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Background: Hereditary spastic paraplegias type 5 (SPG5) is an inherited neurodegenerative disease with 27-hydroxycholesterol abnormal accumulation. Imaging and pathologic manifestations remain poorly understood due to the rare incidence. This study reveals the MRI features of SPG5, and aims to investigate a promising imaging diagnostic biomarker for SPG5.Methods: We prospectively recruited SPG5 patients and matched healthy controls from Neurogenetic Diseases Centers of Fujian province in China, clinical and MRI data of whom were collected. Abnormalities of spinal cord and brain were characterized and quantified by conventional and quantitative MRI. Comparisons were conducted between MRI and cerebrospinal fluid (CSF) bioindicators.Results: Seventeen SPG5 patients were enrolled (11 men, 6 women; age range, 13–49 years; median disease duration, 14 years). For the first time, T2 hyperintensities with “+” form (cross sign) in atrophic spinal cord was found among all SPG5 patients. To grade severity of this sign, we set up a scoring scale (cross-sign scores) in cervical spinal cords. Unexpectedly, total cross-sign scores showed a strong positive correlation with disability scale scores (r = 0.687, P = 0.002) and disease duration (r = 0.520, P = 0.032). Although total spinal cord area was reduced (cervical levels: 12-27%; thoracic levels 41-60%), no correlation was found between spinal cord atrophy and disease severity. In CSF, a positive correlation was identified between 27-hydroxycholesterol and neurofilament light (r = 0.468, P = 0.049), although 27-hydroxycholesterol and neurofilament light were unrelated to disease severity.Conclusion: Cross sign of spinal cord was established as a potentially diagnostic biomarker linked to SPG5 that can guide genetic testing and interpret genetic results. Moreover, cross-sign scoring scale is more sensitive than spinal cord area and CSF markers for monitoring SPG5 progress in our research. more...