Aetiology

Many causes of dementia are irreversible, so the diagnostic emphasis is placed on detecting the treatable subgroup of patients. There are three common causes of dementia:

• Alzheimer's disease (AD) - the cause of most cases of dementia, accounting for about 60% of all cases (1)
  • is a degenerative cerebral disease, with insidious onset, which is characterised by a slow progressive decline in cognition and ability to function
• vascular dementia (VaD) and dementia with Lewy bodies (DLB) are responsible for most other cases of dementia (15 to 20% of cases in each) (1)
  • vascular dementia usually arises from multiple infarcts or generalised small vessel disease - has a more sudden onset than Alzheimer's disease.
  • DLB is slowly progressive - DLB shares many of the features of Alzheimer's disease and Parkinson's disease

Mixed cases (e.g. Alzheimer's disease and vascular dementia or Alzheimer's disease and dementia with Lewy bodies) are recognized increasingly, especially in older people (1).

In young onset dementia, frontotemporal dementia (FTD) is an important cause (after Alzheimer’s disease) (1).

Other causes of dementia include (accounts for less than 5%):

• other degenerative diseases - Huntington’s disease
• prion diseases - Creutzfeldt-Jakob Disease
• reversible causes
  • psychiatric disorders - ‘pseudodementia’ of depression
  • space-occupying lesions
  • toxic and metabolic disorders - alcohol-related dementia, vitamin B12 or folate deficiency
  • endocrine abnormalities – hypothyroidism (1)

Notes:

• smoking as a risk factor for dementia (1)
  • elderly smokers have increased risks of dementia and cognitive decline
• Type 2 diabetes (T2D) has been associated with a modest increased risk in cognitive dysfunction across all cognitive domains
  • effect appears to be consistent across all age groups and mimics an accelerated ageing of brain function - however there is also an increased risk of more severe impairment of cognition and developing dementia in older age groups that would appear to be a different phenomenon (3)

Reference:

• (1) National Collaborating center for mental health 2007: Dementia: A NICE-SCIE guideline on supporting people with dementia and their carers in health and social care
• (2) Anstey KJ et al. Smoking as a risk factor for dementia and cognitive decline: a meta-analysis of prospective studies.Am J Epidemiol. 2007 Aug 15;166(4):367-78
• (3) Pal K et al. Mild cognitive impairment and progression to dementia in people with diabetes, prediabetes and metabolic syndrome: a systematic review and meta-nalysis. Social Psychiatry and Psychiatric Epidemiology 2018;53:1149–1160.