Mitochondrial DNA depletion syndrome-7 is an autosomal recessive severe neurodegenerative disorder characterized primarily by hypotonia, ataxia, ophthalmoplegia, hearing loss, seizures, and sensory axonal neuropathy. Although originally classified as a form of spinocerebellar ataxia (see, e.g., SCA1, 164400) (Koskinen et al., 1994), it has been reclassified as a mitochondrial DNA depletion syndrome (Hakonen et al., 2008) based on the finding of mtDNA depletion in the brain and liver of affected individuals. For a discussion of genetic heterogeneity of autosomal recessive mtDNA depletion syndromes, see MTDPS1 (603041).

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the GRIK2 gene.

A dystonia characterized by autosomal dominant inheritance of recurrent brief involuntary hyperkinesias triggered by sudden movements that has material basis in variation in the chromosome region 16q13-q22.1.

Dystonia 16 is one of many forms of dystonia, which is a group of conditions characterized by involuntary movements, twisting (torsion) and tensing of various muscles, and unusual positioning of affected body parts. Dystonia 16 can appear at any age from infancy through adulthood, although it most often begins in childhood.\n\nThe signs and symptoms of dystonia 16 vary among people with the condition. In many affected individuals, the disorder first affects muscles in one or both arms or legs. Tensing (contraction) of the muscles often sets the affected limb in an abnormal position, which may be painful and can lead to difficulty performing tasks, such as walking. In others, muscles in the neck are affected first, causing the head to be pulled backward and positioned with the chin in the air (retrocollis).\n\nIn dystonia 16, muscles of the jaw, lips, and tongue are also commonly affected (oromandibular dystonia), causing difficulty opening and closing the mouth and problems with swallowing and speech. Speech can also be affected by involuntary tensing of the muscles that control the vocal cords (laryngeal dystonia), resulting in a quiet, breathy voice or an inability to speak clearly. Dystonia 16 gradually gets worse, eventually involving muscles in most parts of the body.\n\nSome people with dystonia 16 develop a pattern of movement abnormalities known as parkinsonism. These abnormalities include unusually slow movement (bradykinesia), muscle rigidity, tremors, and an inability to hold the body upright and balanced (postural instability). In dystonia 16, parkinsonism is relatively mild if it develops at all.\n\nThe signs and symptoms of dystonia 16 usually do not get better when treated with drugs that are typically used for movement disorders.

Combined oxidative phosphorylation deficiency-6 (COXPD6) is an X-linked recessive severe encephalomyopathic disorder with onset in utero or in infancy. Affected patients have hypotonia and severely impaired psychomotor development associated with variably decreased enzymatic activity of mitochondrial respiratory complexes in skeletal muscle or fibroblasts. More variable features may include sensorimotor neuropathy, seizures, severe muscle weakness, abnormal signals in the basal ganglia, hypertrophic cardiomyopathy, deafness, swallowing difficulties, and respiratory insufficiency. Death in childhood may occur (summary by Berger et al., 2011). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Pontocerebellar hypoplasia type 8 is an autosomal recessive neurodevelopmental disorder characterized by severe psychomotor retardation, abnormal movements, hypotonia, spasticity, and variable visual defects. Brain MRI shows pontocerebellar hypoplasia, decreased cerebral white matter, and a thin corpus callosum (summary by Mochida et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Parkinson disease-20 is an autosomal recessive neurodegenerative disorder characterized by young adult-onset of parkinsonism. Additional features may include seizures, cognitive decline, abnormal eye movements, and dystonia (summary by Krebs et al., 2013 and Quadri et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of Parkinson disease, see PD (168600).

Hyperphosphatasia with impaired intellectual development syndrome-4 (HPMRS4) is an autosomal recessive neurologic disorder characterized by severely delayed psychomotor development, impaired intellectual development, lack of speech acquisition, seizures, and dysmorphic facial features. Laboratory studies show increased serum alkaline phosphatase (summary by Howard et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of HPMRS, see HPMRS1 (239300). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Developmental and epileptic encephalopathy-25 with amelogenesis imperfecta (DEE25) is an autosomal recessive neurologic disorder characterized by the onset of refractory seizures in early infancy. Most patients present with seizures in the neonatal period, which is often associated with status epilepticus. However, there is phenotypic variability, and some patients have onset of seizures later in infancy. Affected individuals show global developmental delay with intellectual disability and poor speech and communication. The seizures may remit somewhat with age, but there are persistent neurologic symptoms, including ataxia, spasticity, and abnormal involuntary movements. In addition to neurologic deficits, patients also have dental anomalies with amelogenesis imperfecta (summary by Thevenon et al., 2014 and Schossig et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

MECR-related neurologic disorder is characterized by a progressive childhood-onset movement disorder and optic atrophy; intellect is often – but not always – preserved. The movement disorder typically presents between ages one and 6.5 years and is mainly dystonia that can be accompanied by chorea and/or ataxia. Over time some affected individuals require assistive devices for mobility. Speech fluency and intelligibility are progressively impaired due to dysarthria. Optic atrophy typically develops between ages four and 12 years and manifests as reduced visual acuity, which can include functional blindness (also known as legal blindness) in adulthood. Because only 13 affected individuals are known to the authors, and because nearly half of them were diagnosed retrospectively as adults, the natural history of disease progression and other aspects of the phenotype have not yet been completely defined.

Neurodevelopmental disorder with hypotonia and impaired expressive language and with or without seizures (NEDHELS) is an autosomal recessive disorder characterized by hypotonia, poor feeding, and global developmental delay apparent from infancy. Most patients have poor overall growth, poor eye contact, sleep disturbances, and severely impaired expressive language. Affected individuals also tend to have behavioral problems, microcephaly, and variable dysmorphic features; many develop seizures. Brain imaging may show enlarged ventricles, thin corpus callosum and brainstem, and white matter abnormalities. The phenotype is variable (summary by Nabais Sa et al., 2019).

GNAO1-related disorder encompasses a broad phenotypic continuum that includes hyperkinetic movement disorders and/or epilepsy and is typically associated with developmental delay and intellectual disability. Viewed by age of onset, three clusters in this continuum can be observed: (1) infantile-onset developmental and epileptic encephalopathy (DEE) with or without prominent movement disorder; (2) infantile- or early childhood-onset prominent movement disorder and neurodevelopmental disorder with or without childhood-onset epilepsy with varying seizure types; (3) later childhood- or adult-onset movement disorder with variable developmental delay and intellectual disability. Epilepsy can be either DEE (onset typically within the first year of life of drug-resistant epilepsy in which developmental delays are attributed to the underlying diagnosis as well as the impact of uncontrolled seizures) or varying seizure types (onset typically between ages three and ten years of focal or generalized tonic-clonic seizures that may be infrequent or well controlled with anti-seizure medications). Movement disorders are characterized by dystonia and choreoathetosis, most commonly a mixed pattern of persistent or paroxysmal dyskinesia that affects the whole body. Exacerbations of the hyperkinetic movement disorder, which can be spontaneous or triggered (e.g., by intercurrent illness, emotional stress, voluntary movements), can last minutes to weeks. Hyperkinetic crises (including status dystonicus) are characterized by temporarily increased and nearly continuous involuntary movements or dystonic posturing that can be life-threatening. Deaths in early childhood have been reported due to medically refractory epilepsy or hyperkinetic crises, but the phenotypic spectrum includes milder presentations, including in adults. As many adults with disabilities have not undergone advanced genetic testing, it is likely that adults with GNAO1-related disorder are underrecognized and underreported.

NEDMIAL is a neurodevelopmental disorder characterized by delayed psychomotor development and hypotonia apparent from early infancy, resulting in feeding difficulties, ataxic gait or inability to walk, delayed or absent speech development, and impaired intellectual development, sometimes with behavioral abnormalities, such as hand-flapping. Additional common features may include sleep disorder, nonspecific dysmorphic facial features, and joint hyperlaxity (summary by Lessel et al., 2017 and Mannucci et al., 2021).

GRIN1-related neurodevelopmental disorder (GRIN1-NDD) is characterized by mild-to-profound developmental delay / intellectual disability (DD/ID) in all affected individuals. Other common manifestations are epilepsy, muscular hypotonia, movement disorders, spasticity, feeding difficulties, and behavior issues. A subset of individuals show a malformation of cortical development consisting of extensive and diffuse bilateral polymicrogyria. To date, 72 individuals with GRIN1-NDD have been reported.

Baker-Gordon syndrome (BAGOS) is a neurodevelopmental disorder characterized by infantile hypotonia, ophthalmic abnormalities, moderate to profound global developmental delay, poor or absent speech, behavioral abnormalities, hyperkinetic movements, and EEG abnormalities in the absence of overt seizures (summary by Baker et al., 2018).

Lissencephaly-9 with complex brainstem malformation (LIS9) is an autosomal dominant neurologic disorder characterized by global developmental delay apparent since infancy, impaired intellectual development with poor or absent speech, and sometimes abnormal or involuntary movements associated with abnormal brain imaging that typically shows pachygyria, lissencephaly, and malformation of the brainstem consistent with a neuronal migration defect (summary by Dobyns et al., 2018). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

Combined oxidative phosphorylation deficiency-39 (COXPD39) is an autosomal recessive multisystem disorder resulting from a defect in mitochondrial energy metabolism. Affected individuals show global developmental delay, sometimes with regression after normal early development, axial hypotonia with limb spasticity or abnormal involuntary movements, and impaired intellectual development with poor speech. More variable features may include hypotonia, seizures, and features of Leigh syndrome (256000) on brain imaging. There are variable deficiencies of the mitochondrial respiratory chain enzyme complexes in patient tissues (summary by Glasgow et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

ADCY5 dyskinesia is a hyperkinetic movement disorder (more prominent in the face and arms than the legs) characterized by infantile to late-adolescent onset of chorea, athetosis, dystonia, myoclonus, or a combination of these. To date, affected individuals have had overlapping (but not identical) manifestations with wide-ranging severity. The facial movements are typically periorbital and perioral. The dyskinesia is prone to episodic or paroxysmal exacerbation lasting minutes to hours, and may occur during sleep. Precipitating factors in some persons have included emotional stress, intercurrent illness, sneezing, or caffeine; in others, no precipitating factors have been identified. In some children, severe infantile axial hypotonia results in gross motor delays accompanied by chorea, sometimes with language delays. The overall tendency is for the abnormal movements to stabilize in early middle age, at which point they may improve in some individuals; less commonly, the abnormal movements are slowly progressive, increasing in severity and frequency.

Mitochondrial DNA depletion syndrome-20 (MTDPS20) is an autosomal recessive multisystem disorder with variable manifestations and severity. Most patients develop symptoms in childhood, although the onset can range from infancy to the teenage years. Prominent features include severe gastrointestinal dysmotility often requiring parenteral nutrition, neurogenic bladder, and muscle weakness and atrophy. Neurologic involvement manifests as headaches, stroke-like episodes, seizures, pyramidal signs, and learning difficulties or cognitive decline. Brain imaging usually shows diffuse leukoencephalopathy and may show cerebellar atrophy. The disorder results from a defect in the maintenance and repair of mitochondrial DNA, resulting in mtDNA depletion and impaired mitochondrial function (summary by Bonora et al., 2021). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).

Episodic kinesigenic dyskinesia-3 (EKD3) is an autosomal dominant form of paroxysmal kinesigenic dyskinesia (PKD), an episodic involuntary movement disorder characterized by dystonia, chorea, athetosis, and other hyperkinetic movements. The age at onset is around 9 to 12 years of age and symptoms are usually triggered by sudden movement or stress. Most patients have spontaneous resolution of episodes in their early twenties or later. Brain imaging is normal. There is a favorable response to treatment with carbamazepine (Li et al., 2021; Tian et al., 2022; Wang et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of episodic kinesigenic dyskinesia (EKD), see EKD1 (128200).

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).

X-linked syndromic intellectual developmental disorder-37 (MRXS37) is a developmental disorder showing phenotypic variability and variable severity. Male mutation carriers tend to be more severely affected than female mutation carriers, some of whom may even be asymptomatic. In general, the disorder is characterized by global developmental delay with delayed walking, speech delay, impaired intellectual development that ranges from borderline low to moderate, and behavioral abnormalities, such as autism and sleeping difficulties. Many patients are able to attend mainstream schools with assistance and work under supervision. Additional more variable features include sensorineural hearing loss, ocular anomalies, feeding difficulties, dysmorphic facial features, inguinal and umbilical hernia, genitourinary defects, congenital heart defects, musculoskeletal anomalies, and endocrine dysfunction, such as hypogonadism or hyperparathyroidism (Shepherdson et al., 2024).

Individuals with NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG) typically display a clinical tetrad of developmental delay / intellectual disability in the mild to profound range, hypo- or alacrima, elevated liver transaminases that may spontaneously resolve in childhood, and a complex hyperkinetic movement disorder that can include choreiform, athetoid, dystonic, myoclonic, action tremor, and dysmetric movements. About half of affected individuals will develop clinical seizures. Other findings may include obstructive and/or central sleep apnea, oral motor defects that affect feeding ability, auditory neuropathy, constipation, scoliosis, and peripheral neuropathy.