Nutcracker syndrome: a potentially underdiagnosed cause of proteinuria in children with familial Mediterranean fever

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Abstract
Introduction Familial Mediterranean fever (FMF) is the most common hereditary autoinflammatory disease with an increased risk for secondary amyloidosis. Since lifelong colchicine has been the treatment of choice that prevents renal amyloidosis, non-amyloid kidney diseases are more frequently considered in the differential diagnosis of proteinuria. Nutcracker syndrome (NCS) can be one of the confounding causes. This long-term retrospective study aimed to evaluate the causes of proteinuria in a pediatric cohort of patients with FMF and discuss changing trends in recent years . Methods Demographic, clinic, and laboratory data were extracted from electronic medical records of patients with FMF. All urine tests of the study population were reviewed. Patients were evaluated for persistent proteinuria and grouped according to the etiology of proteinuria. Results A total of 576 patients with FMF were identified with a mean follow-up of 6.3 years in the last 10 years; 8% had persistent proteinuria. The etiology was NCS in 67.5% of the patients with proteinuria, and renal amyloidosis was less commonly encountered (15%) without any new diagnosis for the last 8 years. Non-amyloid kidney diseases were also diagnosed in 17.5% of the patients. Patients with NCS had significantly lower BMI than other patients in the cohort and less subclinical inflammation, higher hemoglobin concentration, and milder levels of proteinuria with normal serum albumin and eGFR than other patients with proteinuria. Conclusion Nutcracker syndrome is the leading cause of proteinuria in children with FMF nowadays, and it should be kept in mind during the evaluation of proteinuria in these patients. Graphical Abstract A higher resolution version of the Graphical abstract is available as Supplementary information.

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