F198S Gerstmann-Sträussler-Scheinker Syndrome With Parkinsonism, Dyskinesia, and Abnormal (I-123)-FP-CIT Single-Photon Emission Computed Tomography: A Case Report.

Gerstmann-Sträussler-Scheinker syndrome (GSS) is an autosomal dominant neurodegenerative disease caused by point mutations in the prion protein gene (PRNP) While variable, the clinical presentation typically encompasses progressive cerebellar ataxia, pyramidal signs, and cognitive impairment. Here, we report a case of F198S-associated GSS manifesting levodopa-responsive parkinsonism, levodopa-induced dyskinesia, and an abnormal (I-123)-FP-CIT single-photon emission computed tomography (DaT-SPECT). A 66-year-old male patient presented with six years of progressive recall and language impairment, with an initial impression of primary progressive aphasia. Over time he developed progressive cerebellar ataxia and akinetic parkinsonism. There was a family history of ataxia in multiple family members. Levodopa was prescribed up to 450 mg per day without benefit. Genetic testing at age 69 revealed a heterozygous F198S mutation in the PRNP gene, with MV heterozygosity at codon 129. At age 70, he developed mild generalized choreiform dyskinesia. Levodopa was discontinued, resulting in the resolution of dyskinesia with a concomitant marked worsening of akinetic parkinsonism. DaT-SPECT demonstrated bilaterally reduced putaminal binding. This case highlights that GSS can resemble atypical parkinsonism both clinically and with DaT-SPECT imaging. Taking a salient family history and other clinical features into consideration, GSS should be added to the differential diagnoses of such patients.