Neurodegenerative disease

A neurodegenerative disease is a blanket term encompassing a wide variety of disorders, typically slowly progressive, with variable gradual neurologic dysfunction. 

Over the years numerous classifications schemas have been described, each adding a layer of confusion for students. The main distinction to be aware of is classifications according to clinical presentation and those relying on underlying pathological processes. As expected earlier descriptions wer primarily of the former, whereas as more and more neurodegenerative disease are understood at a biochemical level, classifications have shifted towards focusing on these changes. 

As such a useful approach is to divide them according to underlying pathological process, although even so there is much overlap and confusion with many clinical patterns being caused by different pathological process and each pathology possibly manifesting in a number of clinical patterns ^1,5. What is worse is that as definitive diagnosis currently necessitates histological diagnosis of brain tissue (either antemortum biopsy or postmortum assessment) the actual diagnosis in most cases is never established, further polluting the published data. 

Dividing neurodegenerative disease by the main pathological process involved results in the following classification:

• synucleinopathies
  • diseases with Lewy bodies
    • Parkinson disease
    • Lewy body disease
  • multiple systemic atrophy (MSA)
    • Shy-Drager syndrome
    • striatonigral degeneration
    • olivopontocerebellar atrophy
• taupathies
  • progressive supranuclear palsy (PSP) 
  • frontotemporal lobar degeneration (FTLD) ³ ~ Pick disease 
    • frontal or behavioural variant ⁴
      • frontotemproal dementia
    • temporal or language variant 
      • semantic dementia (SD)
      • progressive nonfluent aphasia (PNFA)
  • corticobasal degeneration
  • NB : although Alzheimer's disease also has neurofibrillary tangles formed by tau proteins, no gene abnormality in the tau protein gene has been identified ². 
• cerebral amyloidoses
  • Alzheimer disease
    • typical / classical Alzheimer's disease
    • variant (e.g. posterior cortical atrophy)
  • cerebral amyloid angiopathy (CAA)
  • transthyretine-associated cerebral amyloidoses
  • human prion diseases (not always included as neurodegenerative)
  • neuronal intranuclear hyaline inclusion disease (NIHID)
• spinocerebellar ataxias
• Huntington disease
• hereditary spastic paraplegia
• amyotrophic lateral sclerosis (ALS)

It should be noted that any such classification is by the very nature of the heterogeneity of the included conditions, not always felt to be complete. For the purpose of this section diseases that are primarily metabolic in origin, are excluded. See primary metabolic encephalopathies.