Parkinson's disease is indeed a different disease than Alzheimer's disease.
Dementia is a symptom of the deterioration of intellectual abilities with AD (Alzheimer's) being just one of those diseases.
Although you will read about the fact that PD patients often have a form of dementia, it is not quite the same as that in the dementia type known as Alzheimer's. Parkinson's patients can have a form of dementia known as Parkinson's dementia. They can also have dementia with Lewy bodies (DBL). Lewy bodies are made up of alpha-synuclein a protein which plays a role in the synaptic response -by signalling dopamine to the outer areas of the cell - prematurely in the case of Parkinson's because the a-syn is misfolded - and therefore abeting the deterioration of the dopamine. But it apparently plays a additional gumming up role in the case of DLB.
Alzheimer's is just one kind of dementia and at present there is a movement to call older age dementia senile dementia rather than Alzheimer's disease at least until a definitive diagnosis can be made.
In Alzheimer's there are very specific microscopic brain abnormalities. Although it can progress slowly and the initial symptoms can vary, in general it manifests as a loss of short-term memory and losing one or more ability in the areas of speech and language, personality, decision-making and judgment awareness and an inability to interact with the environment.
Parkinson's is a neurodegenerative disease, primarily a motor-neuron disorder. In Parkinson's dementia - not to be confused with Parkinson's depression or PD apathy, the patient may exhibit major depression, more impairment of visual reasoning than seen in AD. In AD patients you will also find less awareness/more denial of the situation and more dis-inhibition than in PD patients.
Similarities are in the area of impaired cognition but there are also differences.
While there are psychoses associated with cognition in DBL, there are few in AD and none in PD dementia. Both DBL and PD-D seems to be worse on attentional differences and better on memory tests than AD. As a matter of fact, AD = amnesic syndrome. Moreover, visual memory is lower in DBL than in AD.
One study found that hallucinations were somewhat more common in AD than in PDD but that they were more severe in the PD patients. At the same time they found that aberrant motor behavior, agitation, dis-inhibition, irritability, euphoria and apathy were more severe in AD.
It has also been reported that fewer than 3% of AD patients live longer than 14 years past the diagnosis. In PD the life-span appears to depend upon the progression of the neurodegenerative aspect and the length of time medications are effective.
I don't think you are interested in the actual problems in the brain which distinguish these diseases so we won't go into plaques, neurofibullary tangles, tau. protein misfolding. Suffice it to say that in 1997 one study came to the conclusion that " Although a common feature of metabolic abnormalities in the posterior brain exists in PDD and AD, the presence of regional metabolic differences suggests different degrees and combinations of disease specific underlying pathological and neurochemical processes."
There was one rather exciting study which indicated that there are fewer muscarinic acetylcholine receptor binding sites in the temporal cortex in DLB than in AD. Furthermore there is a different up-regulation which suggests differences in the degeneration of the cholinergic systems between these dementias.
More disparity may be found in speech. PDD patients have more prominent speech abnormalities as associated with motor speech - the act of speaking while AD patients exhibit more profound language associations.
There are some commonalities in that there may be genes which contribute to both diseases...that remains to be established, however.
They seem similar, they have many similarities but the reason that the differences are important is that it is probably in the differences (caused by different chemical processes) that better treatments and cures will ultimately be found.
Good question - I wish that it still didn't have to be answered because we had all of the answers and the cures.