No Neuron Operates in Isolation

Medical students are taught that an exacting neurologic examination is the cornerstone of neurology and permits lesion localization in the nervous system. This is certainly the case for bitemporal hemianopsia, with lesions of the optic chiasm and discreet sensorimotor deficits with compressive neuropathies in the peripheral nervous system. In the CNS, classically and most commonly, the tetrad of dyscalculia, dysgraphia, finger agnosia, and left-right disorientation (Gerstmann syndrome) is associated with damage to the angular and supramarginal gyri of the dominant hemisphere. Also classically, but often incorrectly, involuntary movements such as dystonia, tremor, chorea, ballism, athetosis, dyskinesia, and tics are linked to dysfunction of the basal ganglia. In this issue of Neurology®, Ekmen et al.¹ used voxel-based morphometry, fixel-based analysis, and spectral dynamic causal modeling in a well-defined cohort of patients with paroxysmal kinesigenic dyskinesias (PKD) due to mutations in PRRT2 to provide rigorous support for the concept that abnormal cerebellar output can drive dystonia. Dystonia was the sole involuntary movement in this PKD cohort. Patients with PRRT2 had decreased gray matter volume in cerebellar lobule V and medial prefrontal cortex, as well as microstructural alterations of white matter in the cerebellum and tracts connecting the cerebellum to the striatum and cortical motor areas. On the basis of these structural findings in combination with the results of transcranial magnetic stimulation of the cerebellum and spectral dynamic causal modeling, the authors propose that striatal dysfunction in PKD results from upstream abnormalities of cerebellar signaling.