Diagnostic Instability Over Time in the Late-Onset Frontal Lobe Syndrome: When Can We Say it's FTD?

Objectives: Distinguishing sporadic behavioral variant of frontotemporal dementia (bvFTD) from late-onset primary psychiatric disorders (PPD) remains challenging with the lack of robust biomarkers. An early bvFTD misdiagnosis in PPD cases and vice-versa is common. Little is known about diagnostic (in)stability over longer period of time. We investigated diagnostic instability in a neuropsychiatric cohort up to 8 years after baseline visit and identified which clinical hallmarks contribute to diagnostic instability. Design: Diagnoses of participants of the late-onset frontal lobe (LOF) study were collected from the baseline visit (T0) and the 2-year follow-up visit (T2). Clinical outcomes were retrieved 5–8 years after baseline visit (T_final). Endpoint diagnoses were categorized into bvFTD, PPD and other neurological disorders (OND). We calculated the total amount of participants that switched diagnosis between T0-T2 and T2-T_final. Clinical records of participants that switched diagnosis were assessed. Results: Of the 137 patients that were included in the study, the final diagnoses at T_final were bvFTD 24.1% (n = 33), PPD 39.4% (n = 54), OND 33.6% (n = 46) and unknown 2.9% (n = 4). Between T0 and T2, a total of 29 (21.2%) patients switched diagnosis. Between T2 and T_final, 8 (5.8%) patients switched diagnosis. Prolonged follow-up identified few cases with diagnostic instability. Major contributors to diagnostic instability where a nonconverting diagnosis of possible bvFTD and a probable bvFTD diagnosis based on informant-based history and an abnormal FDG-PET scan whilst having a normal MRI. Conclusion: Considering these lessons, a FTD diagnosis remains stable enough to conclude that 2 years is sufficient to say if a patient with late-life behavioral disorder has FTD.