Mucopolysaccharidosis type III (MPS III) is a multisystem lysosomal storage disease characterized by progressive central nervous system degeneration manifest as severe intellectual disability (ID), developmental regression, and other neurologic manifestations including autism spectrum disorder (ASD), behavioral problems, and sleep disturbances. Disease onset is typically before age ten years. Disease course may be rapidly or slowly progressive; some individuals with an extremely attenuated disease course present in mid-to-late adulthood with early-onset dementia with or without a history of ID. Systemic manifestations can include musculoskeletal problems (joint stiffness, contractures, scoliosis, and hip dysplasia), hearing loss, respiratory tract and sinopulmonary infections, and cardiac disease (valvular thickening, defects in the cardiac conduction system). Neurologic decline is seen in all affected individuals; however, clinical severity varies within and among the four MPS III subtypes (defined by the enzyme involved) and even among members of the same family. Death usually occurs in the second or third decade of life secondary to neurologic regression or respiratory tract infections.

Delayed sleep phase syndrome (DSPD) is a circadian rhythm sleep disorder characterized by sleep-onset insomnia and difficulty in awakening at the desired time. Patients with DSPD have chronic difficulty in adjusting their sleep-onset and wake-up times to occupational, school, and social activities. Although psychosocial and environmental factors sometimes induce this kind of disorder, most patients with DSPD appear to have an abnormal circadian pacemaker and/or an abnormal entrainment system (summary by Hohjoh et al., 2003).

Neurodevelopmental disorder with hyperkinetic movements, seizures, and structural brain abnormalities (NEDMSB) is a severe autosomal recessive disorder characterized by failure to thrive in infancy, global developmental delay, hypotonia, motor abnormalities with inability to walk, involuntary movements, impaired intellectual development, absent speech, seizures, and structural brain abnormalities (Alkhater et al., 2018; Dafsari et al., 2022).

Neuroocular syndrome-1 (NOC1) encompasses a broad spectrum of overlapping anomalies, with developmental delay or impaired intellectual development as a consistent finding. Eye abnormalities show marked variability in the type and severity of defects, and include anophthalmia, microphthalmia, and coloboma. Other common systemic features include congenital heart and kidney defects, hypotonia, failure to thrive, and microcephaly (summary by Chowdhury et al., 2021). Genetic Heterogeneity of Neuroocular Syndrome See also NOC2 (168885), caused by mutation in the DAGLA gene (614015) on chromosome 11q12.