ATL1 and hereditary spastic paraplegia: We have calculated cross-sectional SPRS disease progressions for SPG4, SPG3A and SPG11; which are valuable information considering the lack of clinical course and natural history data on validated instruments for HSP; however, future longitudinal studies are required to establish progression rates in HSP in a more reliable way and with lower chance of recall bias related to information regarding age at onset and disease duration.