Alterations of thoraco-abdominal volumes and asynchronies in patients with spinal muscle atrophy type III

• In SMA motor neurons degenerate and determine muscle wasting and weakness.
• In SMA II and IIIA, intercostal muscles are weakened and the diaphragm is spared.
• In SMA II and IIIA, diaphragm leads inspiration with possible thoracic inwarding.
• In SMA IIIB all inspiratory muscles are preserved.
• SMA III sub-classification is valid both for motor and respiratory function.

Spinal muscular atrophy (SMA) is characterized by degeneration of motor neurons resulting in muscle weakness. For the mild type III form, a sub-classification into type IIIA and IIIB, based on age of motor impairment, was recently proposed. To investigate if SMA IIIA (more severe) and IIIB differ also in terms of respiratory function, thoracoabdominal kinematics was measured during quiet breathing, inspiration preceding cough and inspiratory capacity on 5 type IIIA and 9 type IIIB patients. Four patients with SMA II (more severe than types III) and 19 healthy controls were also studied.Rib cage motion was similar in SMA IIIB and controls. Conversely, in SMA IIIA and SMA II it was significantly reduced and sometime paradoxical during quiet breathing in supine position. Our results suggest that in SMA IIIA intercostal muscles are weakened and the diaphragm is preserved similarly to SMA II, while in SMA IIIB the action of all inspiratory muscles is maintained. Sub-classification of type III seems feasible also for respiratory function.