MGCA8 is an autosomal recessive metabolic disorder resulting in death in infancy. Features include hypotonia, abnormal movements, respiratory insufficiency with apneic episodes, and lack of developmental progress, often with seizures. Brain imaging is variable, but may show progressive cerebral atrophy. Laboratory studies show increased serum lactate and 3-methylglutaconic aciduria, suggesting a mitochondrial defect (summary by Mandel et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

Susceptibility to acute infection-induced encephalopathy-9 (IIAE9) is an autosomal recessive disorder characterized by episodic acute neurodegeneration and developmental regression associated with infections and febrile illness. Patients present in the first months or years of life, often after normal or only mildly delayed early development. Some patients may have partial recovery between episodes, such as transient ataxia, but the overall disease course is progressive, resulting in global developmental delay, abnormal movements, refractory seizures, microcephaly, and cerebellar atrophy (summary by Fichtman et al., 2019). For a discussion of genetic heterogeneity of susceptibility to acute infection-induced encephalopathy, see 610551.

Congenital central hypoventilation syndrome-2 and autonomic dysfunction (CCHS2) is an autosomal recessive disorder characterized by shallow breathing and apneic spells apparent in the neonatal period. Affected infants require mechanical ventilation due to impaired ventilatory response to hypercapnia, as well as tube feeding due to poor swallowing, aspiration, and gastrointestinal dysmotility. Some patients have other features of autonomic dysfunction, including bladder dysfunction, sinus bradycardia, and temperature dysregulation. Although mild global developmental delay with learning difficulties and seizures were present in the single family reported, it was unclear if these features were related to the hypoventilation phenotype (Spielmann et al., 2017). For a discussion of genetic heterogeneity of CCHS, see CCHS1 (209880).

Joubert syndrome-39 (JBTS39) is an autosomal recessive neurodevelopmental disorder with variable manifestations. Most affected individuals have developmental delay with poor speech and retinal dystrophy with abnormal eye movements. Brain imaging shows the pathognomonic 'molar tooth sign,' which reflects abnormal cerebellar formation (Van De Weghe et al., 2021). For a discussion of genetic heterogeneity of Joubert syndrome, see JBTS1 (213300).