Myasthenia Gravis

Question 1

Definition

Answer 1

Myasthenia gravis is an acquired, organ-specific autoimmun disorder, of unknown cause in which antibodies are directed aginst the posty synaptic acetylcholine receptor.

Disorder of neuromuscular junction

Question 2

Epediomiology

Answer 2

peak incidence

• females in the second and third decades
• another peak affecting mainly males in the 6th and 7th decade

Question 3

Pathophysiology

Answer 3

• Decrease in the number of nicotinic AChRs at the neuromuscular junction
• Also conformation change, simplicfication and increase gap at the NMJ
• as a result of the decrease in receptors, the endplate potentials (EPPS) decrease in amplitude and fail to trigger a muscle action potential. Neuromuscular fatigue occurs as increasing numbers of fibres fail to fire with repeated contractions

Question 4

Clinical features

Answer 4

• Clinical hallmark - painless muscle weakness increase with exercise
• Occular muscles - ptosis and or diploploa
• Limb weakness - proximal, occular and bulbar muscles (muscles of mouth and throat)
• Neck weakness- Neck flexion and extension- patients have difficulty lifting head up
• Respiratory muscle weakness- Shortness of breath
• Other

Question 5

Investigations

Answer 5

• Acetylcholine receptor antibodies (90%)
• Electromyography (EMG) - decreased muscle action potential with repetitive motor nerve stimulation
• tensilon test- intravenous edrophonium assesses immediate and short acting effect of anticholinesterases
• CT thorax - associated thymoma

Question 6

Treatment

Answer 6

Pyridostigmine (cholinesterase inhibitor), lifelong

thymectomy

immunosuppresion