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.

McLeod neuroacanthocytosis syndrome (designated as MLS throughout this review) is a multisystem disorder with central nervous system (CNS), neuromuscular, cardiovascular, and hematologic manifestations in males: CNS manifestations are a neurodegenerative basal ganglia disease including movement disorders, cognitive alterations, and psychiatric symptoms. Neuromuscular manifestations include a (mostly subclinical) sensorimotor axonopathy and muscle weakness or atrophy of different degrees. Cardiac manifestations include dilated cardiomyopathy, atrial fibrillation, and tachyarrhythmia. Hematologically, MLS is defined as a specific blood group phenotype (named after the first proband, Hugh McLeod) that results from absent expression of the Kx erythrocyte antigen and weakened expression of Kell blood group antigens. The hematologic manifestations are red blood cell acanthocytosis and compensated hemolysis. Alloantibodies in the Kell and Kx blood group system can cause strong reactions to transfusions of incompatible blood and severe anemia in affected male newborns of Kell-negative mothers. Females heterozygous for XK pathogenic variants have mosaicism for the Kell and Kx blood group antigens. Although they usually lack CNS and neuromuscular manifestations, some heterozygous females may develop clinical manifestations including chorea or late-onset cognitive decline.

A rare pure or complex hereditary spastic paraplegia with characteristics of variable onset of slowly progressive lower limb spasticity, hyperreflexia and extensor plantar responses, that may be associated with sensorimotor polyneuropathy, decreased vibration sense, lower limb distal muscle wasting, dysarthria and mild to moderate intellectual disability.

Spinocerebellar ataxia type 14 (SCA14) is characterized by slowly progressive cerebellar ataxia, dysarthria, and nystagmus. Axial myoclonus, cognitive impairment, tremor, and sensory loss may also be observed. Parkinsonian features including rigidity and tremor have been described in some families. Findings seen in other ataxia disorders (e.g., dysphagia, dysphonia) may also occur in SCA14. The average age of onset is in the 30s, with a range from childhood to the seventh decade. Life span is not shortened.

Distal hereditary motor neuronopathy (dHMN or HMN) is a heterogeneous group of neuromuscular disorders caused by anterior horn cell degeneration and characterized by progressive distal motor weakness and muscular atrophy of the peripheral nervous system without sensory impairment. Distal HMN is also referred to as spinal Charcot-Marie-Tooth disease (spinal CMT). Distal HMN is often referred to as a 'neuronopathy' instead of a 'neuropathy' based on the hypothesis that the primary pathologic process resides in the neuron cell body and not in the axons (Irobi et al., 2006). Genetic Heterogeneity of Autosomal Dominant Distal Hereditary Motor Neuronopathy Harding (1993) proposed a classification of distal HMN into 7 phenotypic subtypes according to age at onset, mode of inheritance, and presence of additional features. Those that show autosomal dominant inheritance include distal HMN type I, and II (HMN2A, 158590 and HMN2B, 608634), characterized by juvenile and adult onset, respectively; HMN type V (HMN5A, 600794, HMN5B, 614751, and HMN5C, 619112), characterized by upper limb involvement; HMN VII (HMN7A, 158580 and HMN7B, 607641), with vocal cord paralysis; HMN8 (600175); HMN9 (617721); and HMN10 (620080). HMN2A is caused by mutation in the HSPB8 gene (608014), HMN2B by mutation in the HSPB1 gene (602195), HMN2C (613376) by mutation in the HSPB3 gene (604624), and HMN2D (615575) by mutation in the FBXO38 gene (608533). HMN5A is caused by mutation in the GARS gene (600287), HMN5B is caused by mutation in the REEP1 gene (609139), and HMN5C is caused by mutation in the BSCL2 gene (606158). HMN7A is caused by mutation in the SLC5A7 gene (608761); HMN7B is caused by mutation in the DCTN1 gene (601143); HMN8 is caused by mutation in the TRPV4 gene (605427); HMN9 is caused by mutation in the WARS gene (191050); and HMN10 (620080) is caused by mutation in the EMILIN1 gene (130660). See also autosomal dominant ALS4 (602433) and congenital autosomal dominant distal SMA (600175). Genetic Heterogeneity of Autosomal Recessive Distal Hereditary Motor Neuronopathy (Distal Spinal Muscular Atrophy) Harding (1993) classified autosomal recessive distal hereditary motor neuronopathy as dHMN IV (HMN4) and dHMN III (HMN3) (see DSMA3, 607088). HMN has also been referred to as distal spinal muscular atrophy (DSMA). 'Distal' SMA is distinguished from 'proximal' autosomal recessive spinal muscular atrophy (see SMA1, 253300) by the primary muscles involved. DSMA here refers to the autosomal recessive forms of HMN. See DSMA1 (SMARD1; 604320), caused by mutation in the IGHMBP2 gene (600502); DSMA2 (605726), caused by mutation in the SIGMAR1 gene (601978) on chromosome 9p13; DSMA3 (607088), encompassing HMN types III and IV, which maps to chromosome 11q13; DSMA4 (611067), caused by mutation in the PLEKHG5 gene (611101); DSMA5 (614881), caused by mutation in the DNAJB2 gene (604139); and DSMA6 (620011), caused by mutation in the REEP1 gene (609139). See also X-linked SMAX3 (300489), caused by mutation in the ATP7A gene (300011) on chromosome Xq21, and CMS7A (616040), caused by mutation in the SYT2 gene (600104) on chromosome 1q32.

Spastic paraplegia-79B (SPG79B) is an autosomal recessive progressive neurologic disorder characterized by onset of spastic paraplegia and optic atrophy in the first decade of life. Additional features are variable, but may include peripheral neuropathy, cerebellar ataxia, and cognitive impairment (summary by Rydning et al., 2017). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

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

Spinocerebellar ataxia-42 (SCA42) is an autosomal dominant neurologic disorder characterized predominantly by gait instability and additional cerebellar signs such as dysarthria, nystagmus, and saccadic pursuits. The age at onset and severity of the disorder is highly variable. The disorder is slowly progressive (Coutelier et al., 2015). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).