What is the significance of a black line on the nail that resolves without treatment, leaving a crack?

Black Line on Nail That Resolved Leaving a Crack: Significance and Management

A black line on the nail that resolves without treatment and leaves a crack is most likely a subungual hematoma (blood under the nail) caused by trauma, which typically resolves as the nail grows out. This condition is generally benign and self-limiting, requiring no specific treatment in most cases.

Differential Diagnosis

When evaluating a black line on the nail, several conditions should be considered:

1. Subungual hematoma

   • Caused by trauma (single heavy trauma or repeated microtrauma)
   • Often does not form a neat streak 1
   • Resolves as the nail grows out
   • May leave a crack as the damaged nail grows out

2. Melanonychia striata (longitudinal melanonychia)

   • Tan, brown, or black longitudinal streak within the nail plate 2
   • Runs from proximal nail fold to distal part of nail plate
   • Usually doesn't resolve spontaneously
   • Doesn't typically leave a crack

3. Fungal melanonychia

   • Rare cause of nail pigmentation
   • Can mimic longitudinal melanonychia 3
   • Usually requires antifungal treatment to resolve
   • Confirmed by histopathology or mycology

4. Subungual melanoma

   • Most serious condition presenting with melanonychia
   • Warning signs include: abrupt onset after middle age, rapid growth, darkening, pigment variegation, blurry borders, nail plate dystrophy, and Hutchinson sign (periungual spread of pigmentation) 2
   • Does not resolve spontaneously

Clinical Assessment

The fact that the black line resolved without treatment strongly suggests a benign condition, most likely a subungual hematoma. Key features supporting this diagnosis:

• Resolution without treatment
• Left a crack in the nail (consistent with trauma)
• No mention of spreading or worsening of the pigmentation

Management Approach

For a black line that has already resolved leaving only a crack:

1. No further treatment needed

   • The crack will grow out as the nail plate regenerates
   • Complete nail regrowth takes approximately 6 months for fingernails and 12 months for toenails 4

2. Prevention of future occurrences

   • Avoid trauma to nails
   • Wear protective footwear if appropriate
   • Keep nails trimmed to appropriate length

3. When to seek further evaluation

   • If the black line recurs
   • If there is associated pain or nail dystrophy
   • If the pigmentation spreads to surrounding skin (Hutchinson sign)
   • If the line appears without any history of trauma

Important Considerations

• While onychomycosis (fungal nail infection) can cause nail discoloration, it typically presents as white, yellow, or brown discoloration rather than a distinct black line 4
• Diagnosis of persistent nail pigmentation should include microscopic examination with potassium hydroxide (KOH), mycological culture, or nail biopsy in doubtful cases 5
• Melanonychia requires careful evaluation to rule out melanoma, especially in adults with sudden onset of a single pigmented streak 2

Conclusion

The resolution of the black line without treatment and the resulting crack strongly suggest a benign condition, most likely a subungual hematoma from trauma. No further intervention is needed as the crack will grow out with the nail over time.