King's stages of amyotrophic lateral sclerosis: an 18F-FDG-PET study of brain connectivity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting upper and lower motor neurons. TAR DNA-binding protein 43 (TDP-43) proteinopathy is the neuropathological signature of the disease, and 18F-fluorodeoxyglucose PET (18F-FDG-PET) serves as a marker of neurodegeneration in vivo. The aim of the present cross-sectional study was to disentangle 18F-FDG-PET correlates of disease severity assessed through the King's staging system, by exploring connectivity changes across motor stages. Patients with ALS classified as King's stage 1, 2 or 3, who underwent brain 18F-FDG-PET at diagnosis from 2008 to 2022 at the ALS Centre of Turin, were included. A multiple regression analysis to evaluate the relationship between brain metabolism and King's stage was performed. The clusters showing significant results were used as seed regions in an interregional correlation analysis (IRCA), performed for each stage. Of a total of 832 patients with ALS, 337 were classified as King's stage 1, 274 as stage 2 and 221 as stage 3. The three groups significantly differed in age at PET, disease duration and total ALS Functional Rating Scale Revised (ALSFRS-R) score at the time of PET, C9orf72 status and the distribution of cognitive categories. We found a decreasing metabolic gradient from King's stage 1 to King's stage 3 in a cluster encompassing motor and cognitive areas. As King's stage increases, we found a decrease of connectivity within the sensorimotor and cognitive areas. The IRCA also showed the connectivity of motor and cognitive regions with temporal and cerebellar regions. The connectivity with temporal regions found in King's stage 1 decreases in King's stage 2 and finally, disappears in King's stage 3. The connectivity with the cerebellum occurs in King's stage 2 and decreases in King's stage 3. The changes of connectivity of motor and cognitive areas with temporal and cerebellar regions among different King's stages might reflect the spread of TDP-43 proteinopathy or a compensatory mechanism, respectively. The present study suggests that 18F-FDG-PET imaging of the brain may be integrated with the King's staging system to assess the extent of the pathogenic process in the context of clinical trials.