A 58-year-old right-handed man presents with a nine-month history of progressive weakness. It started subtly in his right hand. He noticed difficulty with fine motor tasks like buttoning his shirt and using keys. Over the last few months, the weakness has ascended to his right arm. He now struggles to lift a coffee mug. He has also observed constant muscle twitching in his right arm and shoulder. His wife notes that his speech has become slightly slurred, particularly when tired. He occasionally coughs on liquids. He denies any sensory changes, pain, bowel or bladder incontinence, or visual disturbances. His past medical history is unremarkable. He is not on any regular medications. He works as an accountant and has noticed his handwriting becoming nearly illegible. There is no family history of neurological disease.