Facioscapulohumeral muscular dystrophy diagnosed in childhood: a muscular dystrophy surveillance, tracking and research network cohort.

Reports of clinical characteristics of FSHD in childhood have often focused on the most severely affected. To include children in clinical trials, it is important to understand the full pediatric spectrum. The Muscular Dystrophy Surveillance, Tracking, and Research Network provides a population-based, cross-sectional cohort of children diagnosed with FSHD. Children diagnosed before 19 years of age during 2008-2019 were included (n = 93). Clinical characteristics abstracted from the medical record included motor symptoms, genetic test results, and family history. Of those with known FSHD type (n = 73 of 93, 78.8 %), 72 (98.6 %) had FSHD1. Among those with FSHD1, the most frequent D4Z4 repeat category was 1-3 repeats (n = 33, 45.8 %) and few had 7-10 repeats (n = 7, 9.7 %). Among those with FSHD1 and known symptom onset age (n = 70), 22 (31.4 %) had early onset disease. Overall, most (88.2 %) were ambulatory at last visit. The number of children diagnosed increased steadily across childhood, suggesting a continuous spectrum of age at onset. The most common co-morbidity was hearing loss (n = 19 of 93, 20.4 %); most commonly among children with FSHD1 and 1-3 D4Z4 repeats (n = 13 of 33, 39.4 %). Our series provides data to support the design of pediatric FSHD trials, critical to ensure timely access to future effective treatments.