6

Objective We aimed to determine the prevalence of epilepsy and neurodevelopmental disorders, including autism spectrum disorder, in children and adolescents with hypothalamic hamartoma (HH). We also sought to explore the relationship between these neurodevelopmental comorbidities and epilepsy and to establish the predictive value of structural characteristics of the hamartoma itself. Methods We retrospectively studied a cohort of 62 children with HH, with neuroimaging reviewed at Great Ormond Street Hospital (GOSH) between 2008 and 2018. Clinical records were reviewed, cognitive and language data analysed, and MRI scans studied. Results We confirmed a high burden of epilepsy (56%), autism (19%) and other neurodevelopmental disorders. Although rates of some neurodevelopmental disorders were significantly higher in those with epilepsy, autistic features and/or early developmental concerns often predated the onset of seizures, in particular generalized seizures, or occurred independently of seizures. We found a significant correlation between certain structural characteristics of the hamartoma itself and both epilepsy and certain neurodevelopmental comorbidities. Significance These findings suggest that although seizure burden clearly contributes to the cognitive and behavioural phenotypes seen, the hamartoma itself, and particular characteristics of it, are likely to be primary determinants of both the epilepsy and neurodevelopmental profiles. It is also probable that the underlying aetiology, likely genetic, directly contributes to the clinical profile, with epilepsy, neurodevelopmental impairment and the hamartoma itself representing markers of this aetiology. We propose that atypical neurodevelopmental profiles in HH could best be conceptualized as a developmental and epileptic encephalopathy. These findings have implications for counselling, monitoring and treatment.