Progressive leukoencephalopathy with ovarian failure is an autosomal recessive neurodegenerative disorder characterized by loss of motor and cognitive skills, usually with onset in young adulthood. Some patients may have a history of delayed motor development or learning difficulties in early childhood. Neurologic decline is severe, usually resulting in gait difficulties, ataxia, spasticity, and cognitive decline and dementia. Most patients lose speech and become wheelchair-bound or bedridden. Brain MRI shows progressive white matter signal abnormalities in the deep white matter. Affected females develop premature ovarian failure (summary by Dallabona et al., 2014).

Neonatal intractable myoclonus (NEIMY) is a severe neurologic disorder characterized by the onset of intractable myoclonic seizures soon after birth. Affected infants have intermittent apnea, abnormal eye movements, pallor of the optic nerve, and lack of developmental progress. Brain imaging shows a progressive leukoencephalopathy. Some patients may die in infancy. There is phenotypic and biochemical evidence of mitochondrial dysfunction (summary by Duis et al., 2016).

Neurodevelopmental disorder with progressive microcephaly, spasticity, and brain anomalies (NDMSBA) is an autosomal recessive neurodevelopmental disorder characterized by infantile onset of progressive microcephaly and spasticity and severe global developmental delay resulting in profoundly impaired intellectual development and severely impaired or absent motor function. More variable features include seizures and optic atrophy. Brain imaging may show myelinating abnormalities and white matter lesions consistent with a leukoencephalopathy, as well as structural anomalies, including thin corpus callosum, gyral abnormalities, and cerebral or cerebellar atrophy. Some patients die in early childhood (summary by Falik Zaccai et al., 2017 and Hall et al., 2017).

Mitochondrial complex II deficiency is an autosomal recessive multisystemic metabolic disorder with a highly variable phenotype. Some patients have multisystem involvement of the brain, heart, and muscle with onset in infancy, whereas others have only isolated cardiac or muscle involvement. Measurement of complex II activity in muscle is the most reliable means of diagnosis; however, there is no clear correlation between residual complex II activity and severity or clinical outcome. In some cases, treatment with riboflavin may have clinical benefit (summary by Jain-Ghai et al., 2013). Complex II, also known as succinate dehydrogenase, is part of the mitochondrial respiratory chain. Genetic Heterogeneity of Mitochondrial Complex II Deficiency See MC2DN2 (619166), caused by mutation in the SDHAF1 gene (612848) on chromosome 19q13; MC2DN3 (619167), caused by mutation in the SDHD gene (602690) on chromosome 11q23; and MC2DN4 (619224), caused by mutation in the SDHB gene (185470) on chromosome 1p36. Fullerton et al. (2020) reviewed the genetic basis of isolated mitochondrial complex II deficiency.