Differential Diagnosis for Dark Brown Line Under Toenail
- Single Most Likely Diagnosis
- Melanonychia: This condition is characterized by a brown or black pigmentation under the nail plate, often appearing as a stripe or band. It's commonly caused by nail matrix melanocyte activation and can be benign or, less commonly, a sign of melanoma. The presentation of a dark brown line under the toenail makes melanonychia a leading consideration due to its typical appearance.
- Other Likely Diagnoses
- Subungual hematoma: This condition occurs when there is bleeding under the nail, often due to trauma. It can appear as a dark brown or red line under the nail and may be painful. The color can change over time as the blood is absorbed.
- Nail fungus (onychomycosis): While more commonly associated with thickening, brittleness, or discoloration of the nail, some types of fungal infections can cause dark pigmentation under the nail.
- Nail trauma: Similar to subungual hematoma, trauma to the nail can cause pigmentation changes, including a dark brown line, due to the disruption of nail cells and possible bleeding.
- Do Not Miss Diagnoses
- Subungual melanoma: Although less common than benign melanonychia, subungual melanoma is a dangerous form of skin cancer that can present with a similar dark pigmentation under the nail. It's crucial to distinguish this from benign conditions due to its potential for metastasis and high mortality if not treated early.
- Squamous cell carcinoma: Though rare under the nail, this type of skin cancer can present with pigmentation changes and should be considered, especially if there's a history of exposure to carcinogens or if the lesion is changing.
- Rare Diagnoses
- Addison's disease: This endocrine disorder can cause hyperpigmentation in various parts of the body, including under the nails, though this is a less common presentation.
- 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 melanocytic macules on the skin and mucous membranes, potentially under the nails.
- Laugier-Hunziker syndrome: A rare condition characterized by hyperpigmentation of the nails, oral mucosa, and sometimes the genital mucosa. It typically presents with longitudinal melanonychia.