Patients and Design

A nationwide, prospective, cross‐sectional study was performed. Eligible patients had clinically suspected FSHD, defined as clinical weakness of the facial and/or upper arm muscles for which the patient had sought medical attention with exclusion of other diagnoses. Patients were 17 years or younger and resided in The Netherlands. Patients were recruited by (1) the database of our tertiary referral center, which was established in 1986; (2) neurologists/pediatricians from all neuropediatric clinics in The Netherlands; (3) the Department of Clinical Genetics of the Leiden University Medical Centre, the single reference center for genetic testing of FSHD in The Netherlands; and (4) by social media groups focusing on FSHD. This extensive recruitment ensured a high degree of inclusion (protocol published previously16). Patients were recruited from December 2015 until August 2017. Clinical and molecular data were collected prospectively. This study protocol has been approved by the Medical Review Ethics Committee region Arnhem‐Nijmegen (NL53213.091.15) and is in accord with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. Written informed consent from participants aged 12 to 18 years and from parents/legal guardians of all participants was obtained.