Which statement best differentiates myasthenia gravis from Lambert-Eaton myasthenic syndrome clinically and on electrodiagnostic testing?

Which statement best differentiates myasthenia gravis from Lambert-Eaton myasthenic syndrome clinically and on electrodiagnostic testing?

• A. MG presents with fatigable weakness with constant factors; LEMS presents with distal sensory changes; EMG shows random patterns.
• B. MG presents with fatigable weakness worsened by use with ptosis; LEMS presents with proximal weakness that improves with exertion; EMG shows decremental response in repetitive nerve stimulation for MG and incremental response for LEMS.
• C. MG presents with non-fatigable weakness; LEMS presents with autonomic failure; EMG shows decremental response for both.
• D. MG has autonomic symptoms; LEMS lacks autonomic features; EMG shows identical patterns in both.

Answer: (D)

The key idea is how MG and LEMS differ in both clinical presentation and electrodiagnostic testing. Clinically, myasthenia gravis typically shows fatigable skeletal weakness that worsens with use, with eye muscle involvement (ptosis, diplopia) being common. Autonomic symptoms are not a hallmark. Lambert-Eaton myasthenic syndrome often presents with proximal weakness, and uniquely, strength tends to improve with continued use or after brief exertion. Autonomic features (like dry mouth, erectile dysfunction, constipation) are relatively common in LEMS. Electrodiagnostically, MG most characteristically shows a decrement in CMAP amplitude on repetitive nerve stimulation. LEMS, by contrast, classically shows an incremental (facilitatory) response after high-frequency stimulation or after brief exercise. So, the distinguishing pattern is: MG has fatigable weakness with ocular involvement and a decremental EMG pattern; LEMS has proximal weakness that improves with exertion, autonomic symptoms are common, and an incremental EMG pattern after stimulation. The claim that MG has autonomic symptoms and LEMS lacks them and that EMG patterns are identical does not fit the typical differences.

The key idea is how MG and LEMS differ in both clinical presentation and electrodiagnostic testing.

Clinically, myasthenia gravis typically shows fatigable skeletal weakness that worsens with use, with eye muscle involvement (ptosis, diplopia) being common. Autonomic symptoms are not a hallmark. Lambert-Eaton myasthenic syndrome often presents with proximal weakness, and uniquely, strength tends to improve with continued use or after brief exertion. Autonomic features (like dry mouth, erectile dysfunction, constipation) are relatively common in LEMS.

Electrodiagnostically, MG most characteristically shows a decrement in CMAP amplitude on repetitive nerve stimulation. LEMS, by contrast, classically shows an incremental (facilitatory) response after high-frequency stimulation or after brief exercise.

So, the distinguishing pattern is: MG has fatigable weakness with ocular involvement and a decremental EMG pattern; LEMS has proximal weakness that improves with exertion, autonomic symptoms are common, and an incremental EMG pattern after stimulation. The claim that MG has autonomic symptoms and LEMS lacks them and that EMG patterns are identical does not fit the typical differences.