- Definition
- Two clinical forms of myasthenia
- Ocular
- Limited to the eyelids and exraocular muscles
- Generalized
- Affects the ocular muscles and variable combinations of bulbar, limb, and respiratory muscles
- Ocular
- Two clinical forms of myasthenia
- Pathogenesis
- Auto-antibodies directed against acetylcholine receptors (AChR)
- Autoantibodies are present in 90% of patients
- Auto-antibodies directed against acetylcholine receptors (AChR)
- Epidemiology
- Can happen at any age, but there is a Bimodal distribution
- Early peak –> 20-30s
- Female predominance
- Late peak –> 60-80s
- Male predominance
- Early peak –> 20-30s
- Can happen at any age, but there is a Bimodal distribution
- Presenting symptoms
- Occular symptoms (Ptosis and/or diplopia) –> 50% of patients
- Bulbar Sx (dysarthria, dysphagia, fatigable chewing –> 15% of patients
- Muscle weakness that worsens throughout the day
- Respiratory muscles
- Most feared symptom –> Can lead to respiratory failure (“myasthenic crisis”)
- Diagnosis
- Bedside tests
- Ice pack test
- Bag of ice is placed on the eyelid for 2 minutes then removed. –> Improvement in ptosis is measured
- 80% sensitivity
- Tensilon (Edrophonium) test
- 2mg doses of edrophonium are given –> improvement in symptoms measured
- Ice pack test
- Lab tests
- Two Auto-antibodies:
- Acetylcholine receptor antibody (AChR-Ab)
- Present in 85% of patients with generalized disease
- Muscle specific receptor tyrosine kinase (MuSK)
- Present in up to 50% of patients who are AChR-Ab negative.
- Acetylcholine receptor antibody (AChR-Ab)
- Two Auto-antibodies:
- Electrophysiological studies
- Repetitive nerve stimulation (RNS)
- 75% sensitive
- Most frequently used due to wide availability
- Single-fiber EMG
- Used if RNS is negative because it is 95% sensitive
- Less widely available
- Repetitive nerve stimulation (RNS)
- Bedside tests
- Associated conditions
- Thymic tumors
- 75% of patients with AChR-antibodies have thymic abnormalities (70% hyperplasia vs 10% thymoma)
- CT vs MRI to evaluate
- Autoimmune disorders
- Thyroid disease (5%)
- RA
- SLE
- Thymic tumors
- Treatment
- Acetylcholinesterase inhibitors
- Pyridostigmine (mestinon)
- Some patients only need this
- Cholinergic side effects
- Pyridostigmine (mestinon)
- Immunomodulator drugs
- Chronic immunomodulators
- Glucocorticoids
- Azathioprine, mycophenolate, cyclosporine
- Rapid
- Useful in myasthenic crisis or preoperatively before thymectomy à not useful long term
- IVIG
- Plasmapharesis
- Useful in myasthenic crisis or preoperatively before thymectomy à not useful long term
- Chronic immunomodulators
- Thymectomy
- Acetylcholinesterase inhibitors