SPG11 and cerebellar ataxia: In contrast, patients with the most common form of autosomal recessive HSP, due to pathogenic loss-of-function mutations in SPG11, encoding spatacsin, have a more severe course with complications that can include thinning of the corpus callosum, CNS white matter changes, early-onset parkinsonism and ataxia, and intellectual disability [1, 2, 4, 6].