MedGen

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. [from GeneReviews]

Myofibrillar myopathy (MFM) is a noncommittal term that refers to a group of morphologically homogeneous, but genetically heterogeneous chronic neuromuscular disorders. The morphologic changes in skeletal muscle in MFM result from disintegration of the sarcomeric Z disc and the myofibrils, followed by abnormal ectopic accumulation of multiple proteins involved in the structure of the Z disc, including desmin, alpha-B-crystallin (CRYAB; 123590), dystrophin (300377), and myotilin (TTID; 604103). Genetic Heterogeneity of Myofibrillar Myopathy Other forms of MFM include MFM2 (608810), caused by mutation in the CRYAB gene (123590); MFM3 (609200), caused by mutation in the MYOT gene (604103); MFM4 (609452), caused by mutation in the ZASP gene (LDB3; 605906); MFM5 (609524), caused by mutation in the FLNC gene (102565); MFM6 (612954), caused by mutation in the BAG3 gene (603883); MFM7 (617114), caused by mutation in the KY gene (605739); MFM8 (617258), caused by mutation in the PYROXD1 gene (617220); MFM9 (603689), caused by mutation in the TTN gene (188840); MFM10 (619040), caused by mutation in the SVIL UNC45B gene (611220); MFM11 (619178), caused by mutation in the UNC45B gene (611220); and MFM12 (619424), caused by mutation in the MYL2 gene (160781). 'Desmin-related myopathy' is another term referring to MFM in which there are intrasarcoplasmic aggregates of desmin, usually in addition to other sarcomeric proteins. Rigid spine syndrome (602771), caused by mutation in the SEPN1 gene (606210), is another desmin-related myopathy. Goebel (1995) provided a review of desmin-related myopathy. [from OMIM]

Fast-channel congenital myasthenic syndrome (FCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor (AChR) channel, specifically from abnormally brief opening and activity of the channel, with a rapid decay in endplate current and a failure to reach the threshold for depolarization. Treatment with pyridostigmine or amifampridine may be helpful; quinine, quinidine, and fluoxetine should be avoided (summary by Sine et al., 2003 and Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). [from OMIM]

Laing distal myopathy is characterized by early-onset weakness (usually before age 5 years) that initially involves the dorsiflexors of the ankles and great toes and then the finger extensors, especially those of the third and fourth fingers. Weakness of the neck flexors is seen in most affected individuals and mild facial weakness is often present. After distal weakness has been present for more than ten years, mild proximal weakness may be observed. Life expectancy is normal. [from GeneReviews]

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction. Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. Endplate acetylcholinesterase deficiency is an autosomal recessive congenital myasthenic syndrome characterized by a defect within the synapse at the neuromuscular junction (NMJ). Mutations in COLQ result in a deficiency of acetylcholinesterase (AChE), which causes prolonged synaptic currents and action potentials due to extended residence of acetylcholine in the synaptic space. Treatment with ephedrine may be beneficial; AChE inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). [from OMIM]

Nemaline myopathy is divided into six types. In order of decreasing severity, the types are: severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset, and adult-onset. The types are distinguished by the age when symptoms first appear and the severity of symptoms; however, there is overlap among the various types. The severe congenital type is the most life-threatening. Most individuals with this type do not survive past early childhood due to respiratory failure. The Amish type solely affects the Old Order Amish population of Pennsylvania and is typically fatal in early childhood. The most common type of nemaline myopathy is the typical congenital type, which is characterized by muscle weakness and feeding problems beginning in infancy. Most of these individuals do not have severe breathing problems and can walk unassisted. People with the childhood-onset type usually develop muscle weakness in adolescence. The adult-onset type is the mildest of all the various types. People with this type usually develop muscle weakness between ages 20 and 50.

Nemaline myopathy is a disorder that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with nemaline myopathy have muscle weakness (myopathy) throughout the body, but it is typically most severe in the muscles of the face; neck; trunk; and other muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. This weakness can worsen over time. Affected individuals may have feeding and swallowing difficulties, foot deformities, abnormal curvature of the spine (scoliosis), and joint deformities (contractures). Most people with nemaline myopathy are able to walk, although some affected children may begin walking later than usual. As the condition progresses, some people may require wheelchair assistance. In severe cases, the muscles used for breathing are affected and life-threatening breathing difficulties can occur. [from MedlinePlus Genetics]

Alpha-B crystallin-related myofibrillar myopathy is an autosomal dominant muscular disorder characterized by adult onset of progressive muscle weakness affecting both the proximal and distal muscles and associated with respiratory insufficiency, cardiomyopathy, and cataracts. There is phenotypic variability both within and between families (Fardeau et al., 1978; Selcen and Engel, 2003). A homozygous founder mutation in the CRYAB gene has been identified in Canadian aboriginal infants of Cree origin who have a severe fatal infantile hypertonic form of myofibrillar myopathy; see 613869. For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419). [from OMIM]

Nemaline myopathy-7 is an autosomal recessive congenital myopathy characterized by very early onset of hypotonia and delayed motor development. Affected individuals have difficulty walking and running due to proximal muscle weakness. The disorder is slowly progressive, and patients may lose independent ambulation. Muscle biopsy shows nemaline rods and may later show minicores, abnormal protein aggregates, and dystrophic changes (summary by Ockeloen et al., 2012). For a discussion of genetic heterogeneity of nemaline myopathy, see 161800. [from OMIM]

Emery-Dreifuss muscular dystrophy is a genetically heterogeneous muscular disease that presents with muscular dystrophy, joint contractures, and cardiomyopathy with conduction defects (summary by Liang et al., 2011). For a discussion of genetic heterogeneity of EDMD, see 310300. [from OMIM]

Congenital myopathy-4B (CMYP4B) is an autosomal recessive disorder of the skeletal muscle characterized by the onset of muscle weakness in infancy or early childhood. The severity and pattern of muscle weakness varies, but most affected individuals show congenital contractures, delayed motor development, hypotonia, generalized muscle weakness, and weakness of the proximal limb muscles and neck muscles, resulting in difficulty walking or inability to walk. Affected individuals have respiratory insufficiency due to muscle weakness, which may be life-threatening. Other common features include myopathic facies, chest deformities, distal joint laxity, and scoliosis. Variable histologic findings on skeletal muscle biopsy are observed, including nemaline rods, type 1 fiber predomination, and centralized nuclei (Tan et al., 1999; Lehtokari et al., 2008). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000). [from OMIM]

Fast-channel congenital myasthenic syndrome (FCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the AChR channel, specifically from abnormally brief opening and activity of the channel, with a rapid decay in endplate current and a failure to reach the threshold for depolarization. Treatment with pyridostigmine or amifampridine may be helpful; quinine, quinidine, and fluoxetine should be avoided (summary by Sine et al., 2003 and Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). [from OMIM]

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. [from OMIM]

Brown-Vialetto-Van Laere syndrome-2 (BVVLS2) is an autosomal recessive progressive neurologic disorder characterized by early childhood onset of sensorineural deafness, bulbar dysfunction, and severe diffuse muscle weakness and wasting of the upper and lower limbs and axial muscles, resulting in respiratory insufficiency. Some patients may lose independent ambulation. Because it results from a defect in riboflavin metabolism, some patients may benefit from high-dose riboflavin supplementation (summary by Johnson et al., 2012; Foley et al., 2014). For discussion of genetic heterogeneity of Brown-Vialetto-Van Laere syndrome, see BVVLS1 (211530). [from OMIM]

LMNA-related congenital muscular dystrophy (L-CMD) is a condition that primarily affects muscles used for movement (skeletal muscles). It is part of a group of genetic conditions called congenital muscular dystrophies, which cause weak muscle tone (hypotonia) and muscle wasting (atrophy) beginning very early in life.

In people with L-CMD, muscle weakness becomes apparent in infancy or early childhood and can worsen quickly. The most severely affected infants develop few motor skills, and they are never able to hold up their heads, roll over, or sit. Less severely affected children may learn to sit, stand, and walk before muscle weakness becomes apparent. First the neck muscles weaken, causing the head to fall forward (dropped-head syndrome). As other skeletal muscles become weaker, these children may ultimately lose the ability to sit, stand, and walk unassisted.

Other features of L-CMD often include spinal rigidity and abnormal curvature of the spine (scoliosis and lordosis); joint deformities (contractures) that restrict movement, particularly in the hips and legs; and an inward-turning foot. People with L-CMD also have an increased risk of heart rhythm abnormalities (arrhythmias).

Over time, muscle weakness causes most infants and children with L-CMD to have trouble eating and breathing. The breathing problems result from restrictive respiratory insufficiency, which occurs when muscles in the chest are weakened and the ribcage becomes increasingly rigid. This problem can be life-threatening, and many affected children require support with a machine to help them breathe (mechanical ventilation). [from MedlinePlus Genetics]

Congenital myasthenic syndrome-12 is an autosomal recessive neuromuscular disorder characterized by onset of proximal muscle weakness in the first decade. EMG classically shows a decremental response to repeated nerve stimulation. Affected individuals show a favorable response to acetylcholinesterase (AChE) inhibitors (summary by Senderek et al., 2011). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). [from OMIM]

Oculopharyngeal muscular dystrophy (OPMD) is characterized by ptosis and dysphagia due to selective involvement of the muscles of the eyelids and pharynx, respectively. For the vast majority of individuals with typical OPMD, the mean age of onset of ptosis is usually 48 years and of dysphagia 50 years; in 5%-10% of individuals with severe OPMD, onset of ptosis and dysphagia occur before age 45 years and is associated with lower limb girdle weakness starting around age 60 years. Swallowing difficulties, which determine prognosis, increase the risk for potentially life-threatening aspiration pneumonia and poor nutrition. Other manifestations as the disease progresses can include limitation of upward gaze, tongue atrophy and weakness, chewing difficulties, wet voice, facial muscle weakness, axial muscle weakness, and proximal limb girdle weakness predominantly in lower limbs. Some individuals with severe involvement will eventually need a wheelchair. Neuropsychological tests have shown altered scores in executive functions in some. [from GeneReviews]

Any autosomal dominant Emery-Dreifuss muscular dystrophy in which the cause of the disease is a mutation in the SYNE1 gene. [from MONDO]

Fast-channel congenital myasthenic syndrome (FCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor channel, specifically from abnormally brief opening and activity of the channel, with a rapid decay in endplate current and a failure to reach the threshold for depolarization. Treatment with pyridostigmine or amifampridine may be helpful; quinine, quinidine, and fluoxetine should be avoided (summary by Sine et al., 2003 and Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). [from OMIM]

Neutral lipid storage disease with myopathy (NLSDM) is an autosomal recessive muscle disorder characterized by adult onset of slowly progressive proximal muscle weakness affecting the upper and lower limbs and associated with increased serum creatine kinase; distal muscle weakness may also occur. About half of patients develop cardiomyopathy later in the disease course. Other variable features include diabetes mellitus, hepatic steatosis, hypertriglyceridemia, and possibly sensorineural hearing loss. Leukocytes and muscle cells show cytoplasmic accumulation of triglycerides (summary by Reilich et al., 2011). Neutral lipid storage disease with myopathy belongs to a group of disorders termed neutral lipid storage disorders (NLSDs). These disorders are characterized by the presence of triglyceride-containing cytoplasmic droplets in leukocytes and in other tissues, including bone marrow, skin, and muscle. Chanarin-Dorfman syndrome (CDS; 275630) is defined as NLSD with ichthyosis (NLSDI). Patients with NLSDM present with myopathy but without ichthyosis (summary by Fischer et al., 2007). [from OMIM]

ADCY5 dyskinesia is a hyperkinetic movement disorder (more prominent in the face and arms than the legs) characterized by infantile to late-adolescent onset of chorea, athetosis, dystonia, myoclonus, or a combination of these. To date, affected individuals have had overlapping (but not identical) manifestations with wide-ranging severity. The facial movements are typically periorbital and perioral. The dyskinesia is prone to episodic or paroxysmal exacerbation lasting minutes to hours, and may occur during sleep. Precipitating factors in some persons have included emotional stress, intercurrent illness, sneezing, or caffeine; in others, no precipitating factors have been identified. In some children, severe infantile axial hypotonia results in gross motor delays accompanied by chorea, sometimes with language delays. The overall tendency is for the abnormal movements to stabilize in early middle age, at which point they may improve in some individuals; less commonly, the abnormal movements are slowly progressive, increasing in severity and frequency. [from GeneReviews]