A 50-year-old woman complains of clumsiness in her hands while working in the kitchen: She recently burned her hands on the stove without experiencing any pain. Neurologic examination reveals bilateral weakness of the shoulder girdles, arms, and hands as well as a loss of pain and temperature sensation covering the shoulder and upper limb in a cape-like distribution. Severe atrophy is present in the intrinsic muscles of the hands. The most likely diagnosis is: