Introduction

• Cause: two forms: congenital (rare) and acquired (common). Congenital - structural or functional defect of acetylcholine receptors. Acquired - immune-mediated or paraneoplastic effect.
• Signs: episodic, localized or generalized muscle weakness, worsen by activity and might be relieved by rest, regurgitation (megaesophagus); dysphagia (pharyngeal weakness); voice change (laryngeal weakness); facial muscle group weakness; may be lameness, drooling, tremors. Can present so weak that rest does not improve.
• Diagnosis: presumptive diagnosis by clinical signs and edrophonium chloride (Tensilon ) challenge test; acetylcholine receptor antibody test is confirmatory.
• Treatment: treat underlying cause if present, anticholinesterases +/- immunomodulatory therapy.
• Prognosis: overall guarded (overall 1-year mortality rate as high as 60%). Aspiration pneumonia main cause of death.
  
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Pathogenesis

Etiology

• Congenital: often inherited.
• Acquired: immune-mediated or paraneoplastic.

Predisposing factors

• Underlying genetic predisposition.
• Stresses including surgical.
• Infectious diseases (triggers?).
• Hormonal.
• Thymic neoplasia Thymoma.

Pathophysiology

• Acquired: aberrant immune process → development of heterogenous group of autoantibodies (mostly immunoglobulin class G) produced against the nicotinic acetylcholine receptor (AChR) antibodies → binds to the alpha-subunit nicotinic acetylcholine receptors at the neuromuscular junction → decreased number of functional receptors by causing endocytosis of AChR, activating complement-mediated destruction of postsynaptic muscle cell membrane near AChR, decreasing synthesis and membrane incorporation of new AChR, and directly interfering with AChR function by bound antibody.
• Congenital: structural or functional abnormality of acetylcholine receptors. Result of a single or combination of presynaptic (mutations in choline acetyltransferase gene), synaptic (mutation of the acetylcholine esterase collagen gene) or postsynaptic (mostly deficient synthesis of AChr) defect(s).
• In both forms of disease decreased number of functional nicotinic acetylcholine receptors → failure or reduction of synaptic transmission → failure or reduction of muscle contraction.

Diagnosis

Treatment

Outcomes