here is what Terrty Pratchett has....courtesy Alzheimers UK...

Posterior cortical atrophy (PCA), also known as Benson's syndrome, is a progressive degenerative condition where damage to brain cells is particularly focused at the back (posterior) of the brain, the region responsible for visual processing. This damage to brain cells leads to cell death and loss of volume or 'atrophy' of the posterior cortical regions − hence the name. In the vast majority of cases the underlying cause is Alzheimer's disease.

The death of brain cells is accompanied by accumulation of toxic proteins, amyloid and tau, that form plaques and tangles in the same way as is seen in typical Alzheimer's disease. PCA may therefore be referred to as an atypical variant of Alzheimer's disease.

Although PCA is caused by Alzheimer's disease, it can also be due to other diseases including dementia with Lewy bodies and Creutzfeld Jacob disease. PCA is a rare condition, thought to affect less than five per cent of people with Alzheimer's disease, although epidemiological studies are lacking and PCA has been under-recognised in the past.