48 y/o woman presented with  right sided foot drop which was noted one year before with no pain or sensory symptoms. Six months later she developed left sided foot-drop, and noted  twitching in the thighs and arms. Family history was significant for ALS in the father and a paternal uncle, and dementia in the paternal grandfather. Exam showed normal mental status and cranial nerves, normal strength aside from dorsiflexion ( 0 on the right, 3 on the left) and plantar flexion (4+ bilaterally). Calf muscles were atrophic bilaterally, and occasional fasciculations were noted in the thigh muscles. Reflexes, including the jaw jerk, were brisk but ankle jerks were trace. Hoffmann reflexes were present.  Sensory exam was normal, and she had steppage gait. MRI of the brain showed subtle subcortical changes in FLAIR sequence, likely the result of corticospinal tract degeneration (see right column). Lumbar, and cervical spine MRIs did not show significant abnormalities. Click here for EMG and nerve conduction study. 

What is the diagnosis? (click here)