A 52-year-old male presents with 2 years of bilateral lower limb stiffness and fatigue, and 6 months of bilateral upper limb fatigue with muscle atrophy. Physical exam: atrophy of thenar and hypothenar eminences, visible muscle fasciculations, spastic hypertonia in all limbs, limb muscle strength grade 4, positive patellar and ankle clonus, bilateral positive Babinski sign. The most likely diagnosis is: