Mucolipidosis IV (MLIV) is an ultra-rare lysosomal storage disorder characterized by severe psychomotor delay, progressive visual impairment, and achlorhydria. Individuals with MLIV typically present by the end of the first year of life with delayed developmental milestones (due to a developmental brain abnormality) and impaired vision (resulting from a combination of corneal clouding and retinal degeneration). By adolescence, all individuals with MLIV have severe visual impairment. A neurodegenerative component of MLIV has become more widely appreciated, with the majority of individuals demonstrating progressive spastic quadriparesis and loss of psychomotor skills starting in the second decade of life. About 5% of individuals have atypical MLIV, manifesting with less severe psychomotor impairment, but still exhibiting progressive retinal degeneration and achlorhydria.

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

PEHO is a severe autosomal recessive neurodevelopmental disorder characterized by extreme cerebellar atrophy due to almost total loss of granule neurons. Affected individuals present in early infancy with hypotonia, profoundly delayed psychomotor development, optic atrophy, progressive atrophy of the cerebellum and brainstem, and dysmyelination. Most patients also develop infantile seizures that are often associated with hypsarrhythmia on EEG, and many have peripheral edema (summary by Anttonen et al., 2017).

Bilateral striatal necrosis (BSN) encompasses a heterogeneous group of neurologic disorders with different causation. Familial infantile striatal degeneration is rare and can be inherited as an autosomal recessive or mitochondrial (see 500003) disorder. The familial form has an insidious onset and a slowly progressive course; the sporadic form is associated with acute systemic illness. Many features of BSN overlap with Leigh syndrome (see 256000) and certain metabolic disorders, including glutaric acidemia I (231670) and methylmalonic aciduria (251000). See also Aicardi-Goutieres syndrome (225750) (Mito et al., 1986; De Meirleir et al., 1995). Genetic Heterogeneity of Striatonigral Degeneration Childhood-onset striatonigral degeneration (617054) is caused by mutation in the VAC14 gene (604632) on chromosome 16q22. See also adult-onset autosomal dominant striatal degeneration (ADSD; 609161), caused by mutation in the PDE8B gene (603390) on chromosome 5q13, and early-onset dystonia-37 with striatal lesions (DYT37; 620427), caused by mutation in the NUP54 gene (607607) on chromosome 4q21.

Hypomyelinating leukodystrophy-12 (HLD12) is an autosomal recessive neurologic disorder characterized by severely delayed or even lack of psychomotor development that becomes apparent in the first months of life. Patients are markedly disabled, with acquired microcephaly, lack of speech, and often lack of spontaneous movement due to hypotonia and spasticity. Brain imaging shows delayed myelination (summary by Edvardson et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080. In a review of the pathogenesis of disorders with prominent dystonia or opisthotonic posturing as a feature, Monfrini et al. (2021) classified HLD12 as belonging to a group of neurologic disorders termed 'HOPS-associated neurologic disorders (HOPSANDs), which are caused by mutations in genes encoding various components of the autophagic/endolysosomal system, including VPS11.

Developmental and epileptic encephalopathy-52 (DEE52) is a severe autosomal recessive seizure disorder characterized by infantile onset of refractory seizures with resultant delayed global neurologic development. Affected individuals have impaired intellectual development and may have other persistent neurologic abnormalities, including axial hypotonia and spasticity; death in childhood may occur (summary by Patino et al., 2009 and Ramadan et al., 2017). Some patients with DEE52 may have a clinical diagnosis of Dravet syndrome (607208), which is characterized by the onset of seizures in the first year or 2 of life after normal early development. Developmental delay, impaired intellectual development, and behavioral abnormalities usually become apparent later between 1 and 4 years of age. Dravet syndrome may also include 'severe myoclonic epilepsy in infancy' (SMEI) (summary by Patino et al., 2009). For a discussion of genetic heterogeneity of DEE, see 308350.

A rare genetic disorder with characteristics of infantile onset of stagnation and regression of motor and language development resulting in complete lack of communication and purposeful movement. Further neurological manifestations include truncal hypotonia, appendicular spasticity, dystonia, optic disc pallor, peripheral neuropathy and neurogenic bladder. Patients also present multiple contractures, late-onset relative macrocephaly, short stature and facial dysmorphism (including coarse facial features, sloping forehead, thick eyebrows, low-set ears, prominent nose, flat philtrum, and prominent lower lip). Brain imaging at advanced stages shows diffuse abnormal white matter signal and severe atrophy. Sural nerve biopsy reveals decreased myelination.

NDPLHS is an autosomal dominant disorder characterized by developmental stagnation or regression apparent in the first years of life and manifest as loss of purposeful hand movements, loss of language, and intellectual disability. Additional features may include stereotypic movements, dystonia, gait abnormalities, sleep disturbances, and small hands and feet. The phenotype is reminiscent of Rett syndrome (RTT; 312750) (summary by Yoo et al., 2017).

Neurodevelopmental disorder with hypotonia and variable intellectual and behavioral abnormalities (NEDHIB) is characterized by early-onset hypotonia, delayed walking, poor speech, and impaired intellectual development. Additional features may include feeding difficulties, dysmorphic features, and visual defects. Brain imaging tends to show delayed myelination, thin corpus callosum, and/or enlarged ventricles. The severity of the disorder is highly variable; initial evidence suggests that the severity may depend on the type of mutation (summary by Haijes et al., 2019).

Neurodevelopmental disorder and structural brain anomalies with or without seizures and spasticity (NEDBASS) is an autosomal recessive neurologic disorder characterized by global developmental delay apparent from early infancy, poor overall growth often with microcephaly, impaired intellectual development with delayed or absent speech, axial hypotonia, and peripheral spasticity. Additional common but variable features include early-onset seizures, optic atrophy with poor visual fixation, and dysmorphic facial features. Brain imaging shows cerebral atrophy, poor or absent myelination with loss of white matter volume, and often hypoplasia of the corpus callosum and/or cerebellum. Early death may occur (summary by Bend et al., 2020).

Infection-induced acute encephalitis-12 (IIAE12) is an autosomal recessive disorder characterized by episodic acute encephalopathy associated with infections and febrile illness. Patients present with neurologic symptoms in the first months or years of life, usually after normal early development. Brain imaging in the acute episodes shows restriction on diffusion-weighted imaging (DWI) and T2-weighted abnormalities in the periventricular and subcortical regions, which progresses to cavitation of previously affected areas. The long-term outcome is highly variable, even within families, ranging from death to severe neurologic deficits to normal outcomes (Shashi et al., 2023). For a discussion of genetic heterogeneity of susceptibility to acute infection-induced encephalopathy, see 610551.