This disorder is a rare, nonfamilial, degenerative disorder, a tauopathy that occurs in middle-aged or elderly persons. It is characterized pathologically by the presence of abnormal intracellular filamentous deposits containing tau protein. It sometimes simulates Parkinson disease when bradykinesia and rigidity are conspicuous features. Postural-action tremor may also occur, but the usual cause of profound disability is limb apraxia and clumsiness. Other clinical features include speech disturbances (aphasic, apraxic, or dysarthric), acalculia, cortical sensory deficits (eg, neglect syndromes), stimulus-sensitive myoclonus, alien limb phenomenon (the tendency for a limb to move semipurposefully, involuntarily, and without the knowledge of its owner), dysphagia, postural disturbances, dystonic features, and ultimately cognitive decline and behavioral changes. What is it