Background: Children and adolescents with intellectual disability are at risk of developing psychiatric symptoms and disorders; yet, the estimates reported in the literature have been inconsistent, presenting a potential barrier for service planning and delivery. Sources of variability could arise from differences in measurement instruments as well as subgroup membership by severity of intellectual disability, gender and age. This systematic review aimed to address these gaps. Method: MEDLINE and PsycINFO databases were searched from inception to 2018 and selected studies were reviewed. Studies were included if they reported point prevalence estimates of mental health symptomology or diagnoses in a general population of 6- to 21-year-old individuals with intellectual disability. The Joanna Briggs Institute Prevalence Critical Appraisal Checklist was applied to eligible papers to appraise their scientific strength. Pooled prevalence for mental health symptomology was determined using a random-effects meta-analysis. Results: A total of 19 studies were included, including 6151 children and adolescents. The pooled prevalence estimate captured by the Developmental Behaviour Checklist was 38% (95% confidence interval = [31, 46]), contrasting with 49% (95% confidence interval = [46, 51]) captured by the Child Behaviour Checklist; both rates were higher than a non-intellectual disability population. Severity of intellectual disability did not significantly influence the Developmental Behaviour Checklist risks. Insufficient data were available to conduct statistical analyses on the effects of age, gender and socioeconomic status. Of diagnosed psychiatric disorders, attention deficit/hyperactivity disorder (30%), conduct disorder (3–21%) and anxiety disorders (7–34%) were the most prevalent conditions. Conclusion: This review consists of the largest sample hitherto evaluated. In the intellectual disability population, mental health comorbidities could be better detected by a symptom phenotype than a psychiatric diagnostic phenotype. Crucially, future research needs to address the effect of measurement validity in the intellectual disability population. Estimated prevalence rates were high compared to the general population, indicating the importance of systematic screening, case detection and appropriate management.

The clinical characteristics of children diagnosed with Rett syndrome are well described. Survival and how these characteristics persist or change in adulthood are less well documented. This study aimed to describe overall survival and adult health in those with Rett syndrome.

Initially described as an early onset seizure variant of Rett syndrome, the CDKL5 disorder is now considered as an independent entity. However, little is currently known about the full spectrum of comorbidities that affect these patients and available literature is limited to small case series. This study aimed to use a large international sample to examine the prevalence in this disorder of comorbidities of epilepsy, gastrointestinal problems including feeding difficulties, sleep and respiratory problems and scoliosis and their relationships with age and genotype. Prevalence and onset were also compared with those occurring in Rett syndrome. Data for the CDKL5 disorder and Rett syndrome were sourced from the International CDKL5 Disorder Database (ICDD), InterRett and the Australian Rett syndrome Database (ARSD). Logistic regression (multivariate and univariate) was used to analyse the relationships between age group, mutation type and the prevalence of various comorbidities. Binary longitudinal data from the ARSD and the equivalent cross-sectional data from ICDD were examined using generalized linear models with generalized estimating equations. The Kaplan-Meier method was used to estimate the failure function for the two disorders and the log-rank test was used to compare the two functions. The likelihood of experiencing epilepsy, GI problems, respiratory problems, and scoliosis in the CDKL5 disorder increased with age and males were more vulnerable to respiratory and sleep problems than females. We did not identify any statistically significant relationships between mutation group and prevalence of comorbidities. Epilepsy, GI problems and sleep abnormalities were more common in the CDKL5 disorder than in Rett syndrome whilst scoliosis and respiratory problems were less prevalent. This study captured a much clearer picture of the CDKL5 disorder than previously possible using the largest sample available to date. There were differences in the presentation of clinical features occurring in the CDKL5 disorder and in Rett syndrome, reinforcing the concept that CDKL5 is an independent disorder with its own distinctive characteristics.

Individuals with the CDKL5 disorder have been described as having severely impaired development. A few individuals have been reported having attained more milestones including walking and running. Our aim was to investigate variation in attainment of developmental milestones and associations with underlying genotype. Data was sourced from the International CDKL5 Disorder Database, and individuals were included if they had a pathogenic or probably pathogenic CDKL5 mutation and information on early development. Kaplan-Meier time-to-event analyses investigated the occurrence of developmental milestones. Mutations were grouped by their structural/functional consequence, and Cox regression was used to investigate the relationship between genotype and milestone attainment. The study included 109 females and 18 males. By 5 years of age, only 75% of the females had attained independent sitting and 25% independent walking whilst a quarter of the males could sit independently by 1 year 3 months. Only one boy could walk independently. No clear relationship between mutation group and milestone attainment was present, although females with a late truncating mutation attained the most milestones. Attainment of developmental milestones is severely impaired in the CDKL5 disorder, with the majority who did attain skills attaining them at a late age. It appears as though males are more severely impaired than the females. Larger studies are needed to further investigate the role of genotype on clinical variability.