18 F-FDG metabolic abnormalities precede perfusion and atrophic changes in diagnosis of early frontotemporal dementia: study from a tertiary care university hospital.

Abstract

Objective: Diagnosis of early onset dementia is critical for initiating management. Although structural MRI is an established procedure for dementia evaluation, early cases may be missed. Neurodegenerative diseases lead to reductions in glucose consumption and grey matter volume loss. Our primary aim was to establish whether metabolic changes precede perfusion abnormalities in early cases of dementia especially, frontotemporal dementia (FTD). Secondly to study if cerebral atrophy using Pasquier visual rating scales can be used reliably to correlate with hypometabolism in this group of patients.

Materials and methods: A total of 56 patients (M:F = 39:17) with memory loss as per the DSM-5 diagnostic criteria were clinically and neurologically examined and referred for 18 F-Fluorodeoxyglucose (FDG) PET brain imaging. A few patients who had a prior (recent, <1 week) MR brain underwent brain 18 F-FDG PET-CT, and all others were considered for simultaneous 18 F-FDG PETMR imaging of brain. T2-weighted images were used to report Pasquier rating scales in all our patients as per recommendation.

Results: Cognitive assessments were analysed along with neuroimaging findings. Highest diagnostic performance was obtained with 18 F-FDG PET for identifying early FTD in our series of patients. Sensitivity, specificity and accuracy of FDG and arterial spin labeling (ASL) using simultaneous PETMR were found to be 96.34%/90.1%/89 : 53.57%/62.12%/78, respectively. Cerebral atrophy rated using Pasquier visual scales showed the lowest diagnostic performance. Our study showed that the earliest phase of cognitive decline was found to be associated with specific patterns of hypometabolism, even in the absence of atrophy, which are currently considered diagnostic biomarkers.

Conclusion: Metabolic derangements indeed precede perfusion changes and cerebral atrophy in the setting of early dementia. Simultaneous FDG PETMR is being recommended as the investigation of choice for the evaluation of early FTD. This pilot study shows that FDG PET outperforms cognitive assessments and anatomical imaging modalities in early dementia evaluation. Although ASL can detect perfusion deficits in dementia, compared with FDG PET, its sensitivity and specificity for discerning early onset dementia from controls remain inferior to FDG PET. Pasquier scales, although easy to implement, cannot replace FDG PET metabolic findings, which start very early in the neuronal disease process.