What is Melanonychia?
Melanonychia is a condition where brown or black discoloration appears on the nail plate. It most commonly presents as a vertical stripe (longitudinal melanonychia) running from the cuticle to the tip of the toenail. This discoloration is caused by the deposition of melanin (pigment) into the hard keratin of the nail.
The ABCDEF Rule for Melanoma
While many cases are benign, a dark stripe is the primary symptom of Subungual Melanoma (nail cancer). Podiatrists use this rule to evaluate the risk:
A: Age & African/Asian/Native American descent (highest risk groups).
B: Band (Brown/Black band breadth is greater than 3mm or borders are blurred).
C: Change (Rapid increase in size or growth rate).
D: Digit (Most common in the big toe or thumb).
E: Extension (Hutchinson's Sign: the pigment bleeds into the cuticle or skin).
F: Family (Personal or family history of melanoma).
Common Benign Causes
Not all dark stripes are cancer. In many cases, it is simply a "freckle" or "mole" in the nail matrix.
- Melanocytic Activation: Normal pigment cells are triggered to produce more melanin due to pregnancy, thyroid disease, or trauma (like tight shoes rubbing the toe).
- Nail Matrix Nevus: A benign mole growing in the root of the nail. As the nail grows, it pulls the pigment forward in a stripe.
- Fungal Infection: Some rare strains of fungus (like Trichophyton rubrum) can produce a dark brown pigment.
When to See a Specialist
You cannot diagnose the cause of a dark nail stripe at home. If you notice a new, unexplained dark stripe on your toenail—especially if it is wider than 3mm, has blurred edges, or if the skin around the nail is darkening—you must see a podiatrist immediately. We will perform a dermoscopy (magnified examination) and, if necessary, a nail matrix biopsy to rule out melanoma.