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.

Neurodegeneration is the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases – including amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's – occur as a result of neurodegenerative processes. Such diseases are incurable, resulting in progressive degeneration and/or death of neuron cells. As research progresses, many similarities appear that relate these diseases to one another on a sub-cellular level. Discovering these similarities offers hope for therapeutic advances that could ameliorate many diseases simultaneously. There are many parallels between different neurodegenerative disorders including atypical protein assemblies as well as induced cell death. Neurodegeneration can be found in many different levels of neuronal circuitry ranging from molecular to systemic.

Symptoms of MJD are memory deficits, spasticity, difficulty with speech and swallowing, weakness in arms and legs, clumsiness, frequent urination and involuntary eye movements. Symptoms can begin in early adolescence and they get worse over time. Eventually, MJD leads to paralysis; however, intellectual functions usually remain the same.

Some of the most prevalent symptom types in people exhibiting CBD pertain to identifiable movement disorders and problems with cortical processing. These symptoms are initial indicators of the presence of the disease. Each of the associated movement complications typically appear asymmetrically and the symptoms are not observed uniformly throughout the body. For example, a person exhibiting an alien hand syndrome (explained later) in one hand, will not correspondingly display the same symptom in the contralateral limb. Predominant movement disorders and cortical dysfunctions associated with CBD include:

- Parkinsonism

- Alien hand syndrome

- Apraxia (ideomotor apraxia and limb-kinetic apraxia)

- Aphasia

Because CBD is progressive, a standard set of diagnostic criteria can be used, which is centered on the disease’s evolution. Included in these fundamental features are problems with cortical processing, dysfunction of the basal ganglia, and a sudden and detrimental onset. Psychiatric and cognitive dysfunctions, although present in CBD, are much less prevalent and lack establishment as common indicators of the presence of the disease.

The most recognizable symptoms in Parkinson's disease are movement ("motor") related. Non-motor symptoms, which include autonomic dysfunction, neuropsychiatric problems (mood, cognition, behavior or thought alterations), and sensory (especially altered sense of smell) and sleep difficulties, are also common. Some of these non-motor symptoms may be present at the time of diagnosis.

The most common first sign of MSA is the appearance of an "akinetic-rigid syndrome" (i.e. slowness of initiation of movement resembling Parkinson's disease) found in 62% at first presentation. Other common signs at onset include problems with balance (cerebellar ataxia) found in 22% at first presentation, followed by genito-urinary problems (9%). For men, the first sign can be erectile dysfunction (inability to achieve or sustain an erection). Women have also reported reduced genital sensitivity. Both men and women often experience problems with their bladders including urgency, frequency, incomplete bladder emptying, or an inability to pass urine (retention). About 1 in 5 MSA patients will fall in their first year of disease.

MSA is characterized by a combination of the following, which can be present in any combination:

- autonomic dysfunction

- parkinsonism (muscle rigidity +/ tremor and slow movement)

- ataxia (Poor coordination / unsteady walking)

A variant with combined features of MSA and Lewy body dementia may also exist. There have also been occasional instances of frontotemporal lobar degeneration associated with MSA.

The movement difficulties found in PD are called "parkinsonism" and a number of different disorders feature parkinsonism. "Parkinsonism" is defined as bradykinesia (slowness in initiating voluntary movements, with progressive reduction in speed and range of repetitive actions such as voluntary finger-tapping) in combination with one of three other physical signs: muscular (lead-pipe or cogwheel) rigidity, tremor at rest, and postural instability.

Parkinson's disease is the most common form of parkinsonism and is sometimes called "idiopathic parkinsonism", meaning parkinsonism with no identifiable cause. Identifiable causes of parkinsonism include toxins, infections, side effects of drugs, metabolic derangement, and brain lesions such as strokes. Several neurodegenerative disorders also may present with parkinsonism and are sometimes referred to as "atypical parkinsonism" or "Parkinson plus" syndromes (illnesses with parkinsonism plus some other features distinguishing them from PD). They include multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration, and dementia with Lewy bodies (DLB).

Scientists sometimes refer to Parkinson’s disease as a synucleiopathy (due to an abnormal accumulation of alpha-synuclein protein in the brain) to distinguish it from other neurodegenerative diseases, such as Alzheimer's disease where the brain accumulates tau protein. Considerable clinical and pathological overlap exists between tauopathies and synucleinopathies. In contrast to Parkinson's disease, Alzheimer's disease presents most commonly with memory loss, and the cardinal signs of Parkinson's disease (slowness, tremor, stiffness, and postural instability) are not normal features of Alzheimer's.

Dementia with Lewy bodies is another synucleinopathy and it has close pathological similarities with PD, especially with the subset of PD cases with dementia. The relationship between PD and DLB is complex and incompletely understood. They may represent parts of a continuum with variable distinguishing clinical and pathological features or they may prove to be separate diseases.

Accurate diagnosis of these Parkinson-plus syndromes is improved when precise diagnostic criteria are used. Since diagnosis of individual Parkinson-plus syndromes is difficult, the prognosis is often poor. Proper diagnosis of these neurodegenerative disorders is important as individual treatments vary depending on the condition. The nuclear medicine SPECT procedure using I-IBZM, is an effective tool in the establishment of the differential diagnosis between patients with PD and Parkinson-plus syndromes.

Symptoms start with slowly developing dysarthria (difficulty speaking) and cerebellar ataxia (unsteadiness) and then the progressive dementia becomes more evident. Loss of memory can be the first symptom of GSS. Extrapyramidal and pyramidal symptoms and signs may occur and the disease may mimic spinocerebellar ataxias in the beginning stages. Myoclonus (spasmodic muscle contraction) is less frequently seen than in Creutzfeldt–Jakob disease. Many patients also exhibit nystagmus (involuntary movement of the eyes), visual disturbances, and even blindness or deafness. The neuropathological findings of GSS include widespread deposition of amyloid plaques composed of abnormally folded prion protein.

Machado–Joseph disease (MJD), also known as Machado–Joseph Azorean disease, Machado's disease, Joseph's disease or spinocerebellar ataxia type 3 (SCA3), is a rare autosomal dominantly inherited neurodegenerative disease that causes progressive cerebellar ataxia, which results in a lack of muscle control and coordination of the upper and lower extremities. The symptoms are caused by a genetic mutation that results in an expansion of abnormal "CAG" trinucleotide repeats in the ATXN3 gene that results in an abnormal form of the protein ataxin which causes degeneration of cells in the hindbrain. Some symptoms, such as clumsiness and rigidity, make MJD commonly mistaken for drunkenness or Parkinson's disease.

Machado–Joseph disease is a type of spinocerebellar ataxia and is the most common cause of autosomal-dominant ataxia. MJD causes ophthalmoplegia and mixed sensory and cerebellar ataxia.

Pick's disease is a term that can be used in two different ways. It has traditionally been used as a term for a group of neurodegenerative diseases with symptoms attributable to frontal and temporal lobe dysfunction. Common symptoms that are noticed early are personality and emotional changes, as well as deterioration of language. This condition is now more commonly called frontotemporal dementia by professionals, and the use of "Pick's disease" as a clinical diagnosis has fallen out of fashion. The second use of the term (and the one now used among professionals) is to mean a specific pathology that is one of the causes of frontotemporal lobar degeneration. These two uses have previously led to confusion among professionals and patients and so its use should be restricted to the specific pathological subtype described below. It is also known as Pick disease and PiD (not to be confused with pelvic inflammatory disease (PID) or Parkinson's disease (PD)). A defining characteristic of the disease is build-up of tau proteins in neurons, accumulating into silver-staining, spherical aggregations known as "Pick bodies".

Neuroferritinopathy has several distinguishing signs and symptoms. These fall into two categories: diagnostic findings and physically visible symptoms.

The presenting symptom of dementia with Lewy bodies is often cognitive dysfunction, though dementia eventually occurs in all individuals with DLB. In contrast to Alzheimer's disease (AD), in which memory loss is the first symptom, those with DLB first experience impaired attention, executive function, and visuospatial function, while memory is affected later. These impairments present as driving difficulty, such as becoming lost, misjudging distances, or as impaired job performance. In terms of cognitive testing, individuals may have problems with figure copying as a result of visuospatial impairment, with clock-drawing due to executive function impairment, and difficulty with serial sevens as a result of impaired attention. Short-term memory and orientation to time and place remain intact in the earlier stages of the disease.

While the specific symptoms in a person with DLB may vary, core features include: fluctuating cognition with great variations in attention and alertness from day to day and hour to hour, recurrent visual hallucinations (observed in 75% of people with DLB), and motor features of Parkinson's disease. Suggestive symptoms are rapid eye movement (REM)-sleep behavior disorder and abnormalities detected in PET or SPECT scans. REM sleep behavior disorder (RBD) often is a symptom first recognized by the patient's caretaker. RBD includes vivid dreaming, with persistent dreams, purposeful or violent movements, and falling out of bed. Benzodiazepines, anticholinergics, surgical anesthetics, some antidepressants, and over-the-counter drug cold remedies may cause acute confusion, delusions, and hallucinations.

Tremors are less common in DLB than in Parkinson's disease. Parkinsonian features may include shuffling gait, reduced arm-swing during walking, blank expression (reduced range of facial expression), stiffness of movements, ratchet-like cogwheeling movements, low speech volume, sialorrhea, and difficulty swallowing. Also, DLB patients often experience problems with orthostatic hypotension, including repeated falls, fainting, and transient loss of consciousness. Sleep-disordered breathing, a problem in multiple system atrophy, also may be a problem.

One of the most critical and distinctive clinical features of the disease is hypersensitivity to neuroleptic and antiemetic medications that affect dopaminergic and cholinergic systems. In the worst cases, a patient treated with these medications could become catatonic, lose cognitive function, or develop life-threatening muscle rigidity. Some commonly used medications that should be used with great caution, if at all, for people with DLB, are chlorpromazine, haloperidol, or thioridazine.

Visual hallucinations in people with DLB most commonly involve perception of people or animals that are not there, and may reflect Lewy bodies or AD pathology in the temporal lobe. Delusions may include reduplicative paramnesia and other elaborate misperceptions or misinterpretations. These hallucinations are not necessarily disturbing, and in some cases, the person with DLB may have insight into the hallucinations and even be amused by them, or be conscious they are not real. People with DLB also may have problems with vision, including double vision, and misinterpretation of what they see, for example, mistaking a pile of socks for snakes or a clothes closet for the bathroom.

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.

Symptoms categorized as medically tested and diagnosed include iron accumulation in the brain, basal ganglia cavitation, and neurodegeneration. Patients who are diagnosed with neuroferritinopathy have abnormal iron accumulation in the brain within the neurons and glia of the striatum and cerebellar cortices. Along with the accumulation of iron in the brain, neuroferritinopathy typically causes severe neuronal loss as well.

Secondary symptoms may also arise. It is possible that the initial iron accumulation will cause additional neuronal damage and neuronal death. The damaged neurons may be replaced by other cells in an effort to reverse the neurodegeneration. These cells often have a higher iron content. The breakdown of the blood brain barrier may also occur due to the loss of neurons and will subsequently allow more iron to access the brain and accumulate over time.

Neuroferritinopathy is mainly seen in those who have reached late adulthood and is generally seen to slowly progress throughout many decades in a lifetime with the mean age of onset being 39 years old. A loss of cognition is generally only seen with late stages of the disease. Diagnosed patients are seen to retain most of their cognitive functioning until the most progressive stages of the illness sets in.

Early signs and symptoms of the disorder usually appear around ages 2–10, with gradual onset of vision problems, or seizures. Early signs may be subtle personality and behavior changes, slow learning or regression, repetitive speech or echolalia, clumsiness, or stumbling. Slowing head growth in the infantile form, poor circulation in lower extremities (legs and feet), decreased body fat and muscle mass, curvature of the spine, hyperventilation and/or breath-holding spells, teeth grinding, and constipation may occur.

Over time, affected children suffer mental impairment, worsening seizures, and progressive loss of sight, speech, and motor skills. Batten disease is a terminal disease; life expectancy varies depending on the type or variation.

Females with juvenile Batten disease show first symptoms a year later than males, but on average die a year sooner.

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.

The symptoms of Pick's disease include difficulty in language and thinking, efforts to dissociate from family, behavioral changes, unwarranted anxiety, irrational fears, CBD (Compulsive buying disorder, or oniomania), impaired regulation of social conduct (e.g., breaches of etiquette, vulgar language, tactlessness, , misperception), passivity, low motivation (aboulia), inertia, over-activity, pacing and wandering. It is a characteristic of Pick’s disease that dysfunctional, argumentative, or hostile social conduct is initially exhibited towards family members and not initially exhibited in a workplace or neutral environment. The changes in personality allow doctors to distinguish between Pick's disease and Alzheimer's disease. Pick's disease is one of the causes of the clinical syndrome of frontotemporal lobar degeneration which has three subtypes. Pick's disease pathology is associated more with the frontotemporal dementia and progressive nonfluent aphasia subtypes than the semantic dementia subtype.

FTD is traditionally difficult to diagnose due to the heterogeneity of the associated symptoms. Signs and symptoms are classified into three groups based on the functions of the frontal and temporal lobes:

- Behavioural variant frontotemporal dementia (BvFTD) is characterized by changes in social behavior and conduct, with loss of social awareness and poor impulse control.

- Semantic dementia (SD) is characterized by the loss of semantic understanding, resulting in impaired word comprehension, although speech remains fluent and grammatically faultless.

- Progressive nonfluent aphasia (PNFA) is characterized by progressive difficulties in speech production.

However, the following abilities in the person with FTD are preserved:

- Perception

- Spatial Skills

- Memory

- Praxis

In later stages of FTD, the clinical phenotypes may overlap. FTD patients tend to struggle with binge eating and compulsive behaviors. These binge eating habits are often associated with abnormal eating behavior including overeating, stuffing oneself with food, changes in food preferences (cravings for more sweets, carbohydrates), eating inedible objects and snatching food from others. Recent findings from structural MRI research have indicated that eating changes in FTD are associated with atrophy (wasting) in the right ventral insula, striatum, and orbitofrontal cortex.

Patients with FTD show marked deficiencies in executive functioning and working memory. Most FTD patients become unable to perform skills that require complex planning or sequencing. In addition to the characteristic cognitive dysfunction, a number of primitive reflexes known as frontal release signs are often able to be elicited. Usually the first of these frontal release signs to appear is the palmomental reflex which appears relatively early in the disease course whereas the palmar grasp reflex and rooting reflex appear late in the disease course.

In rare cases, FTD can occur in patients with motor neuron disease (MND) (typically amyotrophic lateral sclerosis). The prognosis for people with MND is worse when combined with FTD, shortening survival by about a year.

Frontotemporal dementia (FTD) is the clinical presentation of frontotemporal lobar degeneration, which is characterized by progressive neuronal loss predominantly involving the frontal or temporal lobes, and typical loss of over 70% of spindle neurons, while other neuron types remain intact.

It was first described by Arnold Pick in 1892 and was originally called "Pick's disease", a term now reserved for Pick disease, one specific type of frontotemporal dementia. Second only to Alzheimer's disease (AD) in prevalence, FTD accounts for 20% of young-onset dementia cases. Signs and symptoms typically manifest in late adulthood, more commonly between the ages of 55 and 65, approximately equally affecting men and women.

Common signs and symptoms include significant changes in social and personal behavior, apathy, blunting of emotions, and deficits in both expressive and receptive language. Currently, there is no cure for FTD, but there are treatments that help alleviate symptoms.

Symptoms typically begin in childhood and are progressive, often resulting in death by early adulthood. Symptoms of PKAN begin before middle childhood, and most often are noticed before ten years of age. Symptoms include:

- dystonia (repetitive uncontrollable muscle contractions that may cause jerking or twisting of certain muscle groups)

- dysphagia & dysarthria due to muscle groups involved in speech being involved

- rigidity/stiffness of limbs

- tremor

- writhing movements

- dementia

- spasticity

- weakness

- seizures (rare)

- toe walking

- retinitis pigmentosa, another degenerative disease that affects the individual’s retina, often causing alteration of retinal color and progressive deterioration of the retina at first causing night blindness and later resulting in a complete loss of vision.

25% of individuals experience an uncharacteristic form of PKAN that develops post-10 years of age and follows a slower, more gradual pace of deterioration than those pre-10 years of age. These individuals face significant speech deficits as well as psychiatric and behavioral disturbances.

Being a progressive, degenerative nerve illness, PKAN leads to early immobility and often death by early adulthood. Death occurs prematurely due to infections such as pneumonia, and the disease in itself is technically not life limiting.

OPCA is characterized by progressive cerebellar ataxia, leading to clumsiness in body movements, veering from midline when walking, wide-based stance, and falls without signs of paralysis or weakness. Clinical presentation can vary greatly between patients, but mostly affects speech, balance and walking. Other possible neurological problems include spasmodic dysphonia, hypertonia, hyperreflexia, rigidity, dysarthria, dysphagia and neck dystonic posture.

The symptoms of DLB overlap clinically with those of Alzheimer's disease and Parkinson's disease, but are associated more commonly with the latter. Because of this overlap, early DLB is often misdiagnosed. The overlap of neuropathological and presenting symptoms (cognitive, emotional, and motor) may make an accurate differential diagnosis difficult. In fact, DLB often is confused in its early stages with Alzheimer's disease and/or vascular dementia (multi-infarct dementia). However, while Alzheimer’s disease usually begins gradually, DLB frequently has a rapid or acute onset, with an especially rapid cognitive and physical decline in the first few months. Thus, DLB tends to progress more rapidly than Alzheimer’s disease. Despite the difficulty, a prompt diagnosis is important because of the risks of sensitivity to certain neuroleptic (antipsychotic) medications and because appropriate treatment of symptoms may improve life for both the person with DLB and the person's caregivers.

Dementia with Lewy bodies is distinguished from the dementia that sometimes occurs in Parkinson's disease by the time frame in which dementia symptoms appear relative to Parkinson symptoms. Parkinson's disease with dementia (PDD) would be the diagnosis when the onset of dementia is more than a year after the onset of Parkinsonian symptoms. DLB is diagnosed when cognitive symptoms begin at the same time or within a year of Parkinson symptoms.