The Spinal Muscular Atrophy Functional Classification System

Area of Science:

• Neurology
• Clinical Medicine
• Rehabilitation Medicine

Background:

• Spinal muscular atrophy (SMA) is a progressive neuromuscular disorder.
• The advent of disease-modifying agents has altered SMA phenotypes.
• A refined classification system is needed to reflect current clinical realities.

Purpose of the Study:

• To develop a functional mobility classification for SMA, termed the Spinal Muscular Atrophy Functional Classification System (SMAFCS).
• To assess the association of the SMAFCS with key clinical outcomes including hip displacement, scoliosis, and pulmonary function.
• To establish thresholds using the Hammersmith Functional Motor Scale Expanded (HFMSE) for each SMAFCS level.

Main Methods:

• Sixty patients with SMA underwent HFMSE assessments over a mean period of 7.5 years.
• The SMAFCS was constructed using Functional Mobility Scale groupings and HFMSE data.
• Receiver-operating characteristic analysis determined HFMSE thresholds for each SMAFCS level.
• Associations between SMAFCS levels and hip displacement, scoliosis, and pulmonary function (forced vital capacity) were analyzed.

Main Results:

• HFMSE thresholds were established for SMAFCS levels I-IV (AUC 0.62-0.89).
• Age-related motor function trajectories differed significantly across SMAFCS levels I, III, and IV.
• Prevalence of hip displacement and scoliosis increased with SMAFCS level (p<0.008).
• Pulmonary function significantly declined with higher SMAFCS levels (p<0.001).

Conclusions:

• The SMAFCS shows potential as a valuable tool for stratifying gross motor function in SMA patients.
• This classification system may aid in predicting the risk of specific clinical complications.
• Further research is warranted to evaluate the reliability and responsiveness of the SMAFCS to change over time.