A narrow black line that has formed vertically underneath your nail is called a splinter hemorrhage. It occurs for a variety of reasons and may be harmless or a sign of a more serious health condition.

This condition is called a splinter hemorrhage because it may look like a wood splinter under your nail. The condition is caused by damaged small blood vessels underneath your nail. Characteristics include the following:

  • It’s black or reddish brown in color.
  • It doesn’t change appearance when you apply pressure to the nail.
  • It appears on one or more places under your nail.

Thicker lines that create a horizontal stripe on the nail are called Beau’s lines. These are usually not harmful, but they may be a symptom of a cancer called subungual melanoma.

Black lines on the nail can be the result of many conditions.

Trauma is one of the most common causes of splinter hemorrhages. This can occur when something hurts your nail bed and bursts a blood vessel under your nail. You may also experience nail trauma without even realizing it, such as when you squeeze something very tightly.

Endocarditis is a heart condition that may cause splinter hemorrhages. of those with endocarditis have splinter hemorrhages. Endocarditis occurs when the inner chambers or valves of the heart have an infection. You may experience other serious heart conditions as a result of endocarditis. Endocarditis generally occurs in those with preexisting cardiac conditions.

Psoriasis may also cause splinter hemorrhages. Psoriasis is a chronic skin condition that can manifest as silvery plaques, or scales on the skin. of those with psoriasis experience splinter hemorrhages and other nail conditions.

Causes of splinter hemorrhages vary and may include:

Beau’s lines on all 20 nails may be a sign of:

Contact your doctor if you’re sure a black line on your nail wasn’t caused by trauma. If the trauma has caused another condition or doesn’t seem to be healing on its own after several weeks, you should see your doctor.

If you suspect the splinter hemorrhage is the cause of a more serious health condition, you should contact your doctor for an appointment. For example, if you have endocarditis, you may experience symptoms like fatigue, a fever, pale skin, joint pain, and shortness of breath, among others. Psoriasis may appear as a rash, patches, or scales on your skin, particularly on your scalp, face, hands, feet, and skin folds.

Splinter hemorrhages that recur or are on more than one nail should also be reviewed by a doctor.

When you see your doctor, expect them to take a medical history and perform a physical exam. Your doctor may recommend further testing after your appointment if an underlying condition is suspected.

Treatment of the splinter hemorrhage will vary based on the underlying cause of the condition. In some cases, you may not need any treatment, and the splinter hemorrhage will grow out with the nail.

If the splinter hemorrhage is a sign of another medical condition, you should see your doctor for treatment. The health conditions causing splinter hemorrhages are quite different, so there is no standard treatment to improve the nail condition. For example, endocarditis requires antibiotics and possibly surgery. Psoriasis is a lifelong condition that requires various topical and oral treatments as well as prevention strategies.

Splinter hemorrhages may be harmless, or they may be a sign of a more serious health condition. If you can recall a trauma to the nail, it’s likely the splinter hemorrhage will grow out with time. If you experience symptoms in addition to the nail condition, you should see your doctor for a thorough examination and diagnosis.

Nails can be an indicator of your overall health. If you experience splinter hemorrhages or other nail conditions without a reasonable explanation, it may be a sign that you should see your doctor.