Capillary Leak Syndrome: Symptoms, Causes, Treatment (2024)

Capillary leak syndrome is a rare disorder where the plasma in your blood leaks from your capillaries (small blood vessels) into surrounding tissues in your body. It affects fewer than 1,000 people in the United States.

This condition may begin on its own, or it may be triggered by another illness or medication. Regardless of how it starts, capillary leak syndrome generally presents in the same way: a rapid drop in blood pressure, swelling in the arms and legs, and thickened blood.

This article will take a closer look at this rare condition, what causes it, and what treatments are available.

Plasma is the fluid portion of your blood. It makes up 55% of your blood and contains substances like water, protein, fat, and salt.

In capillary leak syndrome, plasma leaks out of the small blood vessels (capillaries) and into the muscles, organs, tissues, and body cavity.

When the plasma leaks into these surrounding areas, your blood pressure can drop quickly. This can lead to a number of dangerous symptoms and complications.

The different types

There are two types of capillary leak syndrome:

  • Primary capillary leak syndrome happens when there are repeat episodes or attacks in a person with no known health issues. This type is also called systemic capillary leak syndrome or Clarkson’s syndrome.
  • Secondary capillary leak syndrome happens when there’s a single attack. Another health issue usually triggers it, such as a virus, autoimmune disorder. It may also be triggered by some types of medication.

Each attack typically involves three phases:

  • Prodromal phase: This phase includes the sensations or symptoms you experience before an attack occurs. This can include flu-like symptoms, dizziness, and fatigue.
  • Capillary leak (resuscitation) phase: This is the attack phase and may last up to 3 days. In this phase, fluids and albumin (protein in blood plasma) leak from the capillaries and into tissue, organs, muscles, or other areas. When this happens, blood pressure falls, blood volume decreases, and the blood thickens. It makes it harder for oxygen to be delivered to the rest of the body.
  • Recovery (recruitment) phase: This phase happens after the attack when the capillaries start to absorb fluid again.

Systemic capillary leak syndrome primarily affects adults older than 50 years old; it’s very rare in children. The condition is more common in men than women. While most studies have focused on white people, the condition appears to affect people of all races.

Secondary capillary leak syndrome may affect people of any age, as it’s related to different health conditions and medications a person may be taking.

Each phase of a capillary leak syndrome attack has unique symptoms. Keep in mind that symptoms may come on suddenly and can vary from one person to another, and even from one attack to the next.

Symptoms may range in intensity from mild to severe.

Prodromal phase symptoms may include:

  • stuffy nose
  • abdominal pain or upset stomach
  • nausea and diarrhea
  • muscle pain (myalgia)
  • fever
  • fatigue
  • headache
  • lightheadedness or dizziness
  • cough or upper respiratory symptoms

Capillary leak phase symptoms may include:

  • low blood pressure (hypotension)
  • diarrhea
  • swelling in the arms, legs, and body (peripheral edema)
  • thickened blood (hypercoagulability)

Between attacks, people with primary capillary leak syndrome do not typically show any symptoms or signs.

Primary/systemic capillary leak syndrome

Researchers have yet to determine the exact cause of primary capillary leak syndrome. What they do know is that it doesn’t appear to have a genetic link, meaning it does not run in families.

According to the National Organization for Rare Disorders (NORD), more than 50% of people with primary capillary leak syndrome have monoclonal “M” protein that can be detected in their blood. That said, more research is needed to understand the significance of this finding.

Usually, the level of this protein is low. Researchers believe autoimmune issues or other factors may be at play.

Another theory NORD points out is that the cells lining the capillaries may get damaged by some factor in the blood during an attack, leading to more episodes in the future.

Secondary capillary leak syndrome

Sepsis most often causes secondary capillary leak syndrome. Other health issues may also cause it, including:

  • autoimmune diseases
  • viral hemorrhagic fevers (infectious viral diseases that can damage the walls of blood vessels)
  • hemophagocytic lymphohistiocytosis (a rare condition that’s caused by an abnormal and overactive immune system response)
  • engraftment syndrome (an inflammatory response that can happen after a bone marrow transplant)
  • ovarian hyperstimulation syndrome
  • snakebite poisoning
  • ricin poisoning

Medications may also cause secondary capillary leak syndrome. Drugs include:

  • interleukins
  • monoclonal antibodies
  • gemcitabine


While recent studies are small, there’s some evidence that COVID-19 may increase the risk of capillary leak syndrome.

Researchers explain that viruses can be a trigger for the condition. Furthermore, COVID-19 has been shown to overproduce cytokines. This can cause a multisystem inflammatory syndrome.

Ultimately, more research is needed to determine the link between COVID-19 and the inflammatory response that may lead to secondary capillary leak syndrome.

There’s no cure for capillary leak syndrome. Instead, treatment focuses on preventing attacks or treating symptoms during an attack.

During an attack, you’ll likely need to stay in the hospital. Doctors treat symptoms and the effects of the attack as they happen.

In particular, treatment involves regulating blood pressure to keep the blood moving to the organs and preventing swelling and excess fluid from accumulating in the arms, legs, and body.

Interventions for capillary leak syndrome include:

  • Fluids: It’s important to maintain a balance between enough fluids to help increase blood pressure and not too much fluid, which can make swelling worse.
  • Surgical decompression: This procedure can help with swelling in the arms and legs if compartment syndrome develops.
  • Diuretics: These drugs can treat extreme fluid buildup during the recovery phase.
  • Steroids: If given in the early stages of an attack, glucocorticoids may help reduce the severity of a capillary leak.
  • Other medications: Doctors may also prescribe medications like:
    • theophylline
    • terbutaline
    • leukotriene inhibitors
    • ACE inhibitors

Prevention involves regularly taking certain medications even when symptoms are not present. This method of treatment is called prophylaxis.

Studies have shown that a high dose of intravenous immunoglobulin (IVIG) each month, for example, may prevent attacks in more than 90% of people.

The outlook for people with capillary leak syndrome varies. It depends on the person: One person may have just one attack in their lifetime, while another may have multiple attacks each year.

In one small 2017 study with 69 adults, the survival rate for people with primary capillary leak syndrome was 78% at 5 years and 69% at 10 years.

Researchers found taking monthly prophylaxis (IVIG) improved people’s outlook:

  • For study participants who took IVIG, the 5-year survival rate was 91% and the 10-year survival rate was 77%.
  • For participants who did not take IVIG, the 5-year survival rate was 47% and the 10-year survival rate was 37%.

Early diagnosis and treatment are key. Without prompt treatment, capillary leak syndrome may lead to:

  • kidney failure
  • compartment syndrome
  • stroke
  • shock
  • death

Capillary leak syndrome is a rare condition. Primary capillary leak syndrome is more common in people who are older than 50. Secondary capillary leak syndrome can happen at any age. Another health issue or medication triggers it.

There’s no cure for this condition. Treatment is aimed at addressing symptoms as they arise. Early diagnosis is important to ensure proper treatment and better outcomes.

If you’ve had capillary leak syndrome in the past, talk with a doctor about taking monthly prophylaxis, as it may significantly reduce the possibility of future attacks.

Capillary Leak Syndrome: Symptoms, Causes, Treatment (2024)
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