Changes in climbing fiber-Purkinje cell (CF-PC) synaptic connections have been found in the essential tremor (ET) cerebellum, and these changes are correlated with tremor severity. Whether these postmortem changes are specific to ET remains to be investigated. We assessed CF-PC synaptic pathology in the postmortem cerebellum across a range of degenerative movement disorders [10 Parkinson’s disease (PD) cases, 10 multiple system atrophy (MSA) cases, 10 spinocerebellar ataxia type 1 (SCA1) cases, and 20 ET cases] and 25 controls. We observed differences in terms of CF pathological features across these disorders. Specifically, PD cases and ET cases both had more CFs extending into the parallel fiber (PF) territory, but ET cases had more complex branching and increased length of CFs in the PF territory along with decreased CF synaptic density compared to PD cases. MSA cases and SCA1 cases had the most severely reduced CF synaptic density and a marked paucity of CFs extending into the PF territory. Furthermore, CFs in a subset of MSA cases formed collateral branches parallel to the PC layer, a feature not seen in other diagnostic groups. Using unsupervised cluster analysis, the cases and controls could all be categorized into four clusters based on the CF pathology and features of PC pathology, including counts of PCs and their axonal torpedoes. ET cases and PD cases co-segregated into two clusters, whereas SCA1 cases and MSA cases formed another cluster, separate from the control cluster. Interestingly, the presence of resting tremor seemed to be the clinical feature that separated the cases into the two ET-PD clusters. In conclusion, our study demonstrates that these degenerative movement disorders seem to differ with respect to the pattern of CF synaptic pathology they exhibit. It remains to be determined how these differences contribute to the clinical presentations of these diseases.

Background Essential tremor (ET) is heterogeneous in nature and cases may be subdivided based on clinical features. ET patients may thus be subdivided by age of onset, family history of tremor, and presence of head tremor. We recently described climbing fiber-Purkinje cell (CF-PC) synaptic abnormalities in ET; however, these CF pathological features have not been studied across different ET subtypes. Objectives To explore whether these CF-PC synaptic abnormalities differ across ET subtypes. Methods We studied two climbing fiber (CF-PC) synaptic pathologies (CF synaptic density and percentage of CFs in the parallel fiber [PF] territory) in the cerebella of 60 ET cases with a range of clinical presentations and 30 age-matched controls. Results Compared to controls, ET cases had lower CF synaptic density and a higher percentage of CFs in the PF territory. ET cases with tremor onset <50 years and tremor onset ≥ 50 years did not differ significantly with respect to CF synaptic density and percentage of CFs in the PF territory. Similar results were found when comparing familial vs. sporadic ET cases, and ET cases with head tremor vs. those without head tremor. Among all ET cases, lower CF synaptic density was associated with lower PC counts and higher torpedo counts. In addition, higher percentage of CFs in the PF territory was associated with lower PC counts and higher torpedo counts. Conclusions These findings support the notion that changes in the distribution of CF-PC synapses are broadly part of the neurodegenerative process in the ET cerebellum.

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