X-linked myotubular myopathy (X-MTM), also known as myotubular myopathy (MTM), is characterized by muscle weakness that ranges from severe to mild. Approximately 80% of affected males present with severe (classic) X-MTM characterized by polyhydramnios, decreased fetal movement, and neonatal weakness, hypotonia, and respiratory failure. Motor milestones are significantly delayed and most individuals fail to achieve independent ambulation. Weakness is profound and often involves facial and extraocular muscles. Respiratory failure is nearly uniform, with most individuals requiring 24-hour ventilatory assistance. It is estimated that at least 25% of boys with severe X-MTM die in the first year of life, and those who survive rarely live into adulthood. Males with mild or moderate X-MTM (~20%) achieve motor milestones more quickly than males with the severe form; many ambulate independently, and may live into adulthood. Most require gastrostomy tubes and/or ventilator support. In all subtypes of X-MTM, the muscle disease is not obviously progressive. Female carriers of X-MTM are generally asymptomatic, although manifesting heterozygotes are increasingly being identified. In affected females, symptoms range from severe, generalized weakness presenting in childhood, with infantile onset similar to affected male patients, to mild (often asymmetric) weakness manifesting in adulthood. Affected adult females may experience progressive respiratory decline and ultimately require ventilatory support.

Brown-Vialetto-Van Laere syndrome is a rare autosomal recessive neurologic disorder characterized by sensorineural hearing loss and a variety of cranial nerve palsies, usually involving the motor components of the seventh and ninth to twelfth (more rarely the third, fifth, and sixth) cranial nerves. Spinal motor nerves and, less commonly, upper motor neurons are sometimes affected, giving a picture resembling amyotrophic lateral sclerosis (ALS; 105400). The onset of the disease is usually in the second decade, but earlier and later onset have been reported. Hearing loss tends to precede the onset of neurologic signs, mostly progressive muscle weakness causing respiratory compromise. However, patients with very early onset may present with bulbar palsy and may not develop hearing loss until later. The symptoms, severity, and disease duration are variable (summary by Green et al., 2010). Genetic Heterogeneity of Brown-Vialetto-Van Laere Syndrome See also BVVLS2 (614707), caused by mutation in the SLC52A2 gene (607882) on chromosome 8q.

Treacher Collins syndrome (TCS) is characterized by bilateral and symmetric downslanting palpebral fissures, malar hypoplasia, micrognathia, and external ear abnormalities. Hypoplasia of the zygomatic bones and mandible can cause significant feeding and respiratory difficulties. About 40%-50% of individuals have conductive hearing loss attributed most commonly to malformation of the ossicles and hypoplasia of the middle ear cavities. Inner ear structures tend to be normal. Other, less common abnormalities include cleft palate and unilateral or bilateral choanal stenosis or atresia. Typically intellect is normal.

Neurodevelopmental disorder with progressive spasticity and brain white matter abnormalities (NEDSWMA) is an autosomal recessive disorder characterized by impaired psychomotor development apparent in infancy. Affected individuals show poor overall growth, progressive microcephaly, and axial hypotonia, with later onset of spasticity. The disorder is progressive. Some patients show normal early development, but later have regression of motor, cognitive, and language skills. More variable features include seizures, joint contractures, ocular disturbances, episodic respiratory failure, and nonspecific dysmorphic facial features. The intellectual impairment is variable, ranging from poor visual contact with inability to walk or speak to milder intellectual disability with the ability to say some words. Brain imaging shows variable white matter abnormalities, including thin corpus callosum and poor myelination (summary by Husain et al., 2020).

Pontocerebellar hypoplasia type 1E (PCH1E) is an autosomal recessive neurologic disorder characterized by severe hypotonia and respiratory insufficiency apparent soon after birth. Virtually all patients die in the first days or weeks of life. Postmortem examination and brain imaging show pontocerebellar atrophy and loss of anterior motor neurons in the spinal cord. Additional more variable features may include optic atrophy, peripheral neuropathy, dysmorphic features, congenital contracture or foot deformities, and seizures (summary by Braunisch et al., 2018). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Megacystis-microcolon-intestinal hypoperistalsis syndrome-2 (MMIHS2) is characterized by prenatal bladder enlargement, neonatal functional gastrointestinal obstruction, and chronic dependence on total parenteral nutrition and urinary catheterization. The majority of cases have a fatal outcome due to malnutrition and sepsis, followed by multiorgan failure (summary by Wang et al., 2019). For a discussion of genetic heterogeneity of MMIHS, see 249210.

Autosomal recessive congenital myopathy-2B (CMYP2B) is a disorder of the skeletal muscle characterized by severe hypotonia with lack of spontaneous movements and respiratory insufficiency, usually leading to death in infancy or early childhood (Agrawal et al., 2004). However, longer survival has also been reported, likely due to the type of mutation and extent of its impact (O'Grady et al., 2015). Mutations in the ACTA1 gene can cause a range of skeletal muscle diseases. About 90% of patients with ACTA1 mutations carry heterozygous mutations, usually de novo (CMYP2A; 161800), whereas 10% of patients carry biallelic ACTA1 mutations (CMYP2B) (Nowak et al., 2007). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).