I'm going to take a stab at this because I also have Parkinson's Disease.
Parkinsonism is an umbrella term that describes many conditions which share some of the symptoms of Parkinson's. The main symptoms of Parkinson's – tremor, rigidity and slowness of movement – are also the main symptoms of a number of conditions that are grouped together under the term parkinsonism. Generally, Parkinson's symptoms are caused by a lack of dopamine in the brain. See more.
The most common form of Parkinson's is Idiopathic Parkinson's disease. Its typical symptoms are tremor, rigidity and slowness of movement, among others, but symptoms and the rate the condition progresses can vary in each patient. The underlying cause of idiopathic Parkinson's is unknown and a diagnosis is usually determined after testing the patient with anti-Parkinson's medicines, such as carbidopa/levodopa. Ibid. This medicine was not developoed until the 1960s and would not have been available to Hitler.
Parkinson's can also include a slow gait (which is how I was first diagnosed five years ago), stooped posture, a voice reduced to a whisper, and a dull stare that does not seem to focus on its surroundings. Those suffering from Parkinson's can also suffer from cognitive disorders that include a lack of imagination and spontaneity, difficulty making decisions and general apathy. These are all symptoms that Hitler reportedly showed in 1945. For example, whereas early in his career he was a tremendous orater who was known for violent gesturing with his hands while speaking, that trait left him in his last year. Similarly, although Hitler was willing to trust his generals with battle plans early in the war, he was less willing to at the end of the war.
Other symptoms were obvious to Hitler's advisors. According to Dr. John Murphy in a Newstime.com article, Hitler's entourage wrote in their memoirs that he walked slowly by the war's end. His voice was reduced to a whisper and both hands shook. He was bent over and shuffling. In his 50s, he looked like a man two or three decades older. "Hitler's left hand trembled and he had a stooped and rigid posture," wrote Gen. Heinz Gudarian, one of Hitler's generals. An intelligence officer, Gustave Boldt, also wrote of Hitler's tremors and shuffling walk. An SS officer wrote that Hitler in 1945 "had become an old man,'' his voice a whisper. Murphy added that handwriting analysis shows that as the war progressed, Hitler's handwriting became small and cramped -- another classic symptom of Parkinson's patients on which I am monitored.
I am reluctant to endorse the opinion, cited in another answer here, of F. Gerstenbrand, E. Karamat to the extent that it appears to associate Parkinson's with anti-social personality disorders. Pope John Paul and Muhammed Ali are two other well-known Parkinson's patients, among many, who were never known for anti-social behavior.