Refining functional phenotypes in an international cohort of untreated paediatric type 2 and 3 SMA patients using the Revised Hammersmith Scale.

Spinal muscular atrophy types 2 and 3 encompass a wide spectrum of motor abilities ranging from non-sitting to sitting and walking. This study refines a functional group termed high functioning sitter-standers, positioned between traditional categories, and examined in relation to both the Revised Hammersmith Scale and a World Health Organization motor milestone-based framework. Among 178 participants completing 618 assessments, 109 were classified as type 2, 59 as type 3a, and 10 as type 3b, with ages ranging from 1 to 17.5 years. Twenty-seven non-sitters completed 54 assessments, 110 sitters completed 347, and 50 walkers completed 169, while the high functioning sitter-standers accounted for 48 assessments of 21 individuals. This newly defined group scored significantly lower than walkers and higher than both sitters and non-sitters, highlighting a distinct and measurable functional profile. Although no significant differences in age distribution were observed between the high functioning sitter-standers and walkers or non-sitters, sitters were notably younger. This intermediate phenotype captures patients with partial standing and assisted walking abilities, often overlooked in previous analyses. Recognition of this group is important for understanding emerging functional trajectories in treated spinal muscular atrophy and for informing future outcome measures and quality of life assessments.