Exercise in middle age may reduce the risk of Parkinson's disease later in life. Caffeine also appears protective with a greater decrease in risk occurring with a larger intake of caffeinated beverages such as coffee. People who smoke cigarettes or use smokeless tobacco are less likely than non-smokers to develop PD, and the more they have used tobacco, the less likely they are to develop PD. It is not known what underlies this effect. Tobacco use may actually protect against PD, or it may be that an unknown factor both increases the risk of PD and causes an aversion to tobacco or makes it easier to quit using tobacco.

Antioxidants, such as vitamins C and E, have been proposed to protect against the disease, but results of studies have been contradictory and no positive effect has been proven. The results regarding fat and fatty acids have been contradictory, with various studies reporting protective effects, risk-increasing effects or no effects. There have been preliminary indications that the use of anti-inflammatory drugs and calcium channel blockers may be protective. A 2010 meta-analysis found that nonsteroidal anti-inflammatory drugs (apart from aspirin), have been associated with at least a 15 percent (higher in long-term and regular users) reduction of incidence of the development of Parkinson's disease.

There is currently no effective treatment or cure for PSP, although some of the symptoms can respond to nonspecific measures. The average age at symptoms onset is 63 and survival from onset averages 7 years with a wide variance. Pneumonia is a frequent cause of death.

Exposure to pesticides and a history of head injury have each been linked with Parkinson disease (PD), but the risks are modest. Never having smoked cigarettes, and never drinking caffeinated beverages, are also associated with small increases in risk of developing PD.

Low concentrations of urate in the blood serum is associated with an increased risk of PD.

Clinical presentation of CBD usually does not occur until age 60, with the earliest recorded diagnosis and subsequent postmortem verification being age 28. Although men and women present with the disease, some analysis has shown a predominant appearance of CBD in women. Current calculations suggest that the prevalence of CBD is approximately 4.9 to 7.3 per 100,000 people. The prognosis for an individual diagnosed with CBD is death within approximately eight years, although some patients have been diagnosed over 17 years ago (2017) and are still in relatively good standing, but with serious debilitation such as dysphagia, and overall limb rigidity. The partial (or total) use of a feeding tube may be necessary and will help prevent aspiration pneumonia, primary cause of death in CBD. Incontinence is common, as patients often can't express their need to go, due to eventual loss of speech. Therefore, proper hygiene is mandatory to prevent urinary tract infections.

Currently, an estimated 60 to 75% of diagnosed dementias are of the Alzheimer's and mixed (Alzheimer's and vascular dementia) type, 10 to 15% are Lewy body type, with the remaining types being of an entire spectrum of dementias, including frontotemporal lobar degeneration (Pick's disease), alcoholic dementia, pure vascular dementia, etc. Dementia with Lewy bodies tends to be under-recognized. Dementia with Lewy bodies is slightly more prevalent in men than women. DLB increases in prevalence with age; the mean age at presentation is 75 years.

Dementia with Lewy bodies affects about one million individuals in the United States.

The cause of PSP is unknown. Fewer than 1% of those with PSP have a family member with the same disorder. A variant in the gene for tau protein called the H1 haplotype, located on chromosome 17, has been linked to PSP. Nearly all people with PSP received a copy of that variant from each parent, but this is true of about two-thirds of the general population. Therefore, the H1 haplotype appears to be necessary but not sufficient to cause PSP. Other genes, as well as environmental toxins, are being investigated as other possible contributors to the cause of PSP.

Parkinson-plus syndromes are usually more rapidly progressive and less likely to respond to antiparkinsonian medication than PD. However, the additional features of the diseases may respond to medications not used in PD.

Current therapy for Parkinson-plus syndromes is centered around a multidisciplinary treatment of symptoms.

These disorders have been linked to pesticide exposure.

The early stages of Alzheimer's disease are difficult to diagnose. A definitive diagnosis is usually made once cognitive impairment compromises daily living activities, although the person may still be living independently. The symptoms will progress from mild cognitive problems, such as memory loss through increasing stages of cognitive and non-cognitive disturbances, eliminating any possibility of independent living, especially in the late stages of the disease.

Life expectancy of people with AD is less. Following diagnosis it typically ranges from three to ten years.

Fewer than 3% of people live more than fourteen years. Disease features significantly associated with reduced survival are an increased severity of cognitive impairment, decreased functional level, history of falls, and disturbances in the neurological examination. Other coincident diseases such as heart problems, diabetes or history of alcohol abuse are also related with shortened survival. While the earlier the age at onset the higher the total survival years, life expectancy is particularly reduced when compared to the healthy population among those who are younger. Men have a less favourable survival prognosis than women.

Pneumonia and dehydration are the most frequent immediate causes of death brought by AD, while cancer is a less frequent cause of death than in the general population.

Two main measures are used in epidemiological studies: incidence and prevalence. Incidence is the number of new cases per unit of person–time at risk (usually number of new cases per thousand person–years); while prevalence is the total number of cases of the disease in the population at any given time.

Regarding incidence, cohort longitudinal studies (studies where a disease-free population is followed over the years) provide rates between 10 and 15 per thousand person–years for all dementias and 5–8 for AD, which means that half of new dementia cases each year are AD. Advancing age is a primary risk factor for the disease and incidence rates are not equal for all ages: every five years after the age of 65, the risk of acquiring the disease approximately doubles, increasing from 3 to as much as 69 per thousand person years. There are also sex differences in the incidence rates, women having a higher risk of developing AD particularly in the population older than 85. The risk of dying from Alzheimer's disease is 26% higher among the non-Hispanic white population than among the non-Hispanic black population, whereas the Hispanic population has a 30% lower risk than the non-Hispanic white population.

Prevalence of AD in populations is dependent upon different factors including incidence and survival. Since the incidence of AD increases with age, it is particularly important to include the mean age of the population of interest. In the United States, Alzheimer prevalence was estimated to be 1.6% in 2000 both overall and in the 65–74 age group, with the rate increasing to 19% in the 75–84 group and to 42% in the greater than 84 group. Prevalence rates in less developed regions are lower. The World Health Organization estimated that in 2005, 0.379% of people worldwide had dementia, and that the prevalence would increase to 0.441% in 2015 and to 0.556% in 2030. Other studies have reached similar conclusions. Another study estimated that in 2006, 0.40% of the world population (range 0.17–0.89%; absolute number , range ) were afflicted by AD, and that the prevalence rate would triple and the absolute number would quadruple by 2050.

Parkinson-plus syndromes, also known as disorders of multiple system degeneration, is a group of neurodegenerative diseases featuring the classical features of Parkinson's disease (tremor, rigidity, akinesia/bradykinesia, and postural instability) with additional features that distinguish them from simple idiopathic Parkinson's disease (PD). Some consider Alzheimer's disease to be in this group. Parkinson-plus syndromes are either inherited genetically or occur sporadically.

The atypical parkinsonian or Parkinson-plus syndromes are often difficult to differentiate from PD and each other. They include multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). Dementia with Lewy bodies (DLB), may or may not be part of the PD spectrum, but it is increasingly recognized as the second-most common type of neurodegenerative dementia after Alzheimer's disease. These disorders are currently lumped into two groups, the synucleinopathies and the tauopathies. They may coexist with other pathologies.

Additional Parkinson-plus syndromes include Pick's disease and olivopontocerebellar atrophy. The latter is characterized by ataxia and dysarthria, and may occur either as an inherited disorder or as a variant of multiple system atrophy. MSA is also characterized by autonomic failure, formerly known as Shy–Drager syndrome.

Clinical features that distinguish Parkinson-plus syndromes from idiopathic PD include symmetrical onset, a lack of or irregular resting tremor, and a reduced response to dopaminergic drugs (including levodopa). Additional features include bradykinesia, early-onset postural instability, increased rigidity in axial muscles, dysautonomia, alien limb syndrome, supranuclear gaze palsy, apraxia, involvement of the cerebellum including the pyramidal cells, and in some instances significant cognitive impairment.

Differentiating some kinds of atypical Parkinson: Northwest Parkinson Foundation

Before Parkinson's disease is diagnosed, the differential diagnoses include:

- AIDS can sometimes lead to the symptoms of secondary parkinsonism, due to commonly causing dopaminergic dysfunction. Indeed, parkinsonism can be a presenting feature of HIV infection.

- Corticobasal degeneration

- Creutzfeldt–Jakob disease

- Dementia pugilistica or "boxer's dementia" is a condition that occurs in athletes due to chronic brain trauma.

- Diffuse Lewy body disease

- Drug-induced parkinsonism ("pseudoparkinsonism") due to drugs such as antipsychotics, metoclopramide, sertraline, fluoxetine or the toxin MPTP

- Encephalitis lethargica

- Essential tremor, an illness which has some diagnostic overlap with Parkinson's disease

- Orthostatic tremor

- MDMA addiction and frequent use has been linked to Parkonsonism. Several cases have been reported where individuals are diagnosed with the syndrome after taking MDMA.

- Multiple system atrophy

- Pantothenate kinase-associated neurodegeneration, also known as neurodegeneration with brain iron accumulation or Hallervorden-Spatz syndrome

- Parkinson plus syndrome

- Progressive supranuclear palsy

- Toxicity due to substances such as carbon monoxide, carbon disulfide, manganese, paraquat, mercury, hexane, rotenone, Annonaceae, and toluene (inhalant abuse: "huffing")

- Vascular parkinsonism, associated with underlying cerebrovascular disease

- Wilson's disease is a genetic disorder in which an abnormal accumulation of copper occurs. The excess copper can lead to the formation of a copper-dopamine complex, which leads to the oxidation of dopamine to aminochrome. The most common manifestations include bradykinesia, cogwheel rigidity and a lack of balance.

- Paraneoplastic syndrome: neurological symptoms caused by antibodies associated with cancers

- Genetic

- Rapid onset dystonia parkinsonism

- Parkin mutation

- X-linked dystonia parkinsonism

- Autosomal recessive juvenile parkinsonism

There are many other medical and neurological conditions in which dementia only occurs late in the illness. For example, a proportion of patients with Parkinson's disease develop dementia, though widely varying figures are quoted for this proportion. When dementia occurs in Parkinson's disease, the underlying cause may be dementia with Lewy bodies or Alzheimer's disease, or both. Cognitive impairment also occurs in the Parkinson-plus syndromes of progressive supranuclear palsy and corticobasal degeneration (and the same underlying pathology may cause the clinical syndromes of frontotemporal lobar degeneration). Although the acute porphyrias may cause episodes of confusion and psychiatric disturbance, dementia is a rare feature of these rare diseases.

Aside from those mentioned above, inherited conditions that can cause dementia (alongside other symptoms) include:

- Alexander disease

- Canavan disease

- Cerebrotendinous xanthomatosis

- Dentatorubral-pallidoluysian atrophy

- Epilepsy

- Fatal familial insomnia

- Fragile X-associated tremor/ataxia syndrome

- Glutaric aciduria type 1

- Krabbe's disease

- Maple syrup urine disease

- Niemann–Pick disease type C

- Neuronal ceroid lipofuscinosis

- Neuroacanthocytosis

- Organic acidemias

- Pelizaeus–Merzbacher disease

- Sanfilippo syndrome type B

- Spinocerebellar ataxia type 2

- Urea cycle disorders

Corticobasal degeneration (CBD) or corticobasal ganglionic degeneration (CBGD) is a rare, progressive neurodegenerative disease involving the cerebral cortex and the basal ganglia. CBD symptoms typically begin in people from 50–70 years of age, and the average disease duration is six years. It is characterized by marked disorders in movement and cognitive dysfunction, and is classified as one of the Parkinson plus syndromes. Clinical diagnosis is difficult, as symptoms of CBD are often similar to those of other disorders, such as Parkinson's disease (PD), progressive supranuclear palsy (PSP), and dementia with Lewy bodies (DLB). Due to the various clinical presentations associated with CBD, a final diagnosis can only be made upon neuropathologic examination.

Parkinsonism is a clinical syndrome characterized by tremor, bradykinesia, rigidity, and postural instability. Parkinsonism is found in Parkinson's disease (after which it is named), however a wide range of other causes may lead to this set of symptoms, including some toxins, a few metabolic diseases, and a handful of neurological conditions other than Parkinson's disease.

About 7% of people with parkinsonism have developed their symptoms following treatment with particular medications. Side effect of medications, mainly neuroleptic antipsychotics especially the phenothiazines (such as perphenazine and chlorpromazine), thioxanthenes (such as flupenthixol and zuclopenthixol) and butyrophenones (such as haloperidol), piperazines (such as ziprasidone), and rarely, antidepressants. The incidence of drug-induced parkinsonism increases with age. Drug-induced parkinsonism tends to remain at its presenting level, not progress like Parkinson's disease.

An average clinical profile from published studies shows that the median onset age for HDLS patients is 44.3 years with a mean disease duration of 5.8 years and mean age of death at 53.2 years. As of 2012, there have been around 15 cases identified with at least 11 sporadic cases of HDLS. HDLS cases have been located in Germany, Norway, Sweden, and the United States, showing an international distribution focusing between Northern Europe and the United States.

Through the study of numerous kindred, it was found that the disease did not occur among just males or females, but rather was evenly distributed indicative of an autosomal rather than a sex-linked genetic disorder. It was also observed that the HDLS cases did not skip generations as it would occur with a recessive inheritance, and as such has been labeled autosomal dominant.

Chronic inflammatory conditions that may affect the brain and cognition include Behçet's disease, multiple sclerosis, sarcoidosis, Sjögren's syndrome, systemic lupus erythematosus, celiac disease, and non-celiac gluten sensitivity. This type of dementias can rapidly progress, but usually have a good response to early treatment. This consists of immunomodulators or steroid administration, or in certain cases, the elimination of the causative agent.

The prognosis and rate of the diseases progression vary considerably among individual patients and genetic kindreds, ranging from life expectancies of several months to several years, and, in exceptional cases, as long as two decades.

The prevalence and incidence remain unknown but FTDP-17 is an extremely rare condition. It is caused by mutations in the MAPT gene, which encodes a microtubule-binding protein. Over 100 families with 38 different mutations in the tau gene have been identified worldwide. The phenotype of FTDP-17 varies not only between families carrying different mutations but also between and within families carrying the same mutations.

Frontotemporal lobar degeneration (FTLD) is a pathological process that occurs in frontotemporal dementia. It is characterized by atrophy in the frontal lobe and temporal lobe of the brain, with sparing of the parietal and occipital lobes.

Common proteinopathies that are found in FTLD include the accumulation of Tau proteins and TARDBPs. Mutations in the C9orf72 gene have been established as a major genetic contribution of FTLD, although defects in the GRN and MAPT genes are also associated with it.

The Huntington's disease-like syndromes (often abbreviated as HD-like or "HDL" syndromes) are a family of inherited neurodegenerative diseases that closely resemble Huntington's disease (HD) in that they typically produce a combination of chorea, cognitive decline or dementia and behavioural or psychiatric problems.

Dementia with Lewy bodies (DLB) is a type of dementia that worsens over time. Additional symptoms may include fluctuations in alertness, visual hallucinations, slowness of movement, trouble walking, and rigidity. Excessive movement during sleep and mood changes such as depression are also common.

The cause is unknown. Typically, no family history of the disease exists among those affected. The underlying mechanism involves the buildup of Lewy bodies, clumps of alpha-synuclein protein in neurons. It is classified as a neurodegenerative disorder. A diagnosis may be suspected based on symptoms, with blood tests and medical imaging done to rule out other possible causes. The differential diagnosis includes Parkinson's and Alzheimer's.

At present there is no cure. Treatments are supportive and attempt to relieve some of the motor and psychological symptoms associated with the disease. Acetylcholinesterase inhibitors, such as donepezil, may provide some benefit. Some motor problems may improve with levodopa. Antipsychotics, even for hallucinations, should generally be avoided due to side effects.

DLB is the most common cause of dementia after Alzheimer's disease and vascular dementia. It typically begins after the age of 50. About 0.1% of those over 65 are affected. Men appear to be more commonly affected than women. In the late part of the disease, people may depend entirely on others for their care. Life expectancy following diagnosis is about eight years. The abnormal deposits that cause the disease were discovered in 1912 by Frederic Lewy.

There is no cure or treatment for GSS. It can, however, be identified through genetic testing. GSS is the slowest to progress among human prion diseases. Duration of illness can range from 3 months to 13 years, with an average duration of 5 or 6 years.

Tauopathy belongs to a class of neurodegenerative diseases associated with the pathological aggregation of tau protein in neurofibrillary or gliofibrillary tangles in the human brain. Tangles are formed by hyperphosphorylation of a microtubule-associated protein known as tau, causing it to aggregate in an insoluble form. (These aggregations of hyperphosphorylated tau protein are also referred to as paired helical filaments). The precise mechanism of tangle formation is not completely understood, and it is still controversial as to whether tangles are a primary causative factor in the disease or play a more peripheral role. Primary tauopathies, i.e., conditions in which neurofibrillary tangles (NFT) are predominantly observed, include:

- Primary age-related tauopathy (PART)/Neurofibrillary tangle-predominant senile dementia, with NFTs similar to AD, but without plaques.

- Chronic traumatic encephalopathy, including dementia pugilistica

- Progressive supranuclear palsy

- Corticobasal degeneration

- Frontotemporal dementia and parkinsonism linked to chromosome 17

- Lytico-Bodig disease (Parkinson-dementia complex of Guam)

- Ganglioglioma and gangliocytoma

- Meningioangiomatosis

- Postencephalitic parkinsonism

- Subacute sclerosing panencephalitis

- As well as lead encephalopathy, tuberous sclerosis, Hallervorden-Spatz disease, and lipofuscinosis

Neurofibrillary tangles were first described by Alois Alzheimer in one of his patients suffering from Alzheimer's disease (AD), which is considered a secondary tauopathy. AD is also classified as an amyloidosis because of the presence of senile plaques.

The degree of NFT involvement in AD is defined by Braak stages. Braak stages I and II are used when NFT involvement is confined mainly to the transentorhinal region of the brain, stages III and IV when there's also involvement of limbic regions such as the hippocampus, and V and VI when there's extensive neocortical involvement. This should not be confused with the degree of senile plaque involvement, which progresses differently.

In both Pick's disease and corticobasal degeneration, tau proteins are deposited as inclusion bodies within swollen or "ballooned" neurons.

Argyrophilic grain disease (AGD), another type of dementia, is marked by an abundance of argyrophilic grains and coiled bodies upon microscopic examination of brain tissue. Some consider it to be a type of Alzheimer's disease. It may co-exist with other tauopathies such as progressive supranuclear palsy and corticobasal degeneration, and also Pick's disease.

Huntington's disease (HD): a neurodegenerative disease caused by a CAG tripled expansion in the Huntington gene is the most recently described tauopathy (Fernandez-Nogales et al. Nat Med 2014). JJ Lucas and co-workers demonstrate that, in brains with HD, tau levels are increased and the 4R/3R balance is altered. In addition, the Lucas study shows intranuclear insoluble deposits of tau; these "Lucas' rods" were also found in brains with Alzheimer's disease.

Tauopathies are often overlapped with synucleinopathies, possibly due to interaction between the synuclein and tau proteins.

The non-Alzheimer's tauopathies are sometimes grouped together as "Pick's complex" due to their association with frontotemporal dementia, or frontotemporal lobar degeneration.

Studies have shown that PCA may be a variant of Alzheimer's disease (AD), with an emphasis on visual deficits. Although in primarily different, but sometimes overlapping, brain regions, both involve progressive neural degeneration, as shown by the loss of neurons and synapses, and the presence of neurofibrillary tangles and senile plaques in affected brain regions; this eventually leads to dementia in both diseases. PCA patients have more cortical damage and gray matter (cell body) loss in posterior regions, especially in the occipital, parietal, and temporal lobes, whereas Alzheimer’s patients typically experience more damage in the prefrontal cortex and hippocampus. PCA tends to impair working memory and anterograde memory, while leaving episodic memory intact, whereas AD patients typically have damaged episodic memory, suggesting some differences still lie in the primary areas of cortical damage.

Over time, however, atrophy in PCA patients may spread to regions commonly damaged in AD patients, leading to common AD symptoms such as deficits in memory, language, learning, and cognition. Although PCA has an earlier onset, many PCA patients have also been diagnosed with Alzheimer’s, suggesting that the degeneration has simply migrated anteriorly to other cortical brain regions.

There is no standard definition of PCA and no established diagnostic criteria, so it is not possible to know how many people have the condition. Some studies have found that about 5 percent of people diagnosed with Alzheimer’s disease have PCA. However, because PCA often goes unrecognized, the true percentage may be as high as 15 percent. Researchers and physicians are working to establish a standard definition and diagnostic criteria for PCA.

PCA may also be correlated with the diseases of Lewy body, Creutzfeldt–Jakob disease, Bálint's syndrome, and Gerstmann syndrome. In addition, PCA may result in part from mutations in the presenilin 1 gene (PSEN1).

Typically, females and older patients with organic brain changes are more likely to develop Pisa syndrome. Organic brain changes are physical changes in the brain which lead to neurological dysfunction, including dementia and frontal lobe syndrome. This includes the presence of neurodegenerative illnesses such as Alzheimer's Disease and Parkinson's Disease.