KIF1C-related disorders: spastic ataxia or hypomyelinating leukodystrophy? A paradigm of classification ambiguity

A 40-year-old man with adult-onset spastic-ataxia and tremor showed a leukoencephalopathy with a hypomyelinating pattern on brain MRI. Whole-exome sequencing identified two novel likely pathogenic variants in KIF1C, a gene associated with spastic-ataxia type 2 (SPAX2). This case supports including KIF1C among the causes of adult-onset hypomyelinating leukodystrophies and highlights the diagnostic overlap between spastic-ataxia, hereditary spastic paraplegia, and hypomyelinating leukodystrophies, underscoring the limitations of current nomenclature.