These subgroups include genetic forms (no-SPG4 vs. SPG4), phenotype presentation (complex vs. pure forms), clinical performances measured with the SPRS (scores directly proportional to mI values and inversely proportional to NAA), and longitudinal variations (increased mI in follow-up compared to baseline values in patients), all indicating a close correlation with the severity of HSP. Here, SPAST is linked to hereditary spastic paraplegia.