summary Myasthenia Gravis is a common autoimmune disorder caused by autoantibodies directed against the acetlycholine receptor of the neuromuscular junction. Patients present with fluctuating proximal muscle weakness, ocular symptoms, and dysphagia. Diagnosis is made with serologic testing for anti-AChR autoantibodies. Treatment is medical management with acetylcholinesterase inhibitors. Epidemiology Incidence has a bimodal distribution more common in younger women (< 40 years of age) and older men (> 50 years of age) Risk factors HLA-B8 medications penicillamine aminoglycosides Etiology Pathogenesis autoantibodies directed against a protein of the neuromuscular junction autoantibodies can be directed against nicotinic acetlycholine receptor (AChR) more common muscle-specific receptor tyrosine kinase (MuSK) categorized as a type II hypersensitivity reaction T-cells play a role as well thought to stimulate B-cell antibody production Associated conditions thymoma thymic hyperplasia Presentation Symptoms general feature fluctuating muscle weakness most common weakness is worse with continued use e.g., worse at the end of the day true muscle fatigue secondary to decreasing contractile muscle force ocular symptoms most common presenting symptoms ptosis diplopia bulbar symptoms dysphagia dysarthria fatigable chewing proximal muscle weakness Physical exam ice-pack test place ice on the patient's ptosis → ptosis improves low temperatures change the kinetics of acetylcholinesterase, decreasing its activity this increases the amount of acetylcholine in the synaptic cleft edrophonium chloride (Tensilon test) only used in patients with ptosis or ophthalmoparesis this allows for improvement in muscle strength to be observed edrophonium is an acetylcholinesterase inhibitor that has a short duration of action this increases the amount of acetylcholine in the synaptic cleft Imaging Computerized tomography (CT) scan indication to rule out a thymoma view chest Studies Labs serologic testing for autoantibodies anti-AChR this is the initial laboratory test to confirm the diagnosis anti-MuSK Electromyogram (EMG) decremental decrease in the compound muscle action potential (CMAP) Differential Lambert-Eaton myasthenic syndrome (LEMS) differentiating factor muscle weakness that improves with use autonomic manifestations Botulism differentiating factor poor or impaired pupillary response to light Thyroid ophthalmopathy Treatment Medical corticosteroids indication a chronic immunotherapy agent drugs oral prednisone acetylcholinesterase inhibitors indication considered first-line for symptomatic management drugs pyridostigmine adverse effects abdominal cramping and diarrhea bradycardia sweating bronchial secretion Procedural intravenous immunoglobulins (IVIg) or plasmapharesis indication rapid immunotherapy for myasthenic crisis Surgical thymectomy indication in patients with a thymoma, irrespective if the patient has myasthenia gravis or not Complications Myasthenic crisis respiratory weakness secondary to myasthenia gravis Prognosis Most patients with ocular involvement progress to generalized myasthenia gravis
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