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.
Yesterday (September 30, 2014), I attended a fascinating lecture by Dr. Abraham Lieberman, M.D., a neurologist who specializes in Parkinson's and has written and lectured extensively on his analysis of Hitler's handwriting, films of his speeches, still photographs, and reports of his behavior. I am rewriting this answer to include his analysis.
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.
Lieberman believes that all evidence points to Hitler having Parkinson's, although some of his behavior may be explained by other factors. However, Lieberman believes that after being gassed in World War I, Hitler contracted encephalitis which was being transmitted at about the same time as the Spanish Flu, and that Hitler had the post-encephalitic brand of Parkinson's, rather than the more common idiopathetic type. One clue for this is that in Mein Kampf, Hitler tells that he had a "relapse" of symptoms he suffered during his gassing. This is extremely unlikely, Lieberman says, and was more likely a cover story to describe his hospitalization for von Economo's encephalitis -- something a rising politician would not want to admit. Hitler's asocial behavior; his obsessions and compulsions, his cruelty and rages can be explained by a diagnosis of von Economo's encephalitis. A. Lieberman, "Hitler Had Post-encephalitic Parkinsonism," Pubmed.com, National Institute of Health, Apr. 1996.
Lieberman says that Hitler may have shown transient Parkinson's symptoms as early 1923 when he was 34, possibly in his handwriting which is beginning to become smaller. Film of his speaking during the 1920s, however, show that he enthusiastically waved both arms during speeches. In contrast, by the mid 1930s, Hitler speaks with his left hand at his side or in a pocket. Where the left hand is shown in film, tremors can be seen. Looking at Hitler's handwriting from the 1920s, 1930s, 1943 and 1945, Lieberman demonstrated that there was a clear trend to micrography -- i.e. the signature becomes smaller and smaller. Photographs in 1945 show Hitler stooped over, his face lacks expression, and according to Albert Speer and others, Hitler dragged his left foot and spoke more softly.See this article.
A common symptom of Parkinson's is that patients show a lack of impulse control, often turning to gambling and other high-risk endeavors. Lieberman notes that even before he showed Parkinsonian symptoms, Hitler was a gambler -- his takeover of the German government, and his earlier attempt at a putsch, being prime examples. He said that while some scholars believe that Parkinson's affected Hitler's judgment, and his failure to take the advice of his generals, that is a difficult conclusion. Parkinson's may have enhanced his risk-taking nature, but one cannot conclude this based on available evidence. He writes of this in this paper.
Lieberman also questioned the conclusion of Prof. Nassir Ghaemeni who believed that Hitler abused amphetamines and was bipolar. Lieberman said in the lecture and in his papers that Ghaemeni could not conclude that based only on hearsay and without actually meeting Hitler in person.