Huntington disease (HD) is a progressive disorder of motor, cognitive, and psychiatric disturbances. The mean age of onset is 35 to 44 years, and the median survival time is 15 to 18 years after onset.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Genetic prion disease generally manifests with cognitive difficulties, ataxia, and myoclonus (abrupt jerking movements of muscle groups and/or entire limbs). The order of appearance and/or predominance of these features and other associated neurologic and psychiatric findings vary. The three major phenotypes of genetic prion disease are genetic Creutzfeldt-Jakob disease (gCJD), fatal familial insomnia (FFI), and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Although these phenotypes display overlapping clinical and pathologic features, recognition of these phenotypes can be useful when providing affected individuals and their families with information about the expected clinical course. The age at onset typically ranges from 50 to 60 years. The disease course ranges from a few months in gCJD and FFI to a few (up to 4, and in rare cases up to 10) years in GSS syndrome.

Pick disease refers to the neuropathologic finding of 'Pick bodies,' which are argyrophilic, intraneuronal inclusions, and 'Pick cells,' which are enlarged neurons. The clinical correlates of Pick disease of brain include those of frontotemporal dementia, which encompass the behavioral variant of FTD, semantic dementia, and progressive nonfluent aphasia (summary by Piguet et al., 2011). Kertesz (2003) suggested the term 'Pick complex' to represent the overlapping syndromes of FTD, primary progressive aphasia (PPA), corticobasal degeneration (CBD), progressive supranuclear palsy (601104), and FTD with motor neuron disease. He noted that frontotemporal dementia may also be referred to as 'clinical Pick disease,' and that the term 'Pick disease' should be restricted to the pathologic finding of Pick bodies.

PLA2G6-associated neurodegeneration (PLAN) comprises a continuum of three phenotypes with overlapping clinical and radiologic features: Infantile neuroaxonal dystrophy (INAD). Atypical neuroaxonal dystrophy (atypical NAD). PLA2G6-related dystonia-parkinsonism. INAD usually begins between ages six months and three years with psychomotor regression or delay, hypotonia, and progressive spastic tetraparesis. Many affected children never learn to walk or lose the ability shortly after attaining it. Strabismus, nystagmus, and optic atrophy are common. Disease progression is rapid, resulting in severe spasticity, progressive cognitive decline, and visual impairment. Many affected children do not survive beyond their first decade. Atypical NAD shows more phenotypic variability than INAD. In general, onset is in early childhood but can be as late as the end of the second decade. The presenting signs may be gait instability, ataxia, or speech delay and autistic features, which are sometimes the only evidence of disease for a year or more. Strabismus, nystagmus, and optic atrophy are common. Neuropsychiatric disturbances including impulsivity, poor attention span, hyperactivity, and emotional lability are also common. The course is fairly stable during early childhood and resembles static encephalopathy but is followed by neurologic deterioration between ages seven and 12 years. PLA2G6-related dystonia-parkinsonism has a variable age of onset, but most individuals present in early adulthood with gait disturbance or neuropsychiatric changes. Affected individuals consistently develop dystonia and parkinsonism (which may be accompanied by rapid cognitive decline) in their late teens to early twenties. Dystonia is most common in the hands and feet but may be more generalized. The most common features of parkinsonism in these individuals are bradykinesia, resting tremor, rigidity, and postural instability.

Frontotemporal dementia (FTD) refers to a clinical manifestation of the pathologic finding of frontotemporal lobar degeneration (FTLD). FTD, the most common subtype of FTLD, is a behavioral variant characterized by changes in social and personal conduct with loss of volition, executive dysfunction, loss of abstract thought, and decreased speech output. A second clinical subtype of FTLD is 'semantic dementia,' characterized by specific loss of comprehension of language and impaired facial and object recognition. A third clinical subtype of FTLD is 'primary progressive aphasia' (PPA), characterized by a reduction in speech production, speech errors, and word retrieval difficulties resulting in mutism and an inability to communicate. All subtypes have relative preservation of memory, at least in the early stages. FTLD is often associated with parkinsonism or motor neuron disease (MND) resembling amyotrophic lateral sclerosis (ALS; 105400) (reviews by Tolnay and Probst, 2002 and Mackenzie and Rademakers, 2007). Mackenzie et al. (2009, 2010) provided a classification of FTLD subtypes according to the neuropathologic findings (see PATHOGENESIS below). Clinical Variability of Tauopathies Tauopathies comprise a clinically variable group of neurodegenerative diseases characterized neuropathologically by accumulation of abnormal MAPT-positive inclusions in nerve and/or glial cells. In addition to frontotemporal dementia, semantic dementia, and PPA, different clinical syndromes with overlapping features have been described, leading to confusion in the terminology (Tolnay and Probst, 2002). Other terms used historically include parkinsonism and dementia with pallidopontonigral degeneration (PPND) (Wszolek et al., 1992); disinhibition-dementia-parkinsonism-amyotrophy complex (DDPAC) (Lynch et al., 1994); frontotemporal dementia with parkinsonism (FLDEM) (Yamaoka et al., 1996); and multiple system tauopathy with presenile dementia (MSTD) (Spillantini et al., 1997). These disorders are characterized by variable degrees of frontal lobe dementia, parkinsonism, motor neuron disease, and amyotrophy. Other neurodegenerative associated with mutations in the MAPT gene include Pick disease (172700) and progressive supranuclear palsy (PSP; 601104), Inherited neurodegenerative tauopathies linked to chromosome 17 and caused by mutation in the MAPT gene have also been collectively termed 'FTDP17' (Lee et al., 2001). Kertesz (2003) suggested the term 'Pick complex' to represent the overlapping syndromes of FTD, primary progressive aphasia (PPA), corticobasal degeneration (CBD), PSP, and FTD with motor neuron disease. He noted that frontotemporal dementia may also be referred to as 'clinical Pick disease' and that the term 'Pick disease' should be restricted to the pathologic finding of Pick bodies. Genetic Heterogeneity of Frontotemporal Lobar Degeneration Mutations in several different genes can cause frontotemporal dementia and frontotemporal lobar degeneration, with or without motor neuron disease. See FTLD with TDP43 inclusions (607485), caused by mutation in the GRN gene (138945) on chromosome 17q21; FTLALS7 (600795), caused by mutation in the CHMP2B gene (609512) on chromosome 3p11; inclusion body myopathy with Paget disease and FTD (IBMPFD; 167320), caused by mutation in the VCP gene (601023) on chromosome 9p13; ALS6 (608030), caused by mutation in the FUS gene (137070) on 16p11; ALS10 (612069), caused by mutation in the TARDBP gene (605078) on 1p36; and FTDALS1 (105550), caused by mutation in the C9ORF72 gene (614260) on 9p21. In 1 family with FTD, a mutation was identified in the presenilin-1 gene (PSEN1; 104311) on chromosome 14, which is usually associated with a familial form of early-onset Alzheimer disease (AD3; 607822).

CHMP2B frontotemporal dementia (CHMP2B-FTD) has been described in a single family from Denmark, in one individual with familial FTD from Belgium, and in one individual with FTD and no family history. It typically starts between ages 46 and 65 years with subtle personality changes and slowly progressive behavioral changes, dysexecutive syndrome, dyscalculia, and language disturbances. Disinhibition or loss of initiative is the most common presenting symptom. The disease progresses over a few years into profound dementia with extrapyramidal symptoms and mutism. Several individuals have developed an asymmetric akinetic rigid syndrome with arm and gait dystonia and pyramidal signs that may be related to treatment with neuroleptic drugs. Symptoms and disease course are highly variable. Disease duration may be as short as three years or longer than 20 years.

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Caused by heterozygous mutation in the VAPB gene on chromosome 20q13.

The spectrum of GRN frontotemporal dementia (GRN-FTD) includes the behavioral variant (bvFTD), primary progressive aphasia (PPA; further subcategorized as progressive nonfluent aphasia [PNFA] and semantic dementia [SD]), and movement disorders with extrapyramidal features such as parkinsonism and corticobasal syndrome (CBS). A broad range of clinical features both within and between families is observed. The age of onset ranges from 35 to 87 years. Behavioral disturbances are the most common early feature, followed by progressive aphasia. Impairment in executive function manifests as loss of judgment and insight. In early stages, PPA often manifests as deficits in naming, word finding, or word comprehension. In late stages, affected individuals often become mute and lose their ability to communicate. Early findings of parkinsonism include rigidity, bradykinesia or akinesia (slowing or absence of movements), limb dystonia, apraxia (loss of ability to carry out learned purposeful movements), and disequilibrium. Late motor findings may include myoclonus, dysarthria, and dysphagia. Most affected individuals eventually lose the ability to walk. Disease duration is three to 12 years.

Spinocerebellar ataxia type 17 (SCA17) is characterized by ataxia, dementia, and involuntary movements, including chorea and dystonia. Psychiatric symptoms, pyramidal signs, and rigidity are common. The age of onset ranges from three to 55 years. Individuals with full-penetrance alleles develop neurologic and/or psychiatric symptoms by age 50 years. Ataxia and psychiatric abnormalities are frequently the initial findings, followed by involuntary movement, parkinsonism, dementia, and pyramidal signs. Brain MRI shows variable atrophy of the cerebrum, brain stem, and cerebellum. The clinical features correlate with the length of the polyglutamine expansion but are not absolutely predictive of the clinical course.

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).

Spinocerebellar ataxia-23 (SCA23) is an adult-onset autosomal dominant neurodegenerative disorder characterized by slowly progressive gait and limb ataxia, with variable additional features, including peripheral neuropathy and dysarthria (Bakalkin et al., 2010). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is an autosomal dominant disorder characterized by progressive epilepsy and dementia. Onset of symptoms ranges from the second to fifth decades of life. Severity is variable (summary by Molinari et al., 2003).

The neuronal ceroid lipofuscinoses (NCL; CLN) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis, see CLN1 (256730).

Combined saposin deficiency (PSAPD), a deficiency of prosaposin and saposins A, B, C, and D, is a fatal infantile storage disorder with hepatosplenomegaly and severe neurologic disease (summary by Hulkova et al., 2001).

Christianson syndrome (referred to as CS in this GeneReview), an X-linked disorder, is characterized in males by cognitive dysfunction, behavioral disorder, and neurologic findings (e.g., seizures, ataxia, postnatal microcephaly, and eye movement abnormalities). Males with CS typically present with developmental delay, later meeting criteria for severe intellectual disability (ID). Behaviorally, autism spectrum disorder and hyperactivity are common, and may resemble the behaviors observed in Angelman syndrome. Hypotonia and oropharyngeal dysphagia in infancy may result in failure to thrive. Seizures, typically beginning before age three years, can include infantile spasms and tonic, tonic-clonic, myoclonic, and atonic seizures. Subsequently, regression (e.g., loss of ambulation and ability to feed independently) may occur. Manifestations in heterozygous females range from asymptomatic to mild ID and/or behavioral issues.

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Caused by heterozygous mutation in the FUS gene on chromosome 16p11.

Generally, Parkinson's disease that begins after age 50 is called late-onset disease. The condition is described as early-onset disease if signs and symptoms begin before age 50. Early-onset cases that begin before age 20 are sometimes referred to as juvenile-onset Parkinson's disease.\n\nParkinson's disease can also affect emotions and thinking ability (cognition). Some affected individuals develop psychiatric conditions such as depression and visual hallucinations. People with Parkinson's disease also have an increased risk of developing dementia, which is a decline in intellectual functions including judgment and memory.\n\nOften the first symptom of Parkinson's disease is trembling or shaking (tremor) of a limb, especially when the body is at rest. Typically, the tremor begins on one side of the body, usually in one hand. Tremors can also affect the arms, legs, feet, and face. Other characteristic symptoms of Parkinson's disease include rigidity or stiffness of the limbs and torso, slow movement (bradykinesia) or an inability to move (akinesia), and impaired balance and coordination (postural instability). These symptoms worsen slowly over time.\n\nParkinson's disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.

Niemann-Pick disease type C (NPC) is a slowly progressive lysosomal disorder whose principal manifestations are age dependent. The manifestations in the perinatal period and infancy are predominantly visceral, with hepatosplenomegaly, jaundice, and (in some instances) pulmonary infiltrates. From late infancy onward, the presentation is dominated by neurologic manifestations. The youngest children may present with hypotonia and developmental delay, with the subsequent emergence of ataxia, dysarthria, dysphagia, and, in some individuals, epileptic seizures, dystonia, and gelastic cataplexy. Although cognitive impairment may be subtle at first, it eventually becomes apparent that affected individuals have a progressive dementia. Older teenagers and young adults may present predominantly with apparent early-onset dementia or psychiatric manifestations; however, careful examination usually identifies typical neurologic signs.

Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

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).

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.

Neuronal ceroid lipofuscinosis-13 (CLN13) is an autosomal recessive neurodegenerative disorder characterized by adult onset of progressive cognitive decline and motor dysfunction leading to dementia and often early death. Some patients develop seizures. Neurons show abnormal accumulation of autofluorescent material (summary by Smith et al., 2013). Adult-onset neuronal ceroid lipofuscinosis is sometimes referred to as Kufs disease (see 204300). In a review of the classification of CLN disease, Gardner and Mole (2021) noted that the CLN13 phenotype corresponds to 'Kufs type B', which is characterized by dementia and a variety of motor signs (Smith et al., 2013). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis (CLN), see CLN1 (256730).

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is characterized most often by frontotemporal dementia (FTD) and upper and lower motor neuron disease (MND); however, atypical presentations also occur. Age at onset is usually between 50 and 64 years (range: 20-91 years) irrespective of the presenting manifestations, which may be pure FTD, pure amyotrophic lateral sclerosis (ALS), or a combination of the two phenotypes. The clinical presentation is highly heterogeneous and may differ between and within families, causing an unpredictable pattern and age of onset of clinical manifestations. The presence of MND correlates with an earlier age of onset and a worse overall prognosis.

The spectrum of BSCL2-related neurologic disorders includes Silver syndrome and variants of Charcot-Marie-Tooth neuropathy type 2, distal hereditary motor neuropathy (dHMN) type V, and spastic paraplegia 17. Features of these disorders include onset of symptoms ranging from the first to the seventh decade, slow disease progression, upper motor neuron involvement (gait disturbance with pyramidal signs ranging from mild to severe spasticity with hyperreflexia in the lower limbs and variable extensor plantar responses), lower motor neuron involvement (amyotrophy of the peroneal muscles and small muscles of the hand), and pes cavus and other foot deformities. Disease severity is variable among and within families.

Combined oxidative phosphorylation deficiency-24 (COXPD24) is an autosomal recessive mitochondrial disorder with wide phenotypic variability. Most patients present in infancy with delayed neurodevelopment, refractory seizures, hypotonia, and hearing impairment due to auditory neuropathy. Less common features may include cortical blindness, renal dysfunction, and/or liver involvement, suggestive of Alpers syndrome (MTDPS4A; 203700). Patients with the severe phenotype tend to have brain abnormalities on imaging, including cerebral atrophy and hyperintensities in the basal ganglia and brainstem, consistent with Leigh syndrome. Laboratory values may be normal or show increased lactate and evidence of mitochondrial respiratory chain defects, particularly in muscle. Some patients achieve little developmental milestones and may die in infancy or early childhood. However, some patients have a less severe phenotype manifest only by myopathy (summary by Sofou et al., 2015, Vanlander et al., 2015, and Mizuguchi et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

PEBAT is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development apparent soon after birth or in infancy, profound intellectual disability, poor or absent speech, and seizures. Most patients are never able to walk due to hypotonia or spasticity. Brain imaging shows cerebral and cerebellar atrophy, thin corpus callosum, and secondary hypomyelination. The disorder shows progressive features, including microcephaly, consistent with a neurodegenerative process (summary by Miyake et al., 2016; Flex et al., 2016).

The spectrum of clinical manifestations of MAPT-related frontotemporal dementia (MAPT-FTD) has expanded from its original description of frontotemporal dementia and parkinsonian manifestations to include changes in behavior, motor function, memory, and/or language. A recent retrospective study suggested that the majority of affected individuals have either behavioral changes consistent with a diagnosis of behavioral variant FTD (bvFTD) or, less commonly, a parkinsonian syndrome (i.e., progressive supranuclear palsy, corticobasal syndrome, or Parkinson disease). Fewer than 5% of people with MAPT-FTD have primary progressive aphasia or Alzheimer disease. Clinical presentation may differ between and within families with the same MAPT variant. MAPT-FTD is a progressive disorder that commonly ends with a relatively global dementia in which some affected individuals become mute. Progression of motor impairment in affected individuals results in some becoming chairbound and others bedbound. Mean disease duration is 9.3 (SD: 6.4) years but is individually variable and can be more than 30 years in some instances.

Mitochondrial complex IV deficiency nuclear type 2 (MC4DN2) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms at birth or in the first weeks or months of life. Affected individuals have severe hypotonia, often associated with feeding difficulties and respiratory insufficiency necessitating tube feeding and mechanical ventilation. The vast majority of patients develop hypertrophic cardiomyopathy in the first days or weeks of life, which usually leads to death in infancy or early childhood. Patients also show neurologic abnormalities, including developmental delay, nystagmus, fasciculations, dystonia, EEG changes, and brain imaging abnormalities compatible with a diagnosis of Leigh syndrome (see 256000). There may also be evidence of systemic involvement with hepatomegaly and myopathy, although neurogenic muscle atrophy is more common and may resemble spinal muscular atrophy type I (SMA1; 253300). Serum lactate is increased, and laboratory studies show decreased mitochondrial complex IV protein and activity levels in various tissues, including heart and skeletal muscle. Most patients die in infancy of cardiorespiratory failure (summary by Papadopoulou et al., 1999). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is characterized by executive dysfunction, memory decline, personality changes, motor impairments, and seizures. A frontal lobe syndrome (e.g., loss of judgment, lack of social inhibitors, lack of insight, and motor persistence) usually appears early in the disease course. The mean age of onset is usually in the fourth decade. Affected individuals eventually become bedridden with spasticity and rigidity. The disease course ranges from two to 30 or more years (mean: 8 years).