Differential Diagnosis for a Small Vertical White or Black Line Underneath the Thumbnail
- Single Most Likely Diagnosis
- Nail Plate Trauma or Injury: A small vertical white or black line underneath the thumbnail, often referred to as a nail plate trauma or injury, is usually caused by a minor injury to the nail bed or matrix. This can result from a direct blow to the nail, repetitive strain, or even a sudden pressure change. The color of the line (white or black) may depend on the nature of the injury and the presence of any debris or blood under the nail.
- Other Likely Diagnoses
- Nail Fungus (Onychomycosis): Although less common for a single vertical line, nail fungus can cause changes in nail color and texture, including white or black streaks. However, these are typically accompanied by other signs such as nail thickening, brittleness, or crumbling.
- Nail Melanonychia: This condition is characterized by a brown or black streak in the nail, resulting from an increase in melanin production. While it can appear as a single vertical line, it's more commonly associated with brown pigmentation.
- Nail Plate Pigmentation: Various conditions can cause pigmentation changes under the nail, including systemic diseases or exposure to certain chemicals. These might result in white or black lines, though they are less common presentations.
- Do Not Miss Diagnoses
- Subungual Melanoma: Although rare, a black line under the nail can be a sign of subungual melanoma, a type of skin cancer. It's crucial to investigate any new or changing pigmentation under the nails, especially if accompanied by other symptoms like nail lifting or splitting.
- Subungual Hematoma: A black line could also indicate bleeding under the nail (subungual hematoma), which might be due to trauma. While often benign, it's essential to rule out more serious causes, especially if the hematoma is large or expanding.
- Rare Diagnoses
- Sarcoidosis: This systemic disease can cause a variety of skin and nail manifestations, including nail pigmentation changes. However, it would be an uncommon cause of a single vertical white or black line under the thumbnail.
- Other Systemic Diseases: Certain systemic conditions, such as Addison's disease or vitamin deficiencies, can lead to changes in nail color or texture. These would be rare causes of a single vertical line under the nail and would likely be accompanied by other systemic symptoms.