Histology · Year 1 · from Histology
Case 2: Myasthenia Gravis
Clinical Image
Source: Wikipedia - Myasthenia Gravis - CC BY-SA 3.0
Case Presentation
A 35-year-old woman presents with drooping eyelids (ptosis) and double vision (diplopia) that worsens throughout the day and improves after rest. Over the past month, she has also developed difficulty chewing during meals and her voice becomes nasal after prolonged speaking. Physical examination reveals bilateral ptosis that worsens with sustained upgaze (fatigable weakness). An ice pack test is performed: application of ice to the eyelids for 2 minutes produces noticeable improvement in ptosis. Laboratory studies reveal positive acetylcholine receptor (AChR) antibodies. Electromyography shows a decremental response to repetitive nerve stimulation. CT chest reveals a thymoma. She is diagnosed with myasthenia gravis. Treatment is initiated with pyridostigmine (an acetylcholinesterase inhibitor that prolongs acetylcholine action at the neuromuscular junction) and she is scheduled for thymectomy.
Key Learning Points
- Myasthenia gravis is an autoimmune disease where antibodies attack nicotinic acetylcholine receptors at the neuromuscular junction
- Receptor destruction reduces the number of available receptors, causing the end plate potential (EPP) to fall below threshold for action potential generation
- Fatigable weakness occurs because with repeated stimulation, acetylcholine stores deplete and fewer receptors are available
- The ice test works because cooling slows acetylcholinesterase activity, prolonging ACh action
- Treatment with acetylcholinesterase inhibitors increases ACh concentration at the remaining receptors