Clinical Impact of a Second FDG-PET in Atypical/Unclear Dementia Syndromes. 
Diagnosis of atypical/unclear dementia is often difficult and this delays treatment initiation.  Several authors have shown that beyond standard dementia workup, 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) reduces the number of unclear diagnoses, leads to earlier treatment, and has a beneficial impact on families.  However, it is not uncommon that the FDG-PET findings are equivocal in this setting.  For those cases, a repeat FDG-PET may clarify the diagnosis and prevent treatment delay.  We retrospectively assessed the clinical impact of a repeat FDG-PET in 59 patients with atypical/unclear dementia syndromes and inconclusive initial FDG-PET.  Changes in primary diagnosis, diagnostic confidence, and management following the second FDG-PET were examined.  Conducting a second FDG-PET reduced the number of unclear diagnoses from 80% to 34% , led to diagnostic change in 24% of cases, and treatment modification in 22% of patients.  Overall, the clinical impact was higher when initial diagnostic confidence was low and the second FDG-PET repeated ≥12 months after the first one.  In tertiary care memory clinic settings, when diagnostic incertitude persists despite extensive evaluation and an equivocal FDG-PET, repeating the FDG-PET 12 months later can greatly clarify the diagnosis and improve management.