Case Report: Sustained Cognitive Improvement and Reversible Brain Hypoperfusion in the Context of Posterior Fossa Arachnoid Cyst Shunting Procedure in KBG Syndrome.

KBG syndrome is a genetic syndrome characterized by developmental delay, mild autism spectrum disorder (ASD), variable cognitive disorders and distinctive craniofacial and skeletal features. Posterior fossa arachnoid cysts (PFACs) have been reported in this condition as in other syndromic forms of ASD, but their impact remains uncertain. Here, we report a ten-year follow-up of a patient with KBG syndrome and PFAC shunted for progressive cyst enlargement and his clinical symptoms, who showed important cognitive and behavior improvement post-drainage. Preoperative neuropsychological assessment revealed severe ASD, heterogeneous cognitive profile with below-average IQ (82, average ≥90), slow processing speed (66, cut-off <70), prominent executive function and memory impairments but normal verbal index and working memory. Whole-brain arterial spin labelling MRI showed a markedly reduced cerebral blood flow in the left superior temporal sulcus (STS), a key region for social cognition. Eighteen-month post cystoperitoneal shunt, cyst volume had decreased (34 vs 10 mL), STS perfusion was no longer different from controls and high cognitive improvements were observed (IQ: +1SD, verbal index: +2SD, executive functions planning and flexibility: +3SD, emotional regulation: +1SD, verbal memory: +3SD). At 22 years, autistic traits had decreased, cognitive gains persisted and executive functions progressed again. Visual-motor processing speed, theory of mind and fine motor skills remained low. Resting-state functional MRI seeded from STS showed strengthened temporal connectivity and diffusion imaging revealed increased fibre density in the left temporal lobe. The parallel trajectories of behavioural and neuroimaging improvements may suggest that PFAC surgery could have enabled functional reorganisation within temporal networks in addition to therapeutic interventions, supporting improved cognitive and social outcomes. These results suggest a possible role of the posterior fossa in modulating neurodevelopmental outcomes in ASD-related genetic conditions.