Autoimmune disorder of neuromuscular junction transmission, characterized by weakness and fatiguability of proximal limb, bulbar and ocular muscles
Aetiology
Autoimmune - antibodies to ACh receptors (AChR) are found in 80-90% of patients
75% of patients have thymic hyperplasia/thymoma
Can occur at any age but there are two peaks of incidence: females in 3rd decade, males in 6th or 7th decade (larger)
Pathophysiology
Autoimmune origin in many cases - autoantibodies against nAChr in the endplate results in a reduction in the number of functional channels → amplitude of endplate potential decreases
Even with normal amounts of ACh transmission becomes inefficient
Leads to muscle weakness and fatiguability - symptoms start when ACh receptors reduced to 30% of normal
Clinical features
Fatiguable weakness of skeletal muscle
Most common presentation with extraocular weakness (60%), facial and bulbar weakness
Weakness in bulbar muscles can result in nasal speech or dysarthria