Idiopathic Systemic Capillary Leak Syndrome (ISCLS), also known as Clarkson\'s Syndrome, is due to primary fluid and protein leak across capillaries that leads to an accumulation of interstitial fluids and cardiovascular collapse from intravascular hypovolemia. Viral infections are a putative trigger of these episodes. ISCLS is typically associated with a monoclonal gammopathy. Here we present four patients with idiopathic systemic capillary leak syndrome. The cohort consists of three men and one woman who range in age from 55 to 72 years old. All of the patients had a monoclonal gammopathy. Two patients had viral triggers. Biopsies of normal skin were examined throughout all phases of the disease. During an acute attack, we identified perivascular mixed CD4+ and CD8+ T cell lymphocytic infiltrates in the superficial dermis. We observed significant microvascular deposits of C5b-9 and upregulation of type I interferon signaling in endothelium along with reduced serum levels of complement during very active disease. We also identified deposits of immunoglobulin along the dermal epidermal junction mirroring the monoclonal immunoglobulin isotype implicated in each patient. During a post treatment recovery or mild disease phase there was reduced inflammation and decreased amounts of C5b-9 and type I interferon expression. Sudden onset capillary leak syndrome reflects enhanced endothelial cell permeability as a unique form of endothelial injury mediated by the combined effects of complement pathway activation and upregulation of type I interferon signaling on endothelium.

Capillary leak syndrome (CLS) is an increasingly acknowledged multifaceted and potentially lethal disease. Initial nonspecific symptoms are followed by the intriguing CLS hallmark: the double paradox associating diffuse severe edema and hypovolemia, along with hemoconcentration and hypoalbuminemia. Spontaneous resolutive phase is often associated with poor outcome due to iatrogenic fluid overload during leak phase. CLS is mainly triggered by drugs (anti-tumoral therapies), malignancy, infections (mostly viruses) and inflammatory diseases. Its idiopathic form is named after its eponymous finder: Clarkson\'s disease. CLS pathophysiology involves a severe, transient and multifactorial endothelial disruption which mechanisms are still unclear. Empirical and based-on-experience treatment implies symptomatic care during the acute phase (with the eventual addition of drugs amplifying cAMP levels in the severest cases), and the prophylactic use of monthly polyvalent immunoglobulins to prevent relapses. As CLS literature is scattered, we aimed to collect and summarize the current knowledge on CLS to facilitate its diagnosis, understanding and management.