Alzheimer’s disease is not an inevitable part of normal ageing, nor even an acceleration of ageing; it is a true disease. But it is a disease, like heart disease and cancer which is increasingly common as we age. The incidence of Alzheimer’s disease approximately doubles for every 5 years of age over the age of 60, so that about 25% of those over 80 years old have the disease.
The symptoms of Alzheimer’s disease are caused by a loss of nerve cells in certain regions of the brain, principally the cerebral cortex, the part that controls our higher mental functions and which makes us unique as humans. The degeneration of these nerve cells leads to a loss of millions of the connections (synapses) between nerve cells; it is the loss of connections in the part of the brain dealing with memory (medial temporal lobe; see diagram) that causes the first symptoms.
The disease progresses and spreads throughout the cerebral cortex, gradually affecting those parts of the cortex that deal with almost all our other higher cognitive functions and our behaviour. Certain parts of the cortex are spared in the early stages: for example, the processing of vision and the control of normal movement are not affected until the later stages.