Differential Diagnosis for Black Line on Big Toe Nails Bilaterally
- Single most likely diagnosis:
- Melanonychia: This condition is characterized by a black or brown streak or stripe on the nail plate, often due to an increase in melanin production. It can be caused by various factors, including trauma, infection, or benign melanocytic activation. The bilateral presentation on the big toes could suggest a systemic or repetitive cause, such as trauma from tight shoes.
- Other Likely diagnoses:
- Nail fungus (Onychomycosis): While typically presenting with discoloration, thickening, and brittleness of the nails, some forms of fungal infections can cause black lines or streaks, especially if there's a secondary bacterial or yeast infection.
- Nail trauma: Repeated trauma to the nail bed can cause bleeding under the nail, leading to a black line appearance as the blood dries and becomes visible through the nail plate.
- Subungual hematoma: Similar to nail trauma, a subungual hematoma can cause a black line or spot under the nail due to blood accumulation.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
- Subungual melanoma: Although rare, this is a potentially deadly form of skin cancer that can present as a black line or streak under the nail. It's crucial to consider this diagnosis, especially if there's a change in the nail's appearance, such as widening of the streak, or if there are other signs like nail lifting or pigmentation on the surrounding skin.
- Rare diagnoses:
- Addison's disease: This rare endocrine disorder can cause hyperpigmentation, including longitudinal melanonychia, due to increased ACTH levels stimulating melanin production.
- Peutz-Jeghers syndrome: A genetic disorder characterized by the development of benign hamartomatous polyps in the gastrointestinal tract and an increased risk of certain cancers. It can also present with mucocutaneous melanin deposits, potentially causing nail pigmentation.
- Laugier-Hunziker syndrome: A rare condition characterized by acquired hyperpigmentation of the nails, oral mucosa, and sometimes the lips, without any systemic associations.