Parkinson's Disease and Myasthenia Gravis Quiz

See Clinical Features. Option B is correct because the cardinal feature of myasthenia gravis is fatigable weakness: muscle strength declines with repeated or sustained use and recovers with rest. The cause is a failure of transmission at the neuromuscular junction, the synapse where a motor nerve passes its signal to a muscle fibre using the chemical messenger acetylcholine. Autoantibodies (antibodies directed against the body's own tissue) have damaged the acetylcholine receptors on the muscle side, so there is little spare capacity: the first few contractions work, then the signal fails to activate enough receptors and the muscle tires. Think of a battery that runs flat with use and recharges with rest. The ocular muscles are most often affected first, causing a drooping eyelid (ptosis) and double vision (diplopia). Bulbar muscles, which control speech and swallowing, may also be involved, so a patient may slur their words or tire part-way through a meal, and the limb weakness is typically proximal and symmetrical. Sensation and deep tendon reflexes are normal, and the weakness fluctuates from hour to hour and day to day.

• Cardinal feature: Fatigable weakness that worsens with sustained use and improves with rest.
• Cause: Autoantibodies damage acetylcholine receptors at the neuromuscular junction.
• Distribution: Ocular muscles are often affected first, causing ptosis and diplopia, and bulbar muscles may be involved.
• Examination: Sensation and deep tendon reflexes are normal.