I have had a number of e-mails asking to clarify the difference between various symptoms. I would like to stress I am not a doctor, merely a patient sharing my experiences with you, but hope the following descriptions help explain.
Apathy: Not caring and disinterested, having reduced sensitivity and emotions, a lessened interest in things that used to matter.
Dementia: A decline in intellectual function, unable to retain information, leading to some degree of social impairment as the result of reduced memory or problem-solving abilities.
Depression: A collection of several symptoms, which can easily be confused with other symptoms. Continual sadness or an inability to feel pleasure are the main features. Depression also may include: fatigue, poor sleeping, weight loss or gain, soft voice, lack of patience and irritability.
Fatigue: A sense of immense lack of energy, which is completely different from just feeling sleepy. This is a very common and debilitating problem with Parkinson's.