Myasthenia includes three groups of autoimmune disorders (myasthenia gravis, congenital myasthenic syndromes, Lambert-Eaton myasthenic syndromes) that affect the transmission of signals at the neuromuscular junctions that results in weakness and muscle fatiguability.
- With myasthenia gravis, ptosis, diplopia, dysphagia, and respiratory failure are often the initial symptoms. As the disease progresses, neck and limb-girdle muscle weakness becomes apparent. Diagnosis is confirmed with improvement in strength after intravenous injection of edrophonium. Thymectomy is treatment for young patients. Acetylcholinesterase inhibitors are used to treat symptoms, and plasmapharesis and IVIG are sometimes used to treat crises or to optimize preoperatively. MG is usually caused by antibodies against the nicotinic acetylcholine receptor.
- The congenital syndrome is extremely rare, and caused by inherited mutations in the synaptic vesicles, acetylcholinesterase, or nicotinic acetylcholine receptors resulting in either an increase or decrease in the response to acetylcholine. Immunosuppression and plasmapheresis are not effective treatment strategies.
- Lambert-Eaton myasthenic syndrome is an acquired disorder resulting from autoantibodies targeting the presynaptic voltage-gated calcium channels causing decreased Ach release. It causes proximal muscle weakness in the lower and upper extremities, fatigability, and autonomic dysfunction (but not bulbar or respiratory weakness).