Pyruvate carboxylase (PC) deficiency is characterized in most affected individuals by failure to thrive, developmental delay, recurrent seizures, and metabolic acidosis. Three clinical types are recognized: Type A (infantile form), in which most affected children die in infancy or early childhood. Type B (severe neonatal form), in which affected infants have hepatomegaly, pyramidal tract signs, and abnormal movement and die within the first three months of life. Type C (intermittent/benign form), in which affected individuals have normal or mildly delayed neurologic development and episodic metabolic acidosis.

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle and ornithine degradation pathway. Clinical manifestations and age of onset vary among individuals even in the same family. Neonatal onset (~8% of affected individuals). Manifestations of hyperammonemia usually begin 24-48 hours after feeding begins and can include lethargy, somnolence, refusal to feed, vomiting, tachypnea with respiratory alkalosis, and/or seizures. Infantile, childhood, and adult onset (~92%). Affected individuals may present with: Chronic neurocognitive deficits (including developmental delay, ataxia, spasticity, learning disabilities, cognitive deficits, and/or unexplained seizures); Acute encephalopathy secondary to hyperammonemic crisis precipitated by a variety of factors; and Chronic liver dysfunction (unexplained elevation of liver transaminases with or without mild coagulopathy, with or without mild hyperammonemia and protein intolerance). Neurologic findings and cognitive abilities can continue to deteriorate despite early metabolic control that prevents hyperammonemia.

Allan-Herndon-Dudley syndrome (AHDS), an X-linked disorder, is characterized in males by neurologic findings (hypotonia and feeding difficulties in infancy, developmental delay / intellectual disability ranging from mild to profound) and later-onset pyramidal signs, extrapyramidal findings (dystonia, choreoathetosis, paroxysmal movement disorder, hypokinesia, masked facies), and seizures, often with drug resistance. Additional findings can include dysthyroidism (manifest as poor weight gain, reduced muscle mass, and variable cold intolerance, sweating, elevated heart rate, and irritability) and pathognomonic thyroid test results. Most heterozygous females are not clinically affected but may have minor thyroid test abnormalities.

A form of hereditary spastic paraplegia which usually presents in late adolescence or early adulthood as a pure phenotype of lower limb spasticity with hyperreflexia and extensor plantar responses, as well as mild bladder disturbances and pes cavus. Rarely, it can present as a complex phenotype with additional manifestations including epilepsy, variable peripheral neuropathy and/or memory impairment. Caused by mutations in the NIPA1 gene (15q11.2) encoding the magnesium transporter NIPA1.

A very rare pure form of spastic paraplegia with characteristics of onset in infancy of lower limb spasticity associated with gait disturbances, scissor gait, tiptoe walking, clonus and increased deep tendon reflexes. Mild upper limb involvement may occasionally also be associated.

X-linked spinocerebellar ataxia-6 with or without sideroblastic anemia (SCAX6) is an X-linked recessive disorder characterized by delayed motor development apparent in infancy with delayed walking (often by several years) due to ataxia and poor coordination. Additional features may include dysmetria, dysarthria, spasticity of the lower limbs, hyperreflexia, dysdiadochokinesis, strabismus, and nystagmus. The disorder is slowly progressive, and patients often lose ambulation. Brain imaging usually shows cerebellar atrophy. Most affected individuals have mild hypochromic, microcytic sideroblastic anemia, which may be asymptomatic. Laboratory studies show increased free erythrocyte protoporphyrin (FEP) and ringed sideroblasts on bone marrow biopsy. Female carriers do not have neurologic abnormalities, but may have subtle findings on peripheral blood smear (Pagon et al., 1985; D'Hooghe et al., 2012). For a discussion of genetic heterogeneity of X-linked spinocerebellar ataxia (SCAX), see SCAX1 (302500).

Spastic paraplegia 15 (SPG15), typically an early-onset complex hereditary spastic paraplegia, is characterized by progressive spasticity that begins in the lower extremities and is associated with several manifestations resulting from central and peripheral nervous system dysfunction. While onset of spasticity is typically in mid- to late childhood or adolescence (i.e., between ages 5 and 18 years), other manifestations, such as developmental delay or learning disability, may be present earlier, often preceding motor involvement. Individuals with adult onset have also been reported.

This syndrome is characterized by the association of an axonal sensory and autonomic neuropathy with spastic paraplegia.

2,4-Dienoyl-CoA reductase deficiency (DECRD) is a rare autosomal recessive inborn error of metabolism resulting in mitochondrial dysfunction due to impaired production of NADPH, which is an essential cofactor for several mitochondrial enzymes. Affected individuals have a variable phenotype: some may have severe neurologic symptoms and metabolic dysfunction beginning in early infancy, whereas others may present with more subtle features, such as childhood-onset optic atrophy or intermittent muscle weakness. The variable severity is putatively dependent on the effect of the mutation on the NADK2 enzyme. Biochemical analysis typically shows hyperlysinemia, due to defective activity of the mitochondrial NADP(H)-dependent enzyme AASS (605113), which is usually a benign finding. More severe cases have increased C10:2-carnitine levels, due to defective activity of the enzyme DECR (DECR1; 222745) (summary by Houten et al., 2014 and Pomerantz et al., 2018).

A complex form of hereditary spastic paraplegia characterised by a spastic paraplegia presenting in adolescence, associated with the additional manifestations of sensorial hearing impairment due to auditory neuropathy and persistent vomiting due to a hiatal or paraoesophageal hernia. The phenotype has been mapped to a locus on chromosome 1p31.1-p21.1.

ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts and comprises a clinical continuum of the following three phenotypes: Infantile ascending hereditary spastic paraplegia (IAHSP), characterized by onset of spasticity with increased reflexes and sustained clonus of the lower limbs within the first two years of life, progressive weakness and spasticity of the upper limbs by age seven to eight years, and wheelchair dependence in the second decade with progression toward severe spastic tetraparesis and a pseudobulbar syndrome caused by progressive cranial nerve involvement. Juvenile primary lateral sclerosis (JPLS), characterized by upper motor neuron findings of pseudobulbar palsy and spastic quadriplegia without dementia or cerebellar, extrapyramidal, or sensory signs. Juvenile amyotrophic lateral sclerosis (JALS or ALS2), characterized by onset between ages three and 20 years. All affected individuals show a spastic pseudobulbar syndrome (spasticity of speech and swallowing) together with spastic paraplegia. Some individuals are bedridden by age 12 to 50 years.

Spinocerebellar ataxia-29 (SCA29) is an autosomal dominant neurologic disorder characterized by onset in infancy of delayed motor development and mild cognitive delay. Affected individuals develop a very slowly progressive or nonprogressive gait and limb ataxia associated with cerebellar atrophy on brain imaging. Additional variable features include nystagmus, dysarthria, and tremor (summary by Huang et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Juvenile amyotrophic lateral sclerosis-4 (ALS4) is an autosomal dominant disorder characterized by distal muscle weakness and atrophy, normal sensation, and pyramidal signs, with onset of symptoms before the age of 25 years, a slow rate of progression, and a normal life span (summary by Chen et al., 2004). For a phenotypic description and a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).

Autosomal recessive osteopetrosis-5 (OPTB5) is a form of infantile malignant osteopetrosis, characterized by defective osteoclast function resulting in decreased bone resorption and generalized osteosclerosis. Defective resorption causes development of densely sclerotic fragile bones and progressive obliteration of the marrow spaces and cranial foramina. Marrow obliteration is associated with extramedullary hematopoiesis and hepatosplenomegaly, and results in anemia and thrombocytopenia, whereas nerve entrapment accounts for progressive blindness and hearing loss. Other major manifestations include failure to thrive, pathologic fractures, and increased infection rate. Most affected children succumb to severe bone marrow failure and overwhelming infection in the first few years of life (summary by Quarello et al., 2004).

TSEN54 pontocerebellar hypoplasia (TSEN54-PCH) comprises three PCH phenotypes (PCH2, 4, and 5) that share characteristic neuroradiologic and neurologic findings. The three PCH phenotypes (which differ mainly in life expectancy) were considered to be distinct entities before their molecular basis was known. PCH2. Children usually succumb before age ten years (those with PCH4 and 5 usually succumb as neonates). Children with PCH2 have generalized clonus, uncoordinated sucking and swallowing, impaired cognitive development, lack of voluntary motor development, cortical blindness, and an increased risk for rhabdomyolysis during severe infections. Epilepsy is present in approximately 50%. PCH4. Neonates often have seizures, multiple joint contractures ("arthrogryposis"), generalized clonus, and central respiratory impairment. PCH5 resembles PCH4 and has been described in one family.

A pure form of hereditary spastic paraplegia with late childhood to early adulthood-onset of slowly progressive spastic paraplegia with spastic gait and lower limb hyperreflexia, brisk tendon reflexes and ankle clonus. Lower limb pain and reduced lower limb vibratory sense is also reported in some older adult patients.

Infantile cerebral and cerebellar atrophy with postnatal progressive microcephaly is a rare, central nervous system malformation syndrome characterized by progressive microcephaly with profound motor delay and intellectual disability, associated with hypertonia, spasticity, clonus, and seizures, with brain imaging revealing severe cerebral and cerebellar atrophy, and poor myelination.

PCH2D is an autosomal recessive disorder characterized by progressive microcephaly, postnatal onset of progressive atrophy of the cerebrum and cerebellum, profound mental retardation, spasticity, and variable seizures (summary by Ben-Zeev et al., 2003). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

Lethal neonatal rigidity and multifocal seizure syndrome (RMFSL) is a severe autosomal recessive epileptic encephalopathy characterized by onset of rigidity and intractable seizures at or soon after birth. Affected infants achieve no developmental milestones and die within the first months or years of life (summary by Saitsu et al., 2014).

SPG53 is an autosomal recessive neurologic disorder characterized by onset in infancy of delayed motor development progressing to upper and lower limb spasticity with impaired walking. Affected individuals also show mild to moderate cognitive impairment (summary by Zivony-Elboum et al., 2012).

The overlapping phenotypes of neonatal adrenoleukodystrophy (NALD) and infantile Refsum disease (IRD) represent the milder manifestations of the Zellweger syndrome spectrum (ZSS) of peroxisome biogenesis disorders. The clinical course of patients with the NALD and IRD presentation is variable and may include developmental delay, hypotonia, liver dysfunction, sensorineural hearing loss, retinal dystrophy, and visual impairment. Children with the NALD presentation may reach their teens, and those with the IRD presentation may reach adulthood (summary by Waterham and Ebberink, 2012). For a complete phenotypic description and a discussion of genetic heterogeneity of PBD(NALD/IRD), see 601539. Individuals with mutations in the PEX16 gene have cells of complementation group 9 (CG9, equivalent to CGD). For information on the history of PBD complementation groups, see 214100.

KCNT1-related epilepsy is most often associated with two phenotypes: epilepsy of infancy with migrating focal seizures (EIMFS) and autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). EIMFS is characterized by seizures, typically focal and asynchronous, beginning in the first six months of life with associated developmental plateau or regression. Autonomic manifestations (e.g., perioral cyanosis, flushing, apnea) are common. Seizures are intractable to multiple anticonvulsants and progress to become nearly continuous by age six to nine months. ADNFLE is characterized by clusters of nocturnal motor seizures that vary from simple arousals to hyperkinetic events with tonic or dystonic features. Individuals with KCNT1-related ADNFLE are more likely to develop seizures at a younger age, have cognitive comorbidity, and display psychiatric and behavioral problems than individuals with ADNFLE resulting from other causes. Less common seizure phenotypes in individuals with KCNT1-related epilepsy include West syndrome, Ohtahara syndrome, early myoclonic encephalopathy, leukodystrophy and/or leukoencephalopathy, focal epilepsy, and multifocal epilepsy. Additional neurologic features include hypotonia, microcephaly developing by age 12 months, strabismus, profound developmental delay, and additional movement disorders. Other systemic manifestations including pulmonary hemorrhage caused by prominent systemic-to-pulmonary collateral arteries or cardiac arrhythmia have been reported.

Lipodystrophies are rare disorders characterized by loss of body fat from various regions and predisposition to metabolic complications of insulin resistance and lipid abnormalities. FPLD7 is an autosomal dominant disorder with a highly variable phenotype. Additional features, including early-onset cataracts and later onset of spasticity of the lower limbs, have been noted in some patients (summary by Garg et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of familial partial lipodystrophy (FPLD), see 151660.

Asparagine synthetase deficiency (ASD) mainly presents as a triad of congenital microcephaly, severe developmental delay, and axial hypotonia followed by spastic quadriplegia. Low cerebrospinal fluid (CSF) asparagine level can help the clinician in differentiating this disorder from others. In most cases age of onset of apnea, excessive irritability, and seizures is soon after birth. Affected individuals typically do not acquire any developmental milestones. Spastic quadriplegia can lead to severe contractures of the limbs and neurogenic scoliosis. Feeding difficulties (gastroesophageal reflux disease, frequent vomiting, swallowing dysfunction, and gastroesophageal incoordination) are a significant problem in most affected individuals. A majority have cortical blindness. MRI findings are nonspecific but may include generalized atrophy and simplified gyral pattern.

Myopathy with extrapyramidal signs is an autosomal recessive disorder characterized by early childhood onset of proximal muscle weakness and learning disabilities. While the muscle weakness is static, most patients develop progressive extrapyramidal signs that may become disabling (summary by Logan et al., 2014). Brain MRI in 1 patient showed congenital malformations, including polymicrogyria and cerebellar dysplasia (Wilton et al., 2020).

An extremely rare and complex form of hereditary spastic paraplegia with characteristics of onset in infancy of spastic paraplegia (presenting with delayed walking and a scissors gait) associated with short stature and normal cognition. Periventricular deep white matter changes in the corpus callosum are noted on brain imaging. SPG63 is caused by a homozygous mutation in the AMPD2 gene (1p13.3) encoding AMP deaminase 2.

Pontocerebellar hypoplasia type 9 (PCH9) is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by severely delayed psychomotor development, progressive microcephaly, spasticity, seizures, and brain abnormalities, including brain atrophy, thin corpus callosum, and delayed myelination (summary by Akizu et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Hypomyelinating leukodystrophy-13 (HLD13) is an autosomal recessive neurodegenerative disorder characterized by infantile onset of delayed psychomotor development, axial hypotonia, and spasticity associated with delayed myelination and periventricular white matter abnormalities on brain imaging. More variable neurologic deficits, such as visual impairment, may also occur. Some patients may experience cardiac failure during acute illness (summary by Edvardson et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-2 (PEOB2) is a mitochondrial disorder characterized by adult onset of progressive external ophthalmoplegia, exercise intolerance, muscle weakness, and signs and symptoms of spinocerebellar ataxia, such as impaired gait and dysarthria. Some patients may have respiratory insufficiency. Laboratory studies are consistent with a defect in mtDNA replication (summary by Reyes et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

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).

GLYT1 encephalopathy is characterized in neonates by severe hypotonia, respiratory failure requiring mechanical ventilation, and absent neonatal reflexes; encephalopathy, including impaired consciousness and unresponsiveness, may be present. Arthrogryposis or joint laxity can be observed. Generalized hypotonia develops later into axial hypotonia with limb hypertonicity and a startle-like response to vocal and visual stimuli which should not be confused with seizures. To date, three of the six affected children reported from three families died between ages two days and seven months; the oldest reported living child is severely globally impaired at age three years. Because of the limited number of affected individuals reported to date, the phenotype has not yet been completely described.

NMIHBA is a severe autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by global developmental delay apparent from infancy and profoundly impaired intellectual development. Affected individuals have microcephaly with accompanying dysmorphic features, truncal hypotonia, peripheral spasticity, and lack of independent ambulation or speech acquisition. Brain imaging shows variable abnormalities, including cortical atrophy, thin corpus callosum, cerebellar hypoplasia, and delayed myelination (summary by Zollo et al., 2017).

Developmental and epileptic encephalopathy-55 (DEE55) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first weeks or months of life. Affected individuals have an extremely poor outcome, with profoundly impaired intellectual development, absent speech, spastic quadriplegia, and dyskinetic movements. Most have cortical visual impairment and require a feeding tube. Brain imaging shows nonspecific abnormalities, including cerebral atrophy, thin corpus callosum, and abnormal signals in the white matter. Death in childhood may occur. Biochemically, the disorder is associated with impaired synthesis of GPI-anchored proteins (summary by Vetro et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

3-Methylglutaconic aciduria type IX (MGCA9) is an autosomal recessive disorder characterized by early-onset seizures, severely delayed psychomotor development and intellectual disability. Patients have hypotonia or spasticity, and laboratory investigations show increased serum lactate and 3-methylglutaconic aciduria, suggestive of a mitochondrial defect (summary by Shahrour et al., 2017). For a phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

MRT61 is an autosomal recessive neurodevelopmental disorder characterized by delayed psychomotor development, moderate to severe intellectual disability, and variable dysmorphic facial features. More severely affected patients may develop refractory seizures and have brain abnormalities, including hypoplasia of the corpus callosum (summary by Alwadei et al., 2016).

Neurodevelopmental disorder with spasticity and poor growth (NEDSG) is an autosomal recessive disorder characterized by severe early-onset encephalopathy with progressive microcephaly (Nahorski et al., 2018).

Developmental and epileptic encephalopathy-68 (DEE68) is an autosomal recessive neurologic disorder characterized by onset of twitching and/or myoclonic jerks in infancy. The disorder progresses to refractory generalized tonic-clonic seizures, often resulting in status epilepticus, loss of developmental milestones, and early death. Other features include delayed development, axial hypotonia, spasticity of the limbs, and clonus. Brain imaging may show cortical atrophy (summary by Barel et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Mitochondrial DNA depletion syndrome-18 (MTDPS18) is an autosomal recessive neuromuscular disorder characterized by early-onset progressive weakness and atrophy of the distal limb muscles, resulting in loss of ambulation as well as atrophy of the intrinsic hand muscles with clawed hands. Affected individuals may also develop scoliosis and have hypo- or hyperreflexia and decreased pulmonary vital capacity. Examination of skeletal muscle shows neurogenic atrophy and combined mitochondrial oxidative phosphorylation deficiency associated with mtDNA depletion. The clinical phenotype is reminiscent of spinal muscular atrophy (see SMA, 253300) and the metabolic profile is reminiscent of 2-aminoadipic 2-oxoadipic aciduria (AMOXAD; 204750), which is caused by mutation in the DHTKD1 gene (614984) (summary by Boczonadi et al., 2018). For a discussion of genetic heterogeneity of autosomal recessive mtDNA depletion syndromes, see MTDPS1 (603041).

Childhood-onset neurodegeneration with ataxia, tremor, optic atrophy, and cognitive decline (CONATOC) is an autosomal recessive progressive disorder with onset of symptoms in the first decade. Brain imaging may show variable features, including leukoencephalopathy and cerebellar atrophy (summary by Fagerberg et al., 2020).

Mitochondrial complex IV deficiency nuclear type 12 (MC4DN12) is an autosomal recessive metabolic disorder characterized by the onset of neurologic dysfunction in early infancy. Affected individuals demonstrate hypotonia with poor head control, profoundly delayed global development with inability to fix and follow, poor overall growth, abnormal spasms or myoclonus, and seizures. Most patients die in the first years of life; those that survive have spastic quadriplegia, feeding difficulties necessitating tube feeding, and profoundly impaired intellectual development with poor or absent communication. More variable features include cortical blindness, nystagmus, scoliosis, and hearing impairment. Brain imaging shows abnormalities consistent with Leigh syndrome (see 256000), as well as cystic cavitation. Laboratory studies show lactic acidosis, increased serum creatine kinase, and decreased levels and activity of mitochondrial respiratory complex IV (summary by Lim et al., 2014). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

Infantile-onset myofibrillar myopathy-12 with cardiomyopathy (MFM12) is a severe autosomal recessive disorder affecting both skeletal and cardiac muscle tissue that is apparent in the first weeks of life. Affected infants show tremor or clonus at birth, followed by onset of rapidly progressive generalized muscle weakness and dilated cardiomyopathy and cardiac failure, usually resulting in death by 6 months of age. Skeletal and cardiac muscle tissues show hypotrophy of type I muscle fibers and evidence of myofibrillar disorganization (summary by Weterman et al., 2013). For a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Developmental delay, impaired speech, and behavioral abnormalities (DDISBA) is characterized by global developmental delay apparent from early childhood. Intellectual disability can range from mild to severe. Additional variable features may include dysmorphic facial features, seizures, hypotonia, motor abnormalities such as Tourette syndrome or dystonia, and hearing loss (summary by Cousin et al., 2021).

Individuals with TANGO2-related metabolic encephalopathy and arrhythmias can present in acute metabolic crisis (hypoglycemia, elevated lactate, mild hyperammonemia) or with developmental delay, regression, and/or seizures. The acute presentation varies from profound muscle weakness, ataxia, and/or disorientation to a comatose state. Individuals can present with intermittent acute episodes of rhabdomyolysis. The first episode of myoglobinuria has been known to occur as early as age five months. Acute renal tubular damage due to myoglobinuria can result in acute kidney injury and renal failure. During acute illness, transient electrocardiogram changes can be seen; the most common is QT prolongation. Life-threatening recurrent ventricular tachycardia or torsade de pointes occurs primarily during times of acute illness. Individuals who do not present in metabolic crises may present with gait incoordination, progressively unsteady gait, difficulty with speech, or clumsiness. Intellectual disability of variable severity is observed in almost all individuals. Seizures are observed outside the periods of crises in more than 75% of individuals. Hypothyroidism has been reported in more than one third of individuals.

Childhood-onset neurodegeneration with progressive microcephaly (CONPM) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay apparent from infancy. The phenotype is highly variable: the most severely affected individuals have severe and progressive microcephaly, early-onset seizures, lack of visual tracking, and almost no developmental milestones, resulting in early death. Less severely affected individuals have a small head circumference and severely impaired intellectual development with poor speech and motor delay. Additional features may include poor overall growth, axial hypotonia, limb hypertonia with spasticity, undescended testes, and cerebral atrophy with neuronal loss (Lam et al., 2019 and Vanoevelen et al., 2022).

Autosomal dominant distal hereditary motor neuronopathy-10 (HMND10) is a neurologic disorder of the peripheral nerves characterized clinically by length-dependent motor neuropathy primarily affecting the lower limbs. Affected individuals have onset of distal muscle weakness and atrophy in early childhood that results in walking difficulties and gait abnormalities. Some have pyramidal signs, including hyperreflexia, suggesting the involvement of upper motor neurons. Electrophysiologic studies are consistent with a neurogenic process. More variable features may include mild intellectual disability, minor gyration defects on brain imaging, foot deformities, and connective tissue defects (1 family) (Capuano et al., 2016; Iacomino et al., 2020). For a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).

Congenital disorder of glycosylation type IIz (CDG2Z) is an autosomal recessive disorder characterized by poor overall growth, severe global developmental delay, seizures, contractures, hypotonia, spasticity, and brain imaging abnormalities. Serum transferrin shows a type 2 pattern of glycosylation abnormalities with a combined N- and O-glycosylation defect (Wilson et al., 2022). For a general discussion of CDGs, see CDG1A (212065).

Developmental delay with hypotonia, myopathy, and brain abnormalities (DEDHMB) is an autosomal recessive disorder characterized by global developmental delay and muscle weakness apparent in infancy. Affected individuals show severe motor delay and may not achieve independent walking due to central hypotonia and skeletal muscle myopathy. Some have poor overall growth with microcephaly, subtle dysmorphic features, and delayed language acquisition. Brain imaging shows cerebral atrophy, thinning of the corpus callosum, and delayed myelination (Shamseldin et al., 2016; Kotecha et al., 2021).

Developmental and epileptic encephalopathy-31B (DEE31B) is an autosomal recessive neurologic disorder with early-onset epilepsy, generalized muscular hypotonia, visual impairment, and severe neurodevelopmental delay (Yigit et al., 2022).