Congenital fiber-type disproportion is a condition that primarily affects skeletal muscles, which are muscles used for movement. People with this condition typically experience muscle weakness (myopathy), particularly in the muscles of the shoulders, upper arms, hips, and thighs. Weakness can also affect the muscles of the face and muscles that control eye movement (ophthalmoplegia), sometimes causing droopy eyelids (ptosis). Individuals with congenital fiber-type disproportion generally have a long face, a high arch in the roof of the mouth (high-arched palate), and crowded teeth.\n\nIndividuals with congenital fiber-type disproportion may have joint deformities (contractures) and an abnormally curved lower back (lordosis) or a spine that curves to the side (scoliosis). Approximately 30 percent of people with this disorder experience mild to severe breathing problems related to weakness of muscles needed for breathing. Some people who experience these breathing problems require use of a machine to help regulate their breathing at night (noninvasive mechanical ventilation), and occasionally during the day as well. About 30 percent of affected individuals have difficulty swallowing due to muscle weakness in the throat. Rarely, people with this condition have a weakened and enlarged heart muscle (dilated cardiomyopathy).\n\nThe severity of congenital fiber-type disproportion varies widely. It is estimated that up to 25 percent of affected individuals experience severe muscle weakness at birth and die in infancy or childhood. Others have only mild muscle weakness that becomes apparent in adulthood. Most often, the signs and symptoms of this condition appear by age 1. The first signs of this condition are usually decreased muscle tone (hypotonia) and muscle weakness. In most cases, muscle weakness does not worsen over time, and in some instances it may improve. Although motor skills such as standing and walking may be delayed, many affected children eventually learn to walk. These individuals often have less stamina than their peers, but they remain active. Rarely, people with this condition have a progressive decline in muscle strength over time. These individuals may lose the ability to walk and require wheelchair assistance.

Proud syndrome is an X-linked developmental disorder characterized by agenesis of the corpus callosum, severe mental retardation, seizures, and spasticity. Males are severely affected, whereas females may be unaffected or have a milder phenotype (Proud et al., 1992). Proud syndrome is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from lissencephaly (LISX2; 300215) to Proud syndrome to infantile spasms without brain malformations (DEE1; 308350) to syndromic (309510) and nonsyndromic (300419) mental retardation (Kato et al., 2004; Wallerstein et al., 2008).

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.

Alopecia, neurologic defects, and endocrinopathy syndrome (ANES) is an autosomal recessive disorder characterized by alopecia with skin involvement including multiple facial nevi and flexural hyperpigmentation; moderately to severely impaired intellectual development; progressive motor decline; and endocrine deficiency (summary by Spiegel et al., 2010).

PCH2D is an autosomal recessive disorder characterized by progressive microcephaly, postnatal onset of progressive atrophy of the cerebrum and cerebellum, profound mental retardation, spasticity, and variable seizures (summary by Ben-Zeev et al., 2003). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

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).

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.

SLC39A14 deficiency is characterized by evidence between ages six months and three years of delay or loss of motor developmental milestones (e.g., delayed walking, gait disturbance). Early in the disease course, children show axial hypotonia followed by dystonia, spasticity, dysarthria, bulbar dysfunction, and signs of parkinsonism including bradykinesia, hypomimia, and tremor. By the end of the first decade they develop severe, generalized, pharmaco-resistant dystonia, limb contractures, and scoliosis, and lose independent ambulation. Cognitive impairment appears to be less prominent than motor disability. Some affected children have succumbed in their first decade due to secondary complications such as respiratory infections.

Congenital hypomyelinating neuropathy-3 is an autosomal recessive neurologic disorder characterized by onset of neurogenic muscle impairment in utero. Affected individuals present at birth with severe hypotonia, often causing respiratory insufficiency or failure and inability to swallow or feed properly. They have profoundly impaired psychomotor development and may die in infancy or early childhood. Those that survive are unable to sit or walk. Sural nerve biopsy shows hypomyelination of the nerve fibers, and brain imaging often shows impaired myelination and cerebral and cerebellar atrophy. Nerve conduction velocities are severely decreased (about 10 m/s) or absent due to improper myelination (summary by Vallat et al., 2016 and Low et al., 2018). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

A restrictive dermopathy that has material basis in homozygous or compound heterozygous mutation in the ZMPSTE24 gene on chromosome 1p34.

Developmental and epileptic encephalopathy-101 (DEE101) is a severe autosomal recessive disorder characterized by early infantile epileptic encephalopathy and severe global developmental delay (summary by Blakes et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Congenital disorder of glycosylation type IIz (CDG2Z) is an autosomal recessive disorder characterized by poor overall growth, severe global developmental delay, seizures, contractures, hypotonia, spasticity, and brain imaging abnormalities. Serum transferrin shows a type 2 pattern of glycosylation abnormalities with a combined N- and O-glycosylation defect (Wilson et al., 2022). For a general discussion of CDGs, see CDG1A (212065).

Neurodegeneration with developmental delay, early respiratory failure, myoclonic seizures, and brain abnormalities (NDDRSB) is a severe autosomal recessive disorder characterized by onset of these features in infancy. Affected individuals present with respiratory failure requiring intubation soon after birth; some die due to cardiorespiratory insufficiency. Those that survive show severe global developmental delay, refractory myoclonic seizures, hyperkinetic movements with exaggerated startle response, and microcephaly with dysmorphic features. Additional findings may include sensorineural hearing loss and ocular defects. Brain imaging shows variable abnormalities consistent with progressive neurodegeneration (Cali et al., 2022).

Severe fetal congenital myopathy-22B (CMYO22B) is an autosomal recessive muscle disorder characterized by in utero onset of severe muscle weakness manifest as fetal akinesia. The pregnancies are often complicated by polyhydramnios, and affected individuals develop fetal hydrops with pulmonary hypoplasia, severe joint contractures, and generalized muscle hypoplasia. Those who are born have respiratory failure resulting in death. Dysmorphic facial features may be present. The features in these patients overlap with fetal akinesia deformation sequence (FADS; see 208150) and lethal congenital contractures syndrome (LCCS; see 253310) (Zaharieva et al., 2016). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).