Validation of the shortened Hunter Syndrome-Functional Outcomes for Clinical Understanding Scale (HS-FOCUS)

BACKGROUND: The rare disease, Hunter Syndrome (mucopolysaccharidosis type II; MPS II), characterized by iduronate-2-sulfatase deficiency, has heterogeneous symptoms often including cognitive impairment (CI). To evaluate physical functioning and daily activity limitations of patients with MPS II, the multidomain shortened Hunter Syndrome-Functional Outcomes for Clinical Understanding Scale (HS-FOCUS) questionnaire was previously developed and preliminarily validated. Here we gather data in a dedicated prospective longitudinal observational study using direct responses to the shortened instrument and assess its psychometric properties further. METHODS: Interview data were collected from eligible self-reporting patients (≥ 12 years of age) or caregivers of patients using respective versions of the instrument at baseline and 2–4 weeks later. Internal consistency, test–retest reliability, convergent and discriminant validity, and validity of known groups were assessed. Participants also completed Child Health Questionnaire (CHQ), Health Utilities Index Mark 3, and Global Impression of Severity (GIS) questionnaires. RESULTS: All patients were male, consisting of 31 caregiver-reported patients (aged 3–26 years) and 20 self-reported patients (aged 12–58 years). Most (77.4%) caregiver-reported patients had CI. Both questionnaire versions demonstrated good internal consistency and test–retest reliability; Cronbach’s alpha and intra-class correlation coefficients were > 0.70. Spearman’s correlations demonstrated good convergent validity with moderate (> 0.3) to high (> 0.6) correlations of the HS-FOCUS total score with physical functioning, role/social–physical, and bodily pain domains of CHQ. The tool also differentiated between MPS II severity levels based on GIS scores. CONCLUSIONS: The shortened HS-FOCUS questionnaire was found to be a valid and reliable tool to assess the physical functioning impact of MPS II.