The length of the trinucleotide repeat accounts for 60% of the variation in the age symptoms appear and the rate they progress. A longer repeat results in an earlier age of onset and a faster progression of symptoms. Individuals with more than sixty repeats often develop the disease before age 20, while those with fewer than 40 repeats may not ever develop noticeable symptoms. The remaining variation is due to environmental factors and other genes that influence the mechanism of the disease.

Life expectancy in HD is generally around 20 years following the onset of visible symptoms. Most life-threatening complications result from muscle coordination and, to a lesser extent, behavioral changes induced by declining cognitive function. The largest risk is pneumonia, which causes death in one third of those with HD. As the ability to synchronize movements deteriorates, difficulty clearing the lungs and an increased risk of aspirating food or drink both increase the risk of contracting pneumonia. The second greatest risk is heart disease, which causes almost a quarter of fatalities of those with HD. Suicide is the third greatest cause of fatalities, with 7.3% of those with HD taking their own lives and up to 27% attempting to do so. It is unclear to what extent suicidal thoughts are influenced by behavioral symptoms, as they signify sufferers' desires to avoid the later stages of the disease. Other associated risks include choking, physical injury from falls, and malnutrition.

The families of individuals who have inherited or are at risk of inheriting HD have generations of experience of HD, but may be unaware of recent breakthroughs in understanding the disease, and of the availability of genetic testing. Genetic counseling benefits these individuals by updating their knowledge, seeking to dispel any unfounded beliefs that they may have, and helping them consider their future options and plans. Also covered is information concerning family planning choices, care management, and other considerations.

Its occurrence is very rare. The infantile form from birth to 2 years of age. The average duration of the infantile form of the illness is usually about 3 years. Onset of the juvenile form presents between two and twelve years of age. Duration of this form is in most cases about 6 years. The adult form from twelve years and older. In younger patients, seizures, megalencephaly, developmental delay, and spasticity are usually present. Neonatal onset is also reported. Onset in adults is least frequent. In older patients, bulbar or pseudobulbar symptoms and spasticity predominate. Symptoms of the adult form may also resemble multiple sclerosis.

There are no more than 500 reported cases.

The prognosis is generally poor. With early onset, death usually occurs within 10 years from the onset of symptoms. Individuals with the infantile form usually die before the age of 7. Usually, the later the disease occurs, the slower its course is.

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.

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.

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

Batten disease is a fatal disease of the nervous system that typically begins in childhood. Onset of symptoms is usually between 5 and 10 years of age. Often it is autosomal recessive. It is the most common form of a group of disorders called the neuronal ceroid lipofuscinoses (NCLs).

Although Batten disease is usually regarded as the juvenile form of NCL (or "type 3"), some physicians use the term Batten disease to describe all forms of NCL. Historically, the NCLs were classified by age of disease onset as infantile NCL (INCL), late infantile NCL (LINCL), juvenile NCL (JNCL) or adult NCL (ANCL). At least 20 genes have been identified in association with Batten disease, but juvenile NCL, the most prevalent form of Batten disease, has been linked to mutations in the "CLN3" gene.

It was first described in 1903.

Since the early 1990s, a new class of molecular disease has been characterized based upon the presence of unstable and abnormal expansions of DNA-triplets (trinucleotides). The first triplet disease to be identified was fragile X syndrome, which has since been mapped to the long arm of the X chromosome. At this point, there are from 230 to 4000 CGG repeats in the gene that causes fragile X syndrome in these patients, as compared with 60 to 230 repeats in carriers and 5 to 54 repeats in unaffected individuals. The chromosomal instability resulting from this trinucleotide expansion presents clinically as intellectual disability, distinctive facial features, and macroorchidism in males. The second, related DNA-triplet repeat disease, fragile X-E syndrome, was also identified on the X chromosome, but was found to be the result of an expanded CGG repeat. Identifying trinucleotide repeats as the basis of disease has brought clarity to our understanding of a complex set of inherited neurological diseases.

As more repeat expansion diseases have been discovered, several categories have been established to group them based upon similar characteristics. Category I includes Huntington's disease (HD) and the spinocerebellar ataxias that are caused by a CAG repeat expansion in protein-coding portions of specific genes. Category II expansions tend to be more phenotypically diverse with heterogeneous expansions that are generally small in magnitude, but also found in the exons of genes. Category III includes fragile X syndrome, myotonic dystrophy, two of the spinocerebellar ataxias, juvenile myoclonic epilepsy, and Friedreich's ataxia. These diseases are characterized by typically much larger repeat expansions than the first two groups, and the repeats are located outside of the protein-coding regions of the genes.

Currently, nine neurologic disorders are known to be caused by an increased number of CAG repeats, typically in coding regions of otherwise unrelated proteins. During protein synthesis, the expanded CAG repeats are translated into a series of uninterrupted glutamine residues forming what is known as a polyglutamine tract ("polyQ"). Such polyglutamine tracts may be subject to increased aggregation.

Recent results suggest that the CAG repeats need not always be translated in order to cause toxicity. Researchers at the University of Pennsylvania demonstrated that in fruit flies, a protein previously known to bind CUG repeats ("muscleblind", or "mbl") is also capable of binding CAG repeats. Furthermore, when the CAG repeat was changed to a repeating series of CAACAG (which also translates to polyQ), toxicity was dramatically reduced. The human homolog of "mbl", MBNL1, which was originally identified as binding CUG repeats in RNA, has since been shown to bind CAG (and CCG) repeats as well.

These disorders are characterized by autosomal-dominant mode of inheritance (with the exception of spino-bulbar muscular atrophy, which shows X-linked inheritance), midlife onset, a progressive course, and a correlation of the number of CAG repeats with the severity of disease and the age at onset. Family studies have also suggested that these diseases are associated with anticipation, the tendency for progressively earlier or more severe expression of the disease in successive generations. Although the causative genes are widely expressed in all of the known polyglutamine diseases, each disease displays an extremely selective pattern of neurodegeneration.

Currently, no research has shown a higher prevalence of most leukodsytrophy types in any one place around the world. There is, however, a higher prevalence of the Canavan disease in the Jewish population for unknown reasons. 1 in 40 individuals of Ashkenazi Jewish descent are carriers of Canavan disease. This estimates to roughly 2.5%. Additionally, due to an autosomal recessive inheritance patterns, there is no significant difference found between affected males and affected females for most types of leukodystrophy including, but not limited to, metachromatic leukodystrophy, Krabbe disease, Canavan disease, and Alexander disease. The one exception to this is any type of leukodystrophy carried on a sex chromosome, such as X-linked adrenoleukodystrophy, which is carried on the X-chromosome. Because of the inheritance pattern of X-linked diseases, males are more often affected by this type of leukodystrophy, although female carriers are often symptomatic, though not as severely so as males. To date, there have been no found cases of a leukodystrophy carried on the Y chromosome.

Incidence can vary greatly from type-to-type, and from country-to-country.

In Germany, one study reported an incidence of 1.28 per 100,000.

A study in Italy reported an incidence of 0.56 per 100,000.

A study in Norway reported an incidence of 3.9 per 100,000 using the years from 1978 to 1999, with a lower rate in earlier decades.

NCLs are a family of diseases which are inherited in an autosomal recessive manner. Collectively referred to as Batten disease, NCLs are responsible for the majority of neurodegenerative diseases that affect children. Specifically, the frequency of this disease is about one per 12,500 individuals. The specific type of NCL is characterized by the age of symptomatic onset and genetic mutation involved. Currently, mutations in 10 genes lead to the development of Batten disease.

Progressive myoclonus epilepsy (PME) is a rare epilepsy syndrome caused by a variety of genetic disorders. The syndrome includes myoclonic seizures and tonic-clonic seizures together with progressive neurological decline.

As of 2010, even with the best care, children with infantile Tay–Sachs disease usually die by the age of 4.

In infantile Krabbe disease, death usually occurs in early childhood. A 2011 study found 1, 2, 3 year survival rates of 60%, 26%, and 14%, respectively. A few survived for longer and one was still alive at age 13. Patients with late-onset Krabbe disease tend to have a slower progression of the disease and live significantly longer.

PME accounts for less than 1% of epilepsy cases at specialist centres. The incidence and prevalence of PME is unknown, but there are considerable geography and ethnic variations amongst the specific genetic disorders. One cause, Unverricht Lundborg Disease, has an incidence of at least 1:20,000 in Finland.

Between 1.3% and 10% of cases are of the adult form. The age at onset is variable (6–62 yr). Two main clinical subtypes have been described: progressive myoclonus epilepsy (type A) and dementia with motor disturbances, such as cerebellar, extrapyramidal signs and dyskinesia (type B). Unlike the other NCLs retinal degeneration is absent. Pathologically the ceroid-lipofuscin accumulates mainly in neurons and contains subunit C of the mitochondrial ATP synthase.

Two independent families have been shown to have mutations in the DNAJC5 gene – one with a transvertion and the other with a deletion mutation. The muations occur in a cysteine-string domain, which is required for membrane targeting/binding, palmitoylation and oligomerization of the encoded protein cysteine-string protein alpha (CSPα). The mutations dramatically decrease the affinity of CSPα for the membrane. A second report has also located this disease to this gene.

As Tay–Sachs disease is a deficiency of β-hexosaminidase A, by getting a substance that increases its activity, people affected will not be deteriorating as fast or not at all. While for infantile Tay–Sachs disease, there is no β-hexosaminidase A so then the treatment would be ineffective. However, for people affected by Late-Onset Tay–Sachs disease, they still have β-hexosaminidase A. The drug Pyrimethamine has been shown to increase activity of β-hexosaminidase A. However, the increased levels of β-hexosaminidase A still fall far short of the desired "10% of normal HEXA", above which the phenotypic symptoms begin to disappear.

Researchers do not fully understand what causes PLS, although it is thought it could be due to a combination of environmental and genetic factors. Studies are being done to evaluate the possible causes, although linking causality can be difficult due to the relatively low number of people who are diagnosed with PLS.

Juvenile PLS may be caused by the ALS2 gene, although this condition is very rare.

Juvenile primary lateral sclerosis (JPLS) ", also known as primary lateral sclerois (PLSJ)," is a rare genetic disorder, with a small number of reported cases, characterized by progressive weakness and stiffness of muscles in the arms, legs, and face. The disorder damages motor neurons, which are specialized nerve cells in the brain and spinal cord that control muscle movement.

Kufor–Rakeb syndrome is an autosomal recessive disorder of juvenile onset also known as Parkinson disease-9 (PARK9).

Symptoms include supranuclear gaze palsy, spasticity, and dementia.

It can be associated with "ATP13A2". It is named after Kufr Rakeb in Irbid, Jordan.

Krabbe disease occurs in about one in 100,000 births. A higher incidence, about six in 1,000, has been reported in certain communities in Israel. Scandinavian countries have comparatively high rates of the disease, reported to be one in 50,000 births.

Juvenile Primary Lateral Sclerosis is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered. Most often, parents of affected individuals each carry one copy of the altered gene, but do not show any signs or symptoms.

Mutations in the ALS2 gene, found on Chromosome 2, are responsible for causing Juvenile Primary Lateral Sclerosis. The ALS2 gene provides instructions for making a protein called alsin. Alsin is abundant in motor neurons, but its function is not fully understood. Mutations in the ALS2 gene in this disorder disrupt the instructions for producing alsin. As a result, alsin is unstable and decays rapidly, or it is disabled and cannot function properly. It is currently unknown how the loss of functional alsin protein causes the death of motor neurons and the symptoms of juvenile primary lateral sclerosis.

Specific types of leukodystrophies include the following with their respective ICD-10 codes when available:

- (E71.3) Adrenomyeloneuropathy

- (E75.2) Alexander disease

- (E75.5) Cerebrotendineous xanthomatosis

- Hereditary CNS demyelinating disease

- (E75.2) Krabbe disease

- (E75.2) Metachromatic leukodystrophy

- (E75.2) Pelizaeus–Merzbacher disease

- (E75.2) Canavan disease

- (G93.49) Leukoencephalopathy with vanishing white matter

- (E71.3) Adrenoleukodystrophy

- (G60.1) Refsum disease