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Congenital myasthenic syndrome 2A(CMS2A)

MedGen UID:
908185
Concept ID:
C4225374
Disease or Syndrome
Synonym: Myasthenic syndrome, congenital, 2a, slow-channel
 
Gene (location): CHRNB1 (17p13.1)
 
Monarch Initiative: MONDO:0014581
OMIM®: 616313

Definition

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). [from OMIM]

Additional description

From MedlinePlus Genetics
Some individuals have episodes of breathing problems that may be triggered by fevers or infection. Severely affected individuals may also experience short pauses in breathing (apnea) that can lead to a bluish appearance of the skin or lips (cyanosis).

Congenital myasthenic syndrome is a group of conditions characterized by muscle weakness (myasthenia) that worsens with physical exertion. The muscle weakness typically begins in early childhood but can also appear in adolescence or adulthood. Facial muscles, including muscles that control the eyelids, muscles that move the eyes, and muscles used for chewing and swallowing, are most commonly affected. However, any of the muscles used for movement (skeletal muscles) can be affected in this condition. Due to muscle weakness, affected infants may have feeding difficulties. Development of motor skills such as crawling or walking may be delayed. The severity of the myasthenia varies greatly, with some people experiencing minor weakness and others having such severe weakness that they are unable to walk.  https://medlineplus.gov/genetics/condition/congenital-myasthenic-syndrome

Clinical features

From HPO
Knee flexion contracture
MedGen UID:
98042
Concept ID:
C0409355
Finding
A type of knee joint contracture in which the knee is in a fixed bent (flexed) configuration such that it cannot be straightened actively or passively.
Limb muscle weakness
MedGen UID:
107956
Concept ID:
C0587246
Finding
Reduced strength and weakness of the muscles of the arms and legs.
Poor suck
MedGen UID:
324693
Concept ID:
C1837142
Finding
An inadequate sucking reflex, resulting in the difficult of newborns to be breast-fed.
Abnormally high-pitched voice
MedGen UID:
66836
Concept ID:
C0241703
Finding
A persistent (minutes to hours) abnormal increase in the pitch (frequency) of the voice for the context or social situation or significantly different from baseline of the individual.
Delayed ability to walk
MedGen UID:
66034
Concept ID:
C0241726
Finding
A failure to achieve the ability to walk at an appropriate developmental stage. Most children learn to walk in a series of stages, and learn to walk short distances independently between 12 and 15 months.
Fatigable weakness
MedGen UID:
451076
Concept ID:
C0947912
Disease or Syndrome
A type of weakness that occurs after a muscle group is used and lessens if the muscle group has some rest. That is, there is diminution of strength with repetitive muscle actions.
Easy fatigability
MedGen UID:
373253
Concept ID:
C1837098
Finding
Increased susceptibility to fatigue.
Delayed ability to sit
MedGen UID:
1368737
Concept ID:
C4476710
Finding
A failure to achieve the ability to sit at an appropriate developmental stage. Most children sit with support at 6 months of age and sit steadily without support at 9 months of age.
Hypotonia
MedGen UID:
10133
Concept ID:
C0026827
Finding
Hypotonia is an abnormally low muscle tone (the amount of tension or resistance to movement in a muscle). Even when relaxed, muscles have a continuous and passive partial contraction which provides some resistance to passive stretching. Hypotonia thus manifests as diminished resistance to passive stretching. Hypotonia is not the same as muscle weakness, although the two conditions can co-exist.
Myopathy
MedGen UID:
10135
Concept ID:
C0026848
Disease or Syndrome
A disorder of muscle unrelated to impairment of innervation or neuromuscular junction.
Flexion contracture
MedGen UID:
83069
Concept ID:
C0333068
Anatomical Abnormality
A flexion contracture is a bent (flexed) joint that cannot be straightened actively or passively. It is thus a chronic loss of joint motion due to structural changes in muscle, tendons, ligaments, or skin that prevents normal movement of joints.
Facial palsy
MedGen UID:
87660
Concept ID:
C0376175
Disease or Syndrome
Facial nerve palsy is a dysfunction of cranial nerve VII (the facial nerve) that results in inability to control facial muscles on the affected side with weakness of the muscles of facial expression and eye closure. This can either be present in unilateral or bilateral form.
Muscular atrophy
MedGen UID:
892680
Concept ID:
C0541794
Pathologic Function
The presence of skeletal muscular atrophy (which is also known as amyotrophy).
Poor head control
MedGen UID:
322809
Concept ID:
C1836038
Finding
Difficulty to maintain correct position of the head while standing or sitting. Infant head lag is observed when the head seems to flop around or lags posteriorly behind the trunk. Several articles have maintained that head lag should be absent by age 3 to 4 months.
Muscle fiber splitting
MedGen UID:
322813
Concept ID:
C1836057
Finding
Fiber splitting or branching is a common finding in human and rat skeletal muscle pathology. Fiber splitting refers to longitudinal halving of the complete fiber, while branching originates from a regenerating end of a necrotic fiber as invaginations of the sarcolemma. In fiber branching, one end of the fiber remains intact as a single entity, while the other end has several branches.
Increased variability in muscle fiber diameter
MedGen UID:
336019
Concept ID:
C1843700
Finding
An abnormally high degree of muscle fiber size variation. This phenotypic feature can be observed upon muscle biopsy.
Type 1 muscle fiber predominance
MedGen UID:
344274
Concept ID:
C1854387
Finding
An abnormal predominance of type I muscle fibers (in general, this feature can only be observed on muscle biopsy).
Neonatal hypotonia
MedGen UID:
412209
Concept ID:
C2267233
Disease or Syndrome
Muscular hypotonia (abnormally low muscle tone) manifesting in the neonatal period.
EMG: decremental response of compound muscle action potential to repetitive nerve stimulation
MedGen UID:
892749
Concept ID:
C4021728
Finding
A compound muscle action potential (CMAP) is a type of electromyography (EMG). CMAP refers to a group of almost simultaneous action potentials from several muscle fibers in the same area evoked by stimulation of the supplying motor nerve and are recorded as one multipeaked summated action potential. This abnormality refers to a greater than normal decrease in the amplitude during the course of the investigation.
Anti-acetylcholine receptor antibody positivity
MedGen UID:
868186
Concept ID:
C4022578
Finding
The presence of autoantibodies (immunoglobulins) in the blood circulation that react against neuromuscular junction acetylcholine receptors.
High palate
MedGen UID:
66814
Concept ID:
C0240635
Congenital Abnormality
Height of the palate more than 2 SD above the mean (objective) or palatal height at the level of the first permanent molar more than twice the height of the teeth (subjective).
Long face
MedGen UID:
324419
Concept ID:
C1836047
Finding
Facial height (length) is more than 2 standard deviations above the mean (objective); or, an apparent increase in the height (length) of the face (subjective).
Narrow face
MedGen UID:
373334
Concept ID:
C1837463
Finding
Bizygomatic (upper face) and bigonial (lower face) width are both more than 2 standard deviations below the mean (objective); or, an apparent reduction in the width of the upper and lower face (subjective).
Ptosis
MedGen UID:
2287
Concept ID:
C0005745
Disease or Syndrome
The upper eyelid margin is positioned 3 mm or more lower than usual and covers the superior portion of the iris (objective); or, the upper lid margin obscures at least part of the pupil (subjective).
Ophthalmoplegia
MedGen UID:
45205
Concept ID:
C0029089
Sign or Symptom
Paralysis of one or more extraocular muscles that are responsible for eye movements.

Recent clinical studies

Etiology

Huang K, Luo YB, Bi FF, Yang H
Curr Neuropharmacol 2021;19(5):718-729. doi: 10.2174/1570159X18666200729092332. PMID: 32727330Free PMC Article
Chen Q, Müller JS, Pang PC, Laval SH, Haslam SM, Lochmüller H, Dell A
Biomolecules 2015 Oct 16;5(4):2758-81. doi: 10.3390/biom5042758. PMID: 26501342Free PMC Article

Diagnosis

Cocanougher BT, Liu SW, Francescatto L, Behura A, Anneling M, Jackson DG, Deak KL, Hornik CD, ElMallah MK, Pizoli CE, Smith EC, Tan KGQ, McDonald MT
HGG Adv 2024 Jul 18;5(3):100288. Epub 2024 Apr 1 doi: 10.1016/j.xhgg.2024.100288. PMID: 38566418Free PMC Article
Mishra S, Nair KV, Shukla A
Clin Dysmorphol 2023 Oct 1;32(4):162-167. Epub 2023 Jun 19 doi: 10.1097/MCD.0000000000000465. PMID: 37646703
Chen Q, Müller JS, Pang PC, Laval SH, Haslam SM, Lochmüller H, Dell A
Biomolecules 2015 Oct 16;5(4):2758-81. doi: 10.3390/biom5042758. PMID: 26501342Free PMC Article
Alberca R, Gil-Peralta A, Castilla JM, Rafel E, Bautista J
Eur Neurol 1983;22(1):22-8. doi: 10.1159/000115532. PMID: 6840138

Therapy

Ioos C, Barois A, Richard P, Eymard B, Hantaï D, Estournet-Mathiaud B
Neuropediatrics 2004 Aug;35(4):246-9. doi: 10.1055/s-2004-820993. PMID: 15328566

Prognosis

Cocanougher BT, Liu SW, Francescatto L, Behura A, Anneling M, Jackson DG, Deak KL, Hornik CD, ElMallah MK, Pizoli CE, Smith EC, Tan KGQ, McDonald MT
HGG Adv 2024 Jul 18;5(3):100288. Epub 2024 Apr 1 doi: 10.1016/j.xhgg.2024.100288. PMID: 38566418Free PMC Article
Shapira YA, Sadeh ME, Bergtraum MP, Tsujino A, Ohno K, Shen XM, Brengman J, Edwardson S, Matoth I, Engel AG
Neurology 2002 Feb 26;58(4):603-9. doi: 10.1212/wnl.58.4.603. PMID: 11865139

Clinical prediction guides

Cocanougher BT, Liu SW, Francescatto L, Behura A, Anneling M, Jackson DG, Deak KL, Hornik CD, ElMallah MK, Pizoli CE, Smith EC, Tan KGQ, McDonald MT
HGG Adv 2024 Jul 18;5(3):100288. Epub 2024 Apr 1 doi: 10.1016/j.xhgg.2024.100288. PMID: 38566418Free PMC Article
Chen Q, Müller JS, Pang PC, Laval SH, Haslam SM, Lochmüller H, Dell A
Biomolecules 2015 Oct 16;5(4):2758-81. doi: 10.3390/biom5042758. PMID: 26501342Free PMC Article
Shapira YA, Sadeh ME, Bergtraum MP, Tsujino A, Ohno K, Shen XM, Brengman J, Edwardson S, Matoth I, Engel AG
Neurology 2002 Feb 26;58(4):603-9. doi: 10.1212/wnl.58.4.603. PMID: 11865139

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