Inherited Prion Disease with a 5-octapeptide Repeat Insertion in the <i>PRNP</i> Gene Presenting with Familial Juvenile Dementia

A 47-year-old man with a family history of juvenile dementia in his mother presented with memory loss and cognitive decline. Neuropsychological tests revealed impaired orientation, working memory, and apraxia. Magnetic resonance imaging revealed diffuse brain atrophy, and fluorodeoxyglucose positron emission tomography (PET) showed hypometabolism in the bilateral parietal lobes, posterior cingulate gyri, and precuneus, suggestive of Alzheimer's disease. However, amyloid-beta and tau PET scans were negative. Genetic testing revealed an abnormal repeat insertion in the prion protein gene, confirming inherited prion disease. This case highlights the need to consider inherited prion disease in the differential diagnosis of early-onset familial dementia.