A splinter hemorrhage is a tiny bleed that occurs in the nail bed, the skin directly beneath the nail plate. It gets its distinctive name because the trapped blood forms a thin, longitudinal line that runs in the direction of nail growth, resembling a wood splinter.

To understand why they form this shape, it is helpful to picture the anatomy of the nail bed.

• The nail bed has a rich supply of very small blood vessels called capillaries.
• These capillaries are arranged in parallel ridges and grooves that run along the same direction as the nail grows, from the cuticle toward the fingertip.
• A helpful analogy is to think of the nail bed as a piece of corduroy fabric, with many fine, parallel grooves. Tiny capillaries run along the bottom of these grooves.
• When one of these small vessels is damaged, a tiny amount of blood leaks. This blood becomes trapped in the groove, under the clear nail plate.
• Because of the shape of the groove, the trapped blood forms a thin, straight, linear shape that looks just like a splinter.

As the fingernail grows forward, it will carry this trapped line of blood with it, so a splinter hemorrhage will slowly move toward the tip of the finger over time until it is eventually clipped off.

In my experience, patients often notice these thin, reddish-brown lines under their nails and assume it’s trauma-related but splinter hemorrhages can sometimes signal an underlying systemic issue.

The direct cause of a splinter hemorrhage is always the rupture of a small capillary in the nail bed. The reasons for this rupture, however, can be divided into two main categories: simple trauma and underlying systemic disease.

1. Trauma (The Most Common Cause)

In the vast majority of cases, an isolated splinter hemorrhage is the result of direct physical trauma to the nail. This is the most common and least worrisome cause. The injury damages the delicate capillaries, causing a small bleed. Common sources of trauma include:

• Accidentally hitting your finger or toe (e.g., with a hammer, in a car door).
• A crush injury.
• Even minor, repetitive trauma from certain activities or occupations that involve pressure on the fingertips.

Often, the trauma is so minor that the person does not even remember the specific event that caused it.

2. Systemic Medical Conditions

In a minority of cases, splinter hemorrhages are not caused by an external injury but are a sign that something is happening within the body’s circulatory system. In these situations, the blood vessels in the nail bed either become fragile and prone to spontaneous rupture, or they are being blocked by tiny traveling clots.

The most important systemic condition classically associated with splinter hemorrhages is:

• Infective Endocarditis: This is the most serious potential cause and is a life-threatening heart valve infection. In endocarditis, clumps of bacteria and blood clots, called septic emboli, can form on the damaged heart valves. Tiny pieces of these clumps can break off, travel through the bloodstream, and get lodged in the tiny capillaries of the nail bed. This blockage damages the vessel and causes it to bleed, forming a splinter hemorrhage.

Other potential systemic causes include:

• Vasculitis: An inflammation of blood vessels that can make capillaries brittle and leaky. This can be seen in autoimmune diseases like lupus or rheumatoid arthritis.
• Psoriasis: This skin condition can affect the nails and the nail bed, sometimes causing splinter hemorrhages.
• Certain Blood Disorders that affect clotting.
• Chronic Kidney Disease.

Clinically, I’ve seen trauma as the most common cause, often from nail picking, sports, or manual labor. But I always consider systemic causes in the absence of clear injury.

You “get” a splinter hemorrhage either from a direct physical injury to your nail or as a clinical sign of one of the underlying medical conditions listed above. Risk factors vary by cause. For traumatic splinters, the risk is related to your activities and occupation. For systemic splinters, the risk factors are those associated with the underlying disease, such as having a pre-existing heart valve condition or an artificial heart valve, which increases the risk of developing endocarditis.

In my experience, people develop these from minor unnoticed injuries, especially if they work with their hands a lot but if there’s no trauma history, I dig deeper into medical causes.

The primary sign is the appearance of the hemorrhage itself.

• Appearance: A thin, linear streak under the nail plate.
• Color: It is usually reddish-brown but can appear black as the trapped blood ages.
• Location: It can be on a fingernail or a toenail.
• Movement: It does not move when you press on the nail, and it will slowly grow out toward the tip of the nail as the nail itself grows.

Differentiating Traumatic vs. Systemic Causes

A doctor will look for several clues to help determine if a splinter hemorrhage is likely from simple trauma or a more concerning systemic cause.

• Number and Location: A splinter hemorrhage from trauma usually occurs on only one or two nails and is often on the dominant hand. Splinter hemorrhages related to a systemic disease are more likely to appear on multiple nails on both hands.
• Position: Traumatic splinters are often located further down the nail, closer to the tip. Those from a systemic cause, like endocarditis, are often more “proximal,” meaning they are located closer to the cuticle.
• Associated Symptoms: This is the most important distinction. A traumatic splinter hemorrhage occurs in isolation, with no other symptoms other than perhaps some tenderness at the site of the injury. When splinter hemorrhages are a sign of a systemic disease, the person will almost always have other signs and symptoms of that underlying illness.

For example, a person with splinter hemorrhages due to infective endocarditis would likely also have:

• Fever, chills, and night sweats.
• Profound fatigue and weakness.
• A new or changing heart murmur.
• Other classic skin signs, such as painful nodules on the fingertips (Osler’s nodes) or painless spots on the palms (Janeway lesions).

Clinically, I look for location and symmetry, distal and isolated often means trauma, while proximal or multiple nails may hint at systemic disease.

The diagnosis of a splinter hemorrhage itself is made by simple visual inspection. The entire goal of the diagnostic process is not to diagnose the splinter, but to determine its cause.

1. Medical History and Physical Examination

This is the most important part of the evaluation.

• History: A doctor will first ask about any recent trauma to the hands or fingers. The absence of any remembered injury is a key clue that raises the suspicion of a systemic cause. The doctor will also ask about any other symptoms, like fever, fatigue or joint pain.
• Physical Examination: The doctor will carefully examine all of the fingernails and toenails to see how many are affected. They will then perform a full physical exam, paying very close attention to listening to the heart with a stethoscope to check for any murmurs and examining the skin for any other signs that might suggest endocarditis or vasculitis.

2. The Workup for Suspected Systemic Disease

If a doctor suspects that the splinter hemorrhages are due to an underlying medical condition like endocarditis, they will order a series of tests.

• Blood Cultures: This is a critical test where several samples of blood are taken and sent to the laboratory to see if any bacteria grow, which would confirm an infection in the bloodstream (bacteremia).
• Blood Tests: A complete blood count (CBC) and tests for inflammatory markers like ESR and CRP will be performed.
• Echocardiogram: This is an essential test if endocarditis is suspected. An echocardiogram is a heart ultrasound that allows a doctor to look directly at the heart valves. It can identify the vegetations, the clumps of bacteria and clot that grow on the valves in infective endocarditis.

In my experience, I always take a full history if there’s fever, heart murmur, or signs of systemic illness, I escalate investigation immediately.

The splinter hemorrhage itself is never treated. It is a small amount of trapped, dried blood that is completely harmless. The treatment is always directed at the underlying cause.

• For Traumatic Splinter Hemorrhages: As these are caused by simple injury, no treatment is needed. The splinter hemorrhage will slowly and harmlessly grow out with the nail over a period of several months.
• For Splinter Hemorrhages Caused by an Underlying Disease: The treatment is focused entirely on the specific disease that was diagnosed.
  • If infective endocarditis is the cause, the patient will be hospitalized for a long course of high-dose intravenous (IV) antibiotics. In some cases, surgery may be required to repair or replace the damaged heart valve.
  • If vasculitis from an autoimmune disease is the cause, a rheumatologist will treat the condition with immunosuppressive medications like corticosteroids.
  • If psoriasis is the cause, treatment will be directed at controlling the psoriasis.

I’ve seen traumatic splinter hemorrhages resolve on their own, reassurance is often enough if there’s no underlying condition.

Splinter hemorrhages are thin, dark lines that appear under the fingernails or toenails. While they can be visually surprising, it is essential to remember that the vast majority of these are the simple, harmless result of an accidental knock or bump to the nail and require no treatment whatsoever. Their medical significance lies in the rare instances when they appear without any history of trauma, especially on multiple fingers. In this context, they can be a valuable clue for a physician, signaling the presence of a serious underlying systemic illness, most importantly infective endocarditis. If you notice a single splinter hemorrhage after an injury, you can be reassured that it will simply grow out with your nail. However, if you develop multiple splinter hemorrhages for no apparent reason, especially if you are also feeling unwell or have a fever, it is important to see a doctor for a thorough evaluation.

The American Academy of Dermatology (AAD). (n.d.). Nail Psoriasis: Signs and Symptoms. (Note: Discusses splinter hemorrhages in the context of psoriasis). Retrieved from https://www.aad.org/public/diseases/a-z/nail-psoriasis-symptoms

The Merck Manual Professional Version. (2022). Splinter Hemorrhages. Retrieved from https://www.merckmanuals.com/professional/dermatologic-disorders/nail-disorders/splinter-hemorrhages

American Heart Association. (n.d.). Infective Endocarditis. Retrieved from https://www.heart.org/en/health-topics/infective-endocarditis