Definition
- Autoimmune disorder of the neuromuscular junction
Associated conditions
- Thymoma
- Thymic hyperplasia
Numbers
- 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
Mechanism
- 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
Clinical presentation
- Fluctuating muscle weakness
- Most commonly 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 (see ocular myasthenia)
- Ptosis
- Diplopia
- Bulbar symptoms
- Dysphagia
- Dysarthria
- Fatigable chewing
- Proximal muscle weakness
Test
- 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
- May result in false positives or cause life threatening bradycardia; thus, this test is rarely performed
Differential diagnosis
- 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
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