Horizontal red lines under fingernails are usually tiny broken blood vessels, called splinter hemorrhages, trapped in the nail bed. They run lengthwise in the direction of nail growth, so a line that looks horizontal across the nail is often a single streak of blood sitting sideways. The most common cause is minor trauma, but in some cases they signal a medical condition that needs attention.
What Causes Horizontal Red Lines Under Fingernails?
The nail bed sits directly beneath the hard nail plate, and it is packed with tiny capillaries. When one of those capillaries breaks, a small amount of blood leaks into the tissue. That blood gets trapped between the nail plate and the bone beneath it, and it appears as a thin red or brown streak.
Because the nail grows outward from the base, the streak moves with it. This is why the direction of the line matters. Splinter hemorrhages run along the length of the nail, from cuticle to tip. A truly horizontal line that crosses the nail from side to side is more likely to be a different finding, such as a Beau’s line or a pigmented band.
Doctors group the causes into two broad categories: traumatic and non-traumatic. Trauma is far more common. Non-traumatic causes are rarer but matter more because they can point to a systemic illness.
Is It Trauma or Something More Serious?
Trauma is the leading cause of splinter hemorrhages. Anything that puts repeated pressure or a sharp jolt on the nail can rupture capillaries in the nail bed.
- Hitting the finger against a hard surface
- Closing the finger in a door or drawer
- Repeated typing, piano playing, or gripping tools
- Aggressive manicures or pushing back the cuticle too hard
- Tight shoes pressing on toenails
Traumatic streaks often appear on just one or two nails. They tend to be isolated and may fade as the nail grows out.
Non-traumatic splinter hemorrhages are different. They tend to appear on several nails at once, often without any injury. When this pattern shows up, doctors look for an underlying cause. The two conditions most associated with this pattern are infective endocarditis and psoriasis.
Infective endocarditis is an infection of the inner lining of the heart, usually involving the heart valves. Splinter hemorrhages are one of several skin findings that can appear with it. Others include small painful bumps on the fingertips called Osler nodes and painless spots on the palms or soles called Janeway lesions. These findings are not present in everyone with endocarditis, and no single one of them confirms the diagnosis.
Psoriasis affecting the nails can also produce splinter hemorrhages. In nail psoriasis, the nail bed becomes inflamed and its capillaries are more fragile. This can lead to small bleeds that show up as red or brown streaks. Nail psoriasis often comes with other signs, such as pitting, ridging, or a yellowish discoloration of the nail.
What Is the Difference Between Splinter Hemorrhages and Other Nail Lines?
Not every line on a nail is a splinter hemorrhage. Several findings look similar but have different causes and different implications.
| Finding | Direction | Common Cause |
|---|---|---|
| Splinter hemorrhage | Lengthwise (cuticle to tip) | Trauma, endocarditis, psoriasis |
| Beau’s line | Horizontal (side to side) | Illness, injury, or stress that paused nail growth |
| Mees’ lines | Horizontal white bands | Heavy metal poisoning, chemotherapy, severe illness |
| Longitudinal melanonychia | Lengthwise dark band | Melanin pigment, sometimes melanoma |
The key distinction is direction. Splinter hemorrhages follow the nail’s growth path, so they run vertically. Beau’s lines and Mees’ lines cross the nail horizontally because they mark a moment when nail growth slowed or stopped.
A horizontal line under the nail is more consistent with a Beau’s line than a splinter hemorrhage. Beau’s lines form when something interrupts nail production for a period of time. The result is a groove or ridge that runs across the width of the nail. Causes include high fever, serious illness, chemotherapy, and injury to the nail base.
When Should You See a Doctor?
See a doctor if red lines appear on several nails at once, if they show up without any injury, or if they come with other symptoms. A single streak after a known injury is usually not a concern, but multiple streaks with no clear cause deserve a medical evaluation.
Other symptoms that raise concern include:
- Fever or chills
- Fatigue that does not improve with rest
- Shortness of breath
- A new heart murmur or chest pain
- Joint pain or swelling
- Nail changes such as pitting, thickening, or separation from the nail bed
These symptoms combined with splinter hemorrhages could point to infective endocarditis or another systemic condition. Endocarditis is serious and needs prompt treatment. It is not something to watch at home.
If the lines are on one nail and you remember injuring it, you can usually wait and watch. The streak will grow out with the nail over several months. If it does not move or if new streaks keep appearing, see a doctor.
How Are Splinter Hemorrhages Evaluated?
A doctor starts with a physical exam. They look at all twenty nails, check for other skin findings, and ask about recent injuries, illnesses, and medications. The pattern and number of affected nails guide what happens next.
If trauma is the clear cause and the person is otherwise well, no further testing is needed. The finding is explained and the visit ends there.
If the pattern suggests a systemic cause, the workup may include blood tests and a blood culture to check for infection. An echocardiogram, which uses sound waves to create images of the heart, may be ordered to look at the heart valves. These tests help confirm or rule out endocarditis or another underlying condition.
If nail psoriasis is suspected, a dermatologist may examine the nails and skin more closely. There is no single test for nail psoriasis. The diagnosis is usually made by looking at the pattern of nail changes along with any skin symptoms.
Can You Treat or Prevent Them?
There is no specific treatment for a splinter hemorrhage itself. The streak is trapped blood, and the body clears it as the nail grows. This takes time because nails grow slowly. Fingernails grow about 3 millimeters per month, and toenails grow even more slowly.
Treatment focuses on the underlying cause if there is one. Infective endocarditis requires antibiotics, and sometimes surgery. Nail psoriasis may be treated with topical medications, steroid injections, or systemic treatments depending on severity. These decisions are made by a doctor.
Prevention is mostly about avoiding injury. Keep nails trimmed and avoid aggressive manicures. Do not push back cuticles forcefully or use tools that scrape the nail bed. If you work with your hands, wear gloves when appropriate.
No supplement or home remedy has been shown to resolve splinter hemorrhages faster. The evidence for topical treatments aimed specifically at these streaks is essentially absent. The blood clears on its own timeline.
Frequently Asked Questions
Are horizontal red lines under fingernails dangerous?
They can be. A single streak after an injury is usually harmless, but multiple streaks with no clear cause can signal a systemic condition such as infective endocarditis.
Do splinter hemorrhages go away on their own?
Yes, the trapped blood clears as the nail grows out. This takes several months for fingernails because they grow slowly.
What condition causes red lines under multiple nails?
Infective endocarditis and nail psoriasis are the two conditions most often linked to splinter hemorrhages on several nails. A doctor can determine which, if either, applies.
Should I worry about one red line under one nail?
Usually not, especially if you remember injuring that finger. See a doctor if it does not grow out or if new lines keep appearing.
