Myasthenia gravis (MG) is an autoimmune disease in which antibodies bind to acetylcholine receptors or to functionally related molecules in the postsynaptic membrane at the neuromuscular junction. The antibodies induce weakness of skeletal muscles, which is the sole disease manifestation. The weakness can be generalized or localized, is more proximal than distal, and nearly always includes eye muscles, with diplopia and ptosis. The pattern of involvement is usually symmetric, apart from the eye involvement, which is often markedly asymmetric and involves several eye muscles. The weakness typically increases with exercise and repetitive muscle use (fatigue) and varies over the course of a day and from day to day, often with nearly normal muscle strength in the morning (summary by Gilhus, 2016).

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). 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. CMS6 is an autosomal recessive CMS resulting from a presynaptic defect; patients have onset of symptoms in infancy or early childhood and tend to have sudden apneic episodes. Treatment with acetylcholinesterase inhibitors may be beneficial (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients with mutations in the CHRNE gene may have compensatory increased expression of the fetal subunit CHRNG (100730) and may respond to treatment with cholinergic agents, pyridostigmine, or amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). 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. CMS10 is an autosomal recessive CMS resulting from a postsynaptic defect affecting endplate maintenance of the NMJ. Patients present with limb-girdle weakness in the first decade. Treatment with ephedrine or salbutamol may be beneficial; cholinesterase inhibitors should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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).

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). 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, as well as by pathologic mechanism and electrophysiologic studies (i.e., acetylcholine receptor (AChR) deficiency, slow-channel or fast-channel kinetic defects at the AChR) (summary by Engel et al., 2003; Engel et al., 2015). Approximately 10% of CMS cases are presynaptic, 15% are synaptic, and 75% are postsynaptic, the majority of which are caused by AChR deficiency (Engel et al., 2003). Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic NMJ characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the AChR channel, specifically prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; acetylcholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). Genetic Heterogeneity of Congenital Myasthenic Syndromes Recessive mutations in subunits of the acetylcholine receptor are the most common cause of CMS (Harper, 2004). CMS1A and CMS1B (608930) are caused by mutation in the CHRNA1 gene (100690); CMS2A (616313) and CMS2C (616314) are caused by mutation in the CHRNB1 gene (100710) on 17p12; CMS3A (616321), CMS3B (616322), and CMS3C (616323) are caused by mutation in the CHRND gene (100720) on 2q33; and CMS4A (605809), CMS4B (616324), and CMS4C (608931) are caused by mutation in the CHRNE gene (100725) on 17p13. CMS5 (603034) is caused by mutation in the COLQ gene (603033) on 3p25; CMS6 (254210) is caused by mutation in the CHAT gene (118490) on 10q; CMS7 (616040) is caused by mutation in the SYT2 gene (600104) on 1q32; CMS8 (615120) is caused by mutation in the AGRN gene (103320) on 1p; CMS9 (616325) is caused by mutation in the MUSK gene (601296) on 9q31; CMS10 (254300) is caused by mutation in the DOK7 gene (610285) on 4p; CMS11 (616326) is caused by mutation in the RAPSN gene (601592) on 11p11; CMS12 (610542) is caused by mutation in the GFPT1 gene (138292) on 2p14; CMS13 (614750) is caused by mutation in the DPAGT1 gene (191350) on 11q23; CMS14 (616228) is caused by mutation in the ALG2 gene (607905) on 9q22; CMS15 (616227) is caused by mutation in the ALG14 gene (612866) on 1p21; CMS16 (614198) is caused by mutation in the SCN4A gene (603967) on 17q; CMS17 (616304) is caused by mutation in the LRP4 gene (604270) on 11p12; CMS18 (616330) is caused by mutation in the SNAP25 gene (600322) on 20p11; CMS19 (616720) is caused by mutation in the COL13A1 gene (120350) on 10q22; CMS20 (617143) is caused by mutation in the SLC5A7 gene (608761) on 2q12; CMS21 (617239) is caused by mutation in the SLC18A3 gene (600336) on 10q11; CMS22 (616224) is caused by mutation in the PREPL gene (609557) on 2p21; CMS23 (618197) is caused by mutation in the SLC25A1 gene (190315) on 22q11; CMS24 (618198) is caused by mutation in the MYO9A gene (604875) on 15q22; and CMS25 (618323) is caused by mutation in the VAMP1 gene (185880) on 12p13.

Congenital myasthenic syndrome is a disorder characterized by variable degrees of muscle fatigability caused by impaired transmission of electrical signals at the neuromuscular junction (NMJ) (summary by Arnold et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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).

Congenital myasthenic syndrome-13 (CMS13) 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 Belaya et al., 2012). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome-15 is one of a heterogeneous group of disorders that arise from impaired signal transmission at the neuromuscular synapse and are characterized by fatigable muscle weakness (summary by Cossins et al., 2013). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome-14 is an autosomal recessive neuromuscular disorder characterized by onset of limb-girdle muscle weakness in early childhood. The disorder is slowly progressive, and some patients may become wheelchair-bound. There is no respiratory or cardiac involvement. Treatment with anticholinesterase medication may be beneficial (summary by Cossins et al., 2013). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome-18 (CMS18) is an autosomal dominant presynaptic neuromuscular disorder characterized by early-onset muscle weakness and easy fatigability associated with delayed psychomotor development and ataxia (summary by Shen et al., 2014). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients may show a favorable response to amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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).

Slow-channel congenital myasthenic syndrome (SCCMS) 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 prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; cholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Slow-channel congenital myasthenic syndrome (SCCMS) 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 prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; acetylcholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome-20 is an autosomal recessive neuromuscular disorder characterized by severe hypotonia associated with episodic apnea soon after birth. Patients have muscle weakness resulting in delayed walking, ptosis, poor sucking and swallowing, and generalized limb fatigability and weakness. EMG studies usually show a decremental response to repetitive nerve stimulation, and some patients may show a good response to AChE inhibitors (summary by Bauche et al., 2016). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).