Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease characterized by infantile-onset diarrhea, childhood-onset cataract, adolescent- to young adult-onset tendon xanthomas, and adult-onset progressive neurologic dysfunction (dementia, psychiatric disturbances, pyramidal and/or cerebellar signs, dystonia, atypical parkinsonism, peripheral neuropathy, and seizures). Chronic diarrhea from infancy and/or neonatal cholestasis may be the earliest clinical manifestation. In approximately 75% of affected individuals, cataracts are the first finding, often appearing in the first decade of life. Xanthomas appear in the second or third decade; they occur on the Achilles tendon, the extensor tendons of the elbow and hand, the patellar tendon, and the neck tendons. Xanthomas have been reported in the lung, bones, and central nervous system. Some individuals show cognitive impairment from early infancy, whereas the majority have normal or only slightly impaired intellectual function until puberty; dementia with slow deterioration in intellectual abilities occurs in the third decade in more than 50% of individuals. Neuropsychiatric symptoms such as behavioral changes, hallucinations, agitation, aggression, depression, and suicide attempts may be prominent. Pyramidal signs (i.e., spasticity) and/or cerebellar signs almost invariably become evident between ages 20 and 30 years. The biochemical abnormalities that distinguish CTX from other conditions with xanthomas include high plasma and tissue cholestanol concentration, normal-to-low plasma cholesterol concentration, decreased chenodeoxycholic acid (CDCA), increased concentration of bile alcohols and their glyconjugates, and increased concentrations of cholestanol and apolipoprotein B in cerebrospinal fluid.

Polymicrogyria (PMG) is a malformation of cortical development in which the brain surface is irregular and the normal gyral pattern replaced by multiple small, partly fused gyri separated by shallow sulci. Microscopic examination shows a simplified 4-layered or unlayered cortex. Several patterns of PMG, including bilateral frontal, bilateral perisylvian, and bilateral mesial occipital PMG, have been described on the basis of their topographic distribution. All but the perisylvian form appear to be rare. Bilateral perisylvian PMG (BPP) often results in a typical clinical syndrome that is manifested by mild mental retardation, epilepsy, and pseudobulbar palsy, which causes difficulties with expressive speech and feeding (Kuzniecky et al., 1993). PMG may be a feature of other conditions as well (see, e.g., 300643).

Baraitser-Winter cerebrofrontofacial (BWCFF) syndrome is a multiple congenital anomaly syndrome characterized by typical craniofacial features and intellectual disability. Many (but not all) affected individuals have pachygyria that is predominantly frontal, wasting of the shoulder girdle muscles, and sensory impairment due to iris or retinal coloboma and/or sensorineural deafness. Intellectual disability, which is common but variable, is related to the severity of the brain malformations. Seizures, congenital heart defects, renal malformations, and gastrointestinal dysfunction are also common.

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.

Amyotrophic lateral sclerosis is a neurodegenerative disorder characterized by the death of motor neurons in the brain, brainstem, and spinal cord, resulting in fatal paralysis. ALS usually begins with asymmetric involvement of the muscles in middle adult life. Approximately 10% of ALS cases are familial (Siddique and Deng, 1996). ALS is sometimes referred to as 'Lou Gehrig disease' after the famous American baseball player who was diagnosed with the disorder. Rowland and Shneider (2001) and Kunst (2004) provided extensive reviews of ALS. Some forms of ALS occur with frontotemporal dementia (FTD); see 105500. Ranganathan et al. (2020) provided a detailed review of the genes involved in different forms of ALS with FTD, noting that common disease pathways involve disturbances in RNA processing, autophagy, the ubiquitin proteasome system, the unfolded protein response, and intracellular trafficking. The current understanding of ALS and FTD is that some forms of these disorders represent a spectrum of disease with converging mechanisms of neurodegeneration. Familial ALS is distinct from a form of ALS with dementia reported in cases on Guam (105500) (Espinosa et al., 1962; Husquinet and Franck, 1980), in which the histology is different and dementia and parkinsonism complicate the clinical picture. Genetic Heterogeneity of Amyotrophic Lateral Sclerosis ALS is a genetically heterogeneous disorder, with several causative genes and mapped loci. ALS6 (608030) is caused by mutation in the FUS gene (137070) on chromosome 16p11; ALS8 (608627) is caused by mutation in the VAPB gene (605704) on chromosome 13; ALS9 (611895) is caused by mutation in the ANG gene (105850) on chromosome 14q11; ALS10 (612069) is caused by mutation in the TARDBP gene (605078) on 1p36; ALS11 (612577) is caused by mutation in the FIG4 gene (609390) on chromosome 6q21; ALS12 (613435) is caused by mutation in the OPTN gene (602432) on chromosome 10p13; ALS15 (300857) is caused by mutation in the UBQLN2 gene (300264) on chromosome Xp11; ALS18 (614808) is caused by mutation in the PFN1 gene (176610) on chromosome 17p13; ALS19 (615515) is caused by mutation in the ERBB4 gene (600543) on chromosome 2q34; ALS20 (615426) is caused by mutation in the HNRNPA1 gene (164017) on chromosome 12q13; ALS21 (606070) is caused by mutation in the MATR3 gene (164015) on chromosome 5q31; ALS22 (616208) is caused by mutation in the TUBA4A gene (191110) on chromosome 2q35; ALS23 (617839) is caused by mutation in the ANXA11 gene (602572) on chromosome 10q23; ALS26 (619133) is caused by mutation in the TIA1 gene (603518) on chromosome 2p13; ALS27 (620285) is caused by mutation in the SPTLC1 gene (605712) on chromosome 9q22; and ALS28 (620452) is caused by mutation in the LRP12 gene (618299) on chromosome 8q22. Loci associated with ALS have been found on chromosomes 18q21 (ALS3; 606640) and 20p13 (ALS7; 608031). Intermediate-length polyglutamine repeat expansions in the ATXN2 gene (601517) contribute to susceptibility to ALS (ALS13; 183090). Susceptibility to ALS24 (617892) is conferred by mutation in the NEK1 gene (604588) on chromosome 4q33, and susceptibility to ALS25 (617921) is conferred by mutation in the KIF5A gene (602821) on chromosome 12q13. Susceptibility to ALS has been associated with mutations in other genes, including deletions or insertions in the gene encoding the heavy neurofilament subunit (NEFH; 162230); deletions in the gene encoding peripherin (PRPH; 170710); and mutations in the dynactin gene (DCTN1; 601143). Some forms of ALS show juvenile onset. See juvenile-onset ALS2 (205100), caused by mutation in the alsin (606352) gene on 2q33; ALS4 (602433), caused by mutation in the senataxin gene (SETX; 608465) on 9q34; ALS5 (602099), caused by mutation in the SPG11 gene (610844) on 15q21; and ALS16 (614373), caused by mutation in the SIGMAR1 gene (601978) on 9p13.

LMNB1-related autosomal dominant leukodystrophy (ADLD) is a slowly progressive disorder of central nervous system white matter characterized by onset of autonomic dysfunction in the fourth to fifth decade, followed by pyramidal and cerebellar abnormalities resulting in spasticity, ataxia, and tremor. Autonomic dysfunction can include bladder dysfunction, constipation, postural hypotension, erectile dysfunction, and (less often) impaired sweating. Pyramidal signs are often more prominent in the lower extremities (e.g., spastic weakness, hypertonia, clonus, brisk deep tendon reflexes, and bilateral Babinski signs). Cerebellar signs typically appear at the same time as the pyramidal signs and include gait ataxia, dysdiadochokinesia, intention tremor, dysmetria, and nystagmus. Many individuals have sensory deficits starting in the lower limbs. Pseudobulbar palsy with dysarthria, dysphagia, and forced crying and laughing may appear in the seventh or eighth decade. Although cognitive function is usually preserved or only mildly impaired early in the disease course, dementia and psychiatric manifestations can occur as late manifestations. Affected individuals may survive for decades after onset.

Spinocerebellar ataxia-35 (SCA35) is an autosomal dominant adult-onset neurologic disorder characterized by difficulty walking due to cerebellar ataxia. The age at onset ranges from teenage years to late adulthood, and the disorder is slowly progressive. Additional features may include hand tremor, dysarthria, hyperreflexia, and saccadic eye movements (summary by Guo et al., 2014). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Hypomyelinating leukodystrophy-9 is an autosomal recessive neurologic disorder characterized by onset of delayed psychomotor development, spasticity, and nystagmus in the first year of life. Additional neurologic features such as ataxia and abnormal movements may also occur. Brain imaging shows diffuse hypomyelination affecting all regions of the brain (summary by Wolf et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

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.

Halperin-Birk syndrome (HLBKS) is an autosomal recessive neurodevelopmental disorder characterized by structural brain defects, spastic quadriplegia with multiple contractures, profound developmental delay, seizures, dysmorphism, cataract, and optic nerve atrophy. Death occurs in early childhood (Halperin et al., 2019).

Autosomal recessive SPG9B is a neurologic disorder characterized by early-onset complex spastic paraplegia. Affected individuals had delayed psychomotor development, intellectual disability, and severe motor impairment. More variable features include dysmorphic facial features, tremor, and urinary incontinence (summary by Coutelier et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).