Hip Displacement in Spinal Muscular Atrophy in the Early Disease-Modifying Therapy Era: Association With Hip Pain and Timing of Therapy Initiation

Background: Spinal muscular atrophy (SMA) destroys spinal cord anterior horn cells, causing progressive muscle weakness and atrophy. Despite the introduction of disease-modifying therapies (DMTs), their effects on hip displacement remain unclear. This study investigated the development and progression of hip displacement in SMA, with emphasis on the impact of DMTs. Methods: Patients diagnosed with SMA between 2000 and 2022 who underwent hip surveillance ≥3 times at different ages were included. SMA classification, bilaterality, radiographic parameters, age at DMT initiation, and hip pain were analyzed. Hip displacement was classified by migration percentage. Study-defined early (SDE) DMT use was defined as treatment before 6 months in type 1 SMA or before 2 years in type 2 SMA. Results: Twenty-one patients (10 with type 1 and 11 with type 2 SMA) were included, with a mean follow-up of 31±6 months. In type 1 SMA, 90% of patients developed hip displacement (2 with subluxation and 7 with dislocation), occurring at mean ages of 22.0±7.0 months and 33.0±7.8 months, respectively. In type 2 SMA, 72% of patients developed hip displacement (7 with subluxation and 1 with dislocation), occurring at mean ages of 37.8±16.2 months and 32 months, respectively. SMA classification ( P =0.016), bilaterality ( P <0.001), presence of hip pain ( P =0.006), and final acetabular index ( P =0.002) were associated with final hip displacement. SDE DMT use was not statistically significant ( P =0.163); however, one type 1 SMA patient who initiated DMT at 1 month maintained a contained hip throughout follow-up. Hip pain was absent in contained or subluxated hips but occurred in 75% of dislocated hips. Conclusions: Hip displacement in SMA begins early, with progression patterns differing by disease severity. Hip displacement is also associated with pain, which was observed primarily in dislocated hips. These findings highlight the need for routine hip surveillance from early infancy and pain assessment. DMT initiation may confer only minimal benefit when instituted at a later age, but may yield more substantial benefit when administered presymptomatically soon after birth. Level of Evidence: Level III.