Differential Diagnosis for Dark Discoloration of Great Toenail
Single Most Likely Diagnosis
- Fungal Infection (Onychomycosis): This is the most common cause of toenail discoloration, especially in the absence of injury or pain. Fungal infections can cause a range of color changes, including darkening, and are often seen in the great toenail due to its larger size and increased exposure to moisture.
Other Likely Diagnoses
- Melanonychia: This condition is characterized by a brown or black pigmentation of the nail, which can be caused by an increase in melanin production. It's more common in individuals with darker skin tones and can be benign or, less commonly, associated with melanoma.
- Nail Staining from Dye or Chemicals: Exposure to certain dyes or chemicals can cause discoloration of the nail. This is more likely if the individual has recently used nail polish or has occupational exposure to chemicals.
- Trauma (Even if Not Remembered): Sometimes, minor trauma can cause discoloration without being immediately painful or memorable. The nail may darken due to blood accumulation under the nail plate (subungual hematoma) even if the initial injury was minor.
Do Not Miss Diagnoses
- Subungual Melanoma: Although rare, this is a potentially deadly diagnosis that must be considered, especially if there's a new or changing pigmented band in the nail. Subungual melanoma can present with a dark discoloration of the nail without pain and is more common in individuals with darker skin tones.
- Subungual Hematoma Due to Underlying Disease: In some cases, a subungual hematoma can be a sign of an underlying condition, such as a blood clotting disorder or vascular disease. While these conditions are less likely, they are critical to identify.
Rare Diagnoses
- Addison's Disease: This rare endocrine disorder can cause hyperpigmentation, including nail discoloration, due to increased levels of adrenocorticotropic hormone (ACTH), which stimulates melanin production.
- Wilson's Disease: A genetic disorder that leads to copper accumulation in the body, which can cause a range of symptoms including nail discoloration.
- Arsenic or Heavy Metal Poisoning: Exposure to certain heavy metals can cause nail changes, including discoloration, though this is exceedingly rare and usually associated with other systemic symptoms.