Differential Diagnosis for Longitudinal Black Line on Toe
- Single most likely diagnosis
- Melanonychia: This condition is characterized by a longitudinal pigmented band on the nail plate, often caused by the activation or proliferation of melanocytes in the nail matrix. It's a common and usually benign condition, but it can be a sign of nail melanoma, especially if it's a new onset or changing.
- Other Likely diagnoses
- Nail trauma: A longitudinal black line can appear after an injury to the nail, such as a bruise under the nail (subungual hematoma) that can cause discoloration.
- Nail fungus (onychomycosis): Fungal infections can cause changes in nail color, including black or brown discoloration, especially if the fungus invades the nail plate.
- Nail staining: Exposure to certain chemicals or substances (like nail polish, tobacco, or certain medications) can cause staining of the nail, leading to a black line.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Subungual melanoma: Although rare, this is a type of skin cancer that can present as a longitudinal black line under the nail. It's crucial to distinguish it from benign melanonychia due to its potential for metastasis and high mortality rate if not treated early.
- Squamous cell carcinoma: Though less common than melanoma, squamous cell carcinoma can also present under the nail and may cause similar pigmentation changes.
- Rare diagnoses
- Addison's disease: This rare endocrine disorder can cause hyperpigmentation, including longitudinal melanonychia, due to increased levels of adrenocorticotropic hormone (ACTH).
- 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 cause mucocutaneous melanin deposits, leading to dark blue or brown macules on the skin and mucous membranes, potentially including the nails.