Inborn errors of metabolism (IEM) form a large varied group of disorders that often present in childhood. Movement disorders such as tremor, jerks or involuntary muscle contractions can all be caused by IEM. In this thesis we studied the types of movement disorders that occur in several different IEM, the severity of these movement disorders, and their impact on daily life. We found that many IEM patients, both children and adults, suffer from movement disorders. Depending on the type of IEM (glutaric aciduria type 1, classical galactosemia and Niemann-Pick type C, among others, were studied), movement disorders were found to be present in 49 to 100% of the studied patients. Movement disorders significantly influenced daily functioning; patients with a more severe movement disorder experienced more problems in daily life and reported a lower quality of life. It was further observed that in addition to movement disorders, many patients suffered from psychiatric and behavioural problems, which can also be very bothersome and impact functioning. Despite the frequent occurrence of movement disorders in IEM and their significant impact on daily life, only a few patients are actually treated for their movement disorder. Therefore, movement disorders in patients with IEM deserve more attention. We plead for more awareness by physicians treating patients with IEM for movement disorders and their treatment options. Standardized assessments of the presence of a movement disorder should be included in the routine checks for patients with an IEM that is known to be associated with movement disorders.

Because of the COVID-19 situation, the setting of dr. Kuiper's PhD defence was a little different than usual. Only a small amount of people were allowed to witness the ceremony: prof. Marina de Koning-Tijssen and prof. Tom de Koning, her PhD supervisors, were present. Colleagues and friends were able to watch the defence online.