A Case of Late-Onset Semantic Dementia With Right-Predominant Temporal Lobe Atrophy due to Primary Tauopathy

Semantic dementia (SD) is a type of dementia characterized by a loss of semantic memory, typically presenting with left-predominant temporal lobe atrophy and a frontotemporal lobar degeneration with TDP-43 (FTLD-TDP) type C pathology. However, atypical variants, such as those with a late-onset or right-predominant atrophy, exhibit considerable pathological diversity. We report an 81-year-old woman with atypical SD who presented with impaired naming, mild word comprehension deficits, and prosopagnosia. Brain MRI revealed right-predominant atrophy in the amygdala, hippocampus, and anterior temporal lobes. Cerebrospinal fluid analysis showed a normal Aβ level, suggesting a non-Alzheimer's disease pathology. Tau PET imaging demonstrated a high signal in the right anterior temporal lobe, consistent with primary tauopathy. The patient's clinical presentation and imaging findings, particularly the atrophy in the amygdala and hippocampus, as well as the preserved communication ability, raised the possibility of argyrophilic grain disease (AGD) as the underlying pathology. This case highlights the importance of molecular imaging in diagnosing atypical SD. It suggests that AGD should be considered a possible underlying pathology for atypical SD cases, particularly those with late onset and right-sided predominance. A longer follow-up and eventual autopsy would be necessary to confirm the pathological diagnosis.