Generic diagnosis
Fatigable weakness involving ocular, bulbar and limb muscles since infancy or early childhood
Similarly affected relative
Decremental EMG response at 2- to 3-Hz stimulation
Negative tests for anti-AChR antibodies, MuSK, and P/Q type calcium channels
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Exceptions and caveats
In some CMS the onset is delayed45,37
There may be no similarly affected relatives
The symptoms can be episodic7,13,44
EMG abnormalities may not be present in all muscles, or are present only intermittently7,13
Weakness may not involve cranial muscles40
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Clinical clues pointing to a specific diagnosis*
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Endplate AChE deficiency 35
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Slow-channel CMS45
Repetitive Compound Muscle Fiber Action Potential (CMAPs)
Selectively severe involvement of cervical and wrist and finger extensor muscles in most cases
Dominant inheritance in most cases
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Endplate choline acetyltransferase deficiency7,46
Recurrent apneic episodes, spontaneous or with fever, vomiting, or excitement
No or variable myasthenic symptoms between acute episodes
Stimulation at 10 Hz for 5 min causes marked decrease of CMAP followed by slow recovery
EMG decrement at 2 Hz can be absent at rest but appears after stimulation at 10 Hz for 5 min, then disappears slowly.
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Rapsyn deficiency
Multiple congenital joint contractures in one-fourth of patients
Increased weakness and respiratory insufficiency precipitated by intercurrent infections
EMG decrement can be mild or absent.
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Dok-7 myasthenia 40,15
Proximal greater than distal limb and axial muscle weakness, mild facial weakness and ptosis, and normal ocular ductions in the majority
Bulbar muscles affected in some patients
May deteriorate on exposure to pyridostigmine
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