“Dementia is a chronic decline in cognitive function that causes impairment relative to a person’s previous level of social and occupational functioning”. (G.K. Gouras, in Reference Module in Biomedical Sciences, 2014).
Dementia affects memory, making it difficult to remember who family members are, where they are and what everyday objects are called. It can also make people behave in a way that they’ve never done before, escalating emotions of anger, sadness and confusion. Ultimately it leads to the loss of ability to live and function independently. Dementia can affect people of all different ages, but it usually affects the elderly. As the global population ages and with no cures currently available, it is becoming an epidemic. According to the Alzheimer’s Society, there are around 850,000 people in the UK with dementia. One in 14 people over 65 develop dementia, and this becomes 1 in 6 for people over 80. There are multiple causes of dementia, and it is common for people to have mixed dementia where they suffer from a combination of different types of dementia.
The most common cause of dementia is Alzheimer’s disease. In Alzheimer’s disease, the transmission of information via chemical and electrical signalling between neurones in the brain is disrupted leading to the death of neurones, affecting the processes of communication, metabolism, and repair. Alzheimer’s disease typically starts in the entorhinal cortex and hippocampus, parts of the brain associated with memory. It then progresses to kill neurones in the cerebral cortex which is responsible for language, reasoning, and social behaviour. Alzheimer’s disease is thought to be triggered by the accumulation of proteins in the brain, particularly of amyloid-β peptide. Overproduction and failure of clearance mechanisms of amyloid-β peptide leads to the formation of amyloid oligomers and plaques that collect in the brain’s parenchyma and blood vessels. This blocks synapse signalling by affecting proteasome function, inhibiting mitochondrial activity, altering intracellular Ca2+ levels and stimulating inflammation. A build-up of amyloid-β peptide also interacts with the signalling pathways that regulate the phosphorylation of the protein tau, causing hyperphosphorylation. Tau usually binds to and stabilizes microtubules, however the hyperphosphorylation of tau which causes it to detach from microtubules and stick to other tau molecules, forming threads that join to form neurofibrillary tangles. These tangles block the neuron’s transport system, which harms the synaptic communication between neurons.
Another type of dementia is Lewy body dementia. This disease is caused by deposits of the protein alpha-synuclein, called lewy bodies, inside neurones causing them to work less efficiently and eventually die. Other types of dementia include frontotemporal disorders, and vascular dementia.