A novel clinicopathologic entity causing rapidly progressive cerebellar ataxia?

Sporadic, adult-onset cerebellar ataxia is a disease with multiple etiologies. In addition to cortical cerebellar atrophy (CCA), which is often used for the pathological diagnosis, other terms such as idiopathic late-onset cerebellar ataxia (ILOCA) and sporadic adult-onset ataxia of unknown etiology (SAOA) have been used to refer to this disorder. These names describe key features of the disease, including degeneration limited to the cerebellar cortex (with or without secondary involvement of inferior olivary nuclei), a slowly progressive ataxia, and absence of a clear etiology, such as multiple system atrophy, as well as paraneoplastic, autoimmune, infectious and inherited ataxias. In this Point of View article, we describe two patients with sporadic, adult-onset ataxia with rapidly progressive disease course in addition to extracerebellar symptoms resembling prion disease, including the reevaluation of one patient who was previously reported. Pathological findings are mostly consistent with CCA, but also have degenerative changes in the thalamus. Whole genome sequencing in two patients with rapidly progressive CCA did not reveal any pathogenic variants associated with cerebellar ataxia. Although the underlying etiology behind rapidly progressive CCA is unknown, we suggest that the unique combination of clinical and pathological features of CAA with a short disease course defines a new disease entity, rapidly progressive cerebellar cortical and thalamic degeneration. This viewpoint article draws attention to this rare sporadic cerebellar ataxia with the hope that highlighting clinical and pathologic findings in a typical case will lead to improved recognition and research.