Introduction Rare complex syndromes Patients with complex genetic syndromes, by definition, have combined medical problems affecting multiple organ systems, and intellectual disability is often part of the syndrome. During childhood, patients with rare genetic syndromes receive multidisciplinary and specialized medical care; they usually receive medical care from 3-4 medical specialists.

Increased life expectancy Although many genetic syndromes used to cause premature death, improvement of medical care has improved life expectancy. More and more patients are now reaching adult age, and the complexity of the syndrome persists into adulthood. However, until recently, multidisciplinary care was not available for adults with rare genetic syndromes. Ideally, active and well-coordinated health management is provided to prevent, detect, and treat comorbidities that are part of the syndrome. However, after transition from pediatric to adult medical care, patients and their parents often report fragmented poor quality care instead of adequate and integrated health management. Therefore, pediatricians express the urgent need for adequate, multidisciplinary adult follow up of their pediatric patients with rare genetic syndromes.

Medical guidelines for adults not exist and the literature on health problems in these adults is scarce. Although there is a clear explanation for the absence of adult guidelines (i.e. the fact that in the past patients with rare genetic syndromes often died before reaching adult age), there is an urgent need for an overview of medical issues at adult age, for 'best practice' and, if possible, for medical guidelines.

The aim of this study is to get an overview of medical needs of adults with rare genetic syndromes, including:

1. comorbidities
2. medical and their impact on quality of life
3. medication use
4. the need for adaption of medication dose according to each syndrome

Methods and Results This is a retrospective file study. Analysis will be performed using SPSS version 23 and R version 3.6.0.

Inclusion Criteria:

• Patients with rare syndromes or rare congenital diseases visiting the multidisciplinary outpatient clinic for patients with rare diseases at the department of endocrinology, internal medicine, Erasmus Medical Center.

Exclusion Criteria:

• None

Patients with complex genetic syndromes

Data available upon reasonable request.