Abstract Arginase 1 deficiency (ARG1‐D) is an ultrarare, metabolic disease which may cause spastic paraplegia, cognitive deficiency, seizures, and ultimately severe disability. The aim of this study was to assess disease burden in ARG1‐D by performing a cross‐sectional survey of patients with ARG1‐D and their caregivers in four European countries (France, Portugal, Spain, and the United Kingdom). Patients were enrolled at participating clinics and data were collected using a web‐based questionnaire. The findings indicate that there is a significant share of patients who experience severe cognitive and mobility impairment but also that there is a considerable variance in symptom severity among patients. Disease management was mostly in line with treatment guidelines and self‐reported adherence to treatment was reported to be high among a majority although following diet restrictions was perceived as difficult. However, despite this, since a large share of patients experienced severe cognitive and mobility impairment an unmet need among this patient population is suggested. The introduction of disease‐modifying therapies and early identification and diagnosis may help alleviate the disease burden associated with ARG1‐D in the future.
