Systemic capillary leak syndrome (SCLS) is a condition in which fluid and proteins leak out of tiny blood vessels, into surrounding tissues. This can result in dangerously low blood pressure (hypotensison), hypoalbuminemia, and a decrease in plasma volume (hemoconcentration). Initial symptoms may include fatigue, nausea, abdominal pain, extreme thirst, and sudden increase in body weight. Episodes of SCLS vary in frequency, with some people having one episode in their lifetime, and others having several per year. The severity also varies, and the condition can be fatal.

In many cases the cause is not known (idiopathic SCLS). Some people with SCLS report having had an infection (such as a virus) before the episode. Some cases of capillary leak are associated with various conditions of the heart, kidney, or liver. Treatment during an episode may involve stabilizing the airway and breathing, and infusion of fluids, medications, or blood products. Periodic infusions or the use of certain medications may be used to prevent future episodes.

Periodic infusions or the use of certain medications are used to try to prevent future episodes (prophylactic therapy). Having monthly infusions of intravenous immune globulin (IVIG) has been suggested. A study published in 2017 found that preventive treatment with IVIG was the strongest factor associated with survival in people with SCLS. Those who do not improve with IVIG or who cannot tolerate the therapy may have success with theophylline and terbutaline.

Research into how SCLS develops and factors that affect its course are underway, in hopes that a better understanding of SCLS will lead to more effective treatment options and a lower mortality rate.