Thoracic circumference: A new outcome measure in spinal muscular atrophy type 1?

Since respiratory insufficiency is the first cause of morbidity and mortality in spinal muscular atrophy type 1 (SMA 1), specific respiratory outcome measures are needed to evaluate changes and assess innovative therapies. In this study, thoracic circumference (TC) was used as a proxy for chest growth and an indirect measurement of respiratory function. The anthropometric parameters including TC and head-circumference (HC) were evaluated from birth to 13 months in 19 infants with SMA 1 and 124 control infants. TC was significantly decreased in the SMA 1 group from the first weeks of life. The control group TC/HC ratio = 1 (± 0.04), and was not found to be associated with age. By contrast, it decreased with time in all infants with SMA 1 and those with a TC/HC ratio <0.85 died within 3 months. TC is a simple measurement that provided an index of chest growth and was used as evidence of early, progressive respiratory failure and under-development of the rib-cage in SMA 1. The TC/HC ratio decreased in all patients over time, reflecting the progression of the disease suggesting that TC/HC ratio could be a new measure for SMA 1 for measuring disease severity and prognosis.