Characterizing Hip Instability in Children with Spinal Muscular Atrophy

Research study background

Spinal muscular atrophy (SMA) is a medically complex neuromuscular disorder with a broad variance in severity ranging from type 0 (most severe) to type 5 (least severe). The introduction of nusinersen and other disease-modifying drugs in recent years is improving life expectancy and functional capabilities for many of these patients.

Hip subluxation and dislocation are well known problems in this population, yet the natural progression of hip instability in children across all types of SMA has not been studied and the specific characteristics of the acetabulum (hip socket) in these patients is not well understood. As more children establish or maintain ambulation, the prevalence of hip pain and need for surgical intervention are likely to increase, along with the need for more proactive orthopedic management of hip instability. In this study, experts in the Orthopedics Institute at Children’s Hospital Colorado aimed to address gaps in the existing literature by characterizing the progression of hip instability in SMA patients.

Researchers retrospectively reviewed the medical records of all children diagnosed with SMA at Children’s Colorado before age 25 and between 2013 and 2018. Of the 68 children identified, the final analysis focused on a cohort of 45 children with SMA types 1, 2 and 3. They employed a validated hip surveillance framework for cerebral palsy that effectively assesses the risks of hip subluxation. The team analyzed radiographs to measure longitudinal changes within individual hips across three key metrics. The migration index (MI) quantified femoral head containment, while the acetabular index (AI) and acetabular depth ratio (ADR) assessed acetabular shape.

While all SMA types experience hip subluxation, results revealed that it occurs at an earlier age, is more prevalent and progresses more rapidly in patients with type 1, compared to those with type 2 and 3 as measured by MI. Specifically, in type 1, MI progressed three times faster than in type 2 and six times faster than in type 3. ADR values consistently declined in type 1 and type 2 SMA as age increased, while type 3 values remained relatively stable over time and were not statistically significant. When AI measurements were compared with age-matched, typically developing children, values were higher across all types of SMA. In typically developing children, AI values decrease with age, indicative of femoral head that is well-contained within the hip socket.

“This investigation demonstrated we need to be more mindful of hip instability in this population and more aggressively treat hip pathology to improve quality of life now that individuals live longer due to advancements in disease-modifying drugs,” says Kellen Krajewski, PhD, assistant research professor and one of the study authors.

Clinical implications

Data from this study offers early insight into the natural progression of hip morphology in individuals with SMA, categorized by severity type.

The study authors recommended that clinicians should adopt cerebral palsy monitoring protocols for hip surveillance to guide the standard of care for patients with SMA. Currently, the team is conducting prospective studies to further advance treatment options and guide orthopedic intervention in SMA.