In 1769, the Swiss naturalist and philosopher Charles Bonnet noticed that his nearly-blind grandfather reported vivid hallucinations of things which he knew to be non-existent. As his own sight worsened with age, Bonnet began to experience similar visions: birds, men and women, strange plants, and architectural patterns came and went. As he was the first person to describe this phenomenon, it was named after him: Charles Bonnet Syndrome.
The syndrome did not enter the English medical literature until 1982. Most physicians and nurses had never heard of it, and there was little literature about it even in non-English journals. Millions of people had seriously-impaired vision without experiencing CBS, so clearly it must be a very rare and unusual condition. If elderly people reported seeing things, it was considered to be overwhelmingly likely to be a sign of dementia or psychosis.
Until doctors actually sat down and questioned patients as to whether they had seen peculiar things, with no suggestions that they were insane or demented. Then it was recognized that more than half of all people with impaired vision, particularly those whose loss of sight occurred suddenly, experienced CBS.
Most of them were afraid they’d be considered mad if they mentioned what they saw to people, only about a third revealed the hallucinations to anyone, roughly a third lived in terror that they were losing their minds, and the vast majority did not tell their physicians. Unless actively encouraged to be honest, and given assurances that seeing things wasn’t a sign of insanity, many people simply denied their experiences if asked.
People who had strokes, were developing Alzheimer’s, and so on were indeed more likely to report having visions. Not necessarily because the visions resulted from neurological impairment, but because they were less likely to possess the contextual awareness of others and their expectations and repress mention of them.
This is a superb example of how important it is to always keep in mind the distinction between reality and our knowledge of it. Absence of evidence is not evidence of absence. Relying upon our collective knowledge base is often useful, but it prevents us from checking that base against reality and correcting it when wrong.
If you assumed that CBS must be rare and unusual, because there was no recognition of it in the literature and doctors knew nothing about it, you’d have make the implicit assumption that physicians’ knowledge was valid and complete. In reality, it wouldn’t be – not through any fault of the doctors, mind you. But you’d be ruling out in advance the possibility that the system itself was lacking, and so wouldn’t have been able to notice the problem and correct it.
Only once physicians actively considered the possibility that they were missing something, and sought out evidence from the patients themselves instead of what the profession thought it knew, was the gap between understanding and reality closed.