Hx - 62yo M presents with difficulty walking
PE - He has spastic weakness of the legs. It appears more spastic than weak. The tone is increased, the reflexes brisk with clonus and the plantar responses extensor. Vibration and joint position senses are lost in the lower limbs but no spinothalamic deficits. In the upper limbs there is asymmetrical abnormalities in the biceps and supinator reflexes - there are absent jerks with brisk finger flexion.
What is the likely diagnosis/cause?