can be a pitfall; patients with chronic moisture exposure need guidance on protective measures like wearing gloves while cleaning 1
• For patients over 50 years with a single nail affected, consider biopsy to exclude squamous cell carcinoma, particularly if there is a proliferating mass or significant nail disruption 4, 5