57 Clinical Utility of Neuropsychological Evaluation in the Differential Diagnosis Between Late-Onset Primary Progressive Aphasia Semantic Variant (PPA-SV) Versus Limbic Predominant Age-Related TDP-43 Encephalopathy (LATE): A Case Report

Abstract

Objective:Primary progressive aphasia semantic variant (PPA-SV) is an atypical dementia subtype on the frontotemporal dementia (FTD) spectrum. PPA-SV is clinically defined by naming and semantic deficits, with progressive language decline that generalizes to other domains over time. Typically, it presents as an early-onset dementia with TDP-43 pathology, but 33-46% of PPA-SV cases display initial onset after age 65 with potentially different clinical features. Limbic Predominant Age-Related TDP-43 Encephalopathy (LATE), a more recently discovered neurodegenerative entity, is defined by an older age of onset, hippocampal sclerosis/atrophy, and TDP-43 pathology with a gradually progressive amnestic profile that expands to other cognitive deficits over time. The authors present a case report with overlapping features and suspected TDP-43 neuropathology ante-mortem for two reasons: first, to highlight the need for clinical criteria to formally diagnose LATE, and second, to address a gap in the literature on the possible clinical differences of late-onset PPA-SV.Participants and Methods:The authors present a case of a 78-year-old Indian bilingual man (English dominant) with 18 years of education, noncontributory medical history, and gradually progressive cognitive complaints reported over the past few years who was seen for outpatient neuropsychological evaluation. Prior workup was notable for negative amyloid PET scan, negative p-tau 217 blood test, and abnormal brain MRI revealing marked bilateral hippocampal atrophy with ex vacuo ventriculomegaly and minimal cerebrovascular disease. He scored 23/30 on prior MMSE, was diagnosed with amnestic MCI, prescribed Namenda and Exelon, and complained of minor memory and word-finding difficulties with reportedly intact IADLs and no signs of NPH.Results:Neuropsychological testing revealed a profound dysnomia (RBANS Naming raw score = 1/10), and he provided 0 words on two semantic fluency tasks but 15 words on letter fluency. Receptive vocabulary score was also impaired (PPVT-4, <1st %). Memory performance also demonstrated rapid forgetting of information (RBANS List Recall raw = 0/10, Story Recall raw = 0/12) with no benefit to recognition cues (RBANS List Recognition raw = 12/20), but slightly better visual memory recall, albeit still impaired (RBANS Figure Recall raw = 4/20). Grooved pegboard scores were significantly worse with right-hand than left-hand. Irregular word reading (NAART = 16th %ile) was significantly below expectations and thought to reflect more of a surface dyslexia rather than a cultural confounder given that he has lived in the US for over 50 years and his occupational and educational history was completed in English.Conclusions:Results support the clinical utility of neuropsychological evaluation in the differential diagnosis to support a predominant clinical syndrome of PPA-SV despite overlapping features with LATE and suspected TDP-43 pathology. This case report highlights the diagnostic issue with the lack of literature describing the typical clinical progression or suggested diagnostic criteria of LATE. These findings also indicate that late-onset PPA-SV may include greater memory deficits earlier in the course, but this may also be a clinical masquerade that is more reflective of the extent of language deficits. Further research on late-onset manifestations of PPA-SV and LATE consensus clinical guidelines is advised.