The Inferior Cerebellar Peduncle Sign: A Novel Imaging Marker for Differentiating Multiple System Atrophy Cerebellar Type from Spinocerebellar Ataxia

BACKGROUND AND PURPOSE: The hot cross bun (HCB) sign is a hallmark feature of multiple system atrophy with predominant cerebellar ataxia (MSA-C), typically observed in advanced stages of the disease; however, it can also present in other conditions such as spinocerebellar ataxia (SCA), making the differentiation challenging. The middle cerebellar peduncle (MCP) sign may be observed in various medical conditions and in healthy individuals. We hypothesized that the inferior cerebellar peduncle (ICP), known to be affected in MSA-C, may exhibit hyperintensity on FLAIR imaging, potentially aiding in differentiating MSA-C from SCA. MATERIALS AND METHODS: Medical records of 153 patients with probable MSA-C and 72 genetically confirmed SCAs from a single institution were reviewed retrospectively between January 2012 and June 2023. MRI was performed using 3T scanners. The ICP sign was deemed positive when the bilateral ICP signal intensity exceeded that of the medulla oblongata on axial FLAIR images. MCP and HCB signs were also evaluated. Two independent neuroradiologists evaluated all MRIs, and interobserver agreement was assessed using k statistics. Univariable and multivariable logistic regression analyses identified predictive features, and diagnostic performance was assessed. RESULTS: The ICP sign was more prevalent in patients with MSA-C (65%) compared with those with SCA (6.9%; P, .001). The HCB and MCP signs were more frequent in patients with MSA-C (n = 110 and n = 134) than in those with SCA (n = 19 and n = 30; P, .001). The ICP sign demonstrated the highest specificity (95%) for predicting MSA-C, with an area under the curve (AUC) = 0.82, respectively. The MCP sign exhibited superior sensitivity (87%) but lower specificity and AUC compared with the ICP sign. Combining the ICP and MCP signs improved the AUC to 0.86. Integrating clinical features (age, sex, and disease duration) with imaging features yielded excellent diagnostic performance, with an AUC = 0.98. CONCLUSIONS: The ICP sign on FLAIR imaging has high specificity in distinguishing MSA-C from SCA. Integrating clinical and imaging features further enhances diagnostic accuracy, potentially improving the differential diagnosis in clinical settings of cerebellar ataxia.