An elderly man with history of frequent falls presents to ED with features of dementia, ataxia, difficulty in downward gaze. What is the most likely diagnosis?

Progressive supranuclear palsy (PSP) is a degenerative disease that involves the brainstem, basal ganglia, limbic structures and selected areas of coex. Clinically, this disorder begins with falls and executive or subtle personality changes such as mental rigidity, impulsivity, or apathy. A progressive oculomotor syndrome ensues that begins with square wave jerks, followed by slowed saccades (veical worse than horizontal) before resulting in progressive supranuclear ophthalmoparesis. Dysahria, dysphagia, and symmetric axial rigidity can be prominent features that emerge at any point in the illness. A stiff, unstable posture with hyperextension of the neck and a slow, jerky, toppling gait is characteristic. The oculomotor disorder is supranuclear.The dementia overlaps with Fronto temporal dementia, featuring apathy, frontal-executive dysfunction, poor judgment, slowed thought processes, impaired verbal fluency and difficulty with sequential actions and with shifting from one task to another. Response to l-dopa is limited or absent. Death occurs within 5-10 years of onset. Ref: Harrison's Internal Medicine, 18th Edition, Pages 3311, 3317-3326, 3309, 3305