Role of NO pathway in the clinical picture of idiopathic systemic capillary leak syndrome

Dear Editor,

Clarkson's disease, also known as idiopathic systemic capillary leak syndrome (ISCLS), is a rare and potentially lethal paroxysmal permeability disorder characterized by unpredictable episodes of massive plasma leakage from the intravascular to the extravascular compartment, most often into muscular tissues, leading to shock and ultimately multiple organ failure.

The underlying pathophysiological mechanisms of ISCLS are not well understood. They are believed to involve hypersensitivity of the capillaries to inflammatory or immune system triggers, which result in the transitory derangement of inter-endothelial junctions, causing the abrupt loss of endothelial barrier function. Most ISCLS patients have a monoclonal gammopathy of undetermined significance, which is a characteristic trait in the adult form of ISCLS and has been hypothesized to play a role in the pathophysiology, although not yet characterized. Therefore, the term “monoclonal gammopathy of clinical significance” appears more appropriate with respect to Clarkson's disease. Patients’ sera, collected during the acute and intercritical phases, have been reported to significantly increase the permeability of human umbilical vein endothelial cells (ECs) compared to sera from healthy controls [1].

Recently, it has been demonstrated experimentally that ECs derived from ISCLS patients were functionally hyperresponsive to the two permeabilizing factors vascular endothelial growth (VEGF) and histamine [2]. There is evidence that the main alteration lies in the hyperactivation of the endothelial nitric oxide synthase (eNOS) in ISCLS-derived ECs. Furthermore, eNOS blockade by the administration of NG-nitro-l-arginine methyl ester (NAME) improved vascular leakage in a mouse model of ISCLS after histamine or VEGF challenge [2]. On the other hand, methylene blue, which blocks the nitric oxide (NO) pathway by inhibiting the guanylate cyclase [3] and is generally used in conditions characterized by increased levels of NO, like septic shock [4], did not show efficacy in ISCLS. There is only an anecdotal report of a patient who was successfully treated with methylene blue during an ISCLS crisis. Remarkably, this patient suffered from several attacks per year, and repeated administrations of methylene blue did not yield positive results [5]. Furthermore, the measurement of circulating NO metabolites in both acute and intercritical sera revealed no significant difference in NO levels in samples from ISCLS patients compared to control [1].

Given the suggested central role of the NO pathway in the pathogenesis of ISCLS flares and considering the intrinsic hyperresponsiveness of ISCLS ECs, we decided to assess the endothelial function of ISCLS patients in vivo using a NO-dependent noninvasive functional methodology. Specifically, we employed peripheral arterial tonometry at the fingertip through an automated reactive hyperemia test (EndoPAT 2000, Itamar, Cesarea, Israel). Reactive hyperemia tonometry is strongly modulated by NO synthesis in healthy ECs during an ischemic test. However, the hyperemic response is blunted in certain pathological conditions (such as atherosclerosis, hypertension, heart failure, diabetes mellitus, and chronic kidney disease), as well as by NAME pretreatment [6, 7].

Nine ISCLS patients, 66.6% men, with a median age of 56 years (IQR 47.5–56), were studied during an intercritical phase, and seven non-ISCLS subjects served as a control group.

The median natural logarithm of the reactive hyperemia index (LnRHI) was 0.77 (IQR 0.54–0.81) in ISCLS patients compared to 0.59 (IQR 0.54–0.67) in controls, p = 0.285 (Fig. 1). Only one ISCLS patient had LnRHI below the normal threshold, suggesting endothelial dysfunction with reduced rather than increased NO synthesis. Additionally, a parameter reflecting arterial stiffness, the augmentation index, was not significantly different between ISCLS patients and controls (median of 4.4% [IQR -1.9–31.8] vs. 6% [IQR 0–28], respectively, p = 0.95).

Although eight out of nine ISCLS patients had reactive hyperemia responses within the normal range, this finding does not rule out pathological eNOS activity in ISCLS patients. Reactive hyperemia is not validated for “hyper-response,” as is expected in the case of NO-mediated hyperpermeability in ISCLS ECs.

In conclusion, despite the convincing experimental evidence that NO plays a central role in the pathophysiology of ISCLS, the NO-mediated reactive hyperemia appeared unaffected in a clinical setting during the intercritical period.

This finding does not rule out the role of the NO pathway during the acute phase. It might be consistent with the so-called threshold hypothesis, which suggests that the hyperresponsiveness of ISCLS ECs is also present during intercritical phases but manifests openly only when additional (often multiple) triggers occur. Therefore, the quasi-normal response to the reactive hyperemia test suggests that the involvement of the NO pathway might not be straightforward in vivo. Further research is needed to address several unresolved questions about the clinical scenarios, which likely stem from a complex interplay of different pathways.

The authors declare no conflicts of interest.

This research received no external funding.

The Ethical Committee Area 1 of Milan approved this study and written informed consent has been obtained from the patients.