Scientists peek inside muscles of kids with SMA to see how drugs really work

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Genetic confirmation of SMA with homozygous deletion of SMN1 or compound heterozygous deletion/mutation of SMN1 * Two, three, or four copies of SMN2 * Age 5 to 20 years * Non-ambulatory participants: maximum function sitting or standing with support, HFMSE score at screening between 10 and 45 points. * Ambulatory participants: minimum function of independent walking, able to walk unassisted a minimum of 100 meters at screening, HFMSE score at screening between 40 and 66. * SMN-directed therapy inclusion: * Current Evrysdi prescription (Group 1) * Must have Evrysdi prescription through their treating physician * If initiating combined therapy using Evrysdi with Spinraza or Zolgensma, must have not started Evrysdi treatment OR * Current Spinraza or Zolgensma prescription (Group 2) * For patients on Spinraza, must have been taking Spinraza for at least 12 months at screening (4 loading and 2 maintenance doses) and following the FDA-recommended dosing schedule * For patients on Zolgensma, must have been dosed at least one year prior to screening * Must have Spinraza or Zolgensma prescription through their treating physician OR * Changing from Spinraza or Zolgensma to Evrysdi (Group 3) * For patients on Spinraza, must have been taking Spinraza for at least 12 months at screening (4 loading and 2 maintenance doses) and following the FDA-recommended dosing schedule * For patients on Zolgensma, must have been dosed at least one year prior to screening * Must have voluntarily decided to switch therapies based on discussion with their treating physician * Must have Evrysdi prescription through their treating physician but have not yet initiated treatment OR * Have never received any SMN-directed therapies (Group 4) Exclusion Criteria: * Any chronic medical condition, planned surgery, or treatment with a medication which would impact safety or participation of the study at the investigator's discretion * Inability to perform reliably the motor function testing or the exercise testing in the MR scanner. * Fat fraction \> 35% in calf or bicep at screening MRI * Need for routine non-invasive ventilation support. * Non-oral nutritional support, e.g., gastrostomy tube feeding. * Any ferrous metal implants (e.g., spinal rods) that preclude testing in a MR scanner.