Adult-onset muscle weakness. How to identify the underlying cause

When adults present with muscle weakness as the predominant symptom and sign, the causes can usually be narrowed down to four disorders: amyotrophic lateral sclerosis, Guillain-Barré syndrome, myasthenia gravis, and inflammatory myopathy. Perhaps the most difficult task for the clinician is identifying which of these patients are actually weak and which are describing generalized fatigue. This task should be made easier by keeping in mind the definition of muscle weakness--a loss of power resulting in reduction of motor function--and by performing careful functional muscle testing.