Advanced dementia is itself a medically recognised cause of death.

Advanced dementia has been defined as a Cognitive Performance Scale score of 5-6 (roughly equivalent to MMSE 5.1 +/- 5.3). Mitchell et al (2009) described it as a "terminal illness" with a 6-month mortality of 25% and a median survival of 1.3 years - a life expectancy comparable to metastatic breast cancer or stage IV congestive heart failure.
Dementia and/or senility appeared in the top ten causes of deaths in five-year age groups from 80–84 years and upwards (National End of Life Intelligence Network). Of 4.3 million registered deaths in England (2001-2009), 631,000 listed dementia as a contributing factor; dementia was the primary (underlying) cause of death in 39% of these cases.
Not only is it a cause of death in itself, severe dementia is a relevant factor in deciding whether ancillary investigations or treatment for other symptoms would be too distressing or futile to be warranted.

Causes of dementia include:
  • Alzheimer's dementia (about 50% of cases): characterised by cerebral plaques, beta-amyloid peptides and neurofibiliary tangles: death typically occurs within 3-9 years of diagnosis (Querfurth 2010)
  • Cerebrovascular disease (about 15% of cases) - often coincident with Alzheimer's.
  • Lewy Body Dementia
  • Severe Parkinson's Disease
  • Huntington's Disease.

Some conditions can present with features of cognitive impairment that suggest dementia but are reversible to some degree, including:
  • Subdural haemorrhage
  • Brain tumours
  • Depression
  • Delirium associated with systemic illness


References:
  • Alzheimer Society Canada. "Alzheimer's Disease" http://www.alzheimer.ca/en/About-dementia/Alzheimer-s-disease
  • Mitchell SL, Teno JM, Kiely DK et al, "The clinical course of advanced dementia" NEJM 2009; 361: 1529-38 http://www.nejm.org/doi/full/10.1056/NEJMoa0902234#t=article
  • National End of Life Care Intelligence Network, "Deaths from Alzheimer’s disease, dementia and senility" Nov 2010 www.endoflifecare-intelligence.org.uk/view?rid=88
  • Querfurth HW, Laferia FM, "Alzheimer's disease (Review article)" NEJM 2010; 362: 329-44 http://www.nejm.org/doi/full/10.1056/NEJMra0909142