Differential Diagnosis for Dark Line on Toenail

• Single Most Likely Diagnosis
  • Melanonychia: This condition is characterized by a brown or black line or streak on the nail, caused by an increase in melanin production. It's a common and usually benign condition, but it can be a sign of melanoma in rare cases.
• Other Likely Diagnoses
  • Nail trauma: A dark line on the toenail can be caused by trauma to the nail, such as a blow to the toe or repetitive stress from activities like running.
  • Fungal infection: Fungal infections like onychomycosis can cause discoloration of the nail, including dark lines or streaks.
  • Nail staining: Exposure to certain substances like tobacco, nail polish, or hair dye can cause staining of the nail, resulting in a dark line.
• Do Not Miss Diagnoses
  • Subungual melanoma: Although rare, a dark line on the toenail can be a sign of subungual melanoma, a type of skin cancer that occurs under the nail. Early detection is crucial for effective treatment.
  • Squamous cell carcinoma: This type of skin cancer can also occur under the nail and cause a dark line or other nail changes.
• Rare Diagnoses
  • Addison's disease: This rare endocrine disorder can cause hyperpigmentation, including dark lines on the nails.
  • Peutz-Jeghers syndrome: This genetic disorder is characterized by the development of benign polyps in the digestive tract and can cause melanin deposits in the skin and nails, resulting in dark lines.
  • Laugier-Hunziker syndrome: This rare condition is characterized by hyperpigmentation of the skin and nails, including dark lines on the toenails.