Parkinsonism with Reduced Striatal Uptake on DAT-SPECT in a Patient Harboring a Novel KCND3 Variant

KCND3 encodes the voltage-gated potassium channel Kv4.3, and pathogenic variants in KCND3 are associated with spinocerebellar ataxia type 19/22 (SCA19/22) and related neurological phenotypes. Here, we report a Japanese patient harboring a novel KCND3 missense variant, classified as a variant of uncertain significance, who presented with a clinical phenotype compatible with SCA19/22, including slowly progressive cerebellar ataxia beginning in childhood, accompanied by asymmetric parkinsonism. Dopamine transporter single-photon emission computed tomography demonstrated reduced striatal uptake. Parkinsonian symptoms exhibited a partial response to dopaminergic therapy; however, the improvement was primarily based on clinical observation, with limited objective evidence supporting treatment efficacy. To our knowledge, abnormal dopamine transporter imaging has not been previously reported in patients with SCA19/22-related phenotypes. Although it was difficult to determine whether the parkinsonism was attributable to the KCND3 variant or to coexisting Lewy body disease, we reviewed the literature on parkinsonism associated with SCA19/22 and discussed the potential contributions of both pathologies in the present case.