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.

Hartnup disorder (HND) is characterized by transient manifestations of pellagra, cerebellar ataxia, and psychosis. It is caused by impaired transport of neutral amino acids across epithelial cells in renal proximal tubules and intestinal mucosa (summary by Kleta et al., 2004).

Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by production of autoantibodies against nuclear, cytoplasmic, and cell surface molecules that transcend organ-specific boundaries. Tissue deposition of antibodies or immune complexes induces inflammation and subsequent injury of multiple organs and finally results in clinical manifestations of SLE, including glomerulonephritis, dermatitis, thrombosis, vasculitis, seizures, and arthritis. Evidence strongly suggests the involvement of genetic components in SLE susceptibility (summary by Oishi et al., 2008). Genetic Heterogeneity of Systemic Lupus Erythematosus An autosomal recessive form of systemic lupus erythematosus (SLEB16; 614420) is caused by mutation in the DNASE1L3 gene (602244) on chromosome 3p14.3. An X-linked dominant form of SLE (SLEB17; 301080) is caused by heterozygous mutation in the TLR7 gene (300365) on chromosome Xp22. See MAPPING and MOLECULAR GENETICS sections for a discussion of genetic heterogeneity of susceptibility to SLE.

Phenylalanine hydroxylase (PAH) deficiency results in intolerance to the dietary intake of the essential amino acid phenylalanine and produces a spectrum of disorders. The risk of adverse outcome varies based on the degree of PAH deficiency. Without effective therapy, most individuals with severe PAH deficiency, known as classic PKU, develop profound and irreversible intellectual disability. Affected individuals on an unrestricted diet who have phenylalanine levels above normal but below 1,200 µmol/L (20 mg/dL) are at much lower risk for impaired cognitive development in the absence of treatment.

Prader-Willi syndrome (PWS) is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of morbid obesity (unless eating is externally controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. A distinctive behavioral phenotype (with temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone); characteristic facial features, strabismus, and scoliosis are often present.

X-linked adrenoleukodystrophy (X-ALD) affects the nervous system white matter and the adrenal cortex. Three main phenotypes are seen in affected males: The childhood cerebral form manifests most commonly between ages four and eight years. It initially resembles attention-deficit disorder or hyperactivity; progressive impairment of cognition, behavior, vision, hearing, and motor function follow the initial symptoms and often lead to total disability within six months to two years. Most individuals have impaired adrenocortical function at the time that neurologic disturbances are first noted. Adrenomyeloneuropathy (AMN) manifests most commonly in an individual in his twenties or middle age as progressive stiffness and weakness of the legs, sphincter disturbances, sexual dysfunction, and often, impaired adrenocortical function; all symptoms are progressive over decades. "Addison disease only" presents with primary adrenocortical insufficiency between age two years and adulthood and most commonly by age 7.5 years, without evidence of neurologic abnormality; however, some degree of neurologic disability (most commonly AMN) usually develops by middle age. More than 20% of female carriers develop mild-to-moderate spastic paraparesis in middle age or later. Adrenal function is usually normal.

Variegate porphyria (VP) is both a cutaneous porphyria (with chronic blistering skin lesions) and an acute porphyria (with severe episodic neurovisceral symptoms). The most common manifestation of VP is adult-onset cutaneous blistering lesions (subepidermal vesicles, bullae, and erosions that crust over and heal slowly) of sun-exposed skin, especially the hands and face. Other chronic skin findings include milia, scarring, thickening, and areas of decreased and increased skin pigmentation. Facial hyperpigmentation and hypertrichosis may occur. Cutaneous manifestations may improve in winter and be less prevalent in northern regions and in dark-skinned individuals. Acute neurovisceral symptoms can occur any time after puberty, but less often in the elderly. Acute manifestations are highly variable, but may be similar from episode to episode in a person with recurrent attacks; not all manifestations are present in a single episode; and acute symptoms may become chronic. Symptoms are more common in women than men. The most common manifestations are abdominal pain; constipation; pain in the back, chest, and extremities; anxiety; seizures; and a primarily motor neuropathy resulting in muscle weakness that may progress to quadriparesis and respiratory paralysis. Psychiatric disturbances and autonomic neuropathy can also be observed. Acute attacks may be severe and are potentially fatal.

Norrie disease is an X-linked recessive disorder characterized by very early childhood blindness due to degenerative and proliferative changes of the neuroretina. Approximately 50% of patients show some form of progressive mental disorder, often with psychotic features, and about one-third of patients develop sensorineural deafness in the second decade. In addition, some patients have more complex phenotypes, including growth failure and seizures (Berger et al., 1992). Warburg (1966) noted confusion of the terms 'pseudoglioma' and microphthalmia with Norrie disease in the literature. 'Pseudoglioma' is a nonspecific term for any condition resembling retinoblastoma and can have diverse causes, including inflammation, hemorrhage, trauma, neoplasia, or congenital malformation, and often shows unilateral involvement. Thus, 'pseudoglioma' is not an acceptable clinical or pathologic diagnosis (Duke-Elder, 1958).

Succinic semialdehyde dehydrogenase (SSADH) deficiency is characterized by infantile-onset hypotonia, developmental delay, cognitive impairment, expressive language deficit, and mild ataxia. Epilepsy is present in about half of affected individuals and is more common in adults. Hyperkinetic behavior, aggression, self-injurious behaviors, hallucinations, and sleep disturbances have been reported in nearly half of all affected individuals, more commonly in those who are older. Basal ganglia signs including choreoathetosis, dystonia, and myoclonus have been reported in a few individuals with earlier-onset, more severe disease. Involvement beyond the central nervous system has not been described. Individuals with SSADH deficiency typically have 4-hydroxybutyric aciduria present on urine organic acid analysis. Head MRI reveals T2 hyperintensities in multiple regions, involving the globus pallidi, cerebellar dentate nuclei, subthalamic nuclei, subcortical white matter, and brain stem, as well as cerebral and sometimes cerebellar atrophy. EEG findings include background slowing and spike discharges that are usually generalized.

Virtually all individuals with Woodhouse-Sakati syndrome (WSS) have the endocrine findings of hypogonadism (evident at puberty) and progressive childhood-onset hair thinning that often progresses to alopecia totalis in adulthood. More than half of individuals have the neurologic findings of progressive extrapyramidal movements (dystonic spasms with dystonic posturing with dysarthria and dysphagia), moderate bilateral postlingual sensorineural hearing loss, and mild intellectual disability. To date, more than 40 families (including 33 with a molecularly confirmed diagnosis) with a total of 88 affected individuals have been reported in the literature.

Most infants with short-chain acyl-CoA dehydrogenase deficiency (SCADD) identified through newborn screening programs have remained well, and asymptomatic relatives who meet diagnostic criteria are reported. Thus, SCADD is now viewed as a biochemical phenotype rather than a disease. A broad range of clinical findings was originally reported in those with confirmed SCADD, including severe dysmorphic facial features, feeding difficulties / failure to thrive, metabolic acidosis, ketotic hypoglycemia, lethargy, developmental delay, seizures, hypotonia, dystonia, and myopathy. However, individuals with no symptoms were also reported. In a large series of affected individuals detected on metabolic evaluation for developmental delay, 20% had failure to thrive, feeding difficulties, and hypotonia; 22% had seizures; and 30% had hypotonia without seizures. In contrast, the majority of infants with SCADD have been detected by expanded newborn screening, and the great majority of these infants remain asymptomatic. As with other fatty acid oxidation deficiencies, characteristic biochemical findings of SCADD may be absent except during times of physiologic stress such as fasting and illness. A diagnosis of SCADD based on clinical findings should not preclude additional testing to look for other causes.

A psychotic syndrome caused by damage to the brain by lack of thiamine (vitamin B1). Signs and symptoms include anterograde and retrograde amnesia, confabulation, apathy, ataxia, and coma.

Chorea-acanthocytosis (ChAc) is characterized by a progressive movement disorder, cognitive and behavior changes, a myopathy that can be subclinical, and chronic hyperCKemia in serum. Although the disorder is named for acanthocytosis of the red blood cells, this feature is variable. The movement disorder is mostly limb chorea, but some individuals present with parkinsonism. Dystonia is common and affects the oral region and especially the tongue, causing dysarthria and serious dysphagia with resultant weight loss. Habitual tongue and lip biting are characteristic, as well as tongue protrusion dystonia. Progressive cognitive and behavioral changes resemble those in a frontal lobe syndrome. Seizures are observed in almost half of affected individuals and can be the initial manifestation. Myopathy results in progressive distal muscle wasting and weakness. Mean age of onset in ChAc is about 30 years, although ChAc can develop as early as the first decade or as late as the seventh decade. It runs a chronic progressive course and may lead to major disability within a few years. Life expectancy is reduced, with age of death ranging from 28 to 61 years.

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). The hallmark of CLN3 is the ultrastructural pattern of lipopigment with a 'fingerprint' profile, which can have 3 different appearances: pure within a lysosomal residual body; in conjunction with curvilinear or rectilinear profiles; and as a small component within large membrane-bound lysosomal vacuoles. The combination of fingerprint profiles within lysosomal vacuoles is a regular feature of blood lymphocytes from patients with CLN3 (Mole et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder, is characterized by severe progressive sensorimotor neuropathy with resulting hypotonia, areflexia, and amyotrophy, and by variable degrees of dysgenesis of the corpus callosum. Mild-to-severe intellectual disability and "psychotic episodes" during adolescence are observed. Sensory modalities are moderately to severely affected beginning in infancy. The average age of onset of walking is 3.8 years; the average age of loss of walking is 13.8 years; the average age of death is 33 years.

MED12-related disorders include the phenotypes of FG syndrome type 1 (FGS1), Lujan syndrome (LS), X-linked Ohdo syndrome (XLOS), Hardikar syndrome (HS), and nonspecific intellectual disability (NSID). FGS1 and LS share the clinical findings of cognitive impairment, hypotonia, and abnormalities of the corpus callosum. FGS1 is further characterized by absolute or relative macrocephaly, tall forehead, downslanted palpebral fissures, small and simple ears, constipation and/or anal anomalies, broad thumbs and halluces, and characteristic behavior. LS is further characterized by large head, tall thin body habitus, long thin face, prominent nasal bridge, high narrow palate, and short philtrum. Carrier females in families with FGS1 and LS are typically unaffected. XLOS is characterized by intellectual disability, blepharophimosis, and facial coarsening. HS has been described in females with cleft lip and/or cleft palate, biliary and liver anomalies, intestinal malrotation, pigmentary retinopathy, and coarctation of the aorta. Developmental and cognitive concerns have not been reported in females with HS. Pathogenic variants in MED12 have been reported in an increasing number of males and females with NSID, with affected individuals often having clinical features identified in other MED12-related disorders.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the PAK3 gene.

Familial hemiplegic migraine (FHM) falls within the category of migraine with aura. In migraine with aura (including FHM) the neurologic symptoms of aura are unequivocally localizable to the cerebral cortex or brain stem and include visual disturbance (most common), sensory loss (e.g., numbness or paresthesias of the face or an extremity), and dysphasia (difficulty with speech). FHM must include motor involvement, such as hemiparesis (weakness of an extremity). Hemiparesis occurs with at least one other symptom during FHM aura. Neurologic deficits with FHM attacks can be prolonged for hours to days and may outlast the associated migrainous headache. FHM is often earlier in onset than typical migraine, frequently beginning in the first or second decade; the frequency of attacks tends to decrease with age. Approximately 40%-50% of families with CACNA1A-FHM have cerebellar signs ranging from nystagmus to progressive, usually late-onset mild ataxia.

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.

Developmental and epileptic encephalopathy-9 (DEE9) is an X-linked disorder characterized by seizure onset in infancy and mild to severe intellectual impairment. Autistic and psychiatric features have been reported in some individuals. The disorder affects heterozygous females only; transmitting males are unaffected (summary by Jamal et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of developmental and epileptic encephalopathy, see 308350.

Infantile-onset spinocerebellar ataxia (IOSCA) is a severe, progressive neurodegenerative disorder characterized by normal development until age one year, followed by onset of ataxia, muscle hypotonia, loss of deep-tendon reflexes, and athetosis. Ophthalmoplegia and sensorineural deafness develop by age seven years. By adolescence, affected individuals are profoundly deaf and no longer ambulatory; sensory axonal neuropathy, optic atrophy, autonomic nervous system dysfunction, and hypergonadotropic hypogonadism in females become evident. Epilepsy can develop into a serious and often fatal encephalopathy: myoclonic jerks or focal clonic seizures that progress to epilepsia partialis continua followed by status epilepticus with loss of consciousness.

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.

The Lafora type of progressive myoclonic epilepsy is an autosomal recessive disorder characterized by insidious onset of progressive neurodegeneration between 8 and 18 years of age. Initial features can include headache, difficulties in school work, myoclonic jerks, generalized seizures, and often visual hallucination. The myoclonus, seizures, and hallucinations gradually worsen and become intractable. This is accompanied by progressive cognitive decline, resulting in dementia. About 10 years after onset, affected individuals are in near-continuous myoclonus with absence seizures, frequent generalized seizures, and profound dementia or a vegetative state. Histologic studies of multiple tissues, including brain, muscle, liver, and heart show intracellular Lafora bodies, which are dense accumulations of malformed and insoluble glycogen molecules, termed polyglucosans (review by Ramachandran et al., 2009). There is a slower progression of disease and later age at death in Lafora disease-2 than in Lafora disease-1 (MELF1, EPM2A; 254780); see Genotype/Phenotype Correlations. Myoclonic epilepsy of Lafora-1 is caused by mutation in the EPM2A gene (608072), which encodes laforin, on chromosome 6q24. For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

ACTH-independent macronodular adrenal hyperplasia (AIMAH) is an endogenous form of adrenal Cushing syndrome characterized by multiple bilateral adrenocortical nodules that cause a striking enlargement of the adrenal glands. Although some familial cases have been reported, the vast majority of AIMAH cases are sporadic. Patients typically present in the fifth or sixth decade of life, approximately 10 years later than most patients with other causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). Approximately 10 to 15% of adrenal Cushing syndrome is due to primary bilateral ACTH-independent adrenocortical pathology. The 2 main subtypes are AIMAH and primary pigmented nodular adrenocortical disease (PPNAD; see 610489), which is often a component of the Carney complex (160980) and associated with mutations in the PRKAR1A gene (188830). AIMAH is rare, representing less than 1% of endogenous causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). See also ACTH-independent Cushing syndrome (615830) due to somatic mutation in the PRKACA gene (601639). Cushing 'disease' (219090) is an ACTH-dependent disorder caused in most cases by pituitary adenomas that secrete excessive ACTH. Genetic Heterogeneity of ACTH-Independent Macronodular Adrenal Hyperplasia AIMAH2 (615954) is caused by germline mutation on 1 allele of the ARMC5 gene (615549) coupled with a somatic mutation in the other allele.

ITM2B-related cerebral amyloid angiopathy-2, also known as familial Danish dementia (FDD), is an autosomal dominant neurodegenerative disorder characterized by the progressive development of cataracts and other ocular disorders including ocular hemorrhages, hearing impairment, varying neurologic symptoms, and dementia, usually associated with paranoid reactions and temporal disturbance of consciousness. Most patients die in the fifth to sixth decade of life. Neuropathologic findings include severe widespread cerebral amyloid angiopathy, hippocampal plaques, and neurofibrillary tangles, similar to Alzheimer disease (see 104300) (summary by Vidal et al., 2000).

Primary pigmented nodular adrenocortical disease (PPNAD) is a form of ACTH-independent adrenal hyperplasia resulting in Cushing syndrome. It is usually seen as a manifestation of the Carney complex (CNC1; 160980), a multiple neoplasia syndrome. However, PPNAD can also occur in isolation (Groussin et al., 2002). Genetic Heterogeneity of Primary Pigmented Nodular Adrenocortical Disease See also PPNAD2 (610475), caused by mutation in the PDE11A gene (604961) on chromosome 2q31; PPNAD3 (614190), caused by mutation in the PDE8B gene (603390) on chromosome 5q13; and PPNAD4 (615830), caused by a duplication on chromosome 19p13 that includes the PRKACA gene (601639).

Any primary pigmented nodular adrenocortical disease in which the cause of the disease is a mutation in the PDE11A gene.

3q29 recurrent deletion is characterized by neurodevelopmental and/or psychiatric manifestations including mild-to-moderate intellectual disability (ID), autism spectrum disorder (ASD), anxiety disorders, attention-deficit/hyperactivity disorder (ADHD), executive function deficits, graphomotor weakness, and psychosis/schizophrenia. Age at onset for psychosis or prodrome can be younger than the typical age at onset in the general population. Neurodevelopmental and psychiatric conditions are responsible for the majority of the disability associated with the 3q29 deletion. Other common findings are failure to thrive and feeding problems in infancy that persist into childhood, gastrointestinal disorders (including constipation and gastroesophageal reflux disease [GERD]), ocular issues, dental anomalies, and congenital heart defects (especially patent ductus arteriosus). Structural anomalies of the posterior fossa may be seen on neuroimaging. To date more than 200 affected individuals have been identified.

A rare genetic multiple congenital anomalies/dysmorphic syndrome with the association of severe intellectual disability, strabismus and anterior maxillary protrusion with vertical maxillary excess, open bite and prominent crowded teeth. Mild cochlear hearing loss has been reported in addition.

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.

A heterozygous deletion of chromosome 15q11.2 may increase the susceptibility to neuropsychiatric or neurodevelopmental problems, including delayed psychomotor development, speech delay, autism spectrum disorder, attention deficit-hyperactivity disorder, obsessive-compulsive disorder, and possibly seizures (summary by Doornbos et al., 2009 and Burnside et al., 2011). See also chromosome 15q11.2 duplication syndrome (608636).

Autosomal dominant Wolfram-like syndrome (WFSL) is characterized by the clinical triad of congenital progressive hearing impairment, diabetes mellitus, and optic atrophy. The hearing impairment, which is usually diagnosed in the first decade of life, is relatively constant and alters mainly low- and middle-frequency ranges (summary by Valero et al., 2008). Wolfram syndrome (WFS1; 222300) is an autosomal recessive allelic disorder characterized by optic atrophy, diabetes mellitus, hearing loss, and diabetes insipidus, and is caused by homozygous or compound heterozygous mutation in the WFS1 gene. An autosomal dominant syndrome involving optic atrophy with or without deafness, ophthalmoplegia, myopathy, ataxia, and neuropathy (125250), is caused by heterozygous mutation in the OPA1 gene (605290).

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is characterized by clusters of nocturnal motor seizures, which are often stereotyped and brief (5 seconds to 5 minutes). They vary from simple arousals from sleep to dramatic, often bizarre hyperkinetic events with tonic or dystonic features. Affected individuals may experience aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Onset ranges from infancy to adulthood. About 80% of individuals develop ADNFLE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidity, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADNFLE is lifelong but not progressive. As an individual reaches middle age, attacks may become milder and less frequent.

Mitochondrial complex III deficiency nuclear type 2 is an autosomal recessive severe neurodegenerative disorder that usually presents in childhood, but may show later onset, even in adulthood. Affected individuals have motor disability, with ataxia, apraxia, dystonia, and dysarthria, associated with necrotic lesions throughout the brain. Most patients also have cognitive impairment and axonal neuropathy and become severely disabled later in life (summary by Ghezzi et al., 2011). The disorder may present clinically as spinocerebellar ataxia or Leigh syndrome, or with psychiatric disturbances (Morino et al., 2014; Atwal, 2014; Nogueira et al., 2013). For a discussion of genetic heterogeneity of mitochondrial complex III deficiency, see MC3DN1 (124000).

Macrocephaly refers to an abnormally enlarged head inclusive of the scalp, cranial bones, and intracranial contents. Macrocephaly may be due to megalencephaly (true enlargement of the brain parenchyma), and the 2 terms are often used interchangeably in the genetic literature (reviews by Olney, 2007 and Williams et al., 2008). Autosomal recessive macrocephaly/megalencephaly syndrome is characterized by an enlarged cranium apparent at birth or in early childhood. Affected individuals have intellectual disability and may have dysmorphic facial features resulting from the macrocephaly (summary by Alfaiz et al., 2014).

ADCADN is an autosomal dominant neurologic disorder characterized by adult onset of progressive cerebellar ataxia, narcolepsy/cataplexy, sensorineural deafness, and dementia. More variable features include optic atrophy, sensory neuropathy, psychosis, and depression (summary by Winkelmann et al., 2012).

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.

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).

Progressive myoclonic epilepsy-10 (EPM10) is an autosomal recessive neurodegenerative disorder characterized by onset of progressive myoclonus, ataxia, spasticity, dysarthria, and cognitive decline in the first decade of life. The severity is variable, but some patients may become mute and bedridden with psychosis (summary by Turnbull et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

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.

CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy) is characterized by mid-adult onset of recurrent ischemic stroke, cognitive decline progressing to dementia, a history of migraine with aura, mood disturbance, apathy, and diffuse white matter lesions and subcortical infarcts on neuroimaging.

Hypomyelinating leukodystrophy-16 is an autosomal dominant neurologic disorder characterized by onset of hypotonia, nystagmus, and mildly delayed motor development in infancy. Affected individuals have motor disabilities, including ataxic or broad-based gait, hyperreflexia, intention tremor, dysmetria, and a mild pyramidal syndrome. Some patients have cognitive impairment, whereas others may have normal cognition or mild intellectual disability with speech difficulties. Brain imaging typically shows hypomyelination, leukodystrophy, and thin corpus callosum (summary by Simons et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

Autosomal recessive idiopathic basal ganglia calcification-7 is a neurologic disorder characterized by onset of symptoms in adulthood. Patients present with dysarthria, gait abnormalities, various movement abnormalities, and often cognitive decline. Brain imaging shows abnormal accumulation of calcium deposits in deep brain regions, including the basal ganglia, thalamus, dentate nuclei, cerebellum, and sometimes other areas of the brain and spinal cord. Some patients with brain imaging abnormalities may be clinically asymptomatic (summary by Yao et al., 2018). For a detailed phenotypic description and a discussion of genetic heterogeneity of IBGC, see IBGC1 (213600).

Developmental delay with or without dysmorphic facies and autism (DEDDFA) is a complex neurodevelopmental disorder apparent from infancy or early childhood and associated with variably impaired intellectual development. Some patients may be severely affected with no speech and inability to walk, whereas others may be able to attend special schools or have normal intellectual function associated with autism spectrum disorder and mild speech delay. Genetic analysis has suggested that the phenotype can be broadly categorized into 2 main groups. Patients with TRRAP mutations affecting residues 1031-1159 have a more severe disorder, often with multisystem involvement, including renal, cardiac, and genitourinary systems, as well as structural brain abnormalities. Patients with mutations outside of that region tend to have a less severe phenotype with a higher incidence of autism and usually no systemic involvement. Patients in both groups usually have somewhat similar dysmorphic facial features, such as upslanting palpebral fissures, hypertelorism, low-set ears, and broad or depressed nasal bridge, although these features are highly variable (summary by Cogne et al., 2019).

Neurodevelopmental-craniofacial syndrome with variable renal and cardiac abnormalities (NECRC) is an autosomal dominant disorder characterized by dysmorphic craniofacial features associated with mild developmental delay, mildly impaired intellectual development or learning difficulties, speech delay, and behavioral abnormalities. About half of patients have congenital anomalies of the kidney and urinary tract (CAKUT) and/or congenital cardiac defects, including septal defects (Connaughton et al., 2020).

Developmental delay, hypotonia, musculoskeletal defects, and behavioral abnormalities (DEHMBA) is an early-onset neurodevelopmental disorder characterized by these features. Affected individuals also have nonspecific and variable dysmorphic facial features that do not constitute a recognizable gestalt. Although the disorder is caused by truncating mutations in the SRCAP gene as is FLHS, the DEHMBA phenotype is clinically distinguishable from FLHS by the lack of short stature, brachydactyly, and delayed bone age, as well as absence of a specific facial appearance. There are some overlapping features between the 2 disorders, mainly impaired intellectual development and speech delay (summary by Rots et al., 2021).

Neurodevelopmental disorder with or without variable movement or behavioral abnormalities (NEDMAB) is an autosomal dominant disorder characterized by mildly to severely impaired intellectual development and, in some patients, movement abnormalities consisting of tremors, cerebellar ataxia, or extrapyramidal symptoms. Movement abnormalities have onset in childhood or adolescence. Other variable features include autism spectrum disorder or autistic features and epilepsy.

Combined oxidative phosphorylation deficiency-54 (COXPD54) is an autosomal recessive disorder with pleiotropic multisystem presentations resulting from a disruption in mitochondrial transcription and translation. The phenotype is highly variable. Many patients have early-onset sensorineural hearing loss, sometimes in isolation, and sometimes associated with global developmental delay or primary ovarian failure. Other features may include peripheral hypertonia, seizures, muscle weakness, behavioral abnormalities, and leukoencephalopathy on brain imaging. Serum lactate may or may not be elevated (summary by Hochberg et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Childhood-onset parkinsonism-dystonia-3 (PKDYS3) is an autosomal recessive neurodegenerative disorder with onset in infancy or early childhood. Affected individuals present with progressive movement abnormalities, including parkinsonism with tremor, dystonia, myoclonus ataxia, and hyperkinetic movements such as ballismus. The parkinsonism features may be responsive to treatment with levodopa, although many patients develop levodopa-induced dyskinesia. Some patients may have mild cognitive impairment or psychiatric disturbances (summary by Burke et al., 2018 and Skorvanek et al., 2022). For a discussion of genetic heterogeneity of PKDYS, see 613135.

Tessadori-Bicknell-van Haaften neurodevelopmental syndrome-1 (TEBIVANED1) is characterized by poor overall growth with short stature, microcephaly, hypotonia, profound global developmental delay often with poor or absent speech, and characteristic dysmorphic facial features, including hypertelorism and abnormal nose. Other variable neurologic and systemic features may also occur (Tessadori et al., 2017). Genetic Heterogeneity of Tessadori-van Haaften Neurodevelopmental Syndrome See also TEBIVANED2 (619759), caused by mutation in the H4C11 gene (602826); TEBIVANED3 (619950), caused by mutation in the H4C5 gene (602830); and TEBIVANED4 (619951), caused by mutation in the H4C9 gene (602833).

Autosomal recessive intellectual developmental disorder-75 with neuropsychiatric features and variant lissencephaly (MRT75) is characterized by global developmental delay apparent from infancy or early childhood and moderate to profoundly impaired intellectual development. Most affected individuals have behavioral abnormalities, including aggression and ADHD; a few have psychiatric manifestations, including psychosis. More variable additional features include well-controlled seizures and dysmorphic facial features. Brain imaging often shows frontal predominant pachygyria or other gyri/sulci abnormalities, consistent with a variant of lissencephaly and a malformation of cortical development (MCD) (summary by Zaki et al., 2021).

Developmental delay with variable intellectual disability and dysmorphic facies (DIDDF) is a clinically heterogeneous disorder characterized by neurologic deficits and characteristic dysmorphic facial features apparent from infancy or early childhood. Affected individuals usually show impaired intellectual development, speech delay, learning difficulties, and/or behavioral problems. Additional features may include hypotonia, hand or foot deformities, and palatal defects (Verberne et al., 2021; Verberne et al., 2022).