6. Subungual Melanoma: A Rare but Serious Possibility
One of the most important conditions to recognize is subungual melanoma.
Melanoma is a type of skin cancer that develops from pigment-producing cells called melanocytes.
When melanoma develops in the nail unit, it is sometimes referred to as nail melanoma or subungual melanoma.
It is uncommon, but it can be serious if diagnosis is delayed.
Unlike a typical bruise, a melanoma-related dark area may not have a clear history of injury and may gradually change over time.
It can appear as a dark brown or black longitudinal stripe beneath the nail.
However, nail melanoma does not always look like a neat, obvious stripe. Some cases present as an irregular dark patch or other changes in the nail.
Warning signs that deserve prompt attention
The following features are particularly important:
A new dark stripe in one nail: A new pigmented band affecting a single toenail should be assessed, especially in adults.
Changes in width: A band that becomes wider, particularly near the base of the nail, requires attention.
Irregular pigmentation: Uneven color, blurred borders, or several shades within the same area may be concerning.
Pigment spreading onto surrounding skin: Dark pigment extending from the nail onto the nearby cuticle or skin can be an important warning sign. This is sometimes called Hutchinson's sign.
Nail damage: Splitting, lifting, or destruction of the nail plate can occur in some cases.
Unexplained bleeding: Bleeding or persistent changes without a clear injury should be investigated.
Not every dark stripe is melanoma. Benign pigmentation and other conditions can look similar.
Nevertheless, a suspicious lesion should be examined by a dermatologist rather than monitored indefinitely at home.
How is nail melanoma diagnosed?
A dermatologist may examine the nail using dermoscopy, a specialized magnification tool that helps assess pigment patterns.
If the appearance is concerning, a nail biopsy may be recommended.
The biopsy allows a pathologist to examine the tissue and determine whether abnormal melanocytes or melanoma are present.
Early assessment matters because treatment and prognosis depend heavily on the stage and extent of the disease.
7. Poor Circulation and Peripheral Artery Disease
Changes in circulation can also affect the appearance and health of the toenails.
Peripheral artery disease, or PAD, develops when arteries supplying blood to the legs become narrowed or blocked, often because of atherosclerosis.
Reduced blood flow can affect the feet and toes.
Although a black toenail is not a typical standalone sign of PAD, severe circulation problems can sometimes lead to tissue damage and dark discoloration.
Other symptoms may include:
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Pain in the legs while walking.
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Cold feet or toes.
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Weak or difficult-to-detect foot pulses.
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Slow-healing wounds.
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Changes in skin color.
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Reduced hair growth on the lower legs or feet.
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Numbness or weakness.
In severe cases, insufficient blood supply can cause tissue death, known as gangrene.
Gangrene may make a toe appear dark purple, brown, or black.
This is a medical emergency, particularly when accompanied by severe pain, foul-smelling discharge, swelling, fever, or a wound that is not healing.
Who is at greater risk?
Risk factors for PAD include:
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Smoking.
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Diabetes.
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High blood pressure.
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High cholesterol.
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Older age.
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Chronic kidney disease.
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A history of cardiovascular disease.
People with these risk factors should pay particular attention to new changes in their feet.
A dark or black toe associated with poor circulation should never be dismissed as an ordinary nail bruise without assessment.
8. Diabetes and Black Toenails
Diabetes can increase the risk of several foot and nail problems.
Long-term high blood glucose levels may damage nerves and blood vessels, potentially affecting sensation and circulation in the feet.
This can make minor injuries more dangerous.
For example, a person with diabetic neuropathy may not feel a small injury caused by tight footwear.
Blood may collect beneath the nail, but the person may not notice the trauma because sensation is reduced.
Diabetes can also increase the risk of certain infections, including fungal infections.
Why is this important?
A person with diabetes may have difficulty recognizing a foot injury until swelling, discoloration, or an infection develops.
A black toenail may therefore be more concerning in someone with diabetes, particularly if there is:
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Increasing redness or swelling.
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Warmth around the toe.
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Drainage or pus.
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A wound that does not heal.
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New numbness.
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A cold or unusually pale foot.
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Darkening of the toe beyond the nail.
People with diabetes should seek prompt medical advice for new unexplained nail discoloration, especially when accompanied by a wound or signs of infection.
Regular foot examinations are an important part of diabetes care.