Biochemical studies performed on cultured myoblasts harbouring SPG7 mutations have revealed a reduction in citrate synthase-corrected complex I activity, again suggesting that impaired OXPHOS plays an important role in the pathogenesis of HSP, and by extension the optic neuropathy observed in a subgroup of patients (Wilkinson et al., 2004). This evidence concerns the gene SPG7 and Optic neuropathy.