*3.1. Familial Forms of FMD*

Since 1960s to 1970s, a number of cases of familial FMD, i.e., occurrence of FMD in two or more siblings or relatives, have been reported [26–31]. We report a case of 3 siblings affected by hypertension and renal artery FMD followed at the Grand Hôpital de Charleroi, Gilly, Belgium (Figure 1), as an illustration. Interestingly, while two of them had typical string-of-beads lesions, the third had an isolated stenosis compatible with focal FMD.

In the absence of systematic exploration of first-degree relatives of patients with FMD, the prevalence of familial FMD remains unclear. In a French cohort of 100 unrelated patients with renal artery FMD, FMD lesions were documented by angiography in at least one first-degree relative in 11% of patients [32]. In the first report of the US registry, 7.3% of 447 patients reported a history of FMD in at least another 1st or 2nd degree relative [3]. However, in recent reports this percentage tends to be substantially lower: 2.9% in the ARCADIA study [4] and 2.8% in European registry (A. Persu, personal communication). The prevalence of familial FMD may be higher in paediatric than in adult patients (17.2% of 29 children vs. 4.7% of 864 adult patients with FMD in a recent report of the US registry) [33].

**Figure 1.** Example of familial FMD from Belgian Multicentric FMD Cohort (BEL-FMD). Patient II.2 came to clinical attention at the age of 64 years for de novo arterial hypertension. The abdominal Computed Tomography Angiogram showed an aspect compatible with multifocal FMD of both renal arteries, significant on the right side. She underwent right percutaneous transluminal renal angioplasty (PTRA), and the hypertensive crises regressed. Patient II.4 was hypertensive from the age of 53 years and was referred at the age of 59 years for worsening of blood pressure control, associated to decreased renal function. Abdominal Magnetic Resonance Angiography (MRA) showed mild irregularities suggestive of FMD in the right renal artery, in the absence of significant stenosis. Patient II.6 came to clinical attention at the age of 58 years for renal asymmetry (kidney length: 9.5 cm on the right side vs. 12 cm on the left). The abdominal MRA identified a focal stenosis of 70% of the right renal artery. Patient II.6 underwent renal arteriography, which disclosed an irregular aspect of the arterial wall on the left side and only a 30% stenosis on the right side. Notably, none of these three patients had lesions of cervico-cephalic or others vascular beds. Estimated Glomerular Filtration Rate (eGFR) was calculated using the Modification of Diet in Renal Disease (MDRD) equation.
