Home C Cardiovascular Conditions Capillary leak syndrome: Symptoms, Causes, Diagnosis, and Treatment Options

C Cardiovascular Conditions

Capillary leak syndrome: Symptoms, Causes, Diagnosis, and Treatment Options

January 12, 2026 Modified date: January 12, 2026

Capillary leak syndrome is a serious condition where fluid and proteins escape from the bloodstream into surrounding tissues. When that “leak” becomes widespread, blood pressure can fall quickly, organs may not get enough oxygen, and swelling can become dramatic within hours. Some people experience the syndrome as sudden, recurrent attacks (often called systemic capillary leak syndrome or Clarkson disease), while others develop capillary leak as a complication of infections, cancer therapies, severe inflammation, or critical illness.

This guide explains what is happening inside the body, who is most at risk, and which symptoms should never be ignored. You will also learn how clinicians confirm the diagnosis, what treatment typically looks like in the emergency setting, and what day-to-day prevention and monitoring can involve after recovery.

What capillary leak syndrome does
What causes it and who is at risk
Early symptoms and life-threatening complications
How doctors diagnose it
Treatment and what to expect in hospital
Management, prevention, and when to seek care

What capillary leak syndrome does

Your smallest blood vessels (capillaries) are designed to be selective. They let oxygen and nutrients move into tissues while keeping most fluid and proteins (especially albumin) inside the bloodstream. In capillary leak syndrome (CLS), that selectivity fails. The lining of blood vessels becomes “leaky,” allowing plasma and proteins to shift out of the circulation and into the interstitial space (the area between cells).

That shift creates a distinctive and dangerous paradox:

Outside the vessels: rapid swelling (edema), weight gain over hours to a day, puffy limbs, and sometimes fluid around organs (pleural effusions or abdominal fluid).
Inside the vessels: the circulating blood volume drops, which can cause low blood pressure, reduced kidney perfusion, dizziness, and shock.

Clinicians often describe an attack pattern in systemic forms. During the leak phase, fluid leaves the bloodstream quickly. Lab tests may show hemoconcentration (a higher hematocrit/hemoglobin because the blood is “more concentrated”) and hypoalbuminemia (low albumin). Afterward, many patients enter a recruitment phase, where fluid returns to the bloodstream as the leak shuts down. This second phase is risky in a different way: if large volumes of IV fluids were given earlier, the returning fluid can contribute to pulmonary edema (fluid in the lungs) or heart strain.

It helps to separate CLS into two broad categories:

Idiopathic or primary systemic capillary leak syndrome (SCLS): rare, often recurrent episodes, sometimes associated with a monoclonal protein in the blood (commonly referred to as Clarkson disease).
Secondary capillary leak: occurs due to a recognizable trigger such as severe infection, certain cancer treatments, major inflammation, or critical illness. It may not recur once the underlying cause is controlled.

Because CLS can progress quickly, the most important takeaway is this: sudden swelling plus faintness or low blood pressure is not “just dehydration” or “just edema.” It can represent a dangerous loss of circulating volume that needs urgent care.

What causes it and who is at risk

Capillary leak is not a single disease mechanism with one cause. It is a final pathway—the blood vessel lining becomes overly permeable—reached through several different routes. Understanding the likely cause matters because it shapes both treatment and the chance of recurrence.

Major causes and triggers

Idiopathic SCLS (Clarkson disease)

The exact cause is still being studied, but many patients have an associated monoclonal gammopathy (an abnormal antibody protein detected on blood tests). This does not automatically mean cancer, but it does warrant careful evaluation and follow-up.
Attacks are often preceded by a viral-like prodrome: fatigue, muscle aches, low-grade fever, nausea, or abdominal discomfort. The trigger may be a common infection or sometimes appears unclear.

Secondary capillary leak
Common settings include:

Severe infections and sepsis (a whole-body inflammatory response).
Cancer and blood disorders, including some plasma cell disorders.
Medications and therapies that can disrupt endothelial function, including certain immunotherapies and cytokine-based treatments. Some chemotherapy agents have also been associated with capillary leak.
Major inflammatory states (for example, severe pancreatitis), extensive burns, or trauma.
Post-surgical or critical illness physiology, where inflammation and fluid shifts are already amplified.

Risk factors that raise suspicion

You do not need “classic risk factors” to develop capillary leak, but certain patterns increase concern:

Prior episodes of sudden swelling plus low blood pressure.
A known monoclonal gammopathy or plasma cell disorder.
Recent severe infection or hospitalization for systemic inflammation.
Recent exposure to medications known to affect vascular permeability (often identified by the treating team).
A history of compartment syndrome or unexplained rhabdomyolysis during shock episodes.

Why the endothelium becomes leaky

The endothelium (the inner lining of vessels) behaves like a living filter. In CLS, signaling molecules associated with inflammation and immune activation can loosen junctions between endothelial cells and disturb the glycocalyx (a protective surface layer). The result is protein-rich fluid escaping where it should not, creating both swelling and intravascular collapse.

If you are reading this because you or someone you love has had an episode, it is worth asking one practical question early: Does the team believe this is idiopathic SCLS (recurrent risk) or secondary CLS (trigger-driven)? That distinction often guides long-term prevention strategies.

Early symptoms and life-threatening complications

Capillary leak syndrome can begin subtly and then accelerate. People often describe a “before and after” day: feeling vaguely unwell, then suddenly swollen, weak, and lightheaded.

Early symptoms people notice first

Common early clues include:

Sudden or rapidly worsening swelling in the legs, arms, face, or whole body (anasarca).
Fast weight gain over 12–48 hours.
Lightheadedness, near-fainting, or fainting, especially when standing.
Severe fatigue, muscle aches, or flu-like symptoms.
Nausea, abdominal pain, or diarrhea before the swelling becomes obvious.
Reduced urine output as the kidneys receive less circulating blood.

These symptoms become more concerning when they cluster with signs of low circulating volume:

Cold hands and feet
Rapid heart rate
Confusion or unusual sleepiness
New shortness of breath (especially later in the course)

The complications clinicians worry about most

Shock and organ injury

When blood pressure falls, organs may be underperfused. The kidneys are particularly vulnerable, and acute kidney injury can develop quickly.

Compartment syndrome

As fluid collects in muscles and tight tissue compartments, pressure can rise enough to cut off blood flow. Warning signs include severe limb pain out of proportion, tense swelling, numbness, tingling, or weakness. This is a surgical emergency.

Rhabdomyolysis

Muscle injury can occur from poor perfusion and high compartment pressures, releasing muscle proteins into the blood and worsening kidney injury.

Blood clots

Hemoconcentration and immobility can increase clot risk. Clinicians weigh clot prevention carefully, especially when blood pressure is low and bleeding risk may be present.

Pulmonary edema during the recovery phase

When the leak stops and fluid shifts back into the bloodstream, the lungs can fill with fluid—particularly if large IV fluid volumes were given earlier. This is one reason modern care often emphasizes careful, targeted fluid strategies rather than unlimited “flooding.”

When symptoms should be treated as an emergency

Seek urgent care immediately if any of the following appear:

Fainting or inability to stand due to dizziness
Confusion, severe weakness, or bluish lips
Severe limb pain with tight swelling
Rapidly worsening swelling with low urine output
Shortness of breath, especially if it worsens when lying flat

In CLS, timing matters. Early recognition can reduce downstream complications, especially the cascade of shock, muscle injury, and dangerous fluid overload later.

How doctors diagnose it

Diagnosis usually combines clinical pattern recognition with targeted tests that rule out look-alikes. Because capillary leak can mimic more common conditions—sepsis, allergic reactions, heart failure, kidney failure, liver disease, or severe dehydration—clinicians often diagnose it by asking, “Do the findings fit the leak pattern better than the alternatives?”

Core clinical and lab features

In systemic forms (especially idiopathic SCLS), clinicians look for a triad during an acute episode:

Hypotension (low blood pressure) or shock
Hemoconcentration (elevated hematocrit/hemoglobin)
Hypoalbuminemia (low albumin) without another clear explanation

Other supportive findings can include:

Rapid generalized edema
Elevated lactate (a marker of poor perfusion)
Rising creatinine and reduced urine output
Low total protein
Evidence of fluid around organs on imaging

Tests commonly used

Blood tests

Complete blood count (to assess hemoconcentration and platelets)
Comprehensive metabolic panel (kidneys, electrolytes, liver markers)
Albumin and total protein
Creatine kinase (to check for rhabdomyolysis)
Lactate, blood gases (to assess shock severity)
Inflammatory markers when infection is suspected

Urine tests

Urinalysis and urine output monitoring help assess kidney involvement and alternative causes of swelling.

Cardiac and lung evaluation

Electrocardiogram and sometimes cardiac ultrasound to assess heart function.
Chest imaging if breathing symptoms appear or to check for pulmonary edema or effusions.

Evaluation for monoclonal protein (when SCLS is suspected)

Serum protein electrophoresis, immunofixation, and free light chains may be ordered, because many idiopathic cases are associated with a monoclonal gammopathy.

Ruling out close mimics

Doctors often need to distinguish CLS from:

Anaphylaxis or angioedema: may also cause low blood pressure and swelling, but tends to feature hives, airway symptoms, and a more immediate allergic trigger.
Heart failure: usually causes fluid overload without hemoconcentration and without the same shock profile.
Nephrotic syndrome or liver failure: can cause low albumin and edema, but the onset is usually less abrupt and blood pressure is not typically profoundly low from volume loss.
Sepsis: can overlap significantly; in secondary CLS, sepsis may actually be the trigger, so the distinction becomes a matter of emphasis and management priorities.

A practical point: if a patient is in shock with swelling, clinicians often treat first (stabilize blood pressure and organ perfusion) while continuing the diagnostic workup. CLS is often confirmed by how the data evolve over hours and how well alternative explanations fit.

Treatment and what to expect in hospital

Treatment depends on whether CLS is idiopathic and recurrent or secondary to an identifiable trigger, but the acute priorities are the same: stabilize circulation, protect organs, and prevent the “second-hit” complications that can occur when fluid shifts back into the bloodstream.

Acute treatment goals

1) Stabilize blood pressure and organ perfusion

Many patients require ICU-level monitoring.
Vasopressors (medications that tighten blood vessels and raise blood pressure) may be needed when fluids alone are unsafe or insufficient.

2) Use fluids carefully

Traditional shock care often emphasizes aggressive fluids. In CLS, fluids can be necessary, but they can also worsen tissue swelling and increase the risk of compartment syndrome.
Many teams use a more targeted fluid approach: enough to maintain perfusion, while avoiding very large cumulative volumes.

3) Consider albumin and blood products selectively

Albumin may be used in some cases, but it can also leak out during the active phase. Clinicians weigh timing and response carefully.

4) Treat the trigger (for secondary CLS)

If sepsis is suspected, early antibiotics and source control can be lifesaving.
If a medication reaction is suspected, stopping the offending agent is critical.
If CLS is associated with a malignancy or inflammatory condition, therapy may focus on that underlying driver.

Therapies used in idiopathic SCLS (Clarkson disease)

Intravenous immunoglobulin (IVIG)

IVIG is a cornerstone in many idiopathic cases, used both for prevention and, increasingly in some centers, early in acute episodes in carefully selected patients.
Preventive dosing is commonly given on a regular schedule (often monthly), tailored by specialist teams based on attack history and tolerance.

Other supportive or adjunctive strategies

Some patients have historically received agents that influence vascular tone or endothelial signaling (for example, bronchodilator-related therapies), but practice varies and decisions are individualized.

Managing complications during hospitalization

Compartment syndrome: frequent limb exams; early surgical consultation if pain and pressure signs appear.
Rhabdomyolysis: aggressive monitoring of creatine kinase, kidney function, and electrolytes.
Thrombosis prevention: clot risk is real, but the approach is individualized based on blood pressure stability and bleeding risk.
Recovery-phase fluid overload: careful diuresis (fluid removal) may be needed when fluid returns to the bloodstream, especially if breathing worsens.

The hospital course can feel like a moving target because the physiology changes across phases. A useful way to frame it is: early on, the problem is “not enough blood in the pipes,” and later, the problem can become “too much returning fluid in the pipes.” The treatment strategy shifts accordingly.

Management, prevention, and when to seek care

After an episode, the most important work often happens outside the ICU: preventing recurrence, recognizing early warning signs, and reducing the risk of delayed complications.

Building a prevention plan

If idiopathic SCLS is suspected, long-term care often includes:

Specialist follow-up (commonly allergy-immunology, hematology, rheumatology, cardiology, or nephrology depending on the case).
A relapse-prevention strategy, which may include scheduled therapies such as IVIG in patients with recurrent attacks.
Evaluation and monitoring for monoclonal gammopathy, which may remain stable for years but still needs periodic reassessment.

If secondary CLS occurred, prevention usually focuses on:

Trigger avoidance (for example, documenting a suspected medication association clearly in medical records).
Controlling the underlying disease (infection risk reduction, cancer therapy planning, or autoimmune disease management as appropriate).

Practical day-to-day risk reduction

While there is no one-size-fits-all lifestyle plan, these steps are often sensible:

Keep an up-to-date emergency summary: prior diagnosis, typical lab pattern, medications, and your treating specialist’s contact details. Many patients carry this in a wallet card or phone note.
Track early signals when you have a history of attacks:
daily weight for a short period after infections or travel
swelling changes in rings, shoes, or socks
urine output trends
Act early during viral illnesses: if you have idiopathic SCLS, ask your care team what to do at the first sign of a prodrome. Some plans include early evaluation rather than “wait and see.”

When to seek medical care urgently

Seek emergency care immediately if you have:

New or rapidly worsening swelling plus dizziness or fainting
Very low urine output, confusion, or severe weakness
Severe limb pain with tight swelling, numbness, or weakness
New or worsening shortness of breath, especially after receiving IV fluids

For non-emergency but prompt evaluation, contact your clinician if you notice:

Unexplained weight gain over 24–48 hours
Swelling that is spreading rather than localized
Recurrent “flu-like” prodromes that previously preceded attacks

Outlook

Outcomes vary widely. Secondary CLS may resolve completely once the trigger is treated. Idiopathic SCLS can be recurrent and severe, but many patients achieve fewer and less intense attacks when prevention is optimized and when episodes are treated early with a phase-aware approach that avoids preventable complications.

If you remember just one planning principle, make it this: capillary leak is not only about swelling. It is also about what is happening to circulating volume and organ perfusion underneath the swelling. A clear action plan for early symptoms can be as important as any hospital therapy.

References

Disclaimer

This article is for educational purposes and does not replace individualized medical advice, diagnosis, or treatment. Capillary leak syndrome can be life-threatening and may require urgent emergency care. If you suspect an episode—especially with fainting, low blood pressure, severe swelling, breathing difficulty, confusion, or intense limb pain—seek emergency help immediately. Treatment decisions (including fluids, vasopressors, immunoglobulin therapy, and preventive strategies) must be tailored by qualified clinicians based on your history, exam, and test results.

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