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Foot‐Hand Synkinesis in Corticobasal Syndrome: Single Clinical Feature with Distinct Molecular Imaging Biomarkers

Synkinesis is a subset of motor overflow in which voluntary movements of one part of the body are accompanied by involuntary activation of other, non-mirroring muscles. This disorder has been observed in neurodegenerative diseases such as Parkinson’s disease, Creutzfeldt–Jakob disease, and other parkinsonian disorders. Moreover, it is also observed in healthy older subjects. Their occurrence in patients with corticobasal syndrome (CBS) might be expected, but this phenomenon is still scarcely described in the literature. Here, we report 2 cases of patients with probable CBS presenting ipsilateral and contralateral foothand synkinesis and distinct amyloid imaging biomarkers results. A 52-year-old man (patient 1) was referred to our service with a 2-year history of progressive language impairment and limb rigidity. Neurological examination disclosed hypomimia, asymmetrical right-sided parkinsonism, cervical dystonia, ideomotor apraxia worse on the right side, right arm levitation, and nonfluent aphasia. A diagnosis of CBS was made. It was noteworthy that when we asked him to move his right foot, he started involuntarily to move his left hand, with a similar pattern and pace, presenting a contralateral and ipsilateral foot–hand synkinesis (Video 1, Segment 1). Patient 2 was a 65-year-old man with a 4-year history of progressive cognitive impairment and asymmetric rigidity. His neurological examination demonstrated right-sided parkinsonism, bilateral myoclonus, and ideomotor apraxia (both worse on the right), and cortical sensory deficits. He was diagnosed with probable CBS. When we asked him to move his right foot, he developed involuntary movements in his right hand and later in his left hand (Video 1, Segment 2). Voluntary movements from the hands did not elicit synkinesis in both patients. Both patients underwent a comprehensive investigation that included magnetic resonance imaging (MRI), fluorodeoxyglucose (FDG)-positron emission tomography (PET), and Pittsburgh compound-B (PIB)-PET. Patient 1 showed an asymmetrical hypometabolism at the frontoparietal region contralateral to the affected side at FDG-PET (Fig. 1A) and negative PIB-PET (Fig. 1B). Conversely, patient 2 showed an asymmetrical hypometabolism predominantly at posterior temporoparietal regions at FDG-PET (Fig. 1D) and positive cortical amyloid deposition at PIB-PET (Fig. 1E). Patient 1 was diagnosed as CBS probably related to tauopathy, whereas patient 2 was diagnosed as CBS probably related to Alzheimer’s disease. Both patients also revealed hypometabolism at the supplementary motor area (SMA) and premotor cortex, contralateral to the affected side where synkinesis occurred. Although the brain networks involved in synkinesis are poorly understood, they are likely related to dysfunction in the secondary motor areas, such as the premotor cortex, SMA, cingulate, and their connections to the primary motor cortex. A previous study using functional magnetic resonance imaging (fMRI) data